Physics Department Colloquia

Spring 2019

Organizer: Vito Scarola

These meetings occur on Fridays from 2:30pm to 3:30pm in Robeson 304.
Refreshments are served at 2:15pm (unless otherwise indicated)

January 2019
January 18

Friday 2:30pm
210 Robeson Hall
(poster)

Residential Classes Begin
(No Colloquium)

Host:

January 25

Friday 2:30pm
210 Robeson Hall
(poster)

Reserved

Host:

January 28

"Special Date, Time & Place"
Monday 4:00pm
190 Goodwin Hall
(poster)

Dr. Rui Zhang (University of Chicago)

"Structure and Dynamics of topological Defects in Active Liquid Crystals"

Topological defects in nematic liquid crystals exhibit unique optical and physicochemical properties that have led to emerging applications in directed self-assembly of colloids and macromolecules. Recent experiments have demonstrated that active matter that consists of a dense collection of self-propelled rods can form an active nematic liquid crystal in which defects bind and unbind in a chaotic-like manner. Abundant examples of active nematics are found in different animate and inanimate systems, including flocking animals, bacteria, tissue cells, biopolymer suspensions, and even vibrating granular rods. However, the material properties of and seemingly chaotic-like defect dynamics in these non-equilibrium systems are poorly understood, limiting their applications. In this talk, I will discuss our recent work on unraveling defect behavior in active nematics. Specifically, we have adopted a hydrodynamic model to explain how the morphology, structure and dynamics of defects are determined by the interplay between elasticity and activity. Our model predictions are successfully confirmed by actomyosin-based experiments, shedding light on understanding and further control of topological defects in active liquid crystals.

Host: Shengfeng Cheng

February 2019
February 1

Friday 2:30pm
210 Robeson Hall
(poster)

Dr. Ting Ge (Duke University)

"Rheology and Nanorheology of Entangled Melts of Non-Concatenated Ring Polymers"

Rheology is the branch of science that studies how matter deforms and flows. Rheology of polymers is characterized by viscoelasticity that exhibits both solid-like elastic and liquid-like viscous features depending on the relevant time scales. Understanding and controlling the macromolecular dynamics underlying the viscoelastic response of polymers is one of the great challenges in polymer science. Complementary to experiments, theoretical and computational approaches provide deep physical insight into polymer dynamics. We use scaling theory and molecular dynamics simulation to study the rheology of non-concatenated ring polymers [1], a prominent example of polymers with non-linear architectures. The study of ring polymers sheds light on the effects of polymer architecture on polymer dynamics and rheology. Additionally, the melt of non-concatenated ring polymers serves as a good model for the de-swollen polymer network that possesses super-elasticity and for the organization of chromosomes critical to the gene expression and regulation in cell nucleus. Unlike their linear counterparts, ring polymers do not form long-lived entanglement network. Their viscoelastic response is characterized by a power-law stress relaxation prior to terminal viscous flow. The theoretical description and simulation results agree with each other and are in consistence with experimental observations of ring polymers. We further use molecular simulations to study nanorheology, which employs embedded nanoparticles to explore local viscoelasticity of polymers [2]. Nanorheology is important to the processing of nanoparticle polymer composites and the design of particle-based drug delivery systems in living cells as well. We find that the motion of large nanoparticles is not strongly suppressed in ring polymers as in linear polymers, because there is no long-lived entanglement network that traps nanoparticles. With increasing nanoparticle size, the local viscoelasticity experienced by the particle approaches the bulk viscoelasticity of ring polymers. We estimate that the bulk viscoelasticity is reached for nanoparticle size about twice the average spanning size of ring polymers. References [1]. “Self-Similar Conformations and Dynamics in Entangled Melts and Solutions of Nonconcatenated Ring Polymers”, T. Ge, S. Panyukov, and M. Rubinstein, Macromolecules 49, 708 (2016) [2]. “Nanorheology of Entangled Polymer melts”, T. Ge, G. S. Grest, and M. Rubinstein, Phys. Rev. Lett. 120, 057801 (2018) .

