women in physics talks

Women in Physics Talks

Join the Women in Physics at the University of Chicago for biweekly talks over dinner. Everyone from undergraduates to faculty is welcome.
Email azizas_at_uchicago.edu to be added to our e-list.

Upcoming Talks

November 16, 2017
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Michelle Driscoll, Northwestern University
Mind the Gap: A New Kind of Fingering Instability in Colloidal Rollers
When colloidal particles are rotated adjacent to nearby floor, strong advective flows are generated around them, even quite far away. When a group of these microrollers is driven, the strong hydrodynamic coupling between particles leads to formation of new structures: an initially uniform front of microrollers evolves first into a shock-like structure, which then quickly becomes unstable, emitting fingers of a well-defined wavelength. Our experiments and simulations confirm that this instability is quite different than typical fingering instabilities, where size scale selection is a consequence of competing stresses. Here, this instability arises only due to hydrodynamic interactions, and it is controlled by a single geometric parameter, the particle-floor height. Our measurements of the growth rate in both experiments and simulations agree with results from our continuum model. This instability is a direct consequence of the inward flows created by the interactions between the particles and the nearby solid surface.

Past Talks

November 9, 2017
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Pragallva Barpanda, Shaw Group
Meridional Shift of Extratropical Storm Tracks in Earth's Atmosphere
The midlatitude region of Earth’s atmosphere (~ 45 deg latitude) is dominated by transient cyclones and anticyclones. These propagating weather systems follow preferred geographical paths known as extratropical storm track. In response to external forcings like seasonal cycle, El Nino and climate change, the storm tracks deflect poleward or equatorward. What causes the storm tracks to shift their position? Is there a common dominant factor underlying these shifts? My research aims at answering these questions. I will give a brief background about extratropical cyclones and discuss why they exist in the position as they do now. I will then describe an energetic framework we have developed to draw testable hypothesis that possibly explain the storm-track-shifts and is currently being tested using idealized numerical experiments.

October 26, 2017
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Danielle Scheff, Gardel Lab
The Many Phases of Actin

October 12, 2017
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Chloe Lindeman, Nagel Group
Spooky Action at a Distance in Spring Networks
In traditional solid state physics, materials are thought of as perfect atomic lattices. But what about the opposite end of the spectrum: total disorder? Rather than viewing disorder as a setback, we take advantage of the unique properties of disordered spring networks to create unusual global and local responses to applied stress.

August 30, 2017
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Aziza Suleymanzade, Simon and Schuster Labs
Progress towards engineering strong interactions between optical and microwave photons
I will give an overview of my experimental project on engineering quantum interactions of lights particles. And I will describe in more detail the progress we have made in building a hybrid cryogenic system that brings cold atoms, photonics and circuit Quantum Electrodynamics (QED) together.

August 8, 2017
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Sabrina Berger, Leslie Rogers
Quantifying Thermal Effects on Rocky Exoplanets
Rocky planets can be very diverse in structure and composition compared to the Earth. Their temperature profiles could also differ greatly from Earth’s depending on their mass and distance from their host stars. Interior structure models of rocky exoplanets have not yet studied the full range of possible temperature profiles. We develop a simulation, PyPlanet, for a rocky planet with an arbitrary number of layers and equations of state. We apply this model to explore many possible temperature profiles and quantify the thermal effects on the mass-radius relations of rocky planets. This detailed modeling will be crucial for making robust inferences about rocky planet structure and composition from transit and radial velocity observations.

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Enid M. Cruz-Colón, Dan Fabrycky
Dynamics of multi-planet system TRAPPIST-1
After formation, planetary systems might evolve and get locked in resonances between the orbital periods of adjacent planets in the system. These 2-body resonances can potentially lead to 3-body resonances in the system, in which we relate the coordinates of three adjacent planets. This projects aims to apply an analytical model for resonant chains and 3-body resonances to the planetary system TRAPPIST-1 by means of a numerical model.

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Isabelle Bunge, Dinner Group
Modeling a Liquid Crystal Phase of Cross-Linked Actin Bundles
The study of actin is essential to our understanding of diverse bio-logical processes, such as muscular contraction and cellular division. However, in order to study this protein effectively in vitro we need detailed simulations that can help us predict results of certain experimental setups. Recently, in the Gardel Group here at UChicago, a new phase of cross-linked actin bundles was observed experimentally. The current simulation package for actin filament environments designed by the Dinner Group, titled AFINES, fails to include this liquid-crystal like phase. For the summer, I have been tasked to develop this simulation package to include this newly discovered phase. In this talk, I will discuss the methods used to implement tactoid formations (liquid crystal phase) into this simulation package as well as the results I have seen thus far in my analysis of completed simulations.

