Bistability in electro-kinetic phenomena

In electro-kinetic phenomena, bistability was studied by several groups in a system of following type [35-38] the system was as follows. The plots of / (current) versus A / were found to be typical N-shaped curves (Fig. 8.6). These N-shaped curves have been referred to as flip-flop type. 

A typical example of bistability in electro-kinetic phenomena [35-38] is demonstrated in Fig. 8.6, which is essentially the current-voltage curve obtained when solutions of different concentrations of the electrolyte are kept on the two sides of the membrane. Current is passed and the transmembrane potential A4 is recorded. 

A similar type of curve is obtained when A4 is varied and current is recorded. The current increases with the transmembrane potential. On attaining a certain value, it suddenly decreases (discontinuously) and then again increases linearly with slope much smaller than that found in the initial state. An N-shaped current-voltage curve is obtained which is called the flip-flop type. It is obvious that in the region of bistability, there are two values of A4 corresponding to a particular value of I. 

Recent experiments have shown that bistability is not always observed in electro-kinetic phenomena. Whereas bistability is observed in the case of pyrex sinter, this is not so in the case of millipore filter [37]. In the case of pyrex sinter, the equation relating I and A(f) is non-linear and has cubic terms in A / . Hence, bistability is expected in this case, whereas this is not so in the case of millipore filter, since I versus A f equation is only quadratic in A(f). 

Another point has to be noted in this connection. Electro-kinetic oscillations are observed in the case of both types of membranes. Thus, it follows that bistability is not a necessary condition for oscillation to occur. The former phenomenon corresponds to a situation where A4 varies with change in I when AP is fixed or zero and AC is fixed. 

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