Underdamped modes in a hydrodynamically coupled microparticle system

Abstract

When micron-sized particles are trapped in a linear periodic array,
for example, by using optical tweezers, they interact only through
the hydrodynamic forces between them. This couples the motion of
the spheres and it has been predicted that an extended system might
behave as an elastic medium that could support underdamped propagating
waves. In practice, these underdamped modes can be observed only
with massive particles in very stiff traps and very low viscosity
fluids. We have been able to realize these conditions by trapping
water droplets in air. Even with a system of just two particles we
were able to observe the coupled oscillatory motion predicted: underdamping
of the symmetric (collective) mode and overdamping of the asymmetric
(relative) mode.