Previous Talks: 2017
Jan 2017
4
Wed 12:15

Yali Amit,
email:
Host: Arvind Murugan
Organizer: Glen Hocky
A simple network model for a variety of memory tasks

Delay match to sample experiments have inspired much of the modeling work on attractor neural networks. The basic experiment involves showing a target image, removing it, and after a delay showing a cue image: either the original image or a different one. The subject needs to indicate if the cue is the same or different than the target. Electrophysiological recordings have shown that if the target is a learned one (has been observed multiple times) neurons selective for it maintain activity during the delay between target and cue presentation. This persistent activity is hypothesized to represent `working’ or `short term’ memory. The attractor model posits that learning creates modifications in the synaptic connections such that stimulation with learned patterns leads to sustained activity correlated with these learned patterns. There are a number of variations on the basic DMS paradigm involving distractors in between target and cue, or repetition detection experiments where a sequence of images is shown and one of them chosen at random is repeated. I will present a parsimonious network model of binary neurons and binary synapses and show how many of the phenomena observed in these different experiments can be handled within this framework using simple adjustments of certain global parameters.
Jan 2017
11
Wed 12:15

Dan Lathrop,
email:
Host: William Irvine
Organizer: Stéphane Perrard
Kelvin waves, helicity and visualization of quantum fluid flows

Stemming from visualization studies in superfluid helium, I'll review some basic phenomenology of quantized vortices, reconnection, and Kelvin waves. Some observations about the untangling of vortices lead to predictions regarding the helicity, and some puzzles and questions about the role of invariants like the helicity in the GrossPitaevskii (nonlinear Schrodinger) equation.
Jan 2017
18
Wed 12:15

Nicole Sharp,
email:
Host: Sid Nagel
Organizer: Delphine Coursault
Mixed Nuts, Skipping Rocks, and Molasses Tsunamis: Communicating Fluid Physics to the Public

With the rise of online social media, scientists can communicate their work to the public in ways that were unimaginable fifteen years ago. In this talk, I will discuss my outreach efforts through FYFD, a fluid dynamics blog and YouTube channel with an audience of around a quarter of a million followers. The talk will also present recent work with Harvard University undergraduates to integrate science communication into their fluid dynamics curriculum, and how these undergraduate projects kicked off a joint research effort with Harvard to investigate the physics of the Boston Molasses Flood, an industrial accident from 1919 that flooded Boston’s North End neighborhood with nearly 9000 cubic meters of molasses.
Jan 2017
25
Wed 12:15

Peter B. Littlewood,
email:
Host: William Irvine
Organizer: Yuval Yifat
Metalinsulator transitions in jammed elastic media

The metalinsulator transition driven by strong electronic correlations – generically called the “Mott” transition – is usually described entirely by electronic Hamiltonians, with models designed to exhibit related emergent phenomena such as magnetism and superconductivity. In real solids, the electronic localization also couples to the crystal lattice, and it turns out that these elastic degrees of freedom insert important new entropic phenomena more familiar in soft matter physics.
The coupling to the lattice induces elastic strain fields, which have intrinsic longrange interactions that cannot be screened. When strain fields are produced as a secondary order parameter in phase transitions  as for example in ferroelectrics  this produces unexpected consequences for the dynamics of order parameter fluctuations, including the generation of a gap in what would otherwise have been expected to be Goldstone modes.
A very important class of transition metal oxides – the perovskites – can be thought of as an array of tethered octahedra where the Mott transition produces a shapechange in the unit cell. Coupling of the fundamental order parameter to octahedral rotations gives rise to large entropic effects that can shift the transition temperature by hundreds of degrees K , essentially by exploiting the physics of jammed solids. The insight might offer ways to make better refrigerators by enhancing electrocaloric and magnetoelectric effects. I will also speculate on how this might be relevant for theories of the quantum critical point.
Feb 2017
1
Wed 12:15

Joshua A Frieman,
email:
Host: Daniel Holz
Organizer: Kim Weirich
The Dark Energy Survey

I will overview the Dark Energy Survey (DES) project, highlight its early science results, and discuss its ongoing activities and plans. The DES collaboration built the 570megapixel Dark Energy Camera for the Blanco 4meter telescope at Cerro Tololo InterAmerican Observatory in Chile to carry out a 5year, deep, multiband, optical survey over one eighth of the sky and a timedomain survey that will discover several thousand supernovae. The survey started in Aug. 2013 and is now nearing completion of its fourth observing season. DES was designed to address the questions: why is the expansion of the Universe speeding up? Is cosmic acceleration due to dark energy or does it require a modification of General Relativity? If dark energy, is it the energy density of the vacuum (Einstein's cosmological constant) or something else? DES is addressing these questions by measuring the history of cosmic expansion and the growth of structure through four complementary techniques: galaxy clusters, the largescale galaxy distribution, gravitational lensing, and supernovae, as well as through crosscorrelation with other data sets. I will also discuss how the data are being used to make a variety of other astronomical discoveries, from our Solar System to the most distant quasars.
Feb 2017
8
Wed 12:15

