Computations in Science Seminars

Previous Talks: 2009

Jan 2009
7
Wed 12:30
OPEN *
Jan 2009
14
Wed 12:30
Marcelo Magnasco, Rockefeller University
e-mail:
Host: Leo Kadanoff () *
Organizer: Arnab *
Dynamical and Statistical Criticality in a Model of Neural Tissue

For the nervous system to work at all, a delicate balance of excitation and inhibition must be achieved; too much excitation, and the brain becomes "epileptic"; too little excitation and it goes "flatline". However, when such a balance is sought by global strategies, only few modes remain balanced close to instability, and all other modes are strongly stable. Here we present a simple model of neural tissue in which this balance is sought locally by neurons following `anti-Hebbian' behavior, namely, the strength of synaptic connections between neurons is decreased whenever the activities of the two neurons are correlated. Under this dynamics all degrees of freedom achieve a close balance of excitation and inhibition and the system becomes become "critical" in the dynamical sense, i.e., it has an extensive number of marginal degrees of freedom. At long timescales, the modes of our model oscillate around the instability line, so an extremely complex "breakout" dynamics ensues in which different modes of the system oscillate between prominence and extinction. We show the system develops various anomalous statistical behaviours and hence becomes self-organized critical in the statistical sense.

Jan 2009
21
Wed 12:30
Margaret Gardel, University of Chicago
e-mail:
Regulation of Cellular Traction Stresses by Myosin-II Mediated Assembly of Contractile F-actin Bundles

The ability of adherent cells to regulate traction forces on their extracellular matrix (ECM) is fundamental to tissue morphogenesis and directed cell migration. To a large degree, cellular traction forces are regulated by myosin-II generated cell contractility, which promotes the development of contractile F-actin bundles, the growth of mechanosensitive focal adhesions and large traction stresses exerted on the ECM. In the presence of the myosin-II ATPase inhibitor, blebbistatin, F-actin bundles dissociate, focal adhesions disassemble and traction stress exerted on the ECM is diminished. To address how myosin-II ATPase activity drives the organization of the F-actin cytoskeleton into structures capable of efficient force transmission, we utilized a combination of high resolution confocal microscopy and traction force microscopy. We found that myosin-II-mediated contractility induced an exponential recovery in traction forces with a half time of approximately 1.5 minutes. The force recovery correlated with rapid lengthening of focal adhesions and their associated stress fibers, but was inversely proportional to retrograde F-actin flow speed, which decreased from 4.5 to 0.25 um/min. These relationships indicate that, as focal adhesions assemble, F-actin rapidly reorganizes into compact bundles at timescales consistent with increase in force at adhesion sites. Once focal adhesions and F-actin bundles reached 2-3 and 3-5 um in length, respectively, changes in F-actin flow and traction forces occurred at much slower rates. We propose that enzymatic activity of myosin II promotes rapid reorganization of the F-actin cytoskeleton until myosin-II mediated tensile stresses within the cytoskeleton are balanced with traction stresses exerted on the ECM.

Jan 2009
28
Wed 12:30
Aaron Dinner, University of Chicago
e-mail:
Enhanced sampling algorithms for systems far from equilibrium

Many systems of significant fundamental and applied interest are microscopically irreversible. For theoretical studies of such systems, the steady-state distribution is of central importance because it enables calculation of static averages of observables for comparison to experimental measurements. For systems at equilibrium, low probability states can be explored efficiently in simulations with umbrella sampling methods, in which biasing potentials that are functions of one or more order parameters are used to enhance sampling of selected regions of phase space. What complicates extending umbrella sampling to simulations of non-equilibrium processes is that, by definition, they do not obey detailed balance (microscopic reversibility). As such, one must account for the fact that the steady-state probability of observing particular values of the order parameters can be determined by a balance of flows in phase space through different possible transitions. In this talk, I will describe an algorithm for enforcing equal sampling of different regions of phase space in an ergodic system arbitrarily far from equilibrium, which enables its steady-state probability distribution to be determined with high accuracy. Applications and extensions of the method will be discussed.

Feb 2009
4
Wed 12:30
Douglas MacAyeal, University of Chicago
e-mail:
Organizer: Kostya
The Glaciological Mosh Pit: glacial tsunamigenesis, iceberg calving and the flow of ice melange in an era of collapsing glaciers and ice shelves

The 28-29 February, 2008, break-up of a part of the Wilkins Ice Shelf, Antarctica, (see http://mrsec.uchicago.edu/Comp_in_Sci/macayeal.mov) exemplifies the now-familiar pattern of "explosive" ice-shelf break-up thought to be a consequence of environmental warming in the Antarctic Peninsula region. Key aspects of this "explosive pattern" to be explained by theories of ice-shelf dynamics include:

The abrupt, near simultaneous onset of iceberg calving across a large-scale stretch of ice front

High outward drift velocity (~0.3 m/s) of a leading "phalanx" of tabular icebergs that formerly comprised the seaward edge of the ice shelf prior to the break-up

Efficient "surface coverage" of the ocean surface between the intact ice shelf and the leading phalanx of tabular icebergs by small icebergs, capsized icebergs and dismembered tabular icebergs (small irregular pieces)

Extremely large gravitational potential energy conversion rates, e.g., up to 3 x 1010 W, by the "inverted submarine landslide" process over short periods of time (e.g., hours to days) in the absence of significant ice deformation

The apparent lack of proximal iceberg-calving triggers (e.g., strong atmospheric storms in the local environment) at the time of break-up onset

The capacity for enabling conditions (e.g., presence of melt-water induced "hydrostatic fracture") to make possible multiple break-up events (e.g., a second pulse of the Wilkins Ice Shelf break-up in late May, 2008) distributed through various seasons.

I examine the basic dynamic features of ice-shelf/ocean-wave interaction (including ice-shelf sources of ocean waves) that may explain the above aspects of the explosive ice-shelf disintegration process. Ice-shelf generated surface-gravity waves and flexural-gravity waves associated with initial calving at an arbitrary "seed location" produce stress perturbations capable of stimulating calving on the entire ice front. Waves generated by parting detachment rifts, iceberg capsize and break-up stimulate the "inverted submarine landslide" process, whereby ice shelf of thickness Hi is converted to a field of iceberg rubble of thickness Hf

Feb 2009
11
Wed 12:30
Daniel Rothman, Massachusetts Institute of Technology
e-mail:
Host: Leo Kadanoff ()
Organizer: Kostya
Growth of a Dendritic Channel Network

Dendritic channel networks are a ubiquitous feature of Earth's topography. A half century of work has detailed their scale-invariant geometry. But relatively little is known about how such networks grow, especially in natural settings at geologic time scales.

This talk addresses the growth of a particularly simple class of channel networks: those which drain groundwater. We focus on a pristine field site in the Florida Panhandle, in which channels extending for kilometers have been incised vertically through tens of meters of ancient beach sands.

