LONDON-VDW AND EDL EFFECTS IN THE COALESCENCE OF OIL DROPS

Visual observations of coalescence of n-hexadecane oil drops (70-140 m u m in diameter) suspended inside an aqueous phase of varying pH are r eported. The oil drops are produced in situ and confined inside the aq ueous-phase-filled fine capillary (100-160 mu m i.d.). Rapid, spontane ous coalescence was observed at low pH (less than or equal to 2.5) as soon as two drops were very carefully brought into apparent contact. A t very high pH (greater than or equal to 13.0), coalescence was observ ed some time after the drops made contact. No coalescence was observed at intermediate pH 5.2, 7.1, and 10.9. The ''coalescence times'' at p H 2.2 and 2.4 were less than 0.033 s. At pH 3.2, 12, and 13.1, the coa lescence times were 8, 20, and 1.8 s, respectively. The coalescence ti me in the acidic solution of a certain pH was distinctly lower than th at in the basic solution of equivalent pOH. The coalescence tendency i ncreased with an increase in the acidity or basicity of the aqueous ph ase. Also, in strongly basic or acidic media, freshly formed drops wer e more susceptible to coalescence. Visual observations of collision in teractions are qualitatively described. When two drops were made to co llide, coalescence was rapid at all pH at which coalescence was previo usly observed for the stationary drops. Almost no coalescence was obse rved at the intermediate pH's 5.2, 7.1, and 10.9, even after repeated collisions between the drops. The relative velocity of the collision i nteractions ranged from 100 to 2100 mu m/s. The observed phenomena are explained on the basis of the London-van der Waals and electrical dou ble layer forces, which are important either in stabilizing the thin l iquid film formed between the approaching drops or in drawing the two interfaces close enough to thin the liquid film to the ''critical rupt ure thickness range.'' In this range, it is believed that thin liquid him properties control the rupture process. (C) 1995 Academic Press, I nc.