Orientational instability and spontaneous rotation of active nematic droplets

Abstract

In experiments, an individual chemically active liquid crystal (LC) droplet submerged in the bulk of a surfactant solution may self-propel along a straight, helical, or random trajectory. In this paper, we develop a minimal model capturing all three types of self-propulsion trajectories of a drop in the case of a nematic LC with homeotropic anchoring at LC-fluid interface. We emulate the director field within the drop by a single preferred polarization vector that is subject of two reorientation mechanisms, namely, the internal flow-induced displacement of the hedgehog defect and the droplet's rotation. Within this reduced-order model, the coupling between the nematic ordering of the drop and the surfactant transport is represented by variations of the droplet's interfacial properties with nematic polarization. Our analysis reveals that a novel mode of orientational instability emerges from the competition of the two reorientation mechanisms and is characterized by a spontaneous rotation of the self-propelling drop responsible for helical self-propulsion trajectories. In turn, we also show that random trajectories in isotropic and nematic drops alike stem from the advectiondriven transition to chaos. The succession of the different propulsion modes is consistent with experimentally-reported transitions in the shape of droplet trajectories as the drop size is varied. arXiv:1909.06169v1 [cond-mat.soft]

Instabilities driven by diffusiophoretic flow on catalytic surfaces

“…Besides, in the experiments of active droplets, it is also observed that the droplet can move in a helical or even chaotic trajectory at high Pe (Suga et al 2018;Maass et al 2016). Morozov & Michelin (2019b) also observed the helical and chaotic motion of the catalytic particle. Recently, the stochastic dynamics of active particles was analysed (Gaspard & Kapral 2018;Chamolly & Lauga 2019).…”

Collisions and rebounds of chemically active droplets

“…Also note that Eqs. (37), (41) and (39) indicate that in the lab frame the leading order flow at a distance 1/ from an active droplet is O( 4 ) (recall that at 0 the fluid is motionless), while the droplet's chemical footprint is O( ). That is, at a distance 1/ from both droplets, the advection diffusion equation, Eq.…”

Adsorption inhibition by swollen micelles may cause multistability in active droplets

“…Although velocity of the flow is small, it does not decay far away from the moving drop (recall that we use a coordinate system co-moving with the droplet). This non-vanishing flow makes it impossible to satisfy the far-field boundary conditions (15) in the framework of regular asymptotic expansion. Instead, matched asymptotic expansions are required in this case 28 .…”

“…Robust active behavior and potential biocompatibility 7 of active droplets makes them compelling building blocks for active matter engineering. Accordingly, substantial theoretical effort is aimed at developing reliable models of active droplets 7,[10][11][12][13][14][15][16] . State-of-the-art models attribute spontaneous motion of active droplets to the Marangoni effect, that is, an interfacial flow emerging due to a chemically-induced gradient of the droplet surface tension 7,[10][11][12][13]16 .…”

“…State-of-the-art models attribute spontaneous motion of active droplets to the Marangoni effect, that is, an interfacial flow emerging due to a chemically-induced gradient of the droplet surface tension 7,[10][11][12][13]16 . For simplicity, diffusion-controlled surfactant sorption kinetics is typically postulated, when surfactant Nonlinear Physical Chemistry Unit, Faculté des Sciences, Université libre de Bruxelles (ULB), CP231, 1050 Brussels, Belgium; E-mail: matvey.morozov@ulb.ac.be concentration at the interface is proportional to the bulk concentration 7,[11][12][13][14][15] . Existing models also assume that the chemical reaction at the interface is a 0th 7,[13][14][15][16] or 1st 11,12 order reaction.…”