Microwave is very attractive for the industry because of fast and selective and efficient heating. Although microwave is absorbed on the interface strongly, interfacial behavior under the irradiation has not been understood well. In this research, interfacial tension between two liquid phases (n-Decane and water) with surfactant was measured during microwaves irradiation. The profiles of interfacial tension were compared at different cycle number of irradiation. According to the data, values of interfacial tension at same temperatures before irradiation and after irradiation were different. For example, the tension after the irradiation became lower than original values before the first irradiation. Although the difference depended on the cycle number of the irradiation, the effect almost disappeared around 4 or 5 times irradiation.
In our relevant studies, we found that microwave memory lasts for surface tension of liquid-air interface and network of water molecule and bubble formation during the irradiation plays an important role for the hysteresis. The mechanism for the surface tension reduction was thought as non-thermal effect, and special effect of microwave for interfacial tension can be discussed similarly. Microwave is absorbed at liquid-air interface too much because it can pass through oil phase. Although microwave absorbs in water phase, the absorbance decays exponentially because of the penetration depth. Finally, microwave absorbance energy, that is, rotation of polar molecule, concentrates strongly on the interface.
One of reasons of the reduction might be the fine bubble formation around the interface. The evaporation from water phase was caused at interface by non-equilibrium local heating. When water phase includes surfactant, it is adsorbed at new liquid-air interface of the bubble. The stable bubbles, which are located between oil and water interfaces, interrupt the interfacial tension along the liquid-liquid interface. Accordingly, microwave irradiation history was memorized as bubble formation at the liquid-liquid interface, and interfacial tension before and after the irradiation, became different. The number of bubbles, which are placed at the interface, increase with the cycle number of the irradiation. After a few cycles, because the number is saturated, the interfacial became constant value. This microwave history for liquid-liquid interface is interesting to manipulate the property, and detailed mechanism should be resolved by investigating results obtain from further conditions such as concentration and kind of surfactant and microwave power and duration time. This technology for manipulating interfacial property will be expected for microwave application in the industry with two phases.
