  A general kind of Brownian vortexes are demonstrated by applying an external
nonconservative force field to a colloidal particle bound by a conservative
optical trapping force at a liquid-air interface. As the liquid medium is
translated at a constant velocity with the bead trapped at the interface, the
drag force near the surface provide enough rotational component to bias the
particle's thermal fluctuations in a circulatory motion. The interplay between
the thermal fluctuations and the advection of the bead in constituting the
vortex motions is studied, inferring that the angular velocity of the
circulatory motion offers a comparative measure of the interface fluctuations.