Host: Shunsaku Horiuchi

February 8

Friday 2:30pm
210 Robeson Hall

(poster)

Dr. Daniel Sussman (Syracuse University)

"Anomalous Interfaces in Biological Matter"

What can we learn about dense biological tissue by viewing it as a soft, active matter system? The mechanical and dynamical properties of dense collections of cells help govern processes ranging from wound healing to embryonic development to cancer progression, and an outstanding challenge is developing tractable models that can predict and explain the amazing variety of complex phenomena that even simple cellular systems can exhibit. Recent experiments have shown, for example, that many tissues lie close to a collective rigidity transition, and I will briefly discuss how simple coarse-grained models of dense tissue can support unusual forms of mechanical integrity. I will then show that these models exhibit anomalous interfacial properties, with different measurements of the surface tension between two tissues types differing by orders of magnitude. This departure from equilibrium behavior can be understood as a generic consequence of certain topological features of the cell-cell interactions, and I will discuss the potential relevance of this mechanism for both biological processes (such as cell sorting and compartmentalization) and for designing new materials with exotic bulk and boundary behavior.

Host: Michel Pleimling

February 15

Friday 2:30pm
210 Robeson Hall

(poster)

Dr. Antonia Statt (Princeton University)

"Pathways to Structure Formation in Colloid and Polymer Mixtures"

Soft matter is important in technological applications, biology and everyday life. Its behavior on mesoscopic scales is challenging to predict because dominant energy scales are of the magnitude of thermal fluctuations. I will present simulation results for two examples of structure formation in soft matter: colloidal crystal nucleation and inverted stratification in drying polymer mixtures. We developed a novel method to determine nucleation barriers without calculating the anisotropic interfacial tension or locating the interface precisely. By demonstrating the importance of hydrodynamic interactions during evaporation, we show that hydrodynamics need to be incorporated when predicting the structure of drying films.

Host: Uwe Tauber

February 18

"Special Date, Time & Place"
Monday 4:00pm
190 Goodwin Hall
(poster)

Dr. Cihan Nadir Kaplan (Harvard University)

"Morphing hard and soft matter by reaction-transport dynamics"

Engineering next-generation materials that can grow into efficient multitasking agents, move rapidly, or discern environmental cues greatly benefits from inspiration from biological systems. In the first part of my talk, I will present a geometrical theory that explains the biomineralization-inspired growth and form of carbonate-silica microarchitectures in a dynamic reaction-diffusion system. The theory predicts new self-assembly pathways of intricate morphologies and thereby guides the synthesis of light-guiding optical structures. The second part is dedicated to a soft matter analog of controlled actuation and complex sensing in living systems. Specifically, I will introduce a continuum framework of a simple hydrogel system that is activated upon transport and reaction of chemical stimuli. The hydrogel exhibits unique cascades of mechanical and optical responses, suggesting that common gels have a much larger sensing space than currently employed. The theoretical work presented in my talk is intimately connected to modern materials science. The effective convergence of theory and experiment paves the way for optimized hard or soft biomimetic materials for applications ranging from bottom-up manufacturing to soft robotics.

Host: Rana Ashkar

February 22

Friday 2:30pm
210 Robeson Hall

(poster)

Reserved

Host:

February 25

"Special Date, Time & Place"
Monday 4:00pm
190 Goodwin Hall
(poster)

Dr. Trung Dac Nguyen (Northwestern University)

"Engineering materials from bottom up for bioremediation applications"

The accumulation of mismanaged organic waste and toxins in insecticides and warfare agents poses serious environmental threats to the ecosystem and human health. Bioremediation provides an effective means to address these issues, for instance, by using cleaning agents containing enzymes that break down the pollutants into more bio-friendly products. The ability to maximize the catalytic activity of those enzymes out of their native media will enable industry-scale bioremediation applications for a wider variety of pollutants. In this talk, I will discuss a recently proposed bottom-up approach for stabilizing enzymes in organic solvents, where the enzymes are mixed with copolymers that are composed of hydrophobic and hydrophilic monomers arranged in disordered sequences. These so-called random heteropolymers possess a rich diversity in monomer sequences, which plays a vital role in reducing the enzymes’ exposure the unfavorable solvent. This helps explain why the proposed approach is superior to the often-used reverse-micelle techniques in retaining the activity of numerous types of enzymes in toluene. Furthermore, the collectivity in the interaction between the polymers and enzymes upon assembly is predicted to lead to the uniform size of the assembled complexes, which is important for their solubility and delivery. These outstanding features of the random heteropolymer approach suggests new possibilities for engineering protein-based materials for applications much beyond bioremediation.

