Observation of interfacial tension minima in oil–water–surfactant systems with laser manipulation technique

Abstract

A laser beam passing through a liquid interface results in a deformation on the nanometer scale that is inversely proportional to the interfacial tension. Based on this principle, we developed a method to measure the liquid–liquid interfacial tension. The displacement excited by the pump laser is measured with another probe laser in a non-contact and non-destructive manner. The motion of the interface in response to the modulated intensity of the pump laser shows a characteristic spectrum yielding an accurate value of the interfacial tension. The new method is quite appropriate in the measurement of very low interfacial tension, and we applied it to the oil–surfactant–water system. First, we observed the change in the interfacial tension depending on the NaCl concentration in heptane and a water system containing AOT as a surfactant. The interfacial tension had a minimum value, ∼1 μN m⁻¹, for a certain concentration of NaCl. Secondly, the measurement of the ultra-low interfacial tension was carried out changing the temperature of the same system. In this experiment, the critical decrease in the tension was observed near temperature ranges where the microemulsion phase spontaneously appeared. The minimum of the interfacial tension was interpreted as the critical phenomenon close to the second order phase transition. The critical exponent of the interfacial tension obtained from the results is ν ∼1.5, which is the same as that expected from the mean field theory for the binary mixtures. The experiments successfully demonstrated that this method should be a new tool to study various interfacial phenomena. As another demonstration, we measured the response spectra on the colloidal liquid surface. The results show that the surface tension of a colloidal liquid is the same as that of pure water.