We first present new field observations showing how the flow of subsurface water interacts with the geometry of the network. We then show that the growth of groundwater-driven networks is described by two linear response laws. Remarkably, one of these growth laws is reversible, which allows us to reconstruct network history and estimate network age. A particularly striking feature of the Florida network is the existence of a characteristic length scale between channels. Our theory predicts how this length scale evolves, thereby linking network growth to geometric form.

Feb 2009
18
Wed 12:30
Susan Coppersmith, University of Wisconsin
e-mail:
Host: Leo Kadanoff ()
Organizer: Kostya
Understanding the Microarchitecture of Mother-of-Pearl

Nacre, or mother-of-pearl, is a layered biomineral composite. It is widely studied because of its self-assembled, efficient and accurately ordered architecture, its toughness, and its fascinating and poorly understood formation mechanisms.

The aragonite crystal tablets in nacre orient so that their c-axes are aligned perpendicular to the layers. We will investigate this orientational ordering and argue that it is established dynamically, via regulation of the kinetics of crystal nucleation and growth. An analytically solvable model yields substantial insight into the ordering process.

Feb 2009
25
Wed 12:30
Mahesh Bandi, Los Alamos National Lab
e-mail:
Host: Heinrich Jaeger
Organizer: Eric
Power fluctuations in turbulence

We study the local power fluctuations in numerical simulations of stationary, homogeneous, isotropic turbulence in two and three dimensions with Gaussian forcing. Due to near Gaussianity of the one-point velocity distribution, the probability distribution (PDF) of local power is well modeled by the PDF of the product of two joint normally distributed variables. In appropriate units, this distribution is parameterized only by the mean dissipation rate. The large deviation function for the power PDF is calculated exactly and shown to satisfy a Fluctuation Relation (FR) with a rate constant that depends upon the mean dissipation rate. However, this FR is entirely statistical in origin. I will provide a comparison of this model not only with our numerical data but also with related experiments wherever possible. Time permitting, I will also discuss some directions we are keen to explore in future.

Mar 2009
4
Wed 12:30
Michael Dickinson, California Institute of Technology
e-mail:
Host: Leo Kadanoff () *
Organizer: Arnab *
How Flies Find Stuff

The research in my laboratory focuses on the sensory ethology of fruit flies, treating these tiny insects not simply as convenient laboratory models, but as real animals that have evolved a successful life history pattern. The goal of this work is to try to deconstruct the animal's behavior into a sequence of sensory-motor modules. My talk will focus on several visually-mediated components of behavior including take-off, navigation, predator avoidance, landing, and local exploration, as well as components of social behavior that ensue whenever two or more flies alight on the same piece of rotting fruit

Mar 2009
11
Wed 12:30
Kai Huang, Max Planck Institute for Dynamics and Self Organization, Goettingen, Germany
e-mail:
Host: Heinrich Jaeger † ‡
Organizer: Eric † ‡
Dynamics of Wet Granular Matter
(cancelled)

Adding a certain amount of water to a pile of sand increases its mechanical stability dramatically, leading to a material stiff enough for sculpturing sand castles. This is due to the liquid bridges formed between adjacent particles which introduce cohesion into the system. In this talk, I will present our recent investigations on how the cohesive force influences dynamic behaviors of granular matter. First, I will demonstrate with liquid helium-4, a liquid with a surface tension only 1/200 that of pure water, that the increase of mechanical stability by wetting is prominent. With the increase of helium added, the system undergoes dry, asperity wetting and complete wetting regimes. Remarkable difference between superfluid and normal fluid wetting is found in the asperity wetting regime, which we attribute to the enhancement of liquid bridges by the superflow of unsaturated helium film.

Second, I will present the phase diagram of wet granular matter under vertical vibrations focusing on the coexistence(C) regime of fluid(F) and gas(G) phases. Scaling of F-C transition line, dynamics of 'gas bubbles', interfacial tension between F-G phase boundary will be discussed. We also demonstrate that S-F transition is a surface melting process utilizing ruby fluorescence. In addition, a newly built microwave radar setup for particle tracing in 3D will be introduced briefly.

Third, aggregation of two dimensional wet granular matter under shear flow will be discussed. In this experiment, particles floating on a viscous liquid are wetted by an oil film, which gives rise to a hysteretic and short ranged capillary force between neighboring particles. The balance between the capillary force and the viscous drag forces determines the average size of clusters. The cluster size distribution, scaling between average cluster size and shear rate, and fractal dimensions of the aggregates will be presented and compared with simulations.

Mar 2009
12
Thu 12:30
Eran Sharon, Hebrew University of Jerusalem
e-mail:
Host: Wendy Zhang ()
Organizer: Arnab
Shaping via active deformation of elastic sheets
(Special Date: GCIS E123, 12:30)

Many natural structures are made of soft tissue that undergoes complicated continuous shape transformations as a result of the distribution of local /active/ deformation of its "elements". Currently, the ability to mimic this shaping mode in man made structures is poor.

I will present some results of our study of actively deforming thin sheets.

Theoretically, we have formulated an elastic theory for such bodies and derived from it an approximate 2D plate theory for plates with intrinsic non-Euclidean metric.

Experimentally, we use environmentally responsive gel sheets that adopt prescribed metrics upon induction by environmental conditions. With this system we study the shaping mechanism and energy scaling in different cases of imposed metrics.

Finally, we measure growth of wild types and mutants leaves, attempting to link between their local growth tensors and the different evolution of their global shape.

Mar 2009
13
Fri 12:30
Efi Efrati, Hebrew University of Jerusalem
e-mail:
Host: Wendy Zhang ()
Organizer: Kostya
Non-Euclidean plates
(Special Date: KPTC 213, 12:15. Joint seminar with MRSEC)

Naturally growing tissue as well as environmentally responsive material may form thin elastic structures which are neither plates nor shells, and need not have a stress free configuration.

In this lecture I will discuss the elastic theory of non-Euclidean bodies and derive from it a reduced model for describing thin non Euclidean plates valid for large displacements but small strains, and arbitrary intrinsic geometry. I will describe some of the mathematical properties of the reduced model, such as the buckling transition, the convergence to an isometric immersion in the zero-thickness limit, and the appearance of a boundary layer along the free boundary at small, yet finite thickness. Finally, I will discuss the relevance of the theoretical framework to biological systems, and review some surprising solutions for specific elastic non-Euclidean plates such as helical strips of no intrinsic chirality

Mar 2009
18
Wed 12:30
(During APS March Meeting)
Apr 2009
1
Wed 12:30
Erel Levine, University of California, San Diego
e-mail:
Host: David Biron ()
Organizer: Arnab
The instant messenger: gene regulation in and out of equilibrium

Messenger RNAs are unstable intermediates in the process of gene expression, namely the synthesis of proteins in a way that reflect cellular conditions. In recent years it became evident that regulation of gene expression occurs not only by regulating synthesis of the messenger, but also by regulating its properties. Taking a quantitative approach we suggest a unifying view of RNA-mediated regulation, highlighting the effect of processes in and out of equilibrium.

