Realization of a motility-trap for active particles

Abstract

Trapping of atomic and mesoscopic particles with optical fields is a practical technique employed in many research disciplines. Developing similar trapping methods for self-propelled, i.e. active, particles is, however, challenging due to the typical anisotropic material composition of Janus-type active particles. This renders their trapping with magneto-optical fields to be difficult. Here we present the realization of a motility-trap for active particles, which only exploits their self-propulsion properties. By combining experiments, numerical simulations, and theory, we show that, under appropriate conditions, a force-free rotation of the self-propulsion direction towards the trap's center can be achieved, which results in an exponential localization of active particles. Because this trapping mechanism can be applied to any propulsion scheme, we expect such motility-tweezers to be relevant for fundamental studies of self-driven objects as well as for their applications as autonomous microrobots.

Dynamics of active particles with space-dependent swim velocity

“…We fix k = 5/4 to consider a regime of the persistence length large if compared with the spatial period of u(x). The two relaxation times, in the long-time regime and in the small-time one, do not depend on a, in agreement with prediction (21). However, a affects the survival time of the small time-regime, so that larger values of a (close to the maximal value 1) mean that C vv (t) reaches very small values already in the first time regime, approaching zero for a = 1.…”

“…As shown in Fig. 4(a), the profile (21) fairly agrees with numerical simulations after a transient time which depends on the value of k. Moreover, for k { 1, the persistence length is much smaller than S and the particle velocity relaxes before the spatial variation of the swim velocity affects the dynamics of the system and we can again neglect @ @x uðxÞ in eqn (20) to recover the approximation for C vv (t) given by eqn (21) (see the black dashed curve in Fig. 4(b)).…”

“…In other words, the system behaves as a passive system with space-dependent diffusivity equivalent to that studied in ref. 89: this is clear by considering eqn (4b) with _ Z ¼ 0 so that Z % ffiffi ffi 2 p w. From eqn (21), we also infer that the amplitude of the swim velocity oscillations (controlled by a) cannot affect the relaxation of C vv (t), even if it determines the steady-state variance of the distribution. To evaluate in detail the role of a on the velocity relaxation, we study C vv (t)/C vv (0) for several values of a in Fig.…”

“…This experimental advance offers intriguing perspectives in the world of active matter and provides interesting applications, ranging from micro-motors 15,16 and rectification devices 17,18 to motility-ratchets, 19 where an asymmetric spatial profile of the light intensity is used to induce an asymmetric spatial shape of the swim velocity which produces a net directional motion. Spatial motility landscapes have been also used to experimentally trap Janus particles 20,21 and to investigate the occurrence of polarization patterns induced by motility gradients. 22,23 Among the fascinating applications based on light-sensitive active particles, we mention bacteria-based ''painting'', experimentally realized with engineered E. coli, by Arlt et al 9,24 and, independently, by Frangipane et al, 25 through which some images, such as those of Charles Darwin and Albert Einstein, have been reproduced.…”

Which interactions dominate in active colloids?

“…This phenomenon has been termed "chemotaxis in a non-biological colloidal systems". More recently it has also been observed that some Janus-colloids show "artificial" phototaxis and respond to external light gradients [124][125][126][127][128] as well as thermotaxis [129][130][131]) and rheotaxis [132][133][134][135].…”