Sungyon Lee,
Host: Sid Nagel
Organizer: Glen Hocky
Particleinduced viscous fingering

An inclusion of particles in a Newtonian liquid can fundamentally change the interfacial dynamics and even cause interfacial instabilities. For instance, viscous fingering can arise even in the absence of the destabilizing viscosity ratio between invading and defending phases, when particles are added to the viscous invading fluid inside a HeleShaw cell. Our experimental results demonstrate that the onset and characteristics of fingering are most directly affected by the particle volume fraction but also depend on the ratio of the particle to gap size. In particular, the formation and destabilization of a particle band are observed on the interface only when the particle diameter is comparable to the channel gap thickness. The physical mechanism behind the instability and a quantitative model will also be discussed.
Feb 2017
15
Wed 12:15

OPEN

Feb 2017
22
Wed 12:15

Pedro M Reis,
email:
Host: William Irvine
Organizer: Yuval Yifat
‘Eggstreme’ Mechanics of Shells: From buckliphobia to buckliphilia in soft structures

Buckling of slender structures is typically regarded as a first step towards failure that is to be avoided ('Buckliphobia'). Instead, we envision mechanical instabilities in soft structures as opportunities for scalable, reversible, and robust mechanisms that are first to be predictively understood, and then harvested for function ('Buckliphilia'). A series of examples with a focus on thin elastic shells will be provided. I will first revisit the canonical Mechanics problem of sensitivity of shellbuckling to geometric imperfections. I shall then move on to the postbuckling regime of shells where periodic dimpled patterns are observed; for the cases when i) the shell is constrained from within by a rigid mandrel or ii) bound to an equally curved soft substrate (curved wrinkling). This periodic dimpled patterns will be used as a model system to study fundamental properties of curved surface crystals. Finally, taking inspiration from the resemblance of our dimpled wrinkling patterns to golf balls, I will introduce a new class of smart morphable surfaces for switchable and tunable aerodynamic drag.
Mar 2017
1
Wed 12:15

Paul Wiegmann,
Host: William Irvine
Organizer: Kim Weirich
Hydrodynamics of Onsager’s vortex flow

Turbulent flows of incompressible fluid in two dimensions are comprised of dense systems of vortices. In 1949 Onsager suggested to treat vortices as a macroscopical system whose statistical properties are described by Gibbsian statistical ensemble [1]. In the talk I address hydrodynamics of the vortex fluid. The hydrodynamics of the vortex fluid is different from Euler hydrodynamics of the original fluid. It features the anomalous stress absent in Euler's hydrodynamics, which yields a number of interesting effects. Some of them are: a deflection of stream lines, a correction to the Bernoulli law, accumulation of vortices in regions with high curvature in the curved space [1] L. Onsager, Nuovo Cimento, Suppl. 6, 249, 279 (1949)
Mar 2017
8
Wed 12:15

Scott Waitukaitis ,
email:
Host: Heinrich Jaeger
Organizer: Stéphane Perrard
A soft engine powered by coupling the Leidenfrost effect to elastic deformations

Soft materials are rapidly changing our expectations of what machines can do, but mechanically activating these systems remains challenging. We investigate a new physical effect that serves as a powerful strategy to inject mechanical energy into hydrogels, a widelyutilized class of soft materials. By dropping hydrogel spheres onto a hot substrate, we achieve vigorous energy injection in the form of persistent bouncing and intense screeching. This effect is robust, with spheres bouncing several times their height for minutes at a time. The underlying mechanism arises from a synergistic combination of elasticity and the wellknown Leidenfrost effect: vaporization couples with gel deformations to create rapid pressure oscillations that do mechanical work. With the fuel, mechanism, and mechanical output embedded into a single object made from a single material, our results introduce the concept of a soft engine and promise practical ramifications in fields such as active matter, metamaterials, and soft robotics.
Mar 2017
22
Wed 12:15

Sahand Hormoz,
email:
Host: Arvind Murugan
Organizer: Glen Hocky
Uncovering dynamics of cell state transitions using synthetic biology and counterintuitive mathematics

During development, tissue maintenance, and in diseases, cells proliferate and transition between physiologically and functionally distinct states. Despite the centrality of these transitions for diverse biological functions, it has remained challenging to determine which transitions can occur and at what rates without perturbations or cell engineering. I will discuss how quantitative cell state transition dynamics can be inferred from a static snapshot of gene expression in individual cells combined with their lineage history. We have been using synthetic biology, singlecell timelapse microscopy, and singlemolecule imaging to apply this framework to determine the dynamics of embryonic stem cells in culture. I will discuss these efforts and some counterintuitive, but beautiful, mathematical structures that have helped us interpret our experimental observations in stem cells and other biological systems.
Mar 2017
29
Wed 12:15

