Multi-Point Holographic Micro-Velocimetry

We show how holographic optical trapping can be used for the multi-point measurement of fluid flow in microscopic geometries. An array of microprobes can be simultaneously trapped and used to map out the fluid flow in a microfluidic device. The optical traps are alternately turned on and off such that the probe particles are displaced by the flow of the surrounding fluid and then re-trapped. The particles' displacements are monitored by digital video microscopy and directly converted into velocity field values. The validity of the technique is demonstrated for the case of the flow around a spinning sphere and the flow at the outlet of a micro-channel.