A note on free-surface films in fractures

Free-surface films provide a potential explanation for fast transport of water through unsaturated fractured porous media. In air-filled fractures such films propagate much faster than a plug flow or flow through porous matrix. Thus fractures do not slow down the flow as was believed earlier but actually can transport the fluid fast. In this note we generalize the model of Dragila (1999) [4] and Dragila and Wheatcraft (2001) [5] for free-surface films from vertical to non-vertical fractures. That model represents the thin-film as a substrate on top of which solitary waves travel. Our calculations show that free-surface film flow in air-filled fractures is essentially insensitive to the angles of the inclination. Thus the simpler model of Dragila and Wheatcraft can be used in the prediction of transport rates in vertical as well as non-vertical fractures.