Role of Patterned Surface Charge Heterogeneity on Particle Deposition

Abstract

A finite element analysis of the fluid flow and the colloidal particle transport equations near a micropatterned charged substrate under radial impinging jet flow conditions is presented to investigate the charge heterogeneity effects on particle deposition. The particle Sherwood number representing the dimensionless particle deposition flux is obtained as a function of the radial distance from the stagnation point. The charge heterogeneity is modeled as concentric bands bearing positive and negative charges on the substrate. When a negatively-charged particle approaches such a charge heterogeneous substrate, it experiences an alternating attractive and repulsive force due to the presence of different charges on the substrate. Consequently, as the particle moves radially outward from the stagnation point, it experiences a periodic array of favorable (attractive) and unfavorable (repulsive) regions on the substrate, giving rise to an oscillatory trajectory. The numerical results obtained from the finite element model are in excellent agreement with existing theoretical and experimental values of deposition rates on homogeneous collector surfaces. However, the results for particle deposition over a heterogeneous substrate depict a significant deviation from those predicted by the patchwise heterogeneity model due to the coupled influence of hydrodynamic interactions and the surface chemical heterogeneity of the collector. The particles that do not deposit over an unfavorable repulsive band are convected to the next favorable band by the tangential velocity. This increases the particle concentration at the leading edge of each favorable band resulting in an increase in particle deposition over the favorable bands and the overall deposition rate on to the collector. Application of this phenomenon will be discussed in context of developing micropatterned surfaces with engineered particle capture properties.