Shape-Directed Micro-Rotors Powered By Ultrasound

Shape, symmetry and other geometric features are widely used for programming self-assembly of passive colloids. These parameters can also be used as powerful strategies for engineering motion of active colloids and controlling their interactions and collective dynamics. Unlike self-propulsion directed by anisotropic surface (e.g., catalytic, magnetic and light-driven Janus particles, acoustic bimetallic nanorods), shape-directed motion of active colloids is far less explored. A recent experimental study reported that gold (Au) nanorods (length 2µm, diameter 330nm) with convex and concave tips propel linearly toward their concave end at a speed of 200µm/s in megahertz (MHz) acoustic field. Other shapes and symmetry of individual particles can lead to more complex forms of locomotion than simple linear motion. In this talk, we present shape-directed rotational motion of Au micro-gears (spinners) driven by ultrasound. Both clockwise (CW) and counterclockwise (CCW) rotation can be achieved depending on the chirality of the spinners. We investigate the role of asymmetry, size and number of fins on the rotational speed of individual spinners. Shape-directed rotation can also be harnessed in other external stimuli such as chemical fuel and electric field. Shape- and symmetry-dependent actuation offers new opportunities for designing active colloids with complex forms of locomotion for enhanced functionality and studying their collective dynamics.