Host: Daniel Capelluto

March 2019
March 1

Friday, 2:30pm
210 Robeson Hall
(poster)

Prof. Maikel Rheinstadter (McMaster University)

"Neutrons and X-Rays for Health and Disease"

Molecular biology is key to understand the origin of diseases and the molecular mode-of-action of drugs, which is essential for modern drug design. Neutron and x-ray scattering are amongst our most advanced tools to study such molecular structure and dynamics. Together with computer simulations they provide unprecedented capabilities in this respect. I will give examples of applications of neutrons and x-rays to better understand the functioning of drugs and antibiotics and how these experiments shed new light on our understanding of diseases, such as Alzheimer’s disease, and the origin of life, including biomedical applications [1]. References 1. http://www.rheinstaedter.de/maikel/publications/publications.htm

Host: Rana Ashkar

March 8

Friday 2:30pm
210 Robeson Hall

(poster)

APS Meeting
(No Colloquium)

Host:

March 15

Friday 2:30pm
210 Robeson Hall

(poster)

Spring Break
(No Colloquium)

Host:

March 22

Friday 2:30pm
210 Robeson Hall
(poster)

Prof. Matthew Helgeson (UC Santa Barbara)

"Toward “Damascus yogurt”: developing thermal processing strategies for colloidal solids"

The use of thermal processing to control phase separation in atomic and molecular solids is a conserved motif for designing materials with tailored mechanical properties that spans antiquity, modern technology and the natural world. By contrast, thermal processing of colloidal solids (e.g. particle gels and glasses) poses significant challenges, including relatively slow dynamics and a lack of scalable materials in which colloidal interactions can be actively tuned and quenched. To overcome these challenges, we have developed a model thermally processable colloidal system based on nanoparticles in the presence of thermoresponsive polymers that allows for fine control over their interparticle attractions and resulting colloidal behavior. We show how this thermoreversible behavior allows access to a range of gelation mechanisms including dynamic percolation, arrested phase separation and glass formation, whose outcome can be selected through the path taken to the gelled state. In cases of arrested phase separation, we explore the analogy between thermal processing in colloids with that in atomic and molecular systems, including the kinetics and micromechanics of structural coarsening and the concept of a “time temperature transformation” diagram from which to design arrested morphologies. Ultimately, we show how the sophisticated control of gelation and arrested structure afforded by thermal processing can be used to tailor colloidal solids (and other materials from them) with widely varying and superior mechanical properties. Bio. Matt Helgeson is an Associate Professor in the Department of Chemical Engineering at UC Santa Barbara, where he is also a faculty member and IRG co-leader of the Materials Research Laboratory (an NSF MRSEC). He received his B.S. in Chemical Engineering at Carnegie Mellon University in 2004, and his Ph.D. in Chemical Engineering at the University of Delaware in 2009. He performed postdoctoral research at MIT before joining the faculty at UCSB. Prof. Helgeson’s research focuses on designing and processing complex fluids with well-specified nanostructure, especially those involving colloids in structured liquids. Prof. Helgeson’s research has been recognized with a number of awards, including Early Career Awards from both the National Science Foundation (2013) and Department of Energy (2015), a Hellman Foundation Faculty Fellowship (2016), and both the Victor K. LaMer Award (2011) and Unilever Award (2016) from the American Chemical Society.

