Capillary rise dynamics of liquid hydrocarbons in mesoporous silica as explored by gravimetry, optical and neutron imaging: Nano-rheology and determination of pore size distributions from the shape of imbibition fronts

摘要

We present gravimetrical, optical, and neutron imaging measurements of thecapillarity-driven infiltration of mesoporous silica glass by hydrocarbons.Square-root-of-time Lucas-Washburn invasion kinetics are found for linearalkanes from n-decane (C10) to n-hexacontane (C60) and for squalane, a branchedalkane, in porous Vycor with 6.5 nm or 10 nm pore diameter, respectively.Humidity-dependent experiments allow us to study the influence on theimbibition kinetics of water layers adsorbed on the pore walls. Except for thelongest molecule studied, C60, the invasion kinetics can be described by bulkfluidity and bulk capillarity, provided we assume a sticking, pore-walladsorbed boundary layer, i.e. a monolayer of water covered by a monolayer offlat-laying hydrocarbons. For C60, however, an enhanced imbibition speedcompared to the value expected in the bulk is found. This suggests the onset ofvelocity slippage at the silica walls or a reduced shear viscosity due to thetransition towards a polymer-like flow in confined geometries. Both, lightscattering and neutron imaging indicate a pronounced roughening of theimbibition fronts. Their overall shape and width can be resolved by neutronimaging. The fronts can be described by a superposition of independent wettingfronts moving with pore size-dependent square-root-of-time laws and weightedaccording to the pore size distributions obtained from nitrogen gas sorptionisotherms. This finding indicates that the shape of the imbibition front in aporous medium, such as Vycor glass, with interconnected, elongated pores, issolely determined by independent movements of liquid menisci. These aredictated by the Laplace pressure and hydraulic permeability variations and thusthe pore size variation at the invasion front. Our results suggest that poresize distributions can be derived from the broadening of imbibition fronts.