Tactic behaviour of synthetic microswimmers in response to external stimuli

Naturally, the first sept when investigating self-propelled particles is to focus on their dynamical behaviour in homogeneous environments. For such situations, far-reaching insights have been obtained in recent years [1]. Tactic motion is a locomotory movement which is induced by some unidirectional external stimuli. In living systems, this tactic response requires a complex internal feedback mechanism. In my recent work [1], we observe a strong orientational response of active colloids due to diffusiophoretic torques which, in combination with an intensity-dependent particle motility, eventually leads to Phototaxis, i.e. in response to a light gradient. Our results are in excellent agreement with numerical simulations of a minimal model. Since light fields can be easily adjusted in space and time, this also allows extending our approach to dynamical spatio-temporally modulated environments. Their self-motility thus allows them to swim against a traveling tactic wave, whereas a simple fore-rear asymmetry argument would suggest the opposite [3].

[1] I. Buttinoni, G. Volpe, F. Kümmel, G. Volpe & C. Bechinger, Active Brownian motion tunable by light. J. Phys. Condens. Matter 24, 284129 (2012).
[2] C. Lozano, B. ten Hagen, H. Löwen & C. Bechinger. Phototaxis of synthetic microswimmers in optical landscapes. Nature communications 7, 12828 (2016).
[3] C. Lozano & C. Bechinger [in preparation].