Apr 2009
8
Wed 12:30
Jonathan Widom, Northwestern University
e-mail:
Host: Wendy Zhang ()
Organizer: Kostya
The genomic code for nucleosome positioning

Eukaryotic genomes are packaged into nucleosome particles, in which a short stretch of the genomic DNA (~150 base pairs, 50 nm) is tightly wrapped around a tiny (~35 nm radius) spool-shaped protein core. Consecutive nucleosomes are separated by short stretches of unwrapped DNA. Nucleosomes are remarkable from a physical perspective because, in each nucleosome, one persistence length of DNA -- a lengthscale of DNA inflexibility -- is wrapped in nearly two full turns around the protein spool. Nucleosomes are important in biology, because they strongly occlude their wrapped DNA, thus the detailed locations of nucleosomes along the genome strongly impacts the ability of DNA binding proteins to access particular binding sites.

We have discovered that genomes care where their nucleosomes are located, and that genomes manifest this care by encoding an additional layer of genetic information, superimposed (multiplexed) directly on top of other kinds of regulatory and coding information that were previously recognized. The physical basis of the nucleosome DNA sequences preferences lies in the sequence-dependent mechanics of DNA itself. We have developed two different approaches to read this nucleosome positioning code and predict the in vivo locations of nucleosomes. One approach utilizes a knowledge-based mesoscopic model for the sequence-dependent mechanics of DNA, from which we compute an effective elastic free energy cost for wrapping different DNA sequences into a nucleosome. The other approach develops a statistical profile of the nucleosome sequence preferences; we take the log of the likelihood assigned by this profile to a given DNA sequence as an apparent negative free energy for nucleosome formation. We then exactly solve the 1-dimensional equilibrium distribution problem for placing self-avoiding nucleosomes onto the free energy landscape. Our statistical model, trained on nucleosome DNAs isolated in a purified system in vitro reconstitution experiment, is strongly predictive of nucleosome occupancy patterns in living yeast, C. elegans and human cells. Our mechanics model explains key features of the real nucleosome DNA sequence preferences. Our findings suggest that genomes utilize the nucleosome positioning code to facilitate specific chromosome functions and to define the next higher level of chromosome structure itself.

Apr 2009
15
Wed 12:30
Rafael Jaramillo, University of Chicago
e-mail:
Strong Coupling and Quantum Criticality in a Canonical Density Wave System

We present high resolution x-ray diffraction measurements on antiferromagnetic Chromium, which is known to exhibit strong coupling phenomena despite the familiar weak-coupling density wave ground state. We directly measure the spin- and charge-density wave order parameters as the magnetism is suppressed with applied pressure using diamond anvil cell techniques at the Advanced Photon Source. In the low pressure regime, the BCS-like ground state is manifest in the decades-long exponential suppression of the order parameters, while at high pressure this weak-coupling behavior is cut off by a quantum phase transition. We identify quantum confinement as the physics driving the exponential suppression of the antiferromagnetic phase. We find that Cr is unique among stoichiometric itinerant magnets studied to date insofar as the quantum phase transition is continuous in nature, allowing experimental access to the naked quantum singularity where the competition between the destabilized density wave and more strongly coupled ground states is most intense. Our results inform the growing body of evidence that weak- and strong-coupling phenomena indeed coexist in many solid state systems, such as continuously tuneable charge density waves in manganites, unconventional superconductivity in spin density wave Bechgaard salts, and charge density wave characteristics of the checkerboard order in the high-TC cuprates.

Apr 2009
22
Wed 12:30
OPEN † ‡
Apr 2009
29
Wed 12:30
Peko Hosoi, Massachusetts Insitute of Technology
e-mail:
Host: Heinrich Jaeger
Organizer: Eric
Optimizing Locomotion: From Biology to Robotics

In this talk I will discuss various projects in which we take inspiration from nature to advance technology. The key idea behind these bio-inspired design projects is the belief that, thanks to natural selection, if a structure exists in nature that performs a desired function, it is tough for engineers to dramatically improve upon the natural design. Yet, historically, the countless failures in biomimetics have been more notorious than its successes (e.g. airplanes with flapping wings). There are many reasons for these failures -- impractical energy requirements and complexity of controls, among others. To avoid these pitfalls, our biomimetic studies focus on simple biological systems (preferably organisms with primitive or, better yet, non-existant central nervous systems) in which the energy requirements are low and the biological solutions to challenging questions are grounded in mechanics rather than in neurological controls.

May 2009
6
Wed 12:30
V. Ramanathan, Scripps Institution of Oceanography, University of California, San Diego
e-mail:
Host: Ka Yee Lee
Organizer: Ali
The melting of the Hindu Kush-Himalayan-Tibetan Glaciers
(cancelled)

We have moved into an era when: Climate Change is accepted as real; also accepted is the fact that human activities are the major drivers for many of the changes. One of the major, if not the major, changes predicted for this century is the near disappearance of the Hindu Kush-Himalayan-Tibetan (HKHT) glaciers. The glaciers and snow packs in the HKHT region provide the head waters for most major river systems of S. E and SE Asia and thus are vital for the food and water security of the region. Recent studies employing unmanned aircraft and satellites are yielding new insights into how air pollution and greenhouse warming are contributing to the rapid retreat of these glaciers and snow packs.

May 2009
13
Wed 12:30
Salvatore Torquato, Princeton University
e-mail:
Host: Leo Kadanoff ()
Organizer: Eric
Unusual Classical Ground States of Matter

Ground-state problems naturally arise in many fields, including physics, biology, materials science, and mathematics. A classical ground-state configuration of a system of interacting particles is one that minimizes the system potential energy. In the laboratory, for example, such states can be produced by slowly cooling a liquid to a temperature of absolute zero, and usually the ground states are crystal structures. However, our theoretical understanding of ground states is far from complete. For example, it is difficult to prove what are the ground states for realistic interactions. I discuss recent theoretical/computational methods that we have formulated to identify unusual crystal ground states as well as disordered ground states [1,2,3,4]. Although the latter possibility is counterintuitive, there is no fundamental reason why classical ground states cannot be aperiodic or disordered.

1) M. Rechtsman, F. H. Stillinger and S. Torquato, Synthetic Diamond and Wurtzite Structures Self-Assemble with Isotropic Pair Interactions , Physical Review E, vol. 75, 031403 (2007).

2) S. Torquato and F. H. Stillinger, "New Duality Relations for Classical Ground States," Physical Review Letters, vol. 100, 020602 (2008).

3) R. D. Batten, F. H. Stillinger and S. Torquato, "Classical Disordered Ground States: Super-Ideal Gases, and Stealth and Equi-Luminous Materials," Journal of Applied Physics, vol. 104, 033504, (2008).

