Types and Uses of Fields

For operational purposes, we define a field as any external influence extending through space capable of causing the relative displacement of components with respect to their surroundings. The most prominent examples are electrical and sedimentation fields. Other fields exist, but few have been used to any significant extent in static (nonflow) separations. Some have not been used for any separation, static or flow. The most common reason for neglect is the weakness of the forces generated by most fields. We address more specifically the criteria that must be satisfied for effective separation below. 

A partial compilation of field types is shown in Table 8.1. Some static (Sc) methods (or phenomena) associated with such fields are shown. (Processes like photophoresis—based on radiation pressure—hardly deserve to be called methods because the separation power is so weak that no useful methodology exists. However, natural photophoresis fractionated the gases forming our primitive earth as described in Chapter 1.) For completeness, some non-Sc (largely based on flow) separation systems utilizing these fields are shown in the final column of the table. 

Sedimentation Rate-zonal sedimentation, Equilibrium sedimentation, 

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