Nonlinear oscillation of interfacial tension was found for a liquid/liquid interface composed of di(2-ethylhexyl) phosphoric acid/ n -heptane/calcium chloride/water. The range of the oscillations was over several millineutons per meter, with the longest one continuing over a few hours. A wide variety of oscillation patterns was observed, but they were classified into two groups. One group showed that the power spectrum was proportional to f - 2, with f being the frequency, and the distribution function of the amplitudes followed a Gaussian distribution function. This type of oscillation, type A, became slightly weaker with decreasing temperature and looked like a random process which may be associated with thermal energy. Another type, type B, was entirely different from type A. The power spectrum approximately followed a scaling law. The scaling index was almost unity; however, the scaling law was not perfect compared to that of type A. The distribution function of the amplitudes was not a Gaussian distribution. An increase in temperature made the average amplitude smaller. This type of oscillation required a higher interfacial tension than that for type A. Further, a quasi-periodic change of interfacial tension was sometimes observed. Those changes suggested that type B oscillation was caused by an attractive interaction between surfactant molecules. It introduced an unstable region in the interfacial pressure−interfacial area isotherm which caused type B oscillation. A simple model was proposed to explain type B oscillations, which could capture some experimental trends.
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Shioi et al. (2000) studied this question.
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