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.

The laser manipulation technique was effectively used for agarose solutions and the frequency specstrum of the surface response to the periodical laser irradiation yielded shear elasticity G and surface tension sigma in the gel. The laser spot size, from 60 mum to 200 mum in radius, was chosen so that either the Rayleigh waves or the capillary waves, selectively excited, associated with G or sigma , respectively. The result of G showed a dependence on the agarose concentration that is consistent with the theoretical prediction of the percolation model, while sigma has little dependence on the concentration. The surface state of 0.2 wt.% agarose solution was controlled with sodium-dodecyl-sulfate (SDS) additives, and sigma of the gel and the sol was observed at different SDS concentrations: The result showed (i) sigma decreased with increasing SDS concentration up to 39 x 10-3 mol/l and kept a constant value thereafter, and (ii) the gel and the sol have the same value of sigma and the same dependence on the concentration. These results were considered from a viewpoint of surface pressure and a partially quantitative discussion was made on the surface adsorbed with SDS and agarose molecules.

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