| With the increased interest in developing microscale devices for biological and chemical analysis and detection, various measurement techniques have been developed and refined to study the relevant flow physics or address practical issues in such small scale systems. Fluid velocity measurement, in particular, has been of special interest owing to the increased attention to flow and transport near surfaces, liquid-solid slip, and the desire for direct measurement of slip velocity.;We present the Quantum Dot (QD) imaging technique for near-wall velocimetry where QD nanoparticles are tracked within a few hundred nanometers of a surface using evanescent wave illumination. Water soluble quantum dots with a core diameter size of 6 nm and effective hydrodynamic diameter of 16 nm are used in this study. The local fluid velocity is inferred from tracking the QDs in a pressure-driven flow of an aqueous solution inside 200 and 300 micron microchannels.;Several issues, including the high diffusivity of QDs and the non-uniform distribution of the dots near the wall, affect the interpretation of the measurements and how the information could be used to infer slip length at the solid/fluid boundary. The effects of these parameters are addressed both experimentally and by Brownian Dynamics simulations. Appropriate methods to correctly interpret the measurements are presented.;The simulation of the Brownian motion of nanoparticles next to a wall has shown that the mean velocity measured from their displacement would tend to overestimate the actual mean fluid velocity, depending on the separation time between the two successive realizations of particles. Our measurements using nanocrystal QDs have verified this phenomenon and it is shown that the simulation results agree with the experimental data once the appropriate experimental parameters are incorporated in the simulation.;The measured velocities and slip estimates are presented for flow in a quartz microchannel with naturally hydrophilic and hydrophobic (OTS-coated) surfaces. Results indicate a larger slip length of the order of 20--30 nm for the hydrophobic surface.;This work was supported by the CRC Program of the National Science Foundation, Grant Number CHE-0209898. |