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Elyssa Ferguson, Jaeger Lab
Platy Particles’ Influence on the Behavior of Granular Packings.
Particle shape of granular materials plays an important role in affecting an aggregate’s response to its environment. Out of myriads of shapes, platy particles possess the surprising ability to vigorously adapt while supporting great loads; however, the micro particle interactions generating the macro response are not well understood. By analyzing tomographic reconstructions and stress-strain data of compressed packings, we will better understand how flat shape influences granular mixtures’ response to stress.This provides new clues to understanding how aggregates essentially adapt and can lead to controlling granular materials’ shape and response in soft robotics, self-healing materials, and architecture applications.

July 27, 2017
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Kate Cavanaugh, Gardel Lab
Mechanics of Cell Shape Change
In development, cells must coordinately alter their cell-cell junction lengths, surface area, and overall cell shape to sculpt tissues and organs. Thousands of molecular changes, coordinated both in space and time, determine a cell’s material properties that translate into these drastic cell- and tissue-scale deformations. And yet, how does the cell determine where and how hard to pull? When does the cell resist deformation or undergo shape changes? To answer these questions, we use optogenetics to target contractility with focused light. We hypothesize that the molecular mechanisms controlling actin cytoskeletal reorganization determine the local material properties of cell junctions, effectively driving irreversible cell shape change. Surprisingly, we found that constant junction contraction was not sufficient for irreversible shape changes. Rather, junction contraction stalled before elastically returning to its original shape. Multiple contractions showed an adjustable junction length indicative of contractile behavior. These emergent mechanical behaviors point to regulation of actin turnover as an exciting mechanism for effective shape change. These data will directly test our current models of cell mechanics in a model tissue, allowing us to construct dynamic 3D models of multicellular force transmission that underlie shape changes.

July 13, 2017
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Melody Lim, Jaeger Lab
Collisional charging of acoustically levitated grains: contactless manipulation and self-assembly
Repeated frictional contacts between grains can result in the spontaneous formation of large electrostatic charges on individual grains, even if these grains are identical neutral dielectrics. These large electrostatic charges affect other properties of the granular material through long-range interactions, such as its tendency to cluster. However, the process by which individual dielectric grains  tribocharge is not well understood. We develop a method to contactlessly manipulate individual grains using acoustic levitation. We characterize the motion of particles with a range of materials and sizes in an acoustic pressure field, and produce controlled collisions between insulating grains. We also observe the charge-mediated assembly of particle rafts in systems of acoustically levitated grains.

April 18, 2017
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Jiayi Wu, Irvine lab
Nonlinearities in Gyroscopic Materials
We study the nonlinear dynamics of a one dimensional chain of weakly interacting magnetic gyroscopes. It has been found that gyroscopic lattices with hexagonal and kagome geometries are mechanical analogue to a Chern topological insulator. Thus, the dynamics of the 1D gyroscopic chain may help us understand the nonlinear behavior of chiral edge modes in 2D lattices. We demonstrate that gyroscopic lattices with on-site nonlinearities supports propagating breather solutions.

April 4, 2017
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Sam Stam, Gardel Lab
Biological polymer-based materials remodel under active, molecular motor-driven forces to perform diverse physiological roles, such as force transmission and spatial self-organization. Critical to understanding these biomaterials is elucidating the role of microscopic polymer deformations, such as stretching, bending, buckling, and relative sliding, on material remodeling. Here, we report that the shape of motor-driven deformations can be used to identify microscopic deformation modes and determine how they propagate to longer length scales. In cross-linked actin networks with sufficiently low densities of the motor protein myosin II, microscopic network deformations are predominantly uniaxial, or dominated by sliding. However, longer-wavelength modes are mostly biaxial, or dominated by bending and buckling, indicating that deformations with uniaxial shapes do not propagate across length scales significantly larger than that of individual polymers. As the density of myosin II is increased, biaxial modes dominate on all length scales we examine due to buildup of sufficient stress to produce smaller-wavelength buckling. In contrast, when we construct networks from unipolar, rigid actin bundles, we observe uniaxial, sliding-based contractions on 1 to 100 μm length scales. Our results demonstrate the biopolymer mechanics can be used to tune deformation modes which, in turn, control shape changes in active materials.

