No abstract

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.

A technique for viscosity measurement was developed based on the principle of laser-induced surface deformation. Light incident into liquids increases its momentum due to the difference in refractive index and gives the surface an upward force as a reaction. The plane surface thus swells up and deforms, and the shape is determined so that the force is balanced with the surface tension and the gravity. On sudden laser irradiation, the deformation inevitably accompanies a viscous flow and exhibits a relaxational behavior with a delay time, which gives the viscosity. Theoretical prediction of the step-response function was given that takes surface tension waves excited by the laser into consideration. Nd–yttritium–aluminum–garnet laser with 0.6W output was focused to ∼200μm beam waist and used for the pumping. The deformation process was observed sensitively with another probe laser illuminating the activated area. This system was tested with the standard liquids for viscosity ranging from 1 to 106cSt. The results demonstrated the validity of this technique, though a correction for the inertia effect was needed in the range lower than 10cSt. Further, effect of the thermal expansion by a slight optical absorption was discussed. This technique is especially useful at high viscosities since the measurement takes only a few seconds even in the specimen with 106cSt. Besides the rapidity, it has a great advantage of a noncontact feature and is appropriate for measuring the liquids that strongly dislike contamination. It has also potential applications in industries, measurement of liquids isolated in a production line, for instance.