Abstract
Many microorganisms, with phytoplankton and zooplankton as prominent examples, display phototactic behaviour, that is, the ability to perform directed motion within a light gradient. Here we experimentally demonstrate that sensing of light gradients can also be achieved in a system of synthetic photo-activated microparticles being exposed to an inhomogeneous laser field. We observe a strong orientational response of the particles because of diffusiophoretic torques, which in combination with an intensity-dependent particle motility eventually leads to phototaxis. Since the aligning torques saturate at high gradients, a strongly rectified particle motion is found even in periodic asymmetric intensity landscapes. Our results are in excellent agreement with numerical simulations of a minimal model and should similarly apply to other particle propulsion mechanisms. Because light fields can be easily adjusted in space and time, this also allows to extend our approach to dynamical environments.
Highlights
Many microorganisms, with phytoplankton and zooplankton as prominent examples, display phototactic behaviour, that is, the ability to perform directed motion within a light gradient
Active colloids provide an intriguing chance to understand the formation of dynamical structures in living systems[18,19,20,21] but may find use as microrobots which—similar to their biological counterparts— autonomously navigate through complex environments[22]
The reorientation rate of colloidal particles is entirely determined by the rotational diffusion time trot 1⁄4 8pZR3/kBT, which only depends on the particle radius R, the solvent viscosity Z and the thermal energy kBT
Summary
To understand the occurrence of a rectified motion, one has to recall that the time required for the reorientation of a particle inside an intensity gradient, that is, to, decreases but rapidly saturates at large |=I| (Fig. 3c). To,b is smaller than the typical residence times tr,b, which allows the particle to align opposite to the local gradient and leads to a motion to the right. This is in contrast to the situation in the segments a, where usually tr,aoto,a. Hx_ i gradually decreases when a/b-1, that is, when the light landscape becomes more symmetric
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