Fig. 1. Time evolution in dimensionless units of the spreading process under complete
wetting condition of an oil drop over an originally dry porous substrate (cellulose
wafer). First, imbibition starts taking place inside down while spreading over the surface
proceeds up to a maximum value at tmax. Subsequently, the liquid recedes until the process
stops. With l(tmax)=lmax and Lmax=L(t b tmax)we have L(tmax) b Lmax until it goes to zero