MRSEC Seminars

The MRSEC holds seminars presenting research at the frontier of Bioinspired Soft Materials. The seminars are targeted towards graduate students and other researchers in the field, although everyone is invited to attend. As the topic is highly interdisciplinary, seminars are designed to be accessible to a wide range of backgrounds.

The seminars take place on Thursdays at 4PM on Zoom.

Organizer: John Berezney (Dogic/Fraden Lab Postdoc)

Spring 2021

Life and death of topological defects in polar active matter

February 25, 2021

(Note: This special seminar takes place at 10AM EST)

Amélie Chardac, ENS de Lyon

Abstract:  When motile units self-assemble into flocks where all particles propel along the same direction, they realize one of the most stable phase observed in Nature. Unlike in active nematics or passive systems such as ferromagnets or liquid crystals, the long range orientational ordered active fluids formed by flocking units are robust to defect proliferation even in two dimensions. In this talk, building on model experiments based on Quincke rollers, I will present how a flock suppresses its singularities to form an ordered spontaneous flow and explain how one can stabilize topological defects in a polar active fluid. 

The velocity field of a colloidal flock, initially marred by a number of singularities, heals and reaches pristine orientational order. Combining experiments, simulations and theory I will show how to elucidate the elementary excitations of 2D polar active matter and explain their phase ordering dynamics. In particular, I will explain how self-similar dynamics emerges from the annihilation of ±1 vortices along a filamentous network of domain walls with no counterparts in passive systems. Remarkably, the structure of this fully connected network is mainly determined by extended singularity lines growing from -1 vortices. The two body interactions between the defects correctly account for the selfsimilar coarsening of the density and flow excitations of flocking liquids.

Then, combining experiments and theory, I will show that colloidal flocks collectively cruise through disorder without relaxing the topological singularities of their flows, unlike in pure systems where they reach a macroscopic global order. Indeed, introducing colloidal flocks in micro patterns circular chambers, we reveal a state of strongly disordered active matter with no counterparts in equilibrium : a dynamical vortex glass. The resulting state is highly dynamical but the flow patterns, shaped by a finite density of frozen vortices, are stationary and exponentially degenerated. Quenched isotropic disorder acts as a random gauge field turning active liquids into dynamical vortex glasses.

Stochastic dynamics and selection in the one dimensional Kuramoto-Sivashinsky equation/Active nematic in a lid-driven cavity

March 4, 2021

Saloni Saxena, Brown University

Abstract: In the first part of this talk, I will describe recent results in the problem of wavenumber selection in pattern-forming systems. I will focus on the one dimensional stabilized Kuramoto-Sivashinsky equation with additive noise. I will show that the presence of noise leads the system to prefer one of many possible periodic steady states, establishing the critical role of noise in the selection process. I will then explore the detailed mechanism of noise-induced selection by looking at the ensemble averaged growth of each unstable Fourier mode from the spatially homogeneous state, with and without noise. I will show that noise opposes the growth of perturbations with wave numbers in a small band around the critical wave number and boosts the growth of perturbations with wavenumbers much smaller than the critical wave number. This process determines the selected wave number. 

In the second part of the talk, I will describe some preliminary results of Lattice Boltzmann simulations of an active nematic in a lid-driven cavity. I will conclude by mentioning some questions that we can potentially answer through these simulations.

Tissues as active materials: spontaneous flows and topological defects in active cellular nematics

April 20, 2021

(Note: This special seminar takes place at 2PM EST)

Louis Brézin, PhD Soft matter and biophysics at College de France & Institut Curie

Abstract: Biological tissues formed by elongated cells can organize into a nematic phase, similar to liquid crystals. Because cells convert chemical energy into mechanical work, these tissues can be treated as active materials. A key consequence of this activity is the emergence of spontaneous flows due to gradients of orientation. Using a hydrodynamic theory of active nematics, I will present two cases where spontaneous flows arise in cellular nematics. The first one is from confinement of cells in channels, where geometrical cues control the orientation of cells, allowing for the measurement of material parameters. The second case is the flow created by the configuration of topological defects, that have been reported to be preferential sites for extrusion and multilayering. In particular, I will present the effect of the backflow on the configuration of defects, and the effect of cell proliferation and extrusion on the motion of defects.