Powder fluid-related properties

The apparent density 5 (g/cm3) is usually measured using mercury as a pycnometric fluid. Mercury does not wet most of the solids and, thus, does not penetrate pores until pressure is applied. Mercury is not the only choice highly dispersed powders can serve as a guest fluid with the same penetration properties as well [55], Reciprocal to S is the specific apparent volume of PS, which is equal to the sum of the volumes of the pores and solid phase (e.g., the total volume of a granule shown in Figure 9.17a), and is obviously related to the mass of a PS. Relation between true and apparent density and porosity was considered in Problem 4. 

It is noteworthy that the group classification depends not only on the particle but also on the gas properties. Moreover, the above classification is related to the fluidization in the presence of air at ambient conditions. For a different fluid and operating conditions, a powder may appear in a different group. Thus, Figure 3.52 can be helpful in classifying a powder only for ambient conditions and with air as the fluid. 

Modifications of user properties have various origins. First, the process of size reduction itself may change the shape of the particles. Some properties, like taste, are sometimes directly induced by the particle size at the bio-sensor s scale. For example, it is now widely known that the gustative quality of chocolate is related to particle size. Another direct consequence of size reduction is an increase of the external specific surface area, that is widely used to enhance the rate of transport phenomena, for example in the case of dissolutions or in fluid/solid extractions. For a given particle size distribution, powder properties may be a consequence of the particles shape. 

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