Dynamic deformations and forces in soft matter

The ability of soft matter such as drops and bubbles to change shape dynamically during interaction can give rise to counter-intuitive behaviour that may be expected of rigid materials. Here we show that dimple formation on approach and the possibility of coalescence on separation of proximal drops in relative motion are examples of this general dynamic behaviour of soft matter that arises from the coupling between hydrodynamic forces and geometric deformations. The film capillary number Ca(f) equivalent to (mu V(o)/sigma)(R/H(o))(2) is the key parameter that determines different behavior and depends on viscosity mu, interfacial tension sigma, the Laplace radius R, characteristic film thickness H(o) and velocity V(o).