  We present gravimetrical, optical, and neutron imaging measurements of the
capillarity-driven infiltration of mesoporous silica glass by hydrocarbons.
Square-root-of-time Lucas-Washburn invasion kinetics are found for linear
alkanes from n-decane (C10) to n-hexacontane (C60) and for squalane, a branched
alkane, in porous Vycor with 6.5 nm or 10 nm pore diameter, respectively.
Humidity-dependent experiments allow us to study the influence on the
imbibition kinetics of water layers adsorbed on the pore walls. Except for the
longest molecule studied, C60, the invasion kinetics can be described by bulk
fluidity and bulk capillarity, provided we assume a sticking, pore-wall
adsorbed boundary layer, i.e. a monolayer of water covered by a monolayer of
flat-laying hydrocarbons. For C60, however, an enhanced imbibition speed
compared to the value expected in the bulk is found. This suggests the onset of
velocity slippage at the silica walls or a reduced shear viscosity due to the
transition towards a polymer-like flow in confined geometries. Both, light
scattering and neutron imaging indicate a pronounced roughening of the
imbibition fronts. Their overall shape and width can be resolved by neutron
imaging. The fronts can be described by a superposition of independent wetting
fronts moving with pore size-dependent square-root-of-time laws and weighted
according to the pore size distributions obtained from nitrogen gas sorption
isotherms. This finding indicates that the shape of the imbibition front in a
porous medium, such as Vycor glass, with interconnected, elongated pores, is
solely determined by independent movements of liquid menisci. These are
dictated by the Laplace pressure and hydraulic permeability variations and thus
the pore size variation at the invasion front. Our results suggest that pore
size distributions can be derived from the broadening of imbibition fronts.
