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Role of Surface Roughness in Chemical Detachment of Colloids Deposited at Primary Energy Minima

Shen, Chongyang, Wang, Lian-Ping, Li, Baoguo, Huang, Yuanfang, Jin, Yan
Vadose zone journal 2012 v.11 no.1
colloids, ionic strength, latex, models, polystyrenes, primary energy, sand, surface roughness, vadose zone
This study theoretically and experimentally examined effects of surface roughness on detachment of colloids deposited under favorable chemical conditions on reduction of solution ionic strength. A superposition approach based on elemental geometric models was developed to estimate variation of Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energies between a colloid and a rough surface under different solution chemistries. Theoretical analysis showed that most colloids attached at rough surfaces via primary-minimum association are irreversible on reduction of solution ionic strength because primary minima are deeper and the detachment energy barriers are greater at lower ionic strength. A fraction of colloids initially attached at tips of nanoscale protruding asperities, however, will detach from a rough surface at low ionic strength because the net force acting on the colloids can become repulsive (i.e., calculated DLVO interaction energy curves show monotonic decreases of interaction energies with separation distance at low ionic strength). Column experiments were conducted with 1156-nm polystyrene latex particles and rough sand (300-355-μm diameter) to examine the detachment of colloids initially deposited at primary minima. Experimental results confirmed that a fraction of colloids are released at low ionic strengths. Our theoretical and experimental results are consistent with literature observations, adding convincing evidence to challenge the usual belief that colloids attached at primary minima are irreversible on reduction of solution ionic strength. Although the importance of surface heterogeneity on colloid deposition has been widely recognized, our study implies that surface heterogeneity also plays a critical role in colloid detachment under both favorable and unfavorable conditions.