Coalescence coupled with either coagulation or flocculation in dilute emulsions

In current theories [R.G.P. Borwankar, L.A. Lobo, D.T.G. Wasan, Colloids Surf. 69 (1992) 35; K.D. Danov, N.D. Denkov, D.N. Petsev, R. Borwankar, Langmuir 9 (1993) 1731; S.S. Dukhin, J. Sjöblom, J. Dispers. Sci. Technol. 19 (1998) 311], emulsion destabilization is considered as the combined processes of droplet coagulation/flocculation and coalescence. Irreversible aggregation is usually called coagulation, and the term flocculation is used for reversible aggregation. These theories have different conditions of applicability which are not specified. An approximate criterion for discrimination between coalescence coupled with either coagulation or flocculation in dilute emulsions is proposed. Such discrimination is made possible by comparing the characteristic time for coagulation (Smoluchowski time) with that for floc fragmentation (droplet doublet lifetime, τd). As droplet dimension and electrolyte concentration decrease, and the Stern potential increases, τd is reduced and flocculation takes place. The quantity τd enables one to specify the conditions for the two separate cases. The important role of retarded van der Waals forces, screening length and hydration forces in emulsion coalescence is demonstrated.