Ioana Marinescu,
email:
Host: Daniel Holz
Organizer: Delphine Coursault
Mismatch in the labor market

Could we significantly reduce U.S. unemployment by helping job seekers move closer to jobs? Using data from the leading employment board CareerBuilder.com, we show that, indeed, workers dislike applying to distant jobs: job seekers are 35% less likely to apply to a job 10 miles away from their ZIP code of residence. However, because job seekers are close enough to vacancies on average, this distaste for distance is fairly inconsequential: our search and matching model predicts that relocating job seekers to minimize unemployment would decrease unemployment by only 5.3%. Geographic mismatch is thus a minor driver of aggregate unemployment. We will discuss how we have measured mismatch more broadly, including the skills dimension. Finally, we will discuss the National Center for Opportunity Engineering and Analysis, and how we may build a recommendation engine linking education, skills and career opportunities.
Apr 2017
5
Wed 12:15

Massimo Vergassola,
email:
Host: William Irvine
Organizer: Yuval Yifat
Navigation in turbulent environments

Thermal soaring by birds and olfactory searches by insects are biological examples of navigation in the presence of complex orientation cues. The two problems also have technological applications, namely for extending the autonomy of flying vehicles/gliders, as well as for the development of olfactory robots (sniffers). I shall first review the animal behavior, then present the physics of the orientation cues, and finally discuss the computational aspects of navigation.
Apr 2017
12
Wed 12:15

Jonathan Simon,
Organizer: Kim Weirich
Exploring Landau Levels in Curved Space

I will present recent work realizing topological phases of photons, both in curved space, and in lattices. The talk will focus on our recent exploration of Landau levels on a conical surface, generated using a nonplanar (twisted) optical resonator to induce a synthetic magnetic field for optical photons, and employed to validate the famous WenZee action. I will then discuss recent results demonstrating strong photonphoton interactions mediated by resonator Rydbergelectromagnetically induced transparency (EIT), and techniques we are developing to assemble topological fewbody states both photonbyphoton, and through microscopic devices engineered for photon thermalization. I will conclude with our recent observation of timeresolved helical edge dynamics in Z_2 topological circuit lattices, and a Tbroken extension in the microwave domain using arrays of 3D cavities and circuit quantum electrodynamics techniques. This work showcases the unique possibilities for Hamiltonian engineering and control in the photonic sector, a provides a taste of upcoming breakthroughs in engineering quantum materials from photons.
Apr 2017
19
Wed 12:15

Anette Hosoi,
email:
Host: William Irvine
Organizer: Stephane Perrard
Hydrodynamics of Hairy Surfaces

Flexible slender structures in flow are everywhere. While a great deal is known about individual flexible fibers interacting with fluids, considerably less work has been done on fiber ensembles, such as fur or hair, in flow. These hairy surfaces are abundant in nature and perform multiple functions from thermal regulation to water harvesting to sensing. Motivated by these biological systems, we consider two examples of hairy surfaces interacting with flow: (1) air entrainment in the fur of diving mammals and (2) symmetry breaking in hairy microchannels.
In the first example, we take inspiration from semiaquatic mammals (such as fur seals, otters, and beavers) which have specially adapted fur that serves as an effective insulator both above and below water. Many of these animals have evolved pelts that naturally entrap air when they dive. This air: (1) provides additional insulation under water, (2) provides added buoyancy, and (3) facilitates water shedding when the animals resurface. In this study we investigate diving conditions and fur properties which amplify air entrainment in fur. In the second example, we consider a fundamental component in hydraulic systems, the flow rectifier. One of the simplest ways to generate asymmetry in these devices is with a ball valve in which flow is completely obstructed in one direction and free to flow in the other. In this work we seek a variation that: (1) allows the designer to modulate the relative resistances in the rectifier and (2) can be achieved with solid state components (i.e. no moving parts).
Apr 2017
26
Wed 12:15

Lenka Zdeborova,
email:
Host: Arvind Murugan
Organizer: Delphine Coursault
Statistical physics approach to compressed sensing and generalized linear regression

Bayesian inference and statistical physics are formally closely related. Therefore methodology and concepts developed in statistical physics to understand disordered materials such as glasses and spin glasses can be elevated to analyze models of in statistical inference. We will present this approach in a rather general setting that covers analysis of compressed sensing, generalized linear regression, and the perceptron  a kind of a single layer neural network. At the one hand, this approach leads to the approximate message passing algorithm that is gaining its place among other widely used regression and classification algorithms. At the other hand, the related analyses leads to identification of phase transitions in the performance of Bayesoptimal estimators. We will discuss relation between these phase transitions and algorithmic hardness, and in the case of compressed sensing we will show how this understanding leads to a design of optimal measurement protocols.
Based partly on "Statisticalphysicsbased reconstruction in compressed sensing" PRX 2012 and reviewed in "Statistical physics of inference: Thresholds and algorithms" Advances of Physics 2016.