4) S. Torquato and F. H. Stillinger, "New Conjectural Lower Bounds on the Optimal Density of Sphere Packings," Experimental Mathematics, vol. 15, 307 (2006).

May 2009
15
Fri 12:30
Geosci seminars of special interest

,

Hinds 101, 1:30

Adam Sobel, Columbia University

The influence of condensate evaporation on water vapor and its stable isotopes in a global climate model

Hinds 101, 3:00

Victor C. Tsai, Harvard University

Turbulent Hydraulic Fracture Applied to Water Drainage from a Melting Greenland Ice Sheet

May 2009
20
Wed 12:30
Gary Weisel, Penn State Altoona
e-mail:
Host: Leo Kadanoff ()
Organizer: Arnab
Basic Plasma Physics in Postwar America

This talk reviews the changing conceptions of basic physics that the plasma-physics community put forward in postwar America. I give special attention to the tense relationship between fusion research and the more general study of plasmas in astrophysics, space science, and industry. Although fusion research often led to results that were regarded as basic plasma physics, its dominating influence tended to weaken other plasma work. I show that the conceptions of basic physics put forward by the plasma community were not highly regarded or easily understood by science administrators and the general physics community. This difficulty had at least two aspects. First, fusion research so dominated the public conversation about plasma physics that other work on basic properties and phenomena of plasmas did not gain significant attention. Second, none of the conceptions of basic plasma physics (fusion or otherwise) ever gained the prestige of what I call the Big Questions conception of basic physics, the best example of which is the reductionist program of high energy physics. In contrast to the particle physicist's conception of unity in physics, plasma physicists sought to describe the varied behavior of plasmas with the common theoretical machinery of statistical mechanics and to link plasma behaviors on different scales, in the laboratory and in space.

May 2009
27
Wed 12:30
Sheila Patek, University of California, Berkeley
e-mail:
Host: David Biron ()
Organizer: Ali
Airborne ants and smashing shrimp: by-product and natural selection in the evolution of movement

Extremely fast biological movements often inspire notions of "pinnacles of evolution". However, by taking a look at the basic underlying physics of these movements, an alternative perspective emerges. Could some of these phenomena emerge as by-products of another behavior? Do these "accidents" provide the substrate for evolutionary change? In this talk, I will present two remarkable phenomena -- jaw-jumping in ants and cavitation-generating snail-smashing in mantis shrimp -- and probe the roles of by-product and natural selection in the evolution of fast animal movements.

Jun 2009
3
Wed 12:30
Arup Chakraborty, Massachusetts Institute of Technology
e-mail:
Host: Leo Kadanoff ()
Organizer: Eric
A statistical mechanical model for how "terminally" differentiated cells can be reprogrammed to an embryonic stem cell-like state

Cellular states are plastic, and even terminally differentiated cells can be reprogrammed to a pluripotent state (i.e., an embryonic stem cell-like state) by ectopic expression of a few transcription factors. These recent experimental results have raised the possibility of creating patient-specific stem cells for regenerative medicine without the need for embryonic material. One of many challenges that must be confronted to make this exciting possibility a reality is that reprogramming efficiencies are very low. An understanding of the mechanisms that make reprogramming rare, and even possible, can help design more efficient strategies for creating pluripotent cells. The development of such mechanistic principles will also shed light on how cellular identity is maintained and transformed - a basic unresolved issues in biology. I will describe theoretical work that takes a small step toward elucidation of such principles. A phenomenological model for the architecture of the epigenetic and genetic regulatory networks which describes diverse data obtained during reprogramming experiments will be presented. It is a Potts model (with short and long-ranged interactions) coupled to an Ising model (with short-range interactions). Importantly, our studies identify the rare temporal pathways that result in induced pluripotent cells. Further experimental tests of predictions emerging from our model should lead to fundamental advances, or negation of the model.

Jun 2009
10
Wed 12:30
Dirk Morr, University of Illinois at Chicago
e-mail:
Host: Leo Kadanoff ()
Organizer: Arnab
Transport Phenomena in Nanoscopic Systems: Interplay between Confinement, Interactions, and Disorder

Over the last few years, nanoscopic systems, such as quantum corrals, Kondo droplets, or arrays of quantum dots, have provided a plethora of exciting novel phenomena arising from the interplay of quantum confinement, interactions and disorder. In this talk, I will present some recent theoretical results on non-equilibrium transport phenomena in arrays of quantum dots. In particular, I will show that the absence of self-averaging in nanoscopic systems yields scaling laws that are qualitatively different from those of mesoscopic or macroscopic systems. In addition, quantum confinement, giving rise to the presence of well-defined eigenmodes, can lead to interesting spatial current paths, including the possibility for current backflow. Finally, I will demonstrate how interactions can increase the current flow, suppress the effects of disorder, and neutralize the results of quantum confinement.

Jun 2009
17
Wed 12:30
Predrag Cvitanovic, Georgia Institute of Technology
e-mail:
Host: Wendy Zhang ()
Organizer: Jin
Turbulence, and what to do about it?

In the world of moderate Reynolds number, everyday turbulence of fluids flowing across planes and down pipes a velvet revolution is taking place.Experiments are almost as detailed as the numerical simulations, DNS is yielding exact numerical solutions that one dared not dream about a decade ago, and dynamical systems visualization of turbulent fluid's state space geometry is unexpectedly elegant. We shall take you on a tour of this newly breached, hitherto inaccessible territory. Mastery of fluid mechanics is no prerequisite, and perhaps a hindrance: the talk is aimed at anyone who had ever wondered why - if no cloud is ever seen twice - we know a cloud when we see one? And how do we turn that into mathematics?

Jun 2009
24
Wed 12:30
James Duncan, University of Maryland
e-mail:
Host: Wendy Zhang () † ‡
Organizer: Eric † ‡
Nonlinear Gravity-Capillary Wave Patterns Generated by a Slow-Moving Pressure Source.

Nonlinear wave patterns generated by an isolated steady pressure distribution moving across a water surface at speeds (Uc) below the minimum phase speed (Cmin) for linear gravity-capillary waves are explored experimentally. The pressure distribution is created by forcing air toward the water surface through a vertically oriented tube (internal diameter of 3 mm) that is mounted on a carriage that travels over the water surface. It is found that the wave patterns created are sensitive to both the air-flow rate and Uc. Several distinct wave patterns are found. At low values of Uc (< 0.83Cmin), the pattern is an isolated single depression that is centered on the pressure disturbance. The maximum depth of the depression increases slowly with carriage speed and the width of the depression is greater than its length in the flow direction. At a critical value of Uc , which depends on the air-flow rate, the pattern abruptly transitions to a second stable state in which the largest depression of the surface occurs behind the pressure disturbance. The maximum depth of the surface depression increases by a factor of about two over this transition and the following surface pattern looks very much like a freely propagating gravity-capillary lump. In the narrow transition zone between these states, the wave pattern is unsteady. As Uc is increased further, the depth of the depression decreases. The values of the maximum depth of the depression plotted versus Uc fall on a single curve in this region, independent of air flow rate. At still higher carriage speeds, Uc > 0.96Cmin, a third state is found with a V-shaped surface pattern that is similar to the pattern found for cases with Uc a little greater than Cmin. This state appears to be unsteady.

