Computations in Science Seminars
Feb 2016
10
Wed 12:15
OPEN
Feb 2016
17
Wed 12:15
Chris Fang-Yen, University of Pennsylvania
e-mail:
Host: David Biron ()
Organizer: Stéphane Perrard ()
Optical dissection of rhythmic behaviors in a worm

A fundamental goal of neuroscience is to understand how the activity of neurons in networks gives rise to coordinated behaviors. Our laboratory studies brain and behavior in the roundworm Caenorhabditis elegans. Advantages of this 1-mm long creature include a compact and extraordinarily well-mapped nervous system, genetic manipulability, and optical transparency, and an ever-expanding set of tools for measuring and manipulating activity in its nervous system. In particular, we have developed methods for optogenetically manipulating specific neuron types in freely behaving animals. We describe our efforts to understand the neural circuits underlying two fundamental, rhythmic behaviors: feeding and locomotion.

Feb 2016
24
Wed 12:15
Tom Lubensky, University of Pennsylvania
e-mail:
Host: William Irvine ()
Organizer: Kim Weirich ()
Lattices near mechanical collapse: Topological Mechanics

Frames consisting of nodes connected pairwise by rigid rods or central-force springs, possibly with preferred relative angles controlled by bending forces, are useful models for systems as diverse as architectural structures, crystalline and amorphous solids, sphere packings and granular matter, networks of semi-flexible polymers, proteins, origami, and an increasing number of lab-constructed micron-scale metamaterials. The rigidity of these networks depends on the average coordination number z of the nodes: If z is small enough, the frames have internal zero-frequency modes, and they are “floppy”; if z is large enough, they have no internal zero modes and they are rigid. The critical point separating these two regimes occurs at a rigidity threshold that for central forces in d-dimensions occurs at or near coordination number zc = 2d. At and near the rigidity threshold, elastic frames exhibit unique and interesting properties, including extreme sensitivity to boundary conditions, power-law scaling of elastic moduli with (z- zc), and diverging length and time scales.

This talk will explore elastic and mechanical properties and mode structures of model periodic lattices, such as the square, kagome, pyrochlore, and jammed packings with central-force springs, that are just on verge of mechanical instability. It will discuss the origin and nature of zero modes and elasticity of these structures under both periodic (PBC) and free boundary conditions (FBC), and it will investigate lattices (a) whose zero modes under the two boundary conditions are essentially identical, (b) whose phonon modes in the bulk are “gapped” with no zero modes in the periodic spectrum (except at zero wavenumber) but include zero-frequency surface Rayleigh waves in the free spectrum, and (c) whose bulk phonon modes include isolated points or lines where their frequency is zero. In case (a), lattices are generally in a type of critical state that admits states of self-stress in which there can be tension in bars with zero force on any node. Distortions away from that state gap the spectrum and give rise to surface modes under free boundary conditions whose degree of penetration into the bulk diverges at the critical state. The gapped states have a topological characterization, similar to those of polyacetylene and topological insulators, that define the nature of zero-modes at the boundary between systems with different topology. Case (c) is closely analogous to Weyl semi-metals with isolated points in the Brillouin zone where valence and conduction bands meet. These critical lattices generally have macroscopic elastic distortions, called Guest Modes, that cost no energy.

Mar 2016
2
Wed 12:15
Daniel Hooper, Fermilab
e-mail:
Host: Daniel Holz ()
Mar 2016
9
Wed 12:15
John Novembre, University of Chicago
e-mail:
Host: David Biron ()
Mar 2016
16
Wed 12:15
March Meeting--No Seminar
Mar 2016
30
Wed 12:15 PM
Alison Sweeney, University of Pennsylvania
e-mail:
Host: William Irvine ()
Apr 2016
6
Wed 12:15
Randy H. Ewoldt, University of Illinois Urbana-Champaign
e-mail:
Host: TBD
Droplet impacts: when yield-stress fluids do and do not stick

Yield-stress fluids, including gels and pastes, are effectively fluid at high stress and solid at low stress. In liquid-solid impacts, yield-stress fluids can stick and accumulate where they impact, motivating several applications of these rheologically-complex materials (including fire suppression and spray coating). Here we experimentally study yield-stress fluids impacting three types of surfaces where they may (or may not!) stick: pre-coated surfaces, hot surfaces, and permeable surfaces. Using high-speed video and quantitative analysis, we report various regimes of splashing, Leidenfrost effects, and flow-through. Existing dimensionless groups do not adequately characterize all these regimes. Incorporating relevant lengthscales, we demonstrate successful dimensionless groups that organize the dynamics into a lower-dimensional space. This provides insight into the physics of droplet impact problems. Moreover, it potentially allows for fluid design and extrapolation of these results to dynamically and geometrically similar situations beyond the explicit material and parameter values explored here.

Apr 2016
13
Wed 12:15
OPEN
Apr 2016
20
Wed 12:15
OPEN
Apr 2016
27
Wed 12:15
OPEN
May 2016
4
Wed 12:15
Mark Bowick, Syracuse University
e-mail:
Host: William Irvine ()
Sharp Shapes from Soft Systems

What determines shape? Energy minimization in flexible systems with competition between order and shape change can lead to a wide variety of shapes including highly faceted singular structures. I will discuss shape generation and shape shifting in two systems – molecularly thin vesicles with liquid crystalline order and fluid droplet networks with osmolarity gradients.

May 2016
11
Wed 12:15
Michael Nielsen, Recurse Center
e-mail:
Host: David Biron ()
May 2016
18
Wed 12:15
Jeremie Palacci, University of California, San Diego
e-mail:
Host: William Irvine ()
May 2016
25
Wed 12:15
OPEN
Jun 2016
1
Wed 12:15
OPEN
Jun 2016
8
Wed 12:15
OPEN