We report on the capillary rise of water in three-dimensional networks of hydrophilic silica pores with 3.5 nm and 5 nm mean radii, respectively (porous Vycor monoliths). We find classical square root of time Lucas-Washburn laws for the imbibition dynamics over the entire capillary rise times of up to 16 h investigated. Provided we assume two preadsorbed strongly bound layers of water molecules resting at the silica walls, which corresponds to a negative velocity slip length of -0.5 nm for water flow in silica nanopores, we can describe the filling process by a retained fluidity and capillarity of water in the pore center. This anticipated partitioning in two dynamic components reflects the structural-thermodynamic partitioning in strongly silica bound water layers and capillary condensed water in the pore center which is documented by sorption isotherm measurements.

During spontaneous imbibition, a wetting liquid is drawn into a porous medium by capillary forces. In systems with comparable pore length and diameter, such as paper and sand, the front of the propagating liquid forms a continuous interface. Sections of this interface advance in a highly correlated manner due to an effective surface tension, which restricts front broadening. Here we investigate water imbibition in a nanoporous glass (Vycor) in which the pores are much longer than they are wide. In this case, no continuous liquid-vapor interface with coalesced menisci can form. Anomalously fast imbibition front roughening is experimentally observed by neutron imaging. We propose a theoretical pore-network model, whose structural details are adapted to the microscopic pore structure of Vycor glass and show that it displays the same large-scale roughening characteristics as observed in the experiment. The model predicts that menisci movements are uncorrelated, indicating that despite the connectivity of the network the smoothening effect of surface tension on the imbibition front roughening is negligible. These results suggest a new universality class of imbibition behavior, which is expected to occur in any matrix with elongated, interconnected pores of random radii.liquid imbibition | interface roughening | porous media | neutron radiography | computer simulations M any everyday processes involve the flow of a liquid into a porous matrix, for instance, when we dunk a biscuit into coffee, clean the floor with a cloth, or get drenched with rain. The same process is also important in nature (e.g., for water to reach the tips of the tallest trees or to flow through soil) and crucial for different industrial processes, ranging from oil recovery and chromatography to food processing, agriculture, heterogeneous catalysis, and impregnation (for reviews see refs. 1-4).The above processes are examples of imbibition (Fig. 1). Imbibition of a liquid into a porous matrix is governed by the interplay of capillary pressure, viscous drag, volume conservation, and gravity. The porous matrix often has a complex topology. The inhomogeneities result in variations in the local bulk hydraulic permeability and in the capillary pressure at the moving interface. Nevertheless, the invasion front during solely capillarity-driven (i.e., spontaneous) imbibition advances in a simple square-rootof-time manner, according to the Lucas-Washburn law (5, 6). Such behavior is a result of the time-independent mean capillary pressure and the increasing viscous drag in the liquid column behind the advancing front. It is valid down to nanoscopic pore sizes (7-9) and particularly robust with regard to the geometrical complexity of the porous matrix (1, 4, 10, 11). The evolution of the invasion front displays universal scaling features on large length and timescales, which are independent of the microscopic details of the fluid and matrix (12-18), and which parallels the elegance of critical phenomena.Typically imbibition is studied using paper (14-16) or...

Measurements on helium and argon gas flow through an array of parallel, linear channels of 12 nm diameter and 200 microm length in a single crystalline silicon membrane reveal a Knudsen diffusion type transport from 10(2) to 10(7) in Knudsen number Kn. The classic scaling prediction for the transport diffusion coefficient on temperature and mass of diffusing species, D(He) is proportional to square root T, is confirmed over a T range from 40 K to 300 K for He and for the ratio of D(He)/D(Ar) is proportional to square root (m(Ar)/m(He)). Deviations of the channels from a cylindrical form, resolved with electron microscopy down to subnanometer scales, quantitatively account for a reduced diffusivity as compared to Knudsen diffusion in ideal tubular channels. The membrane permeation experiments are described over 10 orders of magnitude in Kn, encompassing the transition flow regime, by the unified flow model of Beskok and Karniadakis.

Neutron spin echo spectra of the melts of n-hexatriacontane (C36) and poly(ethylene oxide) (PEO), 2 kg/mol, confined in circular channels with a mean diameter of 10 and 12 nm, respectively, in porous silicon, were recorded at Q values between 0.05 and 0.15 Å−1. The spectra were successfully analyzed in terms of a two-state model where wall-adsorbed molecules are immobile and free molecules have a bulk-like dynamics. For C36, we find an adsorbed bilayer at 364 K and a monolayer at 435 K (in both cases, the long axis of the molecules is oriented parallel to the surface) and no adsorbed layer at 512 K. For PEO, we find an adsorbed monolayer at 413 K. The results support the existence of a sticky boundary layer inferred from capillary filling experiments.

Abstract. Along with some fundamental concepts regarding imbibition of liquids in porous hosts we present an experimental, gravimetric study on the capillarity-driven invasion dynamics of water and of the rod-like liquid crystal octyloxycyanobiphenyl (8OCB) in networks of pores a few nanometers across in monolithic silica glass (Vycor). We observe, in agreement with theoretical predictions, square-root of time invasion dynamics and a sticky velocity boundary condition for both liquids investigated. Temperature-dependent spontaneous imbibition experiments on 8OCB reveal the existence of a paranematic phase due to the molecular alignment induced by the pore walls even at temperatures well beyond the clearing point. The ever present velocity gradient in the pores is likely to further enhance this ordering phenomenon and prevent any layering in molecular stacks, eventually resulting in a suppression of the smectic in favor of the nematic phase.