Capillary condensation in nano-porous media

The flow of vapors of various substances through porous anodic alumina membranes is investigated. The vapors are in such a state that they may condense, and re-evaporate further downstreams when flowing through the porous membranes. The aim of the research is the precise description and an accurate measurement of the flow process. The experimental part of the research is carrried out by the Russian partner, the theoretical part is done by the Austrian partner. The anodic alumina membranes utilized here have lateral dimensions of about one centimeter and a thickness of 100 m to 200 m. Anodic alumina membranes have the special property to be perforated by very regularly distributed, parallel holes in a direction normal to the surface. The holes all have a very uniform diameter. The membranes used here have pore diameters between 20 nm and 100 nm. The peculiar feature of the flow process under investigation is the condensation and evaporation in the confined space of the pores within the membrane. Because the interfaces between the gaseous and the liquid phase are located in the membrane, they are strongly curved, which causes a large pressure difference across these interfaces. The pressure difference depends on the surface tension, which in turn depends on the temperature. Therefore, for a precise description of the process, it is crucial to correctly describe the temperature within the membrane. In addition, the shape of a liquid-vapor interface in pores of a few tens of nanometers, under conditions of condensation or evaporation, and hence the pressure difference across this interface, is still subject of current research. The regular structure of the porous anodic alumina membranes makes a theoretical description of the process relatively easy. Within this research project, experimental data on the temperature distribution within the membrane and on the water content shall be obtained. The experimental data shall be combined with a theoretical description of the flow process. Thus, it is hoped to extend the knowledge on evaporation and condensation in small pores, and to be able to accurately describe the shape of and the pressure difference across the interface within the membrane, and the temperature distribution in the entire flow process.

Investigation of the flow of vapors through porous anodic aluminum oxide membranes Porous aluminum oxide membranes have cylindrical pores that are arranged very regularly, are all the same size, and are parallel to each other. The pore diameter can be adjusted and is typically between 10 and 200 nm. Vapors flowing through such small pores do not condense in the open air, but in the pores. The flow is driven by a pressure difference; due to the lower pressure, the condensate evaporates again downstream from the location where it condenses. The state at the interface between condensate and vapor was investigated using numerical methods. It was found that the pressure difference across the curved interface corresponds to the pressure difference that would also occur at a drop or bubble, even though, firstly, the space in the pore is smaller and confined by walls, and secondly, liquid evaporates at the interface. The degree of curvature is influenced by the nearby pore walls. However, if the simulated curvature is used, the pressure difference is consistent with existing knowledge. For the calculation of mass transport through such porous membranes, it is important to know that the usual law for pressure difference at curved interfaces can be applied, regardless of the amount of mass actually flowing through the membrane. On the other hand, the investigations show that a film adsorbed on the pore walls must be taken into account when determining the curvature of the interface. Furthermore, the flow of vapors through the pores was investigated. The numerical method, a lattice Boltzmann method, was carefully adjusted so that the calculated pressure differences and mass flows corresponded very closely to existing measurements from the literature. The well-adjusted mass flows indicate that the conditions in the simulations, i.e., the thickness and velocity of an adsorbed liquid film and its contribution to the total transported mass, describe the real conditions well. A lattice Boltzmann method was used for the calculation. The existing method was further developed to simulate flows with both liquid and gaseous phases and to take temperature gradients and heat flows into account. In collaboration with a primarily experimental group, it was shown that heating or cooling the downstream membrane surface has an influence on mass transport through the membrane. Such an influence had previously been postulated and has now been confirmed.