  We propose and demonstrate a novel scheme for optical nanoparticle
characterization, optofluidic force induction (OF2i), which achieves real-time
optical counting with single-particle sensitivity, high throughput, and for
particle sizes ranging from tens of nanometers to several $\mu$m. The particles
to be analyzed flow through a microfluidic channel alongside a weakly focused
laser vortex beam, which accomplishes 2D trapping of the particles in the
transverse directions and size-dependent velocity changes due to the optical
forces in the longitudinal direction. Upon monitoring the trajectories and
velocity changes of each individually tracked particle, we obtain detailed
information about the number based particle size distribution. A parameter-free
model based on Maxwell's equations and Mie theory is shown to provide very good
agreement with the experimental results for standardized particles of spherical
shape. Our results prove that OF2i can provide a flexible work bench for
numerous pharmaceutical and technological applications, as well as for medical
diagnostics.
