Abstract
A recent experiment revealed the existence of an attractive pair potential of colloidal particles trapped on water droplets in an oil phase (Nikolaides M. G. et al., Nature 420, (2002) 299). We show that such a capillary attraction occurs for stronlgy charged particles at a curved interface, due to the incomplete compensation of the interface pressure and the counterforce on the particle. This attractive force varies as Z4 with the bare valency, and as (a0/R)4 with the ratio of particle size a0 and droplet radius R; it vanishes in the limit R → ∞, i.e., for a flat interface.