February 21, 2017
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Anita Gaj, Chin lab
In Rydberg atoms the valence electron is excited to a barely bound orbit in which it is far away from the nucleus. The interaction between a slow Rydberg electron and a separate ground state atom can lead to a bound state and the formation of ultralong-range Rydberg molecules. In my talk I will discuss how to create these molecules and why they are so much different than the conventional molecules. Finally, I will talk about recent experiments involving Rydberg molecules and the future of this new direction of research.

January 31, 2017
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Prof. Abigail Vieregg
Radio Detection of the Highest Energy Neutrinos
Ultra-high energy neutrino astronomy crosses boundaries between particle physics, astrophysics, and cosmology.  I’ll discuss the search for these highest energy observable particles in the universe, how we do it, what we know now, and what we hope to learn in the coming years.  I’ll talk about a balloon-borne experiment called ANITA that just completed a successful flight in Antarctica, as well as a ground-based experiment at the South Pole called ARA, and a new idea we’re working on here at Chicago to improve the efficiency of such detectors.

January 10, 2017
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Polina Navotnaya, Engel Group
Electronic dynamics in bulk GaAs induced by light with orbital angular momentum
Light with orbital angular momentum, known as twisted light, is a topic of growing interest due to the new opportunities it opens for exploring the interaction of light with matter. Twisted light has been explored in communications, where it was transmitted efficiently over the distance over 100km. In addition to that, entangled states created with twisted light may be important in quantum computing and communications. However, detecting information encoded in twisted light can be a challenge because it requires complicated laser setup and stable conditions. Taking this technology from lab bench to commercial applications requires new detection materials. This inspired me to study interactions of twisted light with matter. Introducing orbital angular momentum to light generates corkscrew-shaped helical wavefront that stirs the sample’s charge density in a way different from planar light-matter interaction. The current density induced in the sample induces terms beyond dipolar in the multipolar expansion. In the interaction Hamiltonian, the presence of the multipolar terms creates new selection rules that are governed by the transition probabilities and the overlap between the light mode and charge density of the sample. The states achieved by these excitations are poorly coupled with the environment. Thus, the lifetime of these states is expected to grow as OAM grows. I have performed pump-probe spectroscopy experiment with twisted light and bulk gallium arsenide at liquid helium temperatures. The results have indeed shown the prolonged lifetime of the excited states, opening up the opportunity to study dynamics of the states beyond dipole moment approximation. 

November 28, 2016
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Nicole James, Jaeger Lab
What causes shear jamming of suspensions?
Fluids like water or honey have an intrinsic, constant viscosity at a given temperature. However, concentrated suspensions of solid particles in a liquid can display a surprising, counterintuitive behavior: the harder they are sheared to flow, the higher the viscosity becomes. In rare cases, the fluid even solidifies, then ‘melts’ when the force is removed! This phenomena is called shear jamming, and is commonly seen in cornstarch/water suspension demonstrations.

Previous understanding of granular flows suggests that shear jamming of suspensions ultimately arises from particles being sheared into direct contact, at which point particle-particle friction prevents the flow of particles past each other, and the suspension jams. However, that raises the question: Why do some systems (most notably cornstarch) display prominent shear jamming, while practically no other systems do?

In this talk I will present a new synthetic particle system that displays shear jamming and allows us to pinpoint and characterize the chemical contributions to particle-particle friction that we believe are necessary for shear jamming. This understanding opens the door to designing and optimizing these materials for applications such as stab-resistant liquid armor.

November 14, 2016
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Margaret Gardel

October 31, 2016
Menglu Chen, Guyot-Sionnest Lab

Electronic states in mercury chalcogneide colloidal quantum dots
In the past few years, colloidal quantum dots based on the zinc-blend mercury chalcogenides, Hg(S, Se, Te), have become leaders in efforts to transform mid-IR technologies with solutions based materials.   Understanding the electronic states, the doping, and the relative band positions of the three materials and the colloidal dots is essential for current and future investigations.
I use spectroscopy and electrochemistry to monitor the electronic states, the mobility of electrons in films, the origin of the spontaneous doping in several of the systems, and the effects of the surface.
I will describe how a single cyclic voltammetry curve on one particular sample of quantum dots already reveals much novel information, such as predicting if the system is spontaneously doped, the Fermi level, the electron injection energies, the degree of reversibility, the presence of surface states, and the mobility of electrons hopping from different states.
I will then present our complete set of electrochemical and spectroscopic data collected on the three different chalcogenides, with a systematic range of sizes and surface chemistry, and these unveil an unprecedented view on the materials and of their properties as quantum dots.