This work is being carried out in collaboration with T. Akylas and his research group at MIT who are performing numerical simulations of the flow.

Jul 2009
1
Wed 12:30
OPEN † *
Jul 2009
8
Wed 12:30
James Martin, Sandia National Laboratories
e-mail:
Organizer: Jin *
Formation of an advection lattice with biaxial magnetic fields: A powerful new method of heat and mass transfer

In recent years we have become interested in the dynamics of magnetic particle suspensions subjected to biaxial and triaxial ac magnetic fields. For suspensions of spherical particles we have convinced ourselves (through theory, simulation and experiment) that we understand what is going on, including some pretty weird stuff. We have now started to study suspensions of magnetic platelets and have observed, under some very particular field conditions, the formation of extraordinary flow patterns and surface instabilities. We have no idea of what is going on, but we have made a lot of interesting videos that are fun to puzzle over.

Biaxial and triaxial ac magnetic fields are spatially uniform, time-dependent fields whose direction explores either two or three dimensions, with typical field component frequencies in the range of 100 - 1000 Hz and magnitudes from 0.005 - 0.05 T. In spherical particles suspensions a biaxial field creates negative dipolar interactions that lead to particle sheets, which are static structures of little interest. All of the really interesting phenomena -- fluid mixing [1], molecular-like particle clusterings [2], the formation of composites with exotic structures [3] -- occur in triaxial fields, which create complex many-body interactions.

We have discovered that magnetic platelets behave very differently, probably because of their tendency to rotate so as to confine the field vector to their plane. The resultant particle fluttering apparently creates complex flow because of hydrodynamic coupling. In the simplest case the flow can be described as a diagonal square lattice (relative to the directions of the field components) of "antiferromagnetic" flow cells, with the flow in each cell normal to the field plane. More complex flows can be stimulated, including helical flows, and these depend critically on both the frequency ratios of the field components and their phase relation. Understanding these flows is probably going to be a computational challenge of the first order, but their utility is clear: we can magnetically transport heat and mass at a terrific rate without a thermal gradient or gravity, and without pumps, hoses, connectors, seals or any moving parts. This has significant implications for cooling in space, or in any other circumstance where convection does not occur.

[1] J. E. Martin, Theory of strong intrinsic mixing of particle suspensions in vortex magnetic fields, Phys. Rev. E 79, 011503 (2009).

[2] J. E. Martin, E. Venturini, G. L. Gulley,* and J. Williamson, Using triaxial magnetic fields to create high susceptibility particle composites, Phys. Rev. E 69, 021508 (2004).

[3] J. E. Martin, R. A. Anderson, and R. L. Williamson, Generating strange magnetic and dielectric interactions: Classical molecules and particle foams, J. Chem. Phys. 118, (2003).

*Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the UnitedStates Department of Energy under Contract No. DE-AC04-94AL85000. This work was supported by the Office of Basic Energy Research, DOE.

Jul 2009
15
Wed 12:30
Robert Schroll, University of Chicago
e-mail:
Organizer: Ali *
The Impact of Viscous Liquid Drops

The phenomenon of drop impact displays a rich variety of behaviors throughout its large parameter space. Here, we focus on the specific regime of viscous liquid drops impacting on a smooth dry substrate. After impact, these drops form a thin pancake of uniform thickness. Using Volume-of-Fluid simulations, we find that this thickness is set by the growth of a viscous boundary layer, due to the no-slip conditions at the solid wall. The upper surface of the drop flattens down onto this interface, starting near the rim and moving inwards.

Jul 2009
22
Wed 12:30
Tom Witten, University of Chicago
e-mail:
Organizer: Arnab *
Fadeout profile of a stain, Chiral sedimentation: two novel shaped responses via creeping flow.

Remarkably, the evaporation of a droplet containing a nonvolatile solute creates a singular deposition profile: an arbitrarily large fraction of the solute becomes concentrated at the the perimeter. The density profile has a distinctive form, with a smooth fadeout at the trailing edge. We present a mechanism that predicts a power-law form for this profile. For the simplest and most common case, the predicted power is -7 as recently shown by Rui Zheng. This power can readily be varied. The power law is governed by the stagnation region of the lateral flow as the drop evaporates. It suffices to know two quantities: a) the evaporating flux J(0) at this stagnation point relative to its average over the drop and b) the height at the stagnation point relative to its average. We describe conditions for achieving a range of power laws. For a noncircular drop a single power law characterizes the deposition over the whole perimeter, despite the anisotropy of the stagnation flow. Another kind of novel controlled motion happens when an irregular object falls through a viscous liquid. These objects twist as they fall. Sufficiently extended objects twist in a way that depends on their shape but not their initial conditions, Nathan Krapf, Nathan Keim and I have shown.

Jul 2009
29
Wed 12:30
Fred Streitz, Lawrence Livermore National Laboratory
e-mail:
Host: Robert Rosner
Organizer: Jin
Pushing the limits: Scientific Computing on Extreme Platforms

Each new generation of supercomputer promises the ability to perform simulations on an unprecedented scale. As the size and complexity of supercomputers continues to increase, however, so too does the difficulty associated with effectively utilizing the new capability. I will discuss the results of three simulations that have made profitable use of the largest computers on earth: the solidification of a molten metal, the formation of a fluid instability and the dynamics of hot, dense plasma. In each case, a multidisciplinary team of physicists and computer scientists was able to overcome the substantial technical challenges involved and deliver simulations providing an unparalleled view into the physical behavior.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC5-07NA27344.

Aug 2009
5
Wed 12:30
Michael Hagan, Brandeis University
e-mail:
Host: Leo Kadanoff ()
Organizer: Arnab
Understanding biology's multifunctional containers -- how viral proteins dynamically encapsulate flexible polymers and rigid nanoparticles.

During the replication of many viruses, hundreds to thousands of protein subunits assemble around the viral nucleic acid to form a protein shell called a capsid. Most viruses must form a particular structure to be infectious and do so with astonishing fidelity. However, recent experiments demonstrate that capsids can assemble with different sizes and morphologies to accommodate nucleic acids or synthetic cargoes such as nanoparticles and inorganic polyelectrolytes. The assembly mechanisms that enable this combination of adaptability and precision are poorly understood.

