The percolation transition in filling a nanoporous body by a nonwetting liquid

The paper presents the results of an experimental study of the percolation transition in filling by nonwetting liquids of nanoporous bodies of various natures with different specific surface areas and mean pore and granule sizes. The liquid that we used was an aqueous solution of ethylene glycol. The hysteresis and non-outflow phenomena observed in this transition at various (known) surface energies of liquids were studied by varying the concentration of ethylene glycol. This helped us explain the mechanism of the percolation transition in filling nanoporous bodies with nonwetting liquids. It was shown that, to quantitatively describe the observed dependences in terms of percolation theory taking into account energy barriers to filling, we must use a non-scaling distribution function of clusters of accessible and filled pores that admits the formation of pore clusters of arbitrary dimensions.     

Dispersion transition and the nonergodicity of the disordered nanoporous medium-nonwetting liquid system

The experiments in which a nonwetting liquid does not flow from a disordered nanoporous medium are described. The outflow is shown to depend on the degree of filling of the porous medium and its temperature in a critical manner. A physical mechanism is proposed where the transition of a system of liquid nanoclusters in a confinement into a metastable state in narrow filling and temperature ranges results from the appearance of a potential barrier due to the fluctuations of the collective “multiparticle” interaction of liquid nanoclusters in neighboring pores of different sizes at the shell of a percolation cluster of filled pores. The energy of a metastable state forms a potential relief with numerous maxima and minima in the space of a porous medium. The dispersed liquid volume in a metastable state is calculated with an analytical percolation theory for a ground state with an infinite percolation cluster. The outflow time distribution function of pores is calculated, and a power law is obtained for the decrease in nonwetting liquid volume retained in a porous medium with increasing time. The relaxation of the system under study is a multistage process accompanied by discontinuous equilibrium and overcoming of numerous local maxima of a potential relief. The formation of the metastable state of retained nonwetting liquid results from the nonergodicity properties of a disordered porous medium. The proposed model can describe the detected dependences of dispersed liquid volume on the degree of filling and temperature.     

Observation of a dispersion transition and the stability of a liquid in a nanoporous medium

It has been found that the removal of overpressure is accompanied by a transition of some nonwetting-liquid nanoclusters to the stable state in narrow ranges of the filling factor and temperature. This means that the nonwetting liquid becomes ??wetting.??     

Anomalously slow relaxation of a nonwetting liquid in the disordered confinement of a nanoporous medium

The time evolution of the water–disordered nanoporous medium Libersorb 23 (L23) system has been studied after complete filling at elevated pressure followed by full release of overpressure. It is established that relaxation of the L23 rapidly flows out during the overpressure relief time, following the variation in pressure. At a temperature below that of the dispersion transition (T < T _d = 284 K), e.g., at T = 277 K, the degree of filling θ decreases from 1 to 0.8 within 10 s. The degree of filling varies with time according to the power law θ ~ t ^–α with the exponent α < 0.1 over a period of t ~ 10⁵ s. This process corresponds to slow relaxation of a metastable state of a nonwetting liquid in a porous medium. At times t > 10⁵ s, the metastable state exhibits decay, manifested as the transition to a power dependence of θ(t) with a larger exponent. The relaxation of the metastable state of nonwetting liquid in a disordered porous medium is described in the mean field approximation as a continuous sequence of metastable states with a barrier decreasing upon a decrease in the degree of filling. Using this approach, it is possible to qualitatively explain the observed relaxation process and crossover transition to the stage described by θ(t) with a larger exponent.     

Correlation effects during liquid infiltration into hydrophobic nanoporous media

To explain the thermal effects observed during the infiltration of a nonwetting liquid into a disordered nanoporous medium, we have constructed a model that includes correlation effects in a disordered medium. It is based on analytical methods of the percolation theory. The infiltration of a porous medium is considered as the infiltration of pores in an infinite cluster of interconnected pores. Using the model of randomly situated spheres (RSS), we have been able to take into account the correlation effect of the spatial arrangement and connectivity of pores in the medium. The other correlation effect of the mutual arrangement of filled and empty pores on the shell of an infinite percolation cluster of filled pores determines the infiltration fluctuation probability. This probability has been calculated analytically. Allowance for these correlation effects during infiltration and defiltration makes it possible to suggest a physical mechanism of the contact angle hysteresis and to calculate the dependences of the contact angles on the degree of infiltration, porosity of the medium, and temperature. Based on the suggested model, we have managed to describe the temperature dependences of the infiltration and defiltration pressures and the thermal effects that accompany the absorption of energy by disordered porous medium-nonwetting liquid systems with various porosities in a unified way.     

Investigation of the dynamics of a percolation transition under rapid compression of a nanoporous body-nonwetting liquid system

The dynamics of infiltration of a nanoporous body with a nonwetting liquid under rapid compression is studied experimentally and theoretically. Experiments are carried out on systems formed by a hydrophobic nanoporous body Libersorb 23, water, and an aqueous solution of CaCl₂ at a compression rate of \(\dot p\) ≥ 10⁴ atm/s. It is found that the infiltration begins and occurs at a new constant pressure independent of the compression energy and viscosity of the liquid. The time of infiltration and the filled volume increase with the compression energy. A model of infiltration of a nanoporous body with a nonwetting liquid is constructed; using this model, infiltration is described as a spatially nonuniform process with the help of distribution functions for clusters formed by pores accessible to infiltration and filled ones. On the basis of the proposed system of kinetic equations for these distribution functions, it is shown that under rapid compression, the infiltration process must occur at a constant pressure p _c whose value is controlled by a new infiltration threshold θ _c = 0.28 for the fraction of accessible pores, which is higher than percolation threshold θ _c0 = 0.18. Quantity θ _c is a universal characteristic of porous bodies. In the range θ _c0 < θ < θ _c , infiltration of the porous body should not be observed. It is shown that the solution to the system of kinetic equations leads to a nonlinear response by the medium to an external action (rapid compression), which means the compensation of this action by percolation of the liquid from clusters of filled pores of finite size to an infinitely large cluster of accessible but unfilled pores. As a result of such compensation, infiltration is independent of the viscosity of the liquid. It is found that all experimental results can be described quantitatively in the proposed model.     

Observation of dynamic effects in the percolation transition in a “ nonwetting liquid-nanoporous body” system

Filling of a nanoporous body with liquid metal upon pulsed pressure buildup to a level far exceeding the percolation-threshold critical pressure was experimentally studied. The onset of the oscillating filling regime was observed. It disappears on bringing pressure down to a value lower than the percolation-transition critical pressure and on increasing the characteristic time of pressure buildup above threshold. A model is suggested allowing the explanation of the observed effects.     

Fast Spontaneous Transport of a Non-wetting Fluid in a Disordered Nanoporous Medium

Transport in Porous Media - The experimental study of cooperative fast transport of non-wetting fluid in a disordered nanoporous medium is carried out in this work. New experimental data for...