Modeling thermocapillary microgear rotation and transferring the concept of asymmetric shape to translational particle propulsion

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In this study, we investigate the thermocapillary rotation of microgears at fluid interfaces and extend the concept of geometric asymmetry to the translational propulsion of micrometer-sized particles. We introduce a transient numerical model that couples the Navier–Stokes equations with heat transfer, displaying particle motion through a moving mesh interface. The model incorporates absorbed light illumination as a heat source and predicts both rotational and translational speeds of particles. Our simulations explore the influence of microgear design geometry and determine the scale at which thermocapillary Marangoni motion could serve as a viable propulsion method. A clear correlation between Reynolds number and rotation efficiency can be recognized. To transfer the asymmetry-based propulsion principle from rotational to directed translational motion, various particle geometries are considered. We demonstrate that, under illumination from above, geometrically asymmetric “Christmas tree”-shaped particles move forward. The exploration of breaking geometric symmetry for translational propulsion is mostly ignored in the existing literature, thus warranting further discussion. Therefore, we analyze expected translational speeds in comparison to corresponding microgears to provide insight into this promising propulsion method. Our simulations indicate that translational propulsion speeds of several particle lengths per second can be expected on the micrometer scale.