The assembly of protein subunits into ordered capsid structures is a nonequilibrium process for which kinetics often trumps thermodynamic stability. In this talk, I will use Brownian dynamics simulations and dynamic Monte Carlo simulations with coarse-grained models for capsid proteins to explore the competition between kinetics and thermodynamics in two questions pertaining to capsid assembly and cargo encapsulation: (1) How do capsid proteins that faithfully form empty capsids (containing no cargo) with a single morphology assemble into different icosahedral morphologies around nanoparticles with different diameters? (2) How do capsid proteins assemble while simultaneously encapsulating a flexible polymer, and how much polymer can an assembling capsid package? In addition to suggesting answers to these questions, the simulations offer predictions that can be tested with both bulk and single molecule experiments. Time permitting, I will use a simple theory to investigate a third question: (3) How much can dynamic light scattering experiments reveal about the time scales of capsid assembly?

Aug 2009
12
Wed 12:30
Eric Brown, University of Chicago
e-mail:
Organizer: Eric
Jamming, dilation, and shear thickening in suspensions

Cornstarch suspended in water has the remarkable mechanical property of shear thickening, in which the viscosity increases reversibly with shear rate. Using rheometry measurements we find shear thickening to be a generic property of suspensions of non-attractive particles. The severity of shear thickening, characterized by the slope of the viscosity curve, is found to diverge at a critical packing fraction corresponding to the jamming point. Videos of the shear profile show the shear thickening regime is a phase transition from a low stress phase where the particles remain unsheared to a high stress phase where all particles participate in shear flow. The videos also show dilation which suggests the dramatic increase in stress associated with shear thickening may be due to dilation against the confining pressure from surface tension. These connections between jamming, dilation, and shear thickening in suspensions may provide a way to connect behavior of many-particle systems from the colloid level to macroscopic dry grains.

Aug 2009
19
Wed 12:30
Satish Kumar, University of Minnesota
e-mail:
Host: Wendy Zhang () ‡ †
Organizer: Ali ‡ †
Squishy, Frozen, and Elastic Interfaces: Instabilities and Applications

Hydrodynamic instabilities at fluid interfaces are usually undesirable because of the detrimental effect they have on practical applications such as polymer processing and liquid-film coating. In this talk, two examples will be given in which such instabilities can actually be exploited for scientific and technological purposes. In the first example, we consider an instability that arises when a fluid flows past a soft elastic solid. Experiments and theory suggest that this instability may be responsible for certain rheological phenomena observed in surfactant solutions, and that it can also be useful for enhancing mixing in small-scale flows. In the second example, we gain insight into the surface-freezing phenomenon by considering an instability that occurs when the free surface of a liquid is oscillated vertically. Experiments show an abrupt change in instability characteristics at the onset of surface freezing, and theory indicates that this change corresponds to a transition from a free surface allowing slip to one where slip is absent. We also show that the instability can arise in viscoelastic liquids even in the absence of inertia, and derive a simple Mathieu equation which reveals that elasticity introduces an effective inertia.

Aug 2009
26
Wed 12:30
Marija Vucelja, Weizmann Institute
e-mail:
Host: Wendy Zhang () ‡ †
Organizer: Jin ‡ †
Weak compressibility of surface wave turbulence

Clustering of matter on the surface of lakes and pools and of oil slicks and seaweed on the sea surface is well-known empirically but there is no theory that describes it. Since surface flows are always compressible, such a theory should be based on the description of the development of density of inhomogeneities in a compressible flow. We studied the growth of small-scale inhomogeneities in the density of particles floating in weakly nonlinear small-amplitude surface waves. Despite the small amplitude, the accumulated effect of the long-time evolution may produce a strongly inhomogeneous distribution of the 'floaters': density fluctuations grow exponentially with a small but finite exponent. We have shown that the exponent is of sixth or higher order in wave amplitude. As a result, the inhomogeneities do not form within typical time scales of the natural environment. Thus the turbulence of surface waves is weakly compressible and alone it cannot be a realistic mechanism of the clustering of matter on liquid surfaces. However if besides waves there are also currents, the interplay of waves with currents, might be in some cases responsible for the patchiness of the floaters.

Sep 2009
16
Wed 12:30
Will Ryu, University of Toronto
e-mail:
Host: David Biron ()
Organizer: Ali
Thermal sensorimotor response of E. coli and C. elegans

Swimming Escherichia coli responds to changes in temperature by modifying its motor behavior. Previous studies using populations of cells have shown that E.coli accumulate in spatial thermal gradients, but these experiments did not cleanly separate thermal responses from chemotactic responses. Here we have isolated the thermal behavior by studying the thermal impulse response of single, tethered cells. The motor output of cells was measured in response to small, impulsive increases in temperature, delivered by an infrared laser, over a range of ambient temperature (23 to 43 degrees C). The thermal impulse response at temperatures < 31 degrees C is similar to the chemotactic impulse response: both follow a similar time course, share the same directionality, and show biphasic characteristics. At temperatures > 31 degrees C, some cells show an inverted response, switching from warm- to cold-seeking behavior. The fraction of inverted responses increases nonlinearly with temperature, switching steeply at the preferred temperature of 37 degrees C.

Caenharbditis elegans also is thermotactic. We have developed a video-tracking microscope to follow movements of C. elegans as it freely crawls on the surface of an agar plate, and quantify this behavior using machine vision processing. A major challenge in analyzing animal behavior is to discover some underlying simplicity in complex motor actions. Here, we show that the space of shapes adopted by the nematode Caenorhabditis elegans is low dimensional, with just four dimensions accounting for 95% of the shape variance. These dimensions provide a quantitative description of worm behavior, and we partially reconstruct "equations of motion" for the dynamics in this space. These dynamics have multiple attractors, and we find that the worm visits these in a rapid and almost completely deterministic response to weak thermal stimuli. Stimulus-dependent correlations among the different modes suggest that one can generate more reliable behaviors by synchronizing stimuli to the state of the worm in shape space. We confirm this prediction, effectively "steering" the worm in real time.

Sep 2009
23
Wed 12:30
Stephen Morris, University of Toronto
e-mail:
Host: Wendy Zhang ()
Organizer: Eric
Icicles, washboard road and meandering syrup

This talk will describe three recent experiments on emergent patterns in three diverse physical systems. The overall shape and subsequent rippling instability of icicles is an interesting free-boundary growth problem. It has been linked theoretically to similar phenomena in stalactites. We attempted (with limited success) to grow ideal icicles and determine the motion of their ripples. Washboard road is the result of the instability of a flat granular surface under the action of rolling wheels. The rippling of the road, which is a major annoyance to drivers, sets in above a threshold speed and leads to waves which travel down the road. We studied these waves, which have their own interesting dynamics, both in the laboratory and using 2D molecular dynamics simulation. A viscous fluid, like syrup, falling onto a moving belt creates a novel device called a "fluid mechanical sewing machine." The belt breaks the rotational symmetry of the rope-coiling instability, leading to a rich zoo of states as a function of the belt speed and nozzle height.

