Force field assisted separations

The above observations have been interpreted within the framework of two distinct models, one involving trapping/detrapping of the photogenerated electrons [345, 346] and the other based on electron diffusion (or field-assisted diffusion) not attenuated by electron localization [347, 348]. The millisecond transit times also mean that the transit times are very long compared with equilibration of majority carriers in a bulk semiconductor or electron-hole pair separation within the depletion layer of a flat electrode. The slow transport is rationalized by a weak driving force and by invoking percolation effects [338]. 

A typical example of a resonance assisted device might be a cell separator designed to discriminate between cells which have only a small difference in dielectric properties. The cross over frequencies from positive to negative DEP will be similar but not identical for the two cells. By working at a frequency between the two crossovers it is possible to separate the cells but the forces produced will be small unless very high fields are used (Fig. 10). Controlled resonance can be used to boost the fields at the working frequency. 

Conventional filtration methods for separation generally use physioal devices such as membranes, sieves or filtration beds and force-driven methods suoh as settling, flotation or centrifugation. Some invoive the assistance of external (eleotrio or magnetic) fields. 

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