Archives: Events

The force on a boundary and the EOS for active matter

A touch of non-linearity: active matter in fluids at intermediate Reynolds

The complexity of emergent active-matter behavior has been demonstrated at many length-scales in both biological and artificial systems. However, a whole region of parameter space, that is mesoscale active matter, i.e. active matter of inertial particles in fluids at intermediate Reynolds numbers (Re), remains largely unexplored. The intermediate regime covers at least three orders of magnitude in Re (1-1000), opening up numerous possibilities for materials science, and describing a plethora of organisms, that we can study as model systems. In this talk, I will show how we are building a framework to study mesoscale active matter in fluids, starting with a classification of model inertial swimmers. I will present experiments and simulations of a reciprocal self-propelled swimmer made out of two unequal spheres. I will show what happens at the onset of inertia, where there is a transition from rest to swimming, and then demonstrate how this simple object actually switches direction as Re increases! The switch is a result of the nonlinearities that add up over a cycle at intermediate Re. I will discuss the relation with the pusher-numbers puller models in Stokes flows and the next steps for exploring collective behavior.

Microbes Under Pressure

Microtubule Self-Organization Only Needs a Little Crosslinking

The cell is a complex autonomous machine taking in information, performing computations, and responding to the environment. To enable agile read/write capabilities, much of the molecular biochemistry that performs these computations must be transient and weak, allowing signals to be carried as a function of the concentration of numerous and coupled interactions. Traditionally, biochemical experiments can only measure strongly interacting systems that can last for long times in dilute concentrations. We have developed microscopy measurements to enable to visualization of weak, transient interactions and the resulting emergent behaviors of coupled systems. I will present excerpts from stories where many weak, transient interactions can have strong repercussions on the overall activity and can, in fact, overpower strongly interacting systems. These studies involve the microtubule cytoskeleton and the transport motor, kinesin-1. Our results reveal a fundamentally important aspect of cellular self-organization: weak, transient interacting species can tune their interaction strength directly by tuning the local concentration to act like a rheostat. The tunability of weak, transient interactions is a fundamental activity of biological systems, and our insights will ultimately enable us to learn how to engineer thesis systems to create biological or biomimetic devices.

Topological Defects Control Collective Dynamics of Active Matter

I will present our recent experiments on collective dynamics of active matter such as Janus particles, molecular motors and biological cells, especially focusing on the role of topological defects in such systems.

Phase Equilibria in Motility-Induced Phase Separation

Motility-induced phase separation (MIPS) arises generically in fluids of self-propelled particles when interactions lead to a kinetic slowdown at high densities. We will show how a scalar continuum description of active matter, akin to a generalized Cahn-Hilliard equation, provides a unified description of MIPS. Indeed, it allows to understand the phase equilibria and quantitatively account for the phase diagram and finite size effects of the two models in which MIPS have been most commonly studied: self-propelled particles interacting either through a density-dependent propulsion speed or via direct pairwise forces.

Synthetic Nanomotors

The field of active and self-propelled micro- and nanomotors is seeing major advances. It encompasses externally as well as chemically propelled micro- and nanomotors, which are predicted to show fascinating collective phenomena at very high particle fractions. High densities, however, pose challenges for many experimental systems. We have therefore developed appropriate fabrication and assembly schemes that allow us to grow large numbers of catalytically active colloids, based both on inorganic light-induced reactions as well as enzymatic reactions, which can be operated at high densities. This talk will present our latest results and discuss open questions.

Flow-induced phase separation: Boundaries determine the collective dynamics

How boundaries and confinement determine the collective dynamics of motile populations, such as biological flocks, robotic swarms, and synthetic active particles, remains largely unexplored. Here, combining experiments, theory and numerical simulations, we study the influence of boundaries on the fluid-mediated, dissipative, many-body forces and torques that determine the collective dynamics of self-propelled particles. Using experiments with active emulsion droplets whose motion is fully three-dimensional, we demonstrate that geometric confinement alters the far-field flow of the particles and, thereby, their hydrodynamic interactions. These changes give rise to distinct states of collective organization: two-dimensional crystals arrested at free interfaces, three-dimensional crystals stabilized by vorticity, and one-dimensional quasi-stable lines that travel in both two and three dimensions.
We rationalize these experimental results by computing the slow viscous flow produced by the droplets in the presence of boundaries. Numerical simulations based on the theory are in excellent agreement with experiment. Our work elucidates how macroscopic boundaries, by altering the microscopic interactions between constituents, influence the emergence of long-ranged order and long-lived structures in non-equilibrium systems. Our findings are relevant to the formation of biological aggregates at multiple scales and to the emerging field of geometric and topological synthetic active matter.