Kinetics of spreading over porous substrate

The spreading of small liquid drops over thin and thick porous layers (dry or saturated with the same liquid) has been investigated in the case of both complete wetting (silicone oils of different viscosities) and partial wetting (aqueous SDS solutions of different concentrations). Consideration has been carried out from both experimental and theoretical points of view. Nitrocellulose membranes of different porosity and averaged pore size were used as a model of thin porous layers, glass and metal filters were used as a model of thick porous substrates. It has been shown, that the spreading process follows the power law in time in the case of spreading of silicon oil drops over porous substrate saturated with the same oil. The liquid flow in the spreading drop has been matched with the flow in the porous substrate. Both the exponent and the pre-exponential factor of the power law have been predicted and compared with our experimental data, which shows the good agreement. An effective lubrication coefficient has been introduced, which accounts for an effective slippage of liquids over porous substrates. This coefficient has been both theoretically predicted and experimentally verified. [Continues.]