Laboratoire Matière et Systèmes Complexes

Flowing soft matter at low Reynolds numbers

Holger Stark

Abstract:

Soft matter in flow creates novel emergent behavior and spatio-temporal patterns, which are finger prints of the non-equilibrium. In this talk I explore emergent collective behavior for passive colloids and microswimmers but also the design of flow patterns at photoresponsive fluid interfaces.

First, simulations of dense colloidal suspensions under pressure-driven flow in a narrow microchannel reveal self-organized rarefaction pulses traveling upstream similar to experimental observations. Force chains attached to the channel walls by frictional contacts create transiently jammed regions, which are unjammed by the rarefaction pulses. We rationalize the traveling pulses in a phenomenological model.

Second, light-switchable surfactants create a photoresponsive fluid interface. Its surface tension can be changed locally upon illumination with light. The gradient in surface tension then drives Marangoni flow. We demonstrate how special feedback rules can be used to create regular and irregular flow patterns.

Lastly, we use squirmers as model swimmers and explore their behavior under gravity, where they show exponential sedimentation profiles. For bottom-heavy microswimmers reminiscent of algae a zoo of dynamic structures emerges including convective plumes and spawning clusters, while under strong gravity single mono layers of squirmers form hydrodynamic „Wigner fluids“ or exhibit swarming depending on the swimmer type.

Tuesday, January 29 at 12:00pm in room 454A