PHD Defense - Claire Doré - 15/12/2022 - Active nematic films under confinement: harnessing topological defects, shaping active flows and designing autonomous microfluidic machines

Claire Doré’s PhD Defense will take place on:
December 15th at 3pm
Charpak amphitheater
10 rue Vauquelin, 75005 Paris

Active nematic films under confinement: harnessing topological defects, shaping active flows and designing autonomous microfluidic machines

Living systems rely on a large number of agents converting local chemical energy into motion. Therefore they are inherently out-of-equilibrium. In this thesis, we study a model synthetic active material created by the group of Zvonimir Dogic using cytoskeleton elements, which is typically referred to as « active nematic ». Here, the active unit is an extensile bundle of microtubules driven by kinesin clusters fueled by adenosine triphosphate. At an oil interface, the bundles self-assemble into a dense film that displays nematic order on a scale much larger than the microtubule or the bundle itself. However, unlike passive liquid crystals, here nematic order is disrupted by the presence of motile topological defects that drive the system dynamics into spatio-temporal chaos. In this thesis, we investigate the emergence of ordered spatio-temporal patterns in laterally confined active nematics using both experiments and numerical simulations, and we show that lateral boundaries can be used to control active flows. In the first part, we experimentally study semi-confined active nematics in the vicinity of a lateral boundary. We find that the lateral wall is exclusively populated by negative defects, which exhibit exotic structure and dynamics. We show that geometrical patterning of the lateral boundary can allow for control over defect nucleation and induce directional flows. In the main part, we confine the active nematic to individual, narrow, open channels, and observe the emergence of directional flows along one direction, selected by spontaneous symmetry breaking. When fluctuations are important, the system can reverse the flow direction. We show that the flow amplitude and its stability can be geometrically controlled. For instance, we are able to effectively enforce either shear or directed flow states by shaping the confining wall with a ratchet pattern. Geometrical patterning of the channel wall induces spatial and orientational ordering of the motile topological defects. We perform numerical simulations using nematodynamics equations and a hybrid Lattice Boltzmann method, capturing some main features of our experimental results. Finally, we experimentally build active nematic flow networks where we test a previously published theoretical model. We start with a simple configuration of three channels connected in a bifurcation, and we conclude with the realization of an AND/OR logical gate, demonstrating the potential of active nematics to power autonomous microfluidic networks.


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