Detachment of polystyrene particles from collector surfaces by surface tension forces induced by air-bubble passage through a parallel plate flow chamber

Abstract

By allowing an air-bubble to pass through a parallel plate flow chamber with negatively charged, colloidal polystyrene particles adhering to the bottom collector plate of the chamber, the detachment of adhering particles stimulated by surface tension forces induced by the passage of a liquid-air interface was quantified. The detachment forces originating from the passing interface were calculated to range from 10-9 to 10-7 N and stimulated detachment of a major proportion of the adhering polystyrene particles, regardless of whether a negatively charged, hydrophilic glass or a hydrophobic, dimethyldichlorosilane-coated glass, or a positively charged, 3-(2-aminoethylamino)propyldimethoxysilane-coated glass collector surface was used. Also, aging of the adhesional bonds between the collector surfaces and the adhering particles up to 72 h or variation of the ionic strength of the particle suspension medium, which was a potassium nitrate solution (10-100 mM), did not prevent detachment of adhering polystyrene particles from the collector surfaces. Assuming that in the case of repulsive electrostatic conditions the polystyrene particles adhered through secondary minimum DLVO interactions, it was calculated that the adhesion force per particle ranged between 10-14 and 10-12 N, which is several orders of magnitude insufficient to withstand the detachment force exerted by the passing liquid-air interface. In the case of attractive electrostatic conditions, the DLVO theory does not yield a secondary interaction minimum and an adhesion force was calculated by assuming that the polymer surface structures on the polystyrene particles kept the adhering particles at a distance of 5 nm from the collector surfaces, corresponding to an adhesion force of 10-11-10-10 N. Even when the assumption is made that the polystyrene particles can approach the collector surfaces up to the minimal separation distance between two interacting surfaces of 1.57 Å, an adhesion force of 10-8 N results, which is still of the same order of magnitude as the detachment force originating from the passing liquid-air interface. In summary, this study demonstrates the huge potential of surface tension forces for particle removal and, at the same time, is intended to be a warning for erroneous enumeration of colloidal particles adhering to collector surfaces after 'slight rinsing', 'dipping', or other manipulations said to remove loosely adhering particles.