LASER INTERFEROMETRIC DETERMINATION OF THERMAL DIFFUSIVITY OF TRANSPARENT SOFT GLASSY MATERIALS

摘要

Many soft materials of industrial importance such as pastes, gels, concentrated dispersions and emulsions fail to attain thermodynamic equilibrium over practical time scales due to jamming of constituent entities. Such soft materials, known as soft glassy materials, have very high viscosity and show extremely slow relaxation behavior. In this work heat transport behavior of a model soft glassy material, namely aqueous suspension of laponite is studied. Laponite is synthetic nanoclay with particles of disc-like shape, diameter 25 nm and thickness 1 nm. For concentrations above 1 volume % the aqueous system forms a space filling gel that supports its weight. In the present work, thermal diffusivity of laponite suspension in deionized water is investigated with laser interferometry. A sample is placed in an octagonal cavity comprising fixed copper plates on the top and bottom sides. Initially, the sample is at a uniform temperature. Collimated laser light beam generated from He-Ne laser with 632.8 nm wavelength is passed through the test cell. The interferometer is aligned in the infinite fringe setting, obtained by balancing the refractive index of the laponite suspension against a reference cell filled with glucose solution. At certain time instant (t=0), the temperature of top surface is increased by 1-3°C. The interference patterns are analyzed to obtain the time dependent temperature field. This data is regressed against the analytical solution of the unsteady heat conduction problem to determine the thermal diffusivity of the solution. Results show a strong dependence of thermal diffusivity on laponite concentration.