Measurement of ultralow interfacial tension with a laser interface manipulation technique

The liquid surface or interface is deformed slightly by a laser beam passing through it. Based on this principle, a method to measure the liquid-liquid interfacial tension is developed. The interfacial tension is determined from the deformation, of which the displacement is measured with another probe laser in a noncontact manner. The measurements were available in two different senses: the constant displacement under continuous laser irradiation gives the static value, and the frequency response spectrum of the displacement under modulated excitation gives the dynamic value. To demonstrate the usefulness of this method, a series of experiments were conducted in the interface between heptane and water containing aerosol OT as a surfactant. The interfacial tension was controlled by the concentration of added NaCl, and measurements were made over the range from 1 microN/m to 100 mN/m of the tension. The results were in good agreement with the previous works. This method would be a new tool for the studies of various interfacial phenomena.