here are some natural icicles 'http'://www.flickr.com/photos/nonlin/3566826271/ 'http'://www.flickr.com/photos/nonlin/3566826193/ 'http'://www.flickr.com/photos/nonlin/3587442026/

and lab icicles 'http'://www.flickr.com/photos/nonlin/3572193534/

washboard road in the lab 'http'://www.flickr.com/photos/nonlin/3697595615/ 'http'://www.flickr.com/photos/nonlin/3698410974/

in spacetime 'http'://www.flickr.com/photos/nonlin/3784441887/

Meandering syrup (movie) 'http'://www.flickr.com/photos/nonlin/3585645592/

Sep 2009
30
Wed 12:30
Lenka Zdeborova, Los Alamos National Laboratory
e-mail:
Host: Wendy Zhang ()
Organizer: Arnab
Physics on random graphs

Abstract: In this talk I will discuss the physical behavior of models on random graphs and its relation to real world systems. The most interesting feature, on which I shall focus, is a presence of an ideal glass phase amenable to analytic description. After describing the computational method, I will present two applications:

(1) Random optimization problems where the presence and properties of the glassy phase are related to the average algorithmical hardness.

(2) A particle model motivated by the patchy colloidal particles, that reproduces some interesting features of the observed phase diagrams of colloidal systems.

Oct 2009
7
Wed 12:30
Henry Abarbanel, University of California, San Diego
e-mail:
Organizer: Jin
Dynamical State and Parameter Estimation

In building models of complex systems and networks, one has sparse measurements from which one must infer unknown parameters at the nodes and links as well as the unobserved state of the system if predictions are to be made. When these systems are nonlinear chaotic oscillations of the networks impede the ability to achieve these goals, and one must regularize the procedure to stabilize the transmission of information from the observations to the model. We will discuss this in a general context and demonstrate how the solution works in practice in the context of electronic circuits, and for a small geophysical model.

We also consider the formulation of this problem for when there is noisy data, errors in the model, and uncertain initial conditions. This formualtion is a statistical physics problem, and we demonstrate that the effective action for this problem is a systematic wasy to evaluate all the conditional means and covariances required for the estimations.

Oct 2009
14
Wed 12:30
Philippe Guyot-Sionnest, University of Chicago
e-mail:
Organizer: Ali
Conductivity in solids of nanocrystals

Many potential applications of nanomaterials involve conductivity and possibly coherent wavefunctions across nanometer building blocks. At present, the conductance of solids of quantum dot nanocrystals exhibits hopping with strong effects of temperature, bias and electronic occupation. The Variable Range Hopping model successfully accounts for the data, but a consensus is still lacking. A magnetic field also affects the conductivity by squeezing the wavefunction and blocking spin relaxation.

Oct 2009
21
Wed 12:30
Boris Shraiman, University of California, Santa Barbara
e-mail:
Host: Leo Kadanoff ()
Organizer: Nicholas
Evolution, sex, and statistical physics

Evolution works through natural selection of the fittest versions of genes (a.k.a. alleles) in populations. Yet, the fitness of an individual is determined by its genotype which includes all of its genes so that ``bad" alleles can hide behind the ``good" ones. Sex and recombination can eliminate this hitch-hiking effect, but in its turn has an adverse effect of breaking up beneficial interactions between alleles often resulting in low fitness progeny. This talk will focus on the rather non-trivial dynamics of selection in the presence of gene interactions and recombination and expose connections with the familiar problems of statistical physics. We will see that selection dynamics has two distinct phases and will calculate how recombination accelerates the rate of evolution.

Oct 2009
28
Wed 12:30
Zheng-Tian Lu, Argonne National Laboratory
e-mail:
Host: Wendy Zhang ()
Organizer: Eric
Atom Trap, Krypton-81, and Saharan Water

Since radiocarbon dating was first demonstrated in 1949, the field of trace analyses of long-lived cosmogenic isotopes has seen steady growth in both analytical methods and applicable isotopes. The impact of such analyses has reached a wide range of scientific and technological areas. A new method, named Atom Trap Trace Analysis (ATTA), was developed by our group and used to analyze Kr-81 (t_1/2 = 2.3x10^5 years, isotopic abundance ~ 1x10^-12) in environmental samples. In this method, individual Kr-81 atoms are selectively captured and detected with a laser-based atom trap. Kr-81 is produced by cosmic rays in the upper atmosphere. It is the ideal tracer for dating ice and groundwater in the age range of 10^4-10^6 years. As the first real-world application of ATTA, we have determined the mean residence time of the old groundwater in the Nubian Aquifer located underneath the Sahara Desert. Moreover, this method of capturing and probing atoms of rare isotopes is also applied to experiments that study exotic nuclear structure and test fundamental symmetries.

Nov 2009
4
Wed 12:30
Subir Sachdev, Harvard University
e-mail:
Host: Leo Kadanoff ()
Organizer: Ali
Strong coupling problems in condensed matter, and the AdS/CFT correspondence

I will argue that experimental studies of quantum phase transitions have highlighted a number of theoretical problems which cannot be addressed by the traditional methods of many body theory. Near the superfluid-insulator transition, studied in ultracold atom systems, we have a regime where transport coefficients are determined completely by fundamental constants of nature; however, we have been unable to make this connection numerically precise. In the high temperature superconductors, and a number of other compounds, we are faced with quantum phase transitions involving the breakup of the Fermi surface: traditional methods fail because of the singular effects of the many low energy excitations near the Fermi surface. I will then present an elementary introduction to the AdS/CFT correspondence, which was discovered by string theorists. I will show how it has already made remarkable progress in describing transport near quantum critical points. Recent progress suggests that it may also shed light on quantum phase transitions of systems with Fermi surfaces.

Nov 2009
11
Wed 12:30
Jun Zhang, New York University
e-mail:
Host: Wendy Zhang ()
Organizer: Nicholas
Experimental Attempts to Simulate Large-scale Continental Dynamics
(Room Change: seminar will be in RI 480)

Thermal convection has come to be regarded as one of the most important archetype systems of dynamical systems. It has been extensively studied over the past 3 decades or so. An experimental system often consists of a fluid confined within a rigid box that is heated at the bottom and cooled at the top. Our experimental studies explore the intriguing phenomena when its rigid boundary is partly replaced either by a freely moving, thermally opaque (which reduces local heat loss) "floater" or by a collection of free-rolling spheres. We identify from our table-top experiments several dynamical states, ranging from oscillation to localization and to intermittency. A phenomenological, low-dimensional model seems to reproduce most of the experimental results. Our on-going experiments are inspired by the geophysical process of continental drift, where the mantle is regarded as the convective fluid and the continents as mobile "thermal-blankets.

