  We introduce a method for analyzing the physical properties of nanoparticles
in fluids via the competition between viscous drag and optical forces. By
flowing particles through a microfluidic device containing an optical
microcavity which acts as a combined optical trap and sensor, the variation of
the rate of trapping events with the different forces can be established. A
clear threshold behaviour is observed which provides a measure of a parameter
combining the dielectric polarizability and the hydrodynamic radius. This
technique could be applied in combination with other analytic techniques to
provide a detailed physical characterisation of particles in solution.
