Self-propelled motion switching in nematic liquid crystal droplets in aqueous surfactant solutions

Abstract

The self-propelled motions of micron-sized nematic liquid crystal droplets in an aqueous surfactant solution have been studied by tracking individual droplets over long time periods. Switching between self-propelled modes is observed as the droplet size decreases at a nearly constant dissolution rate: from random to helical and then straight motion. The velocity of the droplet decreases with its size for straight and helical motions but is independent of size for random motion. The switching between helical and straight motions is found to be governed by the self-propelled velocity, and is confirmed by experiments at various surfactant concentrations. The helical motion appears along with a shifting of a point defect from the self-propelled direction of the droplet. The critical velocity for this shift of the defect position is found to be related with the Ericksen number, which is defined by the ratio of the viscous and elastic stresses. In a thin cell whose thickness is smaller than that of the initial droplet size, the droplets show more complex trajectories, including "figure-8s" and zigzags. The appearance of those characteristic motions is attributed to autochemotaxis of the droplet.

Nonlinear dynamics of a chemically-active drop: From steady to chaotic self-propulsion

“…In contrast, a fascinating feature of chemically-active droplets lies in their ability to exhibit complex dynamical behavior at the individual level as well. Solitary active drops were observed to self-propel spontaneously with straight, helical, or chaotic trajectories, where the choice of a particular trajectory depends on the phase of the liquid crystal constituting the drop [5], on the size of the droplet and on the intensity of the chemical reaction fueling the motion [6], as well as on the geometrical constraints [7]. Self-deformation and division were shown to occur when drops are impregnated with surfactant [8], so that active droplets were also recently considered as minimal model for synthetic cells [9].…”

Bouncing, chasing, or pausing: Asymmetric collisions of active droplets

“…Instead, the emergence of spontaneous dynamics stems from the nonlinear coupling of physico-chemical processes and convective transport. Beyond steady and directed selfpropulsion, active droplets may exhibit complex curling trajectories [14][15][16], random diffusive behaviour [15,17] and unsteady self-propulsion [18] at the individual level, and collective dynamics such as avoidance [19], aggregation [20,21], polar alignment [5], crystallization [22] and train formation [23].…”

Orientational instability and spontaneous rotation of active nematic droplets

“…We emphasize that active droplet mobility is typically attributed solely to the Marangoni effect [15,16]; Equation (2) thus represents an alternative approach to modeling of active drop mobility and the secondary goal of this paper is to demonstrate that such a model can successfully capture fundamental dynamical features of active drops, including the onset of chaos. In experiments, the energy required for self-propulsion of active drops is generated in a chemical reaction sustained at the droplet interface [8,[19][20][21]. Specifically, drops undergo gradual micellar dissolution and their dissolution time is orders of magnitude larger than the characteristic time scales associated with their propulsion.…”

“…In addition to straight and helical self-propulsion trajectories, active nematic drops also exhibit chaotic behavior [20]. Chaotic self-propulsion trajectories were also observed in isotropic droplets [8,20,21], suggesting the existence of a universal mechanism of transition to chaos in active drops. It was recently argued that emergence of random behavior of isotropic active drops is linked to the nonlinear effect of surfactant advection and to the precise physico-chemical mechanism responsible for the flow actuation [22].…”

“…Yet, recent numerical simulations indicate that, in the case when the mobility mechanisms from the two categories act simultaneously, the Marangoni effect may hinder the transition to chaos [22]. On the other hand, ubiquity of chaotic regimes in experiments with active droplets hints that some contribution of diffusiophoresis might be important for successful modeling of surfactant-laden interfaces of active LC droplets [8,20,21].…”