Voltage gradient effect

Electrical trees consist of visible permanent hoUow channels, resulting from decomposition of the material, and show up clearly in polyethylene and other translucent soHd dielectrics when examined with an optical microscope. Eresh, unstained water trees appear diffuse and temporary. Water trees consist of very fine paths along which moisture has penetrated under the action of a voltage gradient. Considerable force is required to effect this... 

Figure 3.30 Isotachophoresis. The spontaneous development of different voltage gradients across the two electrolyte solutions has the effect that both types of anion migrate at the same velocity.

The results indicate that dc electric fields (up to 1.14 V/cm) do not have an adverse effect on mixed aerobic cultures. In fact, an increase in the degradation rate may occur due to increasing the voltage gradient up to a certain value (0.58 V/cm in this study), beyond which this increase may diminish. This behavior is similar to electrostimulation by ac electric fields reported in the bioelectrochemistry research area (Fologea et al., 1998). For the conditions of the preliminary study, the window of significant response of bacteria stimulation is somewhere between 0.27 and 1.14 V/cm. Further, the results show that this window of dc range is also affected by time of exposure. This can be concluded as the impact of the 1.14 V/cm field diminished after 24 hours. However, to confirm this conclusion, it is necessary to separate the effects of any abiotic processes. 

It is important to optimise conditions in order to achieve efficient electroporation. The important parameters are the voltage gradient, which should be up to about 4 kV/cm the capacitance and the resistance of the medium all of which affect the shape and decay characteristics of the waveform (an exponential decay with a drop to 1 /e (37%) in about 5 msec may be most effective). In general, a low field strength and a long decay constant seem to be most effective. 

In the higher voltage domain, the concentration-gradient effects are less pronounced due to the large effect of the electric field. In this case, the very-high-field forms of the hopping transport equation can be used with... 

W. Thormann, A. Tsai, J.P. Michaud, R.A. Mosher, and M. Bier, Capillary isoelectric-focusing -effects of capillary geometry, voltage gradient and addition of linear polymer. Journal of Chromatography, 389, 75-86 (1987). 

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