Microengines have shown promise for a variety of applications in
nanotechnology, microfluidics, and nanomedicine, including targeted drug
delivery, microscale pumping, and environmental remediation. However, achieving
precise control over their dynamics remains a significant challenge. In this
study, we introduce a microengine that exploits both optical and thermal
effects to achieve a high degree of controllability. We find that in the
presence of a strongly focused light beam, a gold-silica Janus particle becomes
confined at the equilibrium point between optical and thermal forces. By using
circularly polarized light, we can transfer angular momentum to the particle
breaking the symmetry between the two forces and resulting in a tangential
force that drives directed orbital motion. We can simultaneously control the
velocity and direction of rotation of the particle changing the ellipticity of
the incoming light beam, while tuning the radius of the orbit with laser power.
Our experimental results are validated using a geometrical optics model that
considers the optical force, the absorption of optical power, and the resulting
heating of the particle. The demonstrated enhanced flexibility in the control
of microengines opens up new possibilities for their utilization in a wide
range of applications, encompassing microscale transport, sensing, and
actuation.Comment: 26 pages, 10 figure