Some observations on the contraction behavior of a water-in-oil drop with attached solids

One of the factors responsible for the high stability of water-in-bitumen or water-in-crude oil emulsions is the presence of a complex adsorbed layer of material at the water-oil interface. However, except for its role as a steric barrier, little is known about the nature and activity of this layer. Studying a macroscopic drop, on which interfacial adsorbed particles can be controlled and directly observed, will provide useful information that can be linked to the behavior of real emulsion systems. A macroscopic drop prepared using silicone oil, water, and silica particles was studied in this work. The results of this study indicate that the presence of slightly hydrophobic particles at the water-oil interface is sufficient to cause the interface crumpling that has been observed with water drops in crude oil or bitumen subjected to interfacial area reduction. For crumpling to occur, the particles need to remain strongly attached to the interface (i.e., have intermediate contact angles) as the drop interfacial area is reduced. The observed crumpling behavior is clearly reversible.