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Spontaneous revolution of micro-swimmers in a spherically aberrated optical trap

Artificial micro-swimmers are fast emerging as models to mimic and thereby understand the movement patterns of microorganisms and biological cells which self-propel themselves by generating fields or gradients that cause fluid flow around their surface by phoretic surface effects, such as thermophoresis or electrophoresis. In this paper, we demonstrate that radiation pressure can lead to spontaneous revolution of a micron-sized asymmetric particle inside an annular potential formed due to geometrical aberrations of a Gaussian beam focused into a stratified medium using a high numerical aperture microscopic objective. The rate of revolution can be controlled from a few Hz to tens of Hz by changing the intensity of the trapping light which can be achieved either by modifying the laser power or the annular trap diameter. Theoretical simulations performed using Finite-difference time- domain method in Lumerical verify the experimental observations. The electric field distribution confirms that the particle revolution is the effect of asymmetrical scattering by the particle in the annular potential that gives rise to a tangential force. A proper Maxwell stress-tensor analysis of the problem demonstrates this uniform tangential force acting on the particle inside the ring. The model also shows that particles could be custom designed in order to spontaneously revolve in such annular trapping potentials. Thus, such systems could be used in place of LG beams to apply torque on DNA strands in order to study protein-DNA interaction, or to study the hydrodynamic synchronization among multiple rotating objects.