DETACHMENT OF POLYSTYRENE PARTICLES FROM COLLECTOR SURFACES BY SURFACE-TENSION FORCES INDUCED BY AIR-BUBBLE PASSAGE THROUGH A PARALLEL-PLATE FLOW CHAMBER

By allowing an air-bubble to pass through a parallel plate flow chambe r 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 origi nating from the passing interface were calculated to range from 10(-9) to 10(-7) N and stimulated detachment of a major proportion of the ad hering polystyrene particles, regardless of whether a negatively charg ed, hydrophilic glass or a hydrophobic, dimethyldichlorosilane-coated glass, or a positively charged, 3-(2-aminoethylamino)propyldimethoxysi lane-coated glass collector surface was used. Also, aging of the adhes ional bonds between the collector surfaces and the adhering particles up to 72 h or variation of the ionic strength of the particle suspensi on medium, which was a potassium nitrate solution (10-100 mM), did not prevent detachment of adhering polystyrene particles from the collect or surfaces. Assuming that in the case of repulsive electrostatic cond itions the polystyrene particles adhered through secondary minimum DLV O interactions, it was calculated that the adhesion force per particle ranged between 10(-14) and 10(-12) N, which is several orders of magn itude insufficient to withstand the detachment force exerted by the pa ssing liquid-air interface. In the case of attractive electrostatic co nditions, the DLVO theory does not yield a secondary interaction minim um and an adhesion force was calculated by assuming that the polymer s urface structures on the polystyrene particles kept the adhering parti cles 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 surfac es up to the minimal separation distance between two interacting surfa ces of 1.57 Angstrom, an adhesion force of 10(-8) N results, which is still of the same order of magnitude as the detachment force originati ng from the passing liquid-air interface. In summary, this study demon strates the huge potential of surface tension forces for particle remo val and, at the same time, is intended to be a warning for erroneous e numeration of colloidal particles adhering to collector surfaces after 'slight rinsing', 'dipping', or other manipulations said to remove lo osely adhering particles.