Run-and-tumble-like motion of active colloids in viscoelastic media

Abstract

Run-and-tumble motion is a prominent locomotion strategy employed by many living microorganisms. It is characterized by straight swimming intervals (runs), which are interrupted by sudden reorientation events (tumbles). In contrast, directional changes of synthetic microswimmers (active particles) are caused by rotational diffusion, which is superimposed with their translational motion and thus leads to rather continuous and slow particle reorientations. Here we demonstrate that active particles can also perform a swimming motion where translational and orientational changes are disentangled, similar to run-and-tumble. In our system, such motion is realized by a viscoelastic solvent and a periodic modulation of the self-propulsion velocity. Experimentally, this is achieved using light-activated Janus colloids, which are illuminated by a time-dependent laser field. We observe a strong enhancement of the effective translational and rotational motion when the modulation time is comparable to the relaxation time of the viscoelastic fluid. Our findings are explained by the relaxation of the elastic stress, which builds up during the self-propulsion, and is suddenly released when the activity is turned off. In addition to a better understanding of active motion in viscoelastic surroundings, our results may suggest novel steering strategies for synthetic microswimmers in complex environments.

Random motion of a circle microswimmer in a random environment

“…For active particles, not only is diffusion affected, but ratchet effects [4], negative differential mobility [13,14,15], and clogging [16] emerge. Similar peculiar behavior has been seen for active Janus particles in visco-elastic media, which has been rationalized in terms of retarded torques coupling back to the propulsion force [17,18], or for active particles exposed to external fields as in gravitaxis [19] and chemotaxis [20,21].…”

Transient coarsening and the motility of optically heated Janus colloids in a binary liquid mixture

“…1,2 Ever since, this type of light activated self-propellers has been intensively used for studying active matter. The phenomena, which have been explored this way, range from clustering and phase separation in dense suspensions, 3 over the circular motion 4 and gravitaxis 5 of asymmetric self-propelled objects, to phototactic behavior 6,7 and self-propulsion in viscoelastic fluids [7][8][9][10] and dense colloidal suspensions. 11 Another important direction of research aims at understanding the self-propulsion mechanism in these systems, which is rather complex.…”

Realization of a motility-trap for active particles

“…Microscope images of the APs were taken with a frame rate of 12 fps and for a duration of at least 3600 s using a CCD camera. Particle positions r = (x, y) were obtained by automated in-house tracking routine developed with Matlab image analysis software, yielding a spatial resolution of ∼100 nm 54 . Because of the optical contrast between the light-absorbing carbon cap and the otherwise transparent silica, the orientation of the cap is obtained from the vector connecting the particle center and the intensity centroid of the particle image.…”