Effect of sodium pyrophosphate and polyethylene oxide on thermal diffusivity of Laponite RD colloidal suspension

Abstract

The effect of Sodium pyrophosphate and polyethylene oxide on thermal diffusivity of Laponite RD colloidal suspension is studied with optical technique of Mach-Zehnder interferometer. Laponite RD is synthetic nano-crystalline colloid having a disk shaped monodispersed particle with radius 12.5 nm and edge thickness 1 nm. Aqueous Laponite suspension generally forms complex microstructure with and without addition of externally added salt. At low volume fraction and low concentration of externally added salt it forms dilute gel and at high concentration and high volume fractions, a disconnected Wigner glass. The appropriate concentrations of sodium pyrophosphate in the range of 5–25 mM and Polyethylene oxide (PEO) 0.5–1.5 wt% is mixed with 2–3 wt% Laponite RD suspension. The freshly prepared 10 days old Laponite RD suspension was stirred for half an hour before pouring in an octagonal test cell cavity. The top and bottom sides of octagonal cavity are attached to copper plates. Initially the suspension was kept at uniform temperature. Collimated laser light beam generated from He–Ne laser with 632.8 nm wavelengths is passed through the test cell in which Laponite suspension is kept. The initial alignment of the interferometer is in the infinite fringe setting mode, which is obtained by balancing the density of test cell filled with Laponite suspensions and reference cell filled with dextrose solution. Subsequently, there is step increase of temperature of top plate in the range of 1–3 ° C. Refractive index variation through change in temperature of Laponite suspension leads to a well-defined interference pattern. The interferograms are then processed through fringe thinning procedure and are analyzed to obtain the time dependent temperature field varying with spacing of octagonal cavity. They are then compared with analytical solutions of the heat conduction diffusion equations. The comparison of least square fit of the experimental data with analytical solutions leads to estimation of thermal diffusivity. Interestingly even with very small concentration of sodium pyrophosphate, the Laponite RD suspension (around 2–3 wt%), shows a substantial reduction in thermal diffusivity values as compared to Laponite RD suspension. The aggregation rate of Laponite suspension as a function of pyrophosphate and polyethylene oxide concentrations are compared using static light scattering experiments from literature in order to interpret laser interferometric thermal diffusivity results.