October 3, 2016
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Chen He Heinrich, Hu Lab
Complete Reionization Constraints from Planck 2015 Polarization
I will present a recent analysis of the 2015 cosmic microwave background data from the Planck satellite that is complete in the reionization observables using principal components (PCs). By allowing for an arbitrary ionization history, the PC technique probes a larger space of physical models than in the standard analysis (which assume stantaneous reionization) and tests the robustness of the inferred optical depth. A reliable measurement of the total optical depth is important for the interpretation of many other cosmological parameters such as the dark energy and neutrino mass. We found that Planck 2015 data not only allow a high redshift z>15 component to the optical depth but prefer it at the 2σ level, contributing to a higher total optical depth than in the standard analysis. I will further demonstrate the power of the PC method to efficiently constrain models given predictions of ionization history by applying our effective likelihood code.

October 17, 2016
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Nora Shipp
Exploring the Effect of Dark Matter Self-Interactions on the Subhalo Distribution of Galaxy Clusters

Observations of galaxy clusters indicate that the splashback radius, a density caustic corresponding to the first orbital apocenter of satellite galaxies after accretion, is smaller than predicted by simulations. This decrease in radius could be due to an effective drag force on the dark matter subhalos resulting from scattering between dark matter particles. In order to explore this possibility and place preliminary constraints on the self-interaction cross section, I integrated dark matter subhalo orbits though evolving galaxy cluster potentials and examined the effect of self-interactions on the subhalo distribution.

August 7, 2016
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Lipi Gupta, Innovations in Bright Beam Science group
Synchotrons and Free Electron Lasers
Light sources such as synchrotrons and FELs generate bright, high-energy radiation. These sources can be used to probe extremely small structures due to the ultra-short (order Angstrom) wavelength light they produce. Currntly operating machines include APS at Argonne National Lab and LCLS at SLAC National Lab, as well as many international facilities sucha as SACLA in Japan. Protein imaging, material surface studies, and more are driving the demand for higher energy photons and higher luminosity. By creating high-energy radiation at increased fluxes, scientists hope to be able to construct "movies" of chemical reactions taking place; which could become possible at future light sources. This talk will discuss the basic physics of these machines, as well as some of the challenges faced by current accelerator physicists designing 4th generation light sources. 

August 10, 2016
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Rebecca Cheng, Jaegar Group
Shear thicknening flow curves and viscosity fluctuations in synthetic pMMA suspensions
Under impact or shear, cornstarch and water suspensions have shown to thicken and solidify, allowing a ball to bounce or a person to run across its surface. However, most observations of shear jamming have been on starch suspensions, which are prone to rotting, making practical application of this phenomenon difficult. Here, we study viscosity fluctuations at regions of shear thickening and jamming of synthetic pMMA suspensions. By learning more about the limits of shear thickening and jamming in synthetic suspensions, we can begin to harness this behavior for further application in engineering and industry. 

August 10, 2016
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Analis Lawrence, Gandi Group
Gain calibration of the Hamamatsu R11410 PMT
Hamamatsu R11410 3" photomultiplier tubes (PMTs) are aligned in liquid xenon detectors designed for dark matter searches. The detectors are two-phase xenon time projection chambers to determine detection of a WIMP particle or a dark matter candidate. The role of the PMTs is to amplify photon signals from xenon scintillation light. In this study, a 470 nm blue laser light pulse signal was sent to the Hamamatsu R11410 through a dark box. A single photoelectron peak gain curve was analyzed at varying voltages. Calibration of the PMT helps ensure accurate detection of interaction events as well as rejection of background. 

August 3, 2016
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Marina David, Robert M. Wald Group
Deformation of the Event Horizon of a Schwarzschild Black Hole Induced by Gravitational Waves

Gravitational waves are fluctuations of spacetime curvature generated in gravitational interactions. We study the effects of gravitational radiation on the event horizon of a Schwarzschild black hole. We find that gravitational waves cause a deviation in the horizon generators, which in turn cause a deformation in the black hole. We first consider the metric perturbation and how it propagates through Schwarzschild spacetime, by which we then calculate the curvature perturbation and use the geodesic deviation equation to determine the deviation in the null generators.

July 27, 2016
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Niharika Sravan, Kalogera Group
Preparing for the Era of Time-Domain Big-Data Astronomy: Supernovae and The Transient Sky
In this talk I will present an overview of the advent of time-domain big-data astronomy, its implications, challenges, and connections to my research. I will give a broad summary of my current research interests aimed at helping prepare for this new era in astronomy. Finally, I will present results from my most recent work on supernovae and describe how they fit in context of my broader research goals. 

July 20, 2016
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Ariel Sommer, Simon Lab
Quantum Anomaly in Pair Dissociation of 2D Fermions

Symmetries present in classical systems can in some cases break in their quantum counterparts. These so-called quantum anomalies appear in a wide range of interacting quantum systems, from particle physics to condensed matter. In two-dimensional gases of fermions with contact interactions, the classically expected scale invariance breaks due to regularization in the quantum system. I will present measurements of the excitation spectra in a two-dimensional gas of ultracold lithium-6 atoms in which the quantum anomaly has a strong effect and discuss how these observations fit in with our understand of 2D Fermi gases. 

July 13, 2016
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Rui Zou, Young-Kee Kim lab
FastTracKer (FTK) system and VBF Higgs to Invisible analysis
I will briefly talk about the ATLAS trigger system, with an emphasis on the structure of FastTracKer (FTK), part of the trigger system update, and how FTK can help confront the challenges due to high luminosity in Run II. I will then proceed to give a brief overview of the Vector Boson Fusion produced Higgs to Invisible decay analysis using the ATLAS detector in Run II.

July 6, 2016
Kim Wirich, Gardel lab

Cytoskeletal droplets: a fluids perspective on structural biomaterials
Living cells dynamically change shape as they routinely divide, migrate, and exert forces on their environment. Driving these enormous deformations is a protein-based structural material, the cytoskeleton. This material has fascinating mechanical properties, primarily derived from semi-flexible filaments, called F-actin, which cross-link into bundles and networks traditionally described as elastic solids. In this talk, I will discuss my recent research that suggests these structures can also behave like liquid crystal droplets, as well as possible biological implications of describing this essential material from a fluids perspective.

June 29, 2016
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Danielle Scheff, Gardel lab
Material Properties of the Actin Skeleton
Actin filaments are an essential protein that creates the skeleton of animal cells. They display a wide range of material properties depending on factors such as surrounding proteins. I will talk about the material properties of actin with one such protein, called fascin, as well as the bulk rheology techniques used.

June 22, 2016
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Kaitlin McLean, Clark lab
Control of Antibody Selection in the Immune System

Every day, our immune system has to deal with multiple assaults from a wide host of different pathogens. To help recognize and fight these pathogens, B cells in the immune system use a process called VDJ recombination to create their repertoire of millions of unique antibodies. It is a tightly controlled process, which when gone awry can lead to lymphoma, immunodeficiency and autoimmune disorders. Our lab recently discovered a new epigenetic modifier, Brwd1, which plays a crucial role in chromatin remodeling during VDJ recombination. I will focus on our lab’s use of different sequencing and genome editing techniques to learn more about the role Brwd1 is playing in this process.

June 14, 2016
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Lisa Nash, Irvine Lab
Topological gyroscopic metamaterials
Mechanical metamaterials can have topologically protected states, much like their electronic and optical counterparts. We recently demonstrated an example of this in experiment by building a metamaterial composed of coupled gyroscopes on a honeycomb lattice. This system breaks time-reversal symmetry and exhibits topologically protected one-way edge modes. In this talk I will discuss our theoretical understanding and experimental demonstration of this metamaterial.

June 7, 2016
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Sofia Magkiriadou, Irvine lab
Color and Disorder: Colloidal Photonic Glasses and the Case of Red

When a material has inhomogeneities at a length scale comparable to the wavelength of light, interference can give rise to structural colors: colors that originate from the interaction of the material's microstructure with light and do not require absorbing dyes. A familiar example are opals, whose crystalline microstructure gives rise to Bragg scattering and iridescent structural colors. However, long-ranged order is not necessary for structural color: glasses, which have only short-ranged order, can also be colorful. Unlike those of crystals, the structural colors of photonic glasses are independent of the viewing angle since glasses are isotropic. Nature has been employing this coloration mechanism for millions of years. However, to the best of our knowledge, there is no natural example of a red photonic glass. What is special about red? In this talk, I will share some insights into the absence of red photonic glasses in nature, derived from experiments with colloidal glasses combined with scattering theory. I will further describe experimental methods that we have developed in order to make non-iridescent, structural red color. Finally, I will discuss the color gamut that could potentially be achieved with analogous techniques, and I will entertain the prospect of using photonic glasses as a new type of long-lasting, non-toxic, and dynamically tunable pigment.

May 17, 2016
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Lesya Horyn, Young-Kee Kim lab
Jets in the ATLAS Detector

This talk will give a brief overview of jets and how we study them at the LHC.

May 3, 2016
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Prof. Young-Kee Kim
An Atom as an Onion

Subatomic-particle research has made enormous progress in the 20th Century by looking inside matter at deeper and deeper levels. It is as if we were peeling the layers of an onion in the hopes of finding more basic rules for the structure of nature. Although the concept of the ultimate building blocks of matter has been modified in several essential respects in the last century, Democritus's idea remains at the foundation of modern science. Great experiments of the 20th century have led to the discovery of ever-smaller entities that make up what were once thought to be indivisible particles. Starting with the atom, they uncovered the nucleus, which was made of smaller particles such as protons and neutrons. These have themselves been dissected into even smaller particles. Moreover, this theory of the very small has been shown to be intimately connected to the largest scales imaginable - cosmology and the beginnings of the universe. Despite these considerable successes, this current theory nevertheless has within it the seeds of its own demise and is predicted to break down when probed at even smaller scales. By using our increased understanding we continue to peel away at the more hidden layers of truth with the hope of discovering a more elegant and complete theory. But as is the case with the onion, we must wonder whether there will ever emerge an ultimate layer where the peeling must stop.

May 3, 2016
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Nidhi Pashine, Nagel lab
Designing Networks with unusual responses

Amorphous materials are structurally very different from crystalline materials. Here, we use jamming as a method to create anisotropic spring networks. By selectively removing a small percentage of bonds/springs from such a network, we can tune the network to have a desired response. It could be a change in the global elastic properties of the material, or a localized output corresponding to a local input strain at a distant site. The latter response is similar to the allosteric regulation in proteins where a reaction at one site activates another site of the protein molecule. Experimentally, we have been able to show these properties in 2D and 3D networks.

April 26, 2016
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Lipi Gupta
Raman Spectroscopy
Raman scattering is used to identify the vibrational modes of molecues, and has been used to characterize molecules since 1930 when C.V. Raman won the Nobel Prize in Physics for discovering this method. When light is scattered though a sample, the molecule can be polarized to certain modes, called Raman active modes, which emit light at specific energies. In order to excite these transitions, polarized light from a known frequency laser is scattered through a liquid sample, causing the emission of light from Raman active modes in the molecule. The light is collected and filtered through a monochromator in order to measure the intensity of the light at frequencies near the laser frequency, then amplified in a photomultiplier tube to be detected. By determining the polarization of the scattered light by using a polarizer, the symmetry of the vibrational modes can be determined. In our experiment, we studied the vibrational modes for CCl4, CS2, CHCl3, C6H6, and C6D6, and compared our results to literature values. We were able to determine the transition energies of the Raman active modes, the symmetry of the modes, and the temperature of the CS2 sample. The optical alignment and amplification of the signal from the Raman modes were optimized in order to study these samples. However, in a future study both could be improved to obtain higher resolution spectra.

April 19, 2016
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Aziza Suleymanzade, Simon lab, Schuster lab
Engineering interactions between optical and microwave photons/
In this talk I will describe the motivation and current progress on the experimental effort to engineer effective strong interaction between optical and microwave photons. In particular, I will give some theoretical background for quantum mechanics behind single photon interactions using Rydberg atoms as a mediator. I will go over the experimental setup for our hybrid optical cryogenic system necessary for the science. And, finally, comment on the importance of this project for the fundamental science as well as its potential applications.

 

When colloidal particles are rotated adjacent to nearby floor, strong advective flows are generated around them, even quite far away. When a group of these microrollers is driven, the strong hydrodynamic coupling between particles leads to formation of new structures: an initially uniform front of microrollers evolves first into a shock-like structure, which then quickly becomes unstable, emitting fingers of a well-defined wavelength. Our experiments and simulations confirm that this instability is quite different than typical fingering instabilities, where size scale selection is a consequence of competing stresses. Here, this instability arises only due to hydrodynamic interactions, and it is controlled by a single geometric parameter, the particle-floor height. Our measurements of the growth rate in both experiments and simulations agree with results from our continuum model. This instability is a direct consequence of the inward flows created by the interactions between the particles and the nearby solid surface.