(371c) Particle Interactions with Patchy Surfaces in Confined Microscale Flow: Insights From Modeling

Patchy surfaces with nanoscale features can be used to detect, manipulate, and control the motion signatures of colloidal- and micrometer-scale objects in flow. For example, colloidal silica particles in flowing suspensions exhibit skipping, rolling, and firm adhesion on net-repulsive silica surfaces with randomly-distributed, 10-nm cationic patches. Although governed by non-specific colloidal interactions that vary spatially with fluctuations in the local density of adsorbed patches, this system displays selective adhesion, a flow-sensitive adhesion threshold, and biomimetic behavior. This talk is focused on insights provided from a computational model that includes hydrodynamic, colloidal, Brownian, and frictional interactions. The model predictions are in strong agreement with experiment with no adjustable parameters. The simulations provide information not accessible experimentally, including detailed particle trajectories, the evolving particle-surface separation distance, the local densities of adhesive elements beneath arrested and rolling particles, and the binding strength per element. Furthermore, the computations explain the deviations from a DLVO treatment based on the ?average? charge of the surfaces. Attention in this talk is focused on the influence of the shear rate on the adhesion threshold (a nano-feature density below which particle adhesion does not occur) and the deposition rates of the particles. The model quantitatively predicts the observed 1/3 power law dependence of the adhesion threshold and the non-monotonic dependence of the deposition rate on the shear rate. This behavior is linked to the RMS variation in the particle-surface separation distance as the particles fluctuate about their secondary minima due to the spatially varying colloidal forces as the particles are transported along the surface by the shear flow. The variation in the RMS separation distance also has a power-law dependence on the shear rate. The coupling of the statistical distribution of the nano-features to the increased size of the locally-attractive region required to adhere a particle as the shear rate increases is used to explain the experimental and computational results.