Surface roughness has been reported to both increase as well as decrease colloid retention. In order to better understand the boundaries within which roughness operates, attachment of a range of colloid sizes to glass with three levels of roughness was examined under both favorable (energy barrier absent) and unfavorable (energy barrier present) conditions in an impinging jet system. Smooth glass was found to provide the upper and lower bounds for attachment under favorable and unfavorable conditions, respectively. Surface roughness decreased, or even eliminated, the gap between favorable and unfavorable attachment and did so by two mechanisms: (1) under favorable conditions attachment decreased via increased hydrodynamic slip length and reduced attraction and (2) under unfavorable conditions attachment increased via reduced colloid-collector repulsion (reduced radius of curvature) and increased attraction (multiple points of contact, and possibly increased surface charge heterogeneity). Absence of a gap where these forces most strongly operate for smaller (<200 nm) and larger (>2 μm) colloids was observed and discussed. These observations elucidate the role of roughness in colloid attachment under both favorable and unfavorable conditions.

We herein demonstrate the cause of well-observed variant transport behaviors for apparently identical colloids in porous media under conditions of colloid-collector repulsion (unfavorable attachment conditions). We demonstrate that variant colloid transport behavior under unfavorable conditions can be explained by inherently variable colloid residence times prior to arrest on grains (collectors). We demonstrate that the residence time distributions derived from particle trajectory simulations incorporating representative nanoscale heterogeneity provide quantitative prediction of colloid transport under unfavorable conditions. We quantitatively predict hyper-exponential retention profiles in glass beads from representative nanoscale heterogeneity determined for glass, and we qualitatively predict nonmonotonic retention profiles in quartz sand from an estimated representative nanoscale heterogeneity for quartz. We also demonstrate that the transition from hyper-exponential to nonmonotonic profiles among glass beads versus quartz sand under otherwise equivalent conditions is primarily driven by greater grain angularity and consequent greater length and number of grain to grain contacts in quartz sand relative to glass beads. That continuum-scale transport behaviors emerge from upscaling of simulated pore-scale colloid residence times corroborates the utility of representative nanoscale heterogeneity for quantitative prediction of colloid transport under unfavorable conditions.

The impact of nanoscale surface heterogeneity on retention of nano-to-micro-scale particles (colloids) on surfaces governs colloid transport in the environment where unfavorable conditions (repulsive barrier present) are prevalent.