Phototaxis of synthetic microswimmers in optical landscapes (1609.09814v1)

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.

Phototaxis of Synthetic Microswimmers in Optical Landscapes

The paper explores the phototactic behavior of synthetic microswimmers, specifically light-activated Janus particles, within inhomogeneous laser fields. Phototactic responses, commonly observed in microorganisms such as phytoplankton, are here induced in artificial systems through diffusiophoretic torques. These torques arise due to the asymmetrical distribution of slip velocity around the particles, resulting in orientation and movement along light gradients.

The experimental system employed spherical colloidal particles partly coated with a carbon layer, which, upon illumination, becomes active due to a local demixing effect in a binary fluid mixture. The particle propulsion velocity is contingent on the local light intensity, enabling controlled motion via exposure to a structured optical field. The particles demonstrated motion towards regions of lower light intensity, a phenomenon indicative of synthetic phototaxis.

Key results demonstrated strong alignment and directed motion in periodic asymmetric intensity landscapes, underscoring the potential to steer particle movement through carefully engineered optical environments. The paper precisely quantified the dynamics of orientational response using theoretical models based on Langevin equations, taking into account the particle motility's dependency on position and orientation.

The saturation of aligning torques at higher gradients emerged as a crucial element in rectifying particle motion over long distances. Experimental findings consistent with numerical simulations confirmed the robustness of the model, which employed a minimalistic approach leveraging slip velocity and heat flux coupling for predictive accuracy.

The implications of this research extend to the development of microscopic machines capable of navigating complex optical terrains autonomously, reminiscent of biological entities. The adaptability of light fields for dynamic control over particle behavior is a promising avenue for designing responsive systems, potentially benefiting realms such as targeted drug delivery and environmental sensing.

Future investigations could enhance these insights by exploring diverse propulsion mechanisms and particle compositions, expanding beyond the demonstrated systems. The exploration of non-linear dependencies of aligning torques on light gradients opens new questions about harnessing phototactic or chemotactic capabilities in varied synthetic environments.

This research contributes significantly to the understanding of synthetic microswimmer dynamics, anchoring the potential for applications that measure up to those found in natural systems. The paper sets a precedent for utilizing light-mediated controls to innovate in the field of active colloidal systems.