Transition to Area-Dependent Dissipation in Droplet Spreading

We have studied the spreading dynamics of a liquid diblock copolymer in the pseudo-partial wetting regime, in which disordered droplets coexist with a wetting layer of the same liquid. Using optical microscopy, the relaxation time of the droplets is measured as they evolve towards an equilibrium contact angle. The relaxation is investigated as a function of the contact angle, droplet volume, and viscosity. In this unique system all three parameters can be systematically varied on the same sample and for both advancing and receding motion. The relaxation time of the droplets does not scale with the length of the contact line as is typically the case. Instead, the relaxation time depends on the contact area. We are able to describe the experiments with a model that assumes area-dependent dissipation with only two adjustable parameters.