Studying Interfacial Transport With Evanescent Wave-Based Particle Velocimetry and Thermometry

Abstract

Interfacial phenomena due to surface forces are important in microfluidic devices with their relatively large surface areas and small volumes. This article reviews our recent studies measuring fluid velocities in Poiseuille and electrokinetically driven flows over the first ∼0.4 μm next to the wall from the motion of an ensemble of O(105) fluorescent particles illuminated by evanescent waves. Because the evanescent-wave intensity decays exponentially with wall-normal distance, the particle–wall separation can be determined from the brightness of each particle image, and used to estimate the steady-state distribution of the tracers near the wall. More recently, evanescent-wave illumination has been combined with fluorescence thermometry, to estimate water temperature fields from changes in the fluorescence intensity of aqueous fluorophore solutions. Poiseuille flow of a fluorescein solution at Reynolds numbers of 3.3 and 8.3 through a heated minichannel was illuminated with evanescent waves to measure solution temperatures at an average distance of 74 nm from the wall. The temperature results obtained over three different regions were in good agreement with numerical predictions of the wall surface temperature, even in the presence of temperature variations exceeding 9°C over the 1-mm width of the channel.