We describe an optical beam deflection technique for measurements of the thermal diffusivity of fluid mixtures and suspensions of nanoparticles with a precision of better than 1%. Our approach is tested using the thermal conductivity of ethanol-water mixtures; in nearly pure ethanol, the increase in thermal conductivity with water concentration is a factor of 2 larger than predicted by effective medium theory. Solutions of fullerenes in toluene and suspensions of alkanethiolate-protected Au nanoparticles were measured to maximum volume fractions of 0.6% and , respectively. We do not observe anomalous enhancements of the thermal conductivity that have been reported in previous studies of nanofluids; the largest increase in thermal conