(332f) Dynamic Adhesion Of Micron-Scale Particles In Low Reynolds Number Flow Over Nano-Patterned Surfaces

Duffadar, R. D., University of Massachusetts
Davis, J. M., University of Massachusetts, Amherst

Potential new applications for sensing, separating, and sorting 200 nm to 5 micron objects will arise from the ability to manipulate selectively their dynamic adhesion behavior on surfaces in low Reynolds number flows. A combined experimental and theoretical investigation is used to study the dynamic adhesion (contact, skipping, rolling, and arrest) of micron-scale particles from suspensions flowing over surfaces tailored at the 5-20 nm length scale. The collecting surfaces are generally repulsive (electrostatically) towards silica particles in flowing solution, but the collectors also contain randomly distributed polymeric patches or proteinaceous entities that produce spatially fluctuating attractions. The dependence of the particle capture rate on the density of adhesive groups was studied, and a lower limit on the feature density was found, which yields a selectivity based on particle characteristics.

A theoretical model that predicts and quantifies these experimental observations has been developed. This model combines hydrodynamics and Brownian motion with the spatially varying physicochemical interactions between the particles and patterned surface to yield both adhesion rates and trajectories that describe the motion of individual particles. The inclusion of surface contact and frictional forces into the model further enables it to identify particle-wall interactions such as skipping, rolling, and arrest of motion as observed in the experiments.

While the model was previously used to predict successfully the motion of individual particles in flowing solution, it is now used to provide a quantitative interpretation of recent experimental results, including the velocity distribution in solution near the surface and the statistical distribution of the rolling velocity upon particle-surface contact. Following experimental validation, this model is used to study particle dynamics and selective adhesion over a broad range of parameter space by varying parameters such as the ionic strength, shear rate, particle size, and the size and net coverage of the heterogeneous patches on the surface. This presentation is focused on the important new developments from the experimental and modeling efforts and a fundamental understanding of particle interactions with these surface features in flow.