Void ratio definition

In conclusion to this section on porosity of bulk powders, it seems that the indirect method of calculating its value from other powder properties, namely the bulk and particle densities, is adequate for most practical applications. The fact whether or not the porosity should include the internal pores of the particles is taken care of by selection of a suitable method for particle density measurement (section 2.4). Care must be taken to avoid a possible confusion between the definitions of porosity and void ratio. 

With this definition ti/p represents the coarse pore and can be assumed to correspond to the original void ratio of a dry sample. During the saturation process the solution will be absorbed and transported through the pores. In the meanwhile it can also enter the interlayer space of the particles. Thus, the particles are expanding with fluid intrusion. Consequently, the sample as a whole tends to swell. If the sample is not confined, the particles can expand freely. Diffusion double layers are easy to build up around the particles and within the interlayer space. In this case the maximal interlayer void ratio can be derived according to the CSM model as (Xie et al. 2003) ... 

In geotechnical literature and standards, the terms relative density and density index are often used. In view of the contradicting definitions in various standards, these terms are replaced in this manual by relative void ratio i , and relative porosity R . The term relative density is only used in a general manner and does not refer to a specific definition. 

Pores are found in many solids and the term porosity is often used quite arbitrarily to describe many different properties of such materials. Occasionally, it is used to indicate the mere presence of pores in a material, sometimes as a measure for the size of the pores, and often as a measure for the amount of pores present in a material. The latter is closest to its physical definition. The porosity of a material is defined as the ratio between the pore volume of a particle and its total volume (pore volume + volume of solid) [1]. A certain porosity is a common feature of most heterogeneous catalysts. The pores are either formed by voids between small aggregated particles (textural porosity) or they are intrinsic structural features of the materials (structural porosity). According to the IUPAC notation, porous materials are classified with respect to their sizes into three groups microporous, mesoporous, and macroporous materials [2], Microporous materials have pores with diameters < 2 nm, mesoporous materials have pore diameters between 2 and 50 nm, and macroporous materials have pore diameters > 50 nm. Nowadays, some authors use the term nanoporosity which, however, has no clear definition but is typically used in combination with nanotechnology and nanochemistry for materials with pore sizes in the nanometer range, i.e., 0.1 to 100 nm. Nanoporous could thus mean everything from microporous to macroporous. 

Most solids of high surface area are to some extent porous. The texture of such materials is defined by the detailed geometry of the void and pore space. Porosity, , is a concept related to texture and refers to the pore space in a material. An open pore is a cavity or channel communicating with the surface of a particle, as opposed to a closed pore. Void is the space or interstice between particles. In the context of adsorption and fluid penetration powder porosity is the ratio of the volume of voids plus the volume of open pores to the total volume occupied by the powder. Similarly, particle porosity is the ratio of the volume of open pores to the total volume of the particle. It should be noted that these definitions place the emphasis on the accessibility of pore space to the adsorptive. 

The review published by Ergun (E2) provides a definitive description of pressure drop in packed tubes when the ratio of particle diameter to tube diameter is sufficiently low. In addition, although the complicated relationship between the diameter ratio, the fraction void and the friction factor can not be accurately represented without some explicit dependence of the friction factor on the diameter ratio, Ergun showed that his correlation does work for a wide variety of experimental conditions. The friction factor is calculated from the expression... 

When a liquid or a gas occupies a volume, it may be assumed to fill the volume completely. On the other hand, when solid particles occupy a volume, there are always spaces (voids) among the particles. The porosity or void fraction of a bed of particles is the ratio (void volume)/(totai bed volume). The bulk density of the solids is the ratio (mass of solids)/(iotaI bed volume), and the absolute density of the solids has the usual definition, (mass of solids)/(volume of solids). 

Equation [3.18] assumes that the extra-column volume Vg is negligible. However, there are two definitions of void volume, and thus also of the capacity ratio of a solute. The two void volumes are called the thermodynamic and the dynamic void volumes and they are not equal (Scott, www.chromatography-online.org) the two void volumes and capacity ratios are used for different purposes. Equations [3.16-3.18] incorporate the thermodynamic dead volume and all further discussion in this chapter assumes this definition. 

The definition above describes the total porosity . Amyx et al. (1960) define total porosity as "... the ratio of the total void space in the rock to the bulk volume of the rock effective porosity is the ratio of the interconnected void space in the rock to the bulk volume of the rock... . 

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