Micrometer Scale Adhesion on Nanometer-Scale Patchy Surfaces:  Adhesion Rates, Adhesion Thresholds, and Curvature-Based Selectivity

Abstract

Using a model system based on electrostatics, we probe interactions between spherical particles (negative silica) and planar surfaces that present randomly placed discrete attractive regions, 10 nm in size, in a repulsive background (silica flats carrying cationic surface constructs). Experiments measure the adhesion rates of particles onto the patchy collecting surfaces from flowing dispersions, as a function of the surface loading of the attractive patches, for different particle sizes (0.5 and 1 mum diameter spheres) and different ionic strengths. Surfaces densely populated with patches, such that they present net electrostatic attractions to approaching particles, capture particles at the transport-limited (maximum) rate. Surfaces sparsely loaded with attractive patches (which present a repulsive mean field to approaching particles) are usually still adhesive, but the particle adhesion rate depends on particle size, ionic strength, and patch loading. Most significant is an adhesion threshold, a critical density of patches needed to capture particles. This threshold, which occurs at average patch spacings of 30 nm and larger and which can be tuned through ionic strength, comprises the ability of the patchy surfaces to selectively distinguish particles of different sizes or objects of different local curvature or roughness. The observation of such an adhesion threshold implicates spatial fluctuations in patch arrangement. In addition to experiments, this paper develops arguments for lengthscales that govern adhesion rate behavior, comparing particle geometry and fluctuation lengthscales, and then demonstrating qualitative consistency with the localized colloidal potentials involved.

Dynamic adhesion behavior of micrometer-scale particles flowing over patchy surfaces with nanoscale electrostatic heterogeneity

“…Using the GSI technique, the particle and heterogeneous wall are discretized into small areal elements, and the interaction (force or energy) between each pair of elements is computed and integrated over the surfaces to yield the net electrostatic force or energy between the particle and wall. Duffadar and Davis used calculations based on the GSI technique to demonstrate that spatial fluctuations in the electrostatic force between the particle and heterogeneous wall are responsible for the interesting dynamic adhesion behavior observed by Kozlova and Santore [23,24]. Furthermore, values of Θ crit predicted by the model were in quantitative agreement with the experimental results.…”

“…Duffadar and Davis [26] developed a theoretical model to predict the particle dynamics in solution flowing over a heterogeneous wall for the situation studied by Kozlova and Santore [23,24]. For this system, particle motion is governed by the coupling of hydrodynamics to a heterogeneous electrostatic energy landscape over the nanotextured surface, as shown in Fig.…”

Stabilization of Weakly Charged Microparticles Using Highly Charged Nanoparticles

“…In solutions with longer Debye lengths (low ionic strength), the critical patch percentage was observed around 10%, or an average spacing of about 34 nm between patches, at which the maximum attainable adhesion was quickly reached. Higher ionic strengths resulted in measureable adhesion at lower densities [41,42].…”

“…In systems where the average surface or zeta potential over the whole surface of a colloidal particle would indicate a sufficiently repulsive force to prevent aggregation or deposition, it has been shown that attraction and deposition still occurs [41][42][43][44]. This is typically attributed to heterogeneities in the surface charge, with surface patches that exist where attractive forces dominate over the surface-average repulsion, leading to particle deposition.…”

“…In particular, the work of Santore and coworkers has shown that even comparatively small densities of heterogeneous surface patches can lead to large degrees of particle adsorption on surfaces [41,42]. In these experiments, cationic patches 10 nm in size were placed on negatively charged silica substrates, producing a surface that had a negative charge averaged over the entire surface, but contained randomly distributed attractive patches.…”