Nov 2009
18
Wed 12:30
Penger Tong, Hong Kong University of Science and Technology
e-mail:
Host: Leo Kadanoff () *
Organizer: Justin Burton () *
Scaling laws and coherent oscillations in turbulent Rayleigh-Benard convection under different geometry

The discovery of scaling laws for the global heat transport, large-scale velocity and local temperature statistics in turbulent Rayleigh-Benard convection has stimulated considerable experimental and theoretical efforts, aimed at understanding the universal nature of the observed scaling laws. Because of historical reasons, most of the convection experiments were conducted in upright cylindrical cells with small aspect ratios. An important question one might ask is: To what extend are these scaling laws universal and independent of the cell geometry? Understanding of this question has important implications to large-scale astro/geophysical convection, such as that in the atmosphere and oceans, in which boundary effects are usually less important. In this talk, I will report a systematic experimental study of turbulent convection in a horizontal cylinder with the bottom 1/3 (curved) surface heated and the top 1/3 (curved) surface cooled. The measurements are conducted with varying aspect ratios and Rayleigh numbers. The experiment reveals important geometric effects on the scaling laws and coherent oscillations in turbulent thermal convection.

This work is done in collaboration with Hao Song and is supported by the Research Grants Council of Hong Kong SAR.

Dec 2009
2
Wed 12:30
Ryan Hayward, University of Massachusetts, Amherst
e-mail:
Host: Ka Yee Lee
Organizer: Ali
An elastic creasing instability of soft polymer surfaces

Soft solids placed under compressive stress can undergo a surface buckling instability leading to formation of sharply-folded creases on their free surfaces. We study a particular example of this instability that occurs upon swelling of crosslinked polymer networks on rigid substrates; when placed in contact with solvent, they are subjected to compressive swelling stresses due to the constraint against lateral expansion imposed by the substrate. We have studied a series of surface-attached hydrogels and found the conditions for instability to be rather insensitive to material properties or layer thickness, in accord with simple predictions. Using temperature-responsive gels, we seek to elucidate the mechanism of crease formation and growth, as well as the evolution of crease structures. Finally, we take advantage of this instability as a way to create dynamic polymer surfaces with reconfigurable chemical patterns.

Dec 2009
7
Mon 12:30
Special MRSEC Seminars
(12:30pm and 2:00pm)

,

William Irvine (12:30pm, KPTC 206)

New York University

william.irvine at gmail.com

'Title': Light geometry and soft condensed matter assembly

Charged PMMA colloids on an oil (Cyclohexyl bromide) water interface interact via a screened coulomb interaction and arrange into a crystalline lattice. The interface can be subject to designer optical fields to study the behavior of a lattice in ordered and disordered potentials. Furthermore I will show how light fields can be designed to `tweeze' individual topological defects and study the dance of dislocations involved in the birth and reversal of a grain. By creating curved oil-water interfaces having positive, negative and varying Gaussian curvature as well as different Euler numbers, we study the influence of curvature on the distribution and dynamics of topological defects. The crystals on curved surfaces are imaged and manipulated using a setup capable of simultaneous holographic optical tweezing and confocal imaging. I will also briefly discuss a system of lock and key colloids in which the geometry oft surfaces of colloidal particles can be used to direct their self assembly into a variety of structures.

----------------------------------------------------------

Silke Henkes (2:00pm, GCIS E223)

University of Leiden

henkes at lorentz.leidenuniv.nl

'Title': Friction and Failure: the role of critical contacts in the frictional jamming transistion

How can we relate microscopic failure at the contact level when the frictional force reaches the Coulomb threshold to the global stability properties of frictional packings? We show that these critical contacts modify the constraint counting and lead to an generalized isostatic line, instead of the isostatic point familiar from frictionless jamming. This can explain the range of contact numbers observed at frictional jamming. We probe the dynamics of failure near this transition for two 'situations': isotropic packings and a tilted bed undergoing an avalanche. In the isotropic case, we find that letting critical contacts slip crucially affects linear response. We recover soft modes, and the plateau of the density of states scales with the distance to generalized isostaticity, closely mirroring the frictionless case. For the avalanching bed, we find that correlated clusters of particles with critical contacts are responsible for most of the failure events in the creeping phase before the onset of the avalanche. These clusters are always generalized isostatic, and both their number and typical size show a sharp upturn just before the system avalanches. We conclude that through the generalized isostaticity concept, critical contacts are crucial to understanding the jamming transition in frictional packings.

Dec 2009
8
Tue 12:30
William Irvine, New York University
Linked and knotted beams of light
(12:30pm, room GCIS E223, Special MRSEC Seminar)

Maxwell's equations allow for remarkable solutions having linked and knotted field lines. A particularly striking solution is one characterized by the property that all electric and magnetic field lines are closed loops with any two electric(magnetic) field lines linked. These little known solutions, are based on the Hopf fibration and have a remarkably simple representation in terms of self-dual Chandrasekhar-Kendall curl eigenstates. I will discuss their structure, time evolution, physical properties, generalization and how they may be physically realizable.

Dec 2009
9
Wed 12:30
Benny Davidovitch, University of Massachusetts, Amherst
e-mail:
Host: Ka Yee Lee
Organizer: Nicholas
Instabilities and morphological phases of stressed elastic membranes

Crumpled papers, wrinkled skins, and buckled plant leaves are few familiar examples of the rich variety of patterns that elastic membranes may exhibit under quite featureless constraints. One may ask: Does a morphological "phase space" exist, according to which the many possible membrane patterns are classified? What are the relevant parameters that determine whether a distribution of forces and constraints gives rise to a smooth shape (e.g. periodic wrinkles) or to an irregular one, characterized by localized ridges and vertices (e.g. crumpled sheets)? In this talk I will address these questions, by focusing on an elementary case: highly-symmetric membrane (homogenous, isotropic, of rectangular shape) that is buckled under uniaxial compression and is subjected to a uniform tension in the orthogonal direction. I will show that a surprisingly rich "phase diagram" of distinct morphologies is spanned by a pair of dimensionless parameters that encapsulate the relevant mechanical conditions and geometric constraints. In particular, a novel series of "period fissioning" instabilities gives rise to a new type of wrinkling cascade when the tension is sufficiently large. This instability mechanism is shown to underlie a recently-discovered phenomenon: A smooth cascade of wrinkles, in uniaxialy-compressed ultrathin membranes, floating on liquid and subject to tension and geometric frustration due to strong capillary forces.

Dec 2009
16
Wed 12:30
Tom Haxton, University of Pennsylvania
e-mail:
Host: Wendy Zhang ()
Organizer: Justin Burton ()
Universality of the jamming phase diagram near the zero-temperature jamming transition

We demonstrate that the dynamical jamming transition that occurs for finite-ranged repulsive particles as a function of temperature, packing fraction, and applied stress can be collapsed onto a universal form near the zero-temperature jamming transition. We show that the rheology and the relaxation time for a class of models at arbitrary temperature and shear stress approach hard sphere scaling functions at low pressure. We find the shape of the universal jamming surface as a function of two ratios, the ratio of temperature to the product of pressure and the particle volume, and the ratio of shear stress to pressure.