Active chiral rotors: Hydrodynamics and chemotaxis

The active rotor studied here is a spherical particle that can self-rotate and generate a chiral flow pattern through active-surface slip velocity rather than through external fields. For instance, if the flow around the upper hemisphere of the rotor is in a clockwise direction, then the flow in the lower hemisphere is in the anti-clockwise direction, and vice versa. In this paper, we aim to study the combined behavior of hydrodynamically interacting active rotors in the presence of an external chemical gradient. While a single isolated rotor is stationary in a fluid, a pair of rotors can move in linear or closed orbital paths as they hydrodynamically interact with each other. It is observed that the emergent linear or closed orbital trajectories depend on the type of rotors and the orientation of their rotation axes. The dynamics of the rotors are altered in a more complex environment, such as in an external chemical gradient. Interestingly, we observe two types of motion: chemotaxis and anti-chemotaxis. In the anti-chemotaxis case, both rotors are driven away from the target; in the chemotaxis case, only one rotor reaches the chemical target, as the other loses its propulsion and cannot continue toward it. This behavior reflects cooperative rather than competitive chemotaxis. The study provides insights into the collective behavior of self-propelled microorganisms and artificial swimmers.