Movement through Porous Solids

There are three general theories for interpretation of moisture distribution and rate of moisture movement inside poroizs solids. These theories can be listed as (1) diffusion theory, (2) capillary theory, and (3) vaporization-condensation theory. 

Any model can be used to predict some drying data if enough parameters are used. However, no theory has been able to predict drying times where most of the resistance is in the solid phase. 

To be valid, each theory has its specific requirements. The major factors that decide the mode of liquid movement through porous solids are the nature of the liquid, the structure of the solid, the concentration of liquid, and the temperature and pressure of the system. 

The movement of liquid in a solid is caused by various forces. The possible mechanisms discussed in the literature are summarized as follows  

Industrial Significance of Liquid Movement through Porous Solids 

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A. Industrial Significance of Liquid Movement through Porous Solids. . 253... 

Section III is concerned with moisture movement through porous solids. The general theory of moisture distribution and the rate of moisture movement inside porous media is reviewed. The three theories of condensation— diffusion, capillarity, and vaporization—are discussed. The roles of various mechanisms causing liquid movement in solids are assessed. 

Moisture flows through porous solids by capillarity - - and to some extent by surface diffusion (see Chap. 25, p. 826). A porous material contains a complicated network of interconnecting pores and channels, the cross sections of which vary greatly. At the surface are the mouths of pores of various sizes. As water is removed by vaporization, a meniscus across each pore is formed, which sets up capillary forces by the interfacial tension between the water and the solid. The capillary forces possess components in the direction perpendicular to the surface of the solid. It is these forces that provide the driving force for the movement of water through the pores toward the surface. 

Capillary movement in porous solids. When granular and porous solids such as clays, sand, soil, paint pigments, and minerals are being dried, unbound or free moisture moves through the capillaries and voids of the solids by capillary action, not by diffusion. This mechanism, involving surface tension, is similar to the movement of oil in a lamp wick. 

Penman, H. L. 1940. Gas and vapor movements in the soil 1. The diffusion of vapours through porous solids. Journal of Agricultural Science 30 436 62. 

Electroosmosis — (also called electroendosmosis and endosmosis) The movement of a polar liquid through a capillary tubing or porous solid driven by an electrical potential difference. First described by F. F. Reuss in 1809. In fuel cells, electroosmosis causes protons moving through a proton exchange membrane (PEM) to drag water molecules from one side (anode) to the other (cathode). This phenomenon is utilized for the dessica-tion of different objects, e.g., walls of buildings. 

Irrespective of the conditions ensuring the abnormally rapid movement of a liquid in a capillary under acoustic cavitation effect, it is important to note that the sonocapillary effect follows all the major effects of the ultrasonic treatment of melts. Among such phenomena are wetting and activation of solid nonmetallic impurities in a liquid metal as well as fine filtration of a melt through porous filters under action of the ultrasonic cavitation treatment. For both processes, ultrasonic cavitation and sonocapillary effect with formation of cumulative jets provide the accelerated mass transfer of a melt to slots and cracks in the surface of nonwettable solid particles and into capillary channels of fine filters. 

For example, the determining factors for the filtration are the initial rate of liquid movement (6 to 8 cm s l) and the rate of the liquid rises to several centimeters during the first seconds of the ultrasonic action. On the other hand, due to adhesion of dispersed particles of solid nonmetallic inclusions onto the walls of a capillary, the effective cross-section of the latter continuously changes in the process of filtration. Finally, with actual filtration we deal not with the regularities of the flow through a capillary channel but with the statistics of melt movement through a porous medium with a host of channels. 

Diffusion Independent mixing of gases, liquids, solids in and with each other due to molecular movement. Important for bonding, for example, diffusion of water vapor in adhesive/boundary layers or diffusion of solvents through porous adherends during the setting of adhesives. 

Liquid slip has implications to various macroscopic applications, i.e., flow through porous media, particle aggregation, liquid coating, and lubrication, etc., in addition to small scale, i.e., MEMS and bio-MEMS, applications. The movement of a three-phase contact line between two immiscible fluids and solid on a substrate during the advancing or receding film motion indicates... 

Adsorption by soils and sediments can remove solvents dissolved in water. Furthermore, the rate of movement of dissolved solvents through porous geological material may be retarded by adsorption-desorption reactions between the solvents and the solid phases. The partitioning of solvents between the liquid phase and soil is usually described by an adsorption isotherm. The adsorption of solvents may be described by the Freundlich equation ... 

Surface evaporation can be a limiting factor in the manufacture of many types of products. In the drying of paper, chrome leather, certain types of synthetic rubbers and similar materials, the sheets possess a finely fibrous structure which distributes the moisture through them by capillary action, thus securing very rapid diffusion of moisture from one point of the sheet to another. This means that it is almost impossible to remove moisture from the surface of the sheet without having it immediately replaced by capillary diffusion from the interior. The drying of sheetlike materials is essentially a process of surface evaporation. Note that with porous materials, evaporation may occur within the solid. In a porous material that is characterized by pores of diverse sizes, the movement of water may be controlled by capillarity, and not by concentration gradients. 

The thermal movement of molecules often serves as a prototype of random motion. In fact, molecular diffusion is the result of the random walk of atoms and molecules through gaseous, liquid, solid, or mixed media. This section deals with molecular diffusion of organic substances in gases (particularly air) and in aqueous solutions. Diffusion in porous media (i.e., mixes of gases or liquids with solids) and in other media will be discussed in the following section. 

In electro-osmosis (Fig. 5), when an externally applied electric field gradient operates across the wet clay, water is moved from the anode (the positive electrode) to the cathode (the negative electrode) that is, there is a movement of the liquid phase through the stationary solid phase (a clay, soil, capillary, or porous plug, etc.) in response to an applied electric field, as shown schematically in Fig. 6, taken from Probstein. ... 

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