  Achieving control over the motion of dissolved particles in liquid metals is
of importance for the meticulous realization of hierarchical particle
assemblies in a variety of nanofabrication processes. Brownian forces can
impede the motion of such particles, impacting the degree of perfection that
can be realized in assembled structures. Here we show that light induced
Marangoni flow in liquid metals (i.e., liquid-gallium) with Laguerre-gaussian
(LG) lasers as heating sources, is an effective approach to overcome Brownian
forces on particles, giving rise to predictable assemblies with high degree of
order. We show that by carefully engineering surface tension gradients in
liquid-gallium using non-gaussian LG lasers, the Marangoni and convective flow
that develops in the fluid drives the trajectory of randomly dispersed
particles to assemble into 100 um wide ring-shaped particle assemblies. Careful
control over the parameters of the LG laser (i.e., laser mode, spot size, and
intensity of the electric field) can tune the temperature and fluid dynamics of
the liquid-gallium as well as the balance of forces on the particle. This in
turn can tune the structure of the ring-shaped particle assembly with a high
degree of fidelity. The use of light to control the motion of particles in
liquid metals represents a tunable and rapidly reconfigurable approach to
spatially design surface tension gradients in fluids for more complex assembly
of particles and small-scale solutes. This work can be extended to a variety of
liquid-metals, complementary to what has been realized in particle assembly out
of ferrofluids using magnetic fields.
