Near-surface velocimetry using evanescent wave illumination

Total internal reflection velocimetry (TIRV) is used to measure particle motion in the near-wall region of a microfluidic system. TIRV images are illuminated with the evanescent field of an incident laser pulse and contain only particles that are very close to the channel surface. Sub-micron-sized fluorescent particles suspended in water are used as seed particles and their images are analyzed with a particle tracking velocimetry (PTV) algorithm to extract information about apparent slip velocity. At relatively low shear rates (less than 2,500 s(-1)), a velocity proportional to the shear rate was observed. The statistical difference between velocities measured over hydrophilic and hydrophobic surfaces was found to be minimal. The results suggest that the slip length, if present, is less than 10 nm, but uncertainty regarding the exact character of the illumination field prevents a more accurate measurement at this time. Numerical simulations are presented to help understand the results and to provide insight into the mechanisms that result in the experimentally observed distributions. Issues associated with the accuracy of the experimental technique and the interpretations of the experimental results are also discussed.