Por Ania Maciolek (Max-Planck-Institut für Intelligente Systeme and IV. Institut für Theoretische Physik, Universität Stuttgart, Germany / Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland).

Colloidal particles covered by materials that absorb light receive increasing attention in various fields of research and in many important applications, for example, bio-medical applications. Such particles become hot under illumination by laser. This gives rise to a variety of curious phenomena, which may be of practical use. For instance, an optically heated colloid suspended in a binary liquid mixture may self-propel or, if trapped by a laser beam, it becomes a microscopic engine.

Using a fluid particle dynamics approach we investigate a propulsion mechanism for a gold-capped Janus particle as a function of the wettability contrast between capped and uncapped sides of a Janus particle and of a heating power for symmetric and nonsymmetric binary solvent. We also have immobilized this particle in a narrow channel and studied nonequilibrium dynamics of a binary solvent around it using experiment and theory. Our results demonstrate that a remarkably complex time evolution of the concentration field around the colloid is governed by combined effects of temperature gradient and wettability, and crucially depends on whether the colloid is free to move or trapped. Moreover, we discover an important role of transient dynamics for self-propulsion.