Submersed micropatterned structures control active nematic flow, topology, and concentration.
Proceedings of the National Academy of Sciences of the United States of America 118:38 (2021)
Abstract:Coupling between flows and material properties imbues rheological matter with its wide-ranging applicability, hence the excitement for harnessing the rheology of active fluids for which internal structure and continuous energy injection lead to spontaneous flows and complex, out-of-equilibrium dynamics. We propose and demonstrate a convenient, highly tunable method for controlling flow, topology, and composition within active films. Our approach establishes rheological coupling via the indirect presence of fully submersed micropatterned structures within a thin, underlying oil layer. Simulations reveal that micropatterned structures produce effective virtual boundaries within the superjacent active nematic film due to differences in viscous dissipation as a function of depth. This accessible method of applying position-dependent, effective dissipation to the active films presents a nonintrusive pathway for engineering active microfluidic systems.
Fluid flows on many scales
NATURE PHYSICS 17:6 (2021) 756-756
Extensile stress promotes out-of-plane flows in active layers
ArXiv 2105.10812 (2021)
Abstract:We use numerical simulations and linear stability analysis to study an active nematic layer where the director is allowed to point out of the plane. Our results highlight the difference between extensile and contractile systems. Contractile stress suppresses the flows perpendicular to the layer and favours in-plane orientations of the director. By contrast extensile stress promotes instabilities that can turn the director out of the plane, leaving behind a population of distinct, in-plane regions that continually elongate and divide. Our results suggest a mechanism for the initial stages of layer formation in living systems, and explain the propensity of dislocation lines in three-dimensional active nematics to be of twist-type in extensile or wedge-type in contractile materials.
Activity pulses induce spontaneous flow reversals in viscoelastic environments.
Journal of the Royal Society, Interface The Royal Society 18:177 (2021) ARTN 20210100
Abstract:Complex interactions between cellular systems and their surrounding extracellular matrices are emerging as important mechanical regulators of cell functions, such as proliferation, motility and cell death, and such cellular systems are often characterized by pulsating actomyosin activities. Here, using an active gel model, we numerically explore spontaneous flow generation by activity pulses in the presence of a viscoelastic medium. The results show that cross-talk between the activity-induced deformations of the viscoelastic surroundings and the time-dependent response of the active medium to these deformations can lead to the reversal of spontaneously generated active flows. We explain the mechanism behind this phenomenon based on the interaction between the active flow and the viscoelastic medium. We show the importance of relaxation time scales of both the polymers and the active particles and provide a phase space over which such spontaneous flow reversals can be observed. Our results suggest new experiments investigating the role of controlled pulses of activity in living systems ensnared in complex mircoenvironments.
Morphology of Active Deformable 3D Droplets
PHYSICAL REVIEW X American Physical Society (APS) 11:2 (2021) 21001