Clustering-induced velocity-reversals of active colloids mixed with passive particles

Abstract

Recent experiments have shown that colloidal suspensions can spontaneously self-assemble into dense clusters of various internal structures, sizes and dynamical properties when doped with active Janus particles. Characteristically, these clusters move ballistically during their formation, but dynamically revert their velocity and temporarily move opposite to the self-propulsion direction of the Janus particles they contain. Here we explore a simple effective model of colloidal mixtures which allows reproducing most aspects seen in experiments, including the morphology and the velocity-reversal of the clusters. We attribute the latter to the nonreciprocal phoretic attractions of the passive particles to the active colloids' caps, taking place even at close contact and pushing the active particles backwards. When the phoretic interactions are repulsive, in turn, they cause dynamical aggregation of passive colloids in the chemical density minima produced by the active particles, as recently seen in experiments; in other parameter regimes they induce travelling fronts of active particles pursued by passive ones coexisting with an active gas.arXiv:1909.09578v1 [cond-mat.soft]

Which interactions dominate in active colloids?

“…Eq. ( 8) fits well to experimental data for the time-evolution of the ensemble-averaged relative distance between active Janus-colloids and passive tracers and suggests that the screening length might be on the order of a few particle diameters in Janus-colloids [66,67]. However, in the future a more detailed modeling of the effect of bulk reactions within a multi-species framework would be desirable to understand effective screening in more detail (see open challenges at the end of the article).…”

“…These simulations could successfully reproduce the aggregation dynamics with a near quantitative agreement of the time-dependent mean size of the largest cluster and broadly also the cluster speed as a function of . Velocity-reversals of active Janus particles (red/black disks) seen in experiments [136] and simulations [67]. These reversals are induced by nonreciprocal phoretic (and osmotic) interactions leading to the aggregation of passive colloids (blue/grey disks) at the cap of the Janus-colloids.…”

“…See further comparisons between the 1/r 2 -law and its screened variant see also refs. [66,67]. Here the results of the measurements of the distance between the Janus colloids and the tracers from [88] and [136] has been compared with Eq.…”

“…Even at close contact the passive colloids are nonreciprocally attracted by the active Janus colloids and push them against their self-propulsion direction. Reproduced with permission from [67].…”

Time-dependent properties of interacting active matter: Dynamical behavior of one-dimensional systems of self-propelled particles

“…Certain biological systems such as run-and-tumble bacteria or crawling cells, as well as nonbiological systems such as self-driven colloids or artificial swimmers, commonly referred as active matter, can be described in terms of effective models able to capture their salient features [1][2][3]. Active particles display a very rich phenomenology, such as their accumulation at the boundaries [4][5][6][7] and near rigid obstacles [8][9][10][11][12][13] or a kind of nonequilibrium phase-coexistence, known as motility induced phase separation (MIPS) [14][15][16][17][18] occurring even in the absence of attractive [19][20][21][22][23][24][25][26][27][28] or depletion interactions [29]. Selfpropelled particles are far-from-equilibrium systems, showing several dynamical anomalies which have not a Brownian counterpart [30,31].…”