Shujiro Mitani

Measurement of high viscosity with laser induced surface deformation technique

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 r...

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

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, approximately 1 microN 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 v approximately 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.

Dependence of Transport Mean Free Path on Size of Scatterer.

The transport mean free path l* characterizing the light diffusion phenomenon in highly dispersed media was theoretically predicted and compared with the values observed by the experiments of coherent backscattering. The Mie scattering theory for a spherical particle was used to calculate the scattering cross section, that leads to the l* of the suspension together with the number density of the particles. The anisotropy in a single scattering process was also taken into account. A series of experiments was made in polystyrene latex with 5% and 10% volume fractions and different diameters ranging from 0.1 µm to 3 µm. The experimental values of l* obtained from the coherent backscattering peak were in good agreement with the theoretical predictions. This study is performed to give a solid base to the coherent backscattering phenomenon as a new approach for material evaluation of dispersed media.

Development of submillisecond Brillouin spectroscopy with optical beating technique

We developed a rapid measurement system for Brillouin scattering spectroscopy, which requires the data acquisition time of only 100 μs. The optical beating detection system was improved to directly observe and accumulate the incoherent signal carrying the information of the power spectrum of the scattered light. The temporal evolution of the mechanical properties of material can be measured with high time resolution. It is also possible to apply it to the light absorbing sample, since the short time radiating of the laser beam does not cause serious temperature change in the sample. The performance was actually demonstrated for the light absorbing material.

Laser excitation of high-frequency capillary waves

The laser-induced surface deformation (LISD) technique was applied to generate high-frequency capillary waves on liquid surfaces up to several tens of kHz in a noncontact manner. The dynamic response of the fluid near the surface was theoretically derived under the condition of periodical radiation pressure. The result of the numerical calculation predicts the propagation of induced capillary waves out from the excitation region. The efficiency of the wave generation was experimentally examined by changing the width of the excitation laser beam at the surface. The observed LISD spectra were well reproduced by the theory, showing that the effective frequency band can be extended up to over 100 kHz. The propagation of the optically generated wave was measured with a laser probe sweeping the position of the observation. The spatial profile gives the surface tension and the shear viscosity of the sample liquid. The frequency domain measurement was also carried out and the spectrum obtained at a fixed point agrees with the theory, demonstrating the rapid measurement of frequency-dependent phenomena.

Ripplon spectroscopic study on multilayered liquid surface

Ripplon spectroscopy was used to observe the propagation modes of surface waves on a layered structure. We experimentally prepared a water surface covered with an oil layer of µm-order thickness, on which two surface modes, bending and squeezing, were theoretically predicted to stand independently. Thermally excited capillary waves, called ripplons, propagating on the layered surface were observed with an optical beating light-scattering system developed by us and two modes were clearly observed as the doublet peaks of the Stokes and anti-Stokes components, respectively. The dispersion and the damping constant of ripplons of the two modes as well as their intensity ratio agreed well with the theoretical calculations. The dispersion of the mechanically excited surface waves, which is attributed to the bending mode, was also measured to examine the mode assignment in the light-scattering experiment.

Measurement of surface tension of liquid microdroplets through observation of droplet collision

We investigate the applicability of measuring surface tension of liquid microdroplets by analyzing their oscillatory behavior induced by droplet collision. The eigenfrequency of droplet oscillation reflects surface tension for small-amplitude oscillation. In this study, the oscillation frequencies of ethanol–ethanol and water–water droplet collisions are measured by direct observation. We experimentally verify that surface tension can be measured for deformation smaller than 10% of the droplet radius. The collision between ethanol–water droplets is also analyzed. The eigenfrequency induced by unlike-droplet collision exhibits a peculiar behavior, which may be attributed to the temporal hybrid structure of a combined droplet.

Non-Contact Method for Measurement of Surface/Interfacial Liquid Properties with Laser Manipulation Technique

The laser manipulation technique is a powerful tool for studying the liquid surface properties such as surface tension and viscosity. This method has several remarkable features, for example, non‐contact, wide range and high sensitivity. Ultra‐low interfacial tension, ∼lμN/m, was measured with this method on the water/heptane interface containing surfactant. This method is also applicable to the observation of highly‐viscous and colloidal liquids.