Host: Rana Ashkar

March 29

Friday 2:30pm
210 Robeson Hall

(poster)

Vidya Madhavan (University of Illinois Urbana-Champaign)

Signatures of Dispersing 1D Majorana Fermions in Condensed Matter Systems

Dirac discovered that every fundamental particle must also have an anti-particle which has the opposite charge. When particles and anti-particles meet, they annihilate each other, releasing energy. A Majorana fermion is a special type of fundamental particle which is its own antiparticle. The possible realization of these exotic Majorana fermions as quasiparticle excitations in condensed matter physics has created much excitement. Most recent studies have focused on Majorana bound states which can serve as topological qubits. More generally, akin to elementary particles, Majorana fermions can propagate and display linear dispersion. These excitations have not yet been directly observed, and can also be used for quantum information processing. This talk is focused on our recent work in realizing dispersing Majorana modes. I will describe the conditions under which such states can be realized in condensed matter systems and what their signatures are. Finally, I will describe our scanning tunneling experiments of domain walls in the superconductor FeSe0.45Te0.55, which might potentially be first realization of dispersing Majorana states in 1D.

Host: Kyungwha Park

April 2019
April 5

Friday 2:30pm
210 Robeson Hall

(poster)

Awards Day
(No Colloquium)

Host:

April 12

Friday 2:30pm
210 Robeson Hall

(poster)

Prof. Brian Metzger (Columbia University)

"The Multi-Messenger Picture of a Neutron Star Merger"

On August 17, 2017 the LIGO/Virgo gravitational wave observatories detected the first binary neutron star merger event (GW170817), a discovery followed by the most ambitious electromagnetic (EM) follow-up campaign ever conducted. Within 2 seconds of the merger, a weak burst of gamma-rays was discovered by the Fermi and INTEGRAL satellites. Within 11 hours, a bright but rapidly-fading thermal optical counterpart was discovered in the galaxy NGC 4993 at a distance of only 130 Million light years. The properties of the optical transient match remarkably well predictions for “kilonova” emission powered by the radioactive decay of heavy nuclei synthesized in the expanding merger ejecta by rapid neutron capture nucleosynthesis (r-process). The rapid spectral evolution of the kilonova emission to near-infrared wavelengths demonstrates that a portion of the ejecta contains heavy lanthanide nuclei. Two weeks after the merger, rising non-thermal X-ray and radio emission were detected from the position of the optical transient, consistent with delayed synchrotron afterglow radiation from an initially off-axis relativistic jet (or a shock-heated "cocoon" produced as the ejecta interacts with the kilonova ejecta). I will describe efforts to create a unified scenario for the range of EM counterparts from GW170817 and their implications for the astrophysical origin of the r-process and the properties of neutron stars (particularly their uncertain radii and maximum mass, which are determined by the equation of state of dense nuclear matter). Time permitting, I will preview the upcoming era of multi-messenger astronomy, once Advanced LIGO/Virgo reach design sensitivity and a neutron star merger is detected every few weeks.

Host: Tommy O'Donnell

April 19

Friday 2:30pm
210 Robeson Hall

(poster)

Professor Zheng-Gang Wang (California Institute of Technology)

"Electrostatics beyond Poisson-Boltzmann: Effects of Self-Energy"

Ions are essential in physical chemistry, colloidal science, electrochemistry, biology and many other areas of science and engineering. While their role is commonly described in terms of screening and translational entropy, many phenomena, ranging from some classical experimental observations made many decades ago to some new systems of current interest, cannot be explained, even qualitatively, by these concepts. A key effect that is often ignored or inadequately treated in the main literature on electrolytes and polyelectrolytes is the self-energy of the ions. In this talk, I will discuss several self-energy effects in macromolecular and interfacial systems. First, we show that the preferential solvation energy of the ions provides a significant driving force for phase separation. This concept is used to develop a theory to explain the dramatic shift in the order-disorder transition temperature in PEO-PS diblock copolymers upon the addition of salt. Second, we show that the dielectric contrast between the polymer backbone and the solvent significantly affects the conformation and charge condensation in dilute polyelectrolyte solutions. Third, we show that the image force has qualitative effects on the double layer structure and forces, such as like charge attraction and charge inversion. Finally we present a simple theory for treating charge correlation effects in polyelectrolyte solutions that self-consistently account for the conformation changes of the polyelectrolyte chains.

Host: Shengfeng Cheng

April 26

Friday 2:30pm
210 Robeson Hall

(poster)

Prof. Christopher Jarzynski (University of Maryland)

“Scaling Down the Laws of Thermodynamics”

Thermodynamics provides a robust conceptual framework and set of laws that govern the exchange of energy and matter. Although these laws were originally articulated for macroscopic objects, it is hard to deny that nanoscale systems also exhibit “thermodynamic-­like” behavior – for instance, biomolecular motors convert chemical fuel into mechanical work. To what extent can the laws of thermodynamics be “scaled down” to apply to individual microscopic systems, and what new features emerge at the nanoscale? I will describe some of the recent progress and challenges associated with addressing these questions.

Host: Ed Barnes

May 2019
May 3

Friday 2:30pm
210 Robeson Hall
(poster)

Prof. Mete Atature (Cambridge University)

“Solid-State Quantum Emitters: Old Friends & New”

Optically active spins confined in solids, such as semiconductors, silicon carbide or diamond, are exciting physical systems as quantum-bit candidates for quantum science and its applications. Their inherently mesoscopic nature leads to a multitude of dynamics within the solid-state environment of spins, charges, vibrations and light. Implementing a high level of control on these constituents and their interactions with each other creates exciting opportunities for realizing stationary and flying qubits within the context of spin-based quantum information science. Quantum optics, developed originally for atomic systems, provides a valuable toolbox for this endeavour. In this talk, I will provide a snapshot of the progress and challenges for quantum light sources, spin-photon interfaces and optical interconnection in semiconductor systems.

Host: Sophia Economou

May 8

Wednesday,
210 Robeson Hall
(poster)

Classes End
(No Colloquium)

May 10

Friday, 2:30pm
210 Robeson Hall
(poster)

Exam Day
(No Colloquium)

May 17

Friday, 2:30pm
210 Robeson Hall
(poster)

University and Graduate Commencement
(No Colloquium)

May 18

Saturday
210 Robeson Hall
(poster)

Spring Commencement (College Ceremonies)
(No Colloquium)

Physics Department Colloquia

Fall 2019

Organizer: Vito Scarola

These meetings occur on Fridays from 2:30pm to 3:30pm in Robeson 304.
Refreshments are served at 2:15pm (unless otherwise indicated)

August 2019
August 30

Friday 2:30pm
210 Robeson Hall
(poster)

Host:

September 2019
September 6

Friday 2:30pm
210 Robeson Hall
(poster)

Host:

September 13

Friday 2:30pm
210 Robeson Hall
(poster)

Host:

September 20

Friday 2:30pm
210 Robeson Hall
(poster)

.

Host:

September 27

Friday 2:30pm
210 Robeson Hall

(poster)

Host:

October 2019
October 4

Friday 2:30pm
210 Robeson Hall

(poster)

Host:

October 11

Friday 2:30pm
210 Robeson Hall

(poster)

Host:

October 18

Friday 2:30pm
210 Robeson Hall

(poster)

Host:

October 25

Friday 2:30pm
210 Robeson Hall

(poster)

Host:

November 2019
November 1

Friday 2:30pm
210 Robeson Hall

(poster)

Host:

November 8

Friday 2:30pm
210 Robeson Hall


(poster)

Host:

November 15

Friday 2:30pm
210 Robeson Hall

(poster)

Prof. Arjun Yodh University of Pennsylvania

"TBD"

Host: Shengfeng Cheng

November 22

Friday 2:30pm
210 Robeson Hall

(poster)

Host:

November 29

Friday 2:30pm
210 Robeson Hall

(poster)

Thanksgiving Holiday (No Classes)

Host:

December 2019
December 6

Friday 2:30pm
210 Robeson Hall
(poster)

Host:

December 13

Friday 2:30pm
210 Robeson Hall
(poster)

Exam Day (No Classes)

Host:

December 20

Friday 2:30pm
210 Robeson Hall
(poster)

University and Graduate Ceremonies (No Classes)

Host:
December 27

Friday 2:30pm
210 Robeson Hall
(poster)

Holiday (No Classes)

Host:
December 30

Friday 2:30pm
210 Robeson Hall
(poster)

Holiday (No Classes)

Host: