Micro- and nano-scale colloidal dynamics near surfaces

We present an examination of near-wall colloidal particle dynamics, their associated measurement techniques and applications to micro- and nano-scale fluid flows. The dynamics of small particles in solution exhibit interesting and useful properties. The most popular measurement methods for characterizing their motion are imaging techniques based on evanescent wave illumination, namely total internal reflection fluorescence (TIRF) microscopy, which allows the imaging of small, fluorescent particles (10 nanometer to 10 micron) within a few hundred nanometers of a fluid-solid interface. In some instances, the particle intensity may be used to determine particle's distance from the wall, and thus, the three-dimensional particle motion. In the presence of a solid boundary, the random, Brownian motion and the convective motion of the particles is modified due to interactions between the particle and wall. Some examples of these effects are hindered diffusion, shear-induced rolling, electrostatic repulsion and van der Waals attractive forces. This has immense consequences for fluid velocity measurement techniques that infer fluid motion from the measured particle motion. Evanescent wave illumination was applied to measure single particle dynamics and also to infer the fluid motion in several fluid systems. The dynamics of semiconductor nanocrystals or quantum dots (QDs) and single molecules were examined and applied as tracer particles, extending the resolution of particle velocimetry probes down to about 10 nm. Additionally, the inherent intensity variation of QDs with temperature was used to measure the local fluid temperature within the evanescent field. QDs were also applied to measure high speed micro-flows (near 1 cm/s) within about 200 nm of the fluid-solid interface. A three-dimensional TIRF velocimetry (3D-TIRV) technique was developed to measure slip velocities at microchannel walls with tracer particles on the order of 100 nm. The three-dimensional adhesion dynamics of large (6 micron diameter) particles were examined to determine the kinetics of adhesion ligands and receptors for applications to leukocyte dynamics. Finally, the techniques of fluorescence microscopy and in-line holographic imaging were applied to characterized the structure and dynamics of electrospray droplets.