Spreading of liquid drops from a liquid source

The spreading of liquid drops over solid substrates with a liquid source in the centre of the drop is considered from both theoretical and experimental points of view. For conditions of complete wetting the spreading is an overlapping of two processes: a spontaneous spreading and a forced flow caused by the liquid source in the centre. Both capillary and gravitational regimes of spreading are considered and power laws are deduced. In both cases of small and large droplets the exponent is a sum of two terms: the first term corresponds to the spontaneous spreading and the second term is determined by the intensity of the liquid source. In the case of a constant flow rate from the source the latter gives for the radius of spreading the following law R(t) ∼ t0.4 in the case of the capillary spreading and R(t) ∼ t0.5 in the case of gravitational spreading. In the case of partial wetting droplets spread with a constant advancing contact angle (at small capillary numbers). This yields R(t) ∼ t1/3. Experimental data are in good agreement with the theoretical predictions.