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 stationary point where the
optical and thermal forces balance. 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 phenomenological 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, including microscale transport, sensing,
and actuation.