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

Interfacial phenomena due to surface forces are important in microfluidic devices with their relatively large surface areas and small volumes. Most experimental studies of interfacial transport estimate flow velocities from the motion of tracer particles less than 1 μm in diameter, and assume that the particle displacements over a known interval are due to the fluid velocity field. This talk discusses some of our research on measuring fluid velocities in Poiseuille and electrokinetically driven flows over the first ∼0.5 μm next to the solid wall from the motion of an ensemble of O(10⁵) fluorescent spheres as small as 100 nm 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 fluid-solid interface. Recently, evanescent-wave illumination has been combined with fluorescence thermometry, where water temperature fields are estimated from changes in the intensity of the emissions from aqueous solutions of temperature-sensitive fluorophores. The Poiseuille flow of a fluorescein solution through a heated channel at a Reynolds number of 8.3 was illuminated with evanescent waves to obtain the solution temperature 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 differences exceeding 15 °C over the 1 mm width of the channel.