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

Abstract

Chemically-active droplets exhibit complex avoiding trajectories. While heterogeneity is inevitable in active matter experiments, it is mostly overlooked in their modelling. Exploiting its geometric simplicity, we fully-resolve the head-on collision of two swimming droplets of different radii and demonstrate that even a small contrast in size critically conditions their collision and subsequent dynamics. We identify three fundamentally-different regimes. The resulting high sensitivity of pairwise collisions is expected to profoundly affect their collective dynamics. * Electronic address: sebastien.michelin@ladhyx.polytechnique.fr arXiv:2002.05085v1 [physics.flu-dyn]

Alignment and scattering of colliding active droplets

“…Complex dynamical regimes were identified, including a delay of the droplets' rebound at larger Pe or the emergence of bound states of chasing droplets of different radii. 38 The subdominance of hydrodynamic interactions near the self-propulsion threshold was also confirmed, 37 as a result of the small stresslet signature of the droplets.…”

“…These chemical interactions are repulsive and have been identified as the dominant contribution to the droplets' collective dynamics, both in experiments 19 and from a complete modelling of the two-droplet dynamics. 37,38 Based on this observation, this work proposes a general simplified framework in terms of moving singularities to analyse the collisions of N-droplet collisions. Building upon a detailed understanding of the axisymmetric configuration, for which a full solution of the chemo-hydrodynamic system is available, this model is then exploited to characterise in detail the generic (oblique) planar collisions of two droplets, which is more relevant to experimental conditions.…”

Isotropically active colloids under uniform force fields: from forced to spontaneous motion

“…We note that axisymmetric (Lippera et al. 2020 c ; Lippera, Benzaquen & Michelin 2020 b ) and planar (Lippera, Benzaquen & Michelin 2020 a ) interactions have recently been considered in the different physical context of active droplets, discussed in § 1. These studies also employed a weakly nonlinear analysis near the critical Péclet number (Lippera et al.…”

“…Rather than external forcing, one could also consider perturbations owing to a slight asymmetry in the particle geometry or surface composition, or weak interactions with distant particles and boundaries. We note that axisymmetric (Lippera et al 2020c;Lippera, Benzaquen & Michelin 2020b) and planar (Lippera, Benzaquen & Michelin 2020a) interactions have recently been considered in the different physical context of active droplets, discussed in § 1. These studies also employed a weakly nonlinear analysis near the critical Péclet number (Lippera et al 2020c).…”

Confined self-propulsion of an isotropic active colloid

“…2019), spherical (Michelin et al. 2013) or bi-spherical coordinates (Lippera, Benzaquen & Michelin 2020 a ; Lippera et al. 2020 b ).…”