For sedimenting, immiscible drops at unit viscosity ratio, a cohesive chemical potential is formulated to agree formally with the boundary-integral (BI) equation of Stokes flow. Against conventional wisdom, the resulting 'bulk cohesion' (BC) force field does not give rise to interfacial tension. Nevertheless, particle simulations based upon this concept accurately reproduce complex shape evolutions of trailing drops as calculated with BI. The BC force is easier to compute than volumetric versions of interfacial tension, and the radius of cohesion need not be small. (C) 2004 Elsevier B.V. All rights reserved.