Mykola Tasinkevych (University of Lisbon): Chiral topological active matter powered by ultrasound

Abstract

The creation of colloidal machines – that is, dynamic assemblies of colloidal components that perform useful functions – requires advances in our ability to rationally engineer the dynamics of active colloids operating outside of thermodynamic equilibrium. Owing to their small size, such machines ideally should assemble spontaneously and operate autonomously in response to simple energy inputs. Achieving non-trivial dynamical behaviors and ultimately function therefore demands the use of complex components, into which the desired behaviors can be effectively encoded. In pursuit of this goal, it is instructive to consider the dynamics of even a single particle and how it might be programmed to perform increasingly complex tasks. From this perspective, in this talk we concentrate on the role of particle´s shape and how it can be tuned to achieve desired particle motions. We demonstrate experimentally that gold microplates of appropriate rotational symmetry can rotate in a uniform acoustic field with a direction and speed dictated by their shape.

In particular, we investigate the rotational motion of chiral star-shaped particles – spinners – subject to ultrasonic actuation as a function of the particle size, chiral asymmetry, and rotational order. Particles of opposite chirality rotate in opposite directions; achiral particles do not rotate. Interestingly, the direction of rotation also depends on the rotational order of the particles – for example, 3-fold symmetric particles rotate in the opposite direction as 4-fold particles. These observations are explained by a propulsion mechanism based on asymmetric acoustic streaming, whereby the presence of a particle in the primary oscillatory fluid flow gives rise to a secondary (in Reynolds) steady fluid flows. To this end we perform numerical analysis based on boundary elements method to describe the hydrodynamic flows surrounding acoustically-activated spinners and to calculate how the resulting angular velocity depends on the particle shape. Surprisingly, numerical calculations predict the reversal of the direction of the particle rotation upon increasing of the frequency of the driving acoustic field. The threshold value of the frequency is sensitive to the particle shape.