The interaction of colloidal particles collected at fluid interfaces

Simple approximate expressions are derived for the meniscus forces acting between spherical and cylindrical bodies at a fluid interface using a superposition approximation due to Nicolson (Proc. Cambridge Philos. Soc. 45, 288 (1949)). These expressions are correct to lowest order in the Bond number, B = (ϱB - ϱA)gR2/γAB, and are applicable to bodies that may be dissimilar in Bond number and wetting characteristics. Our results compare very favorably with the exact numerical calculations of Gifford and Scriven (Chem. Eng. Sci. 26, 287 (1971)) for parallel cylinders (for B ≲ 10−1). The small Bond number expressions derived herein are directly applicable to the interaction between particles of colloidal dimensions collected at fluid interfaces. Some sample calculations are given to illustrate the importance of capillary forces in interfacial coagulation processes. The extension of the theory to higher Bond number is discussed briefly.