A molecular dynamics simulation of capillary imbibition

To characterize porous media, use is often made of the Lucas-Washburn equation, which relates the rate of capillary penetration of a given liquid to an effective cylindrical pore radius and the contact angle between the liquid and the medium. Here, we extend previous large-scale molecular dynamics simulations developed for flat substrates to show how this tool can be used to study capillary imbibition in some detail. In particular, we demonstrate that the contact angle depends on the rate of wetting, especially during the early stages of pore filling, and that this leads to a reduction in the rate of penetration. The observed behavior can be modeled by a modified form of the Lucas-Washburn equation which takes specific account of these effects.