Fluorescence correlation spectroscopy for flow rate imaging and monitoring--optimization, limitations and artifacts.

We recently demonstrated a new method for mapping fluid velocities in 3 dimensions and with exceptionally high spatial resolution for the characterization of flow in microfluidic devices. In the method, a colloidal suspension containing fluorescent tracer particles, dye doped polymer spheres, is pumped through a microchannel and confocal microscopy combined with fluorescence correlation spectroscopy is used to measure fluid velocities. In this report, we further characterize the technique and report on optimizations that allow a 5-fold increase in speed of single point velocity measurements. This increase in measurement speed will yield a 25 fold reduction in the time needed to collect a complete velocity image. The precision of measured velocities was characterized as a function of tracer particle concentration, measurement time, and fluid velocity. In addition, we confirm the linearity of the measurement method (velocity vs. applied pressure) over a range of velocities spanning four orders of magnitude. Furthermore, we demonstrate that an artifact in velocity measurements using fluorescence correlation spectroscopy (FCS) that was interpreted by others as being caused by optical trapping forces is actually an artifact caused by detector saturation and can be avoided by careful choice of experimental conditions.