  We study the spatio-temporal dynamics of water uptake by capillary
condensation from unsaturated vapor in mesoporous silicon layers (pore radius
$r_\mathrm{p} \simeq 2$ nm), taking advantage of the local changes in optical
reflectance as a function of water saturation. Our experiments elucidate two
qualitatively different regimes as a function of the imposed external vapor
pressure: for low saturations, equilibration occurs via a diffusion-like
process; for high saturations, an imbibition-like wetting front results in fast
equilibration towards a fully saturated sample. We show that the imbibition
dynamics can be described by a modified Lucas-Washburn equation that takes into
account the liquid stresses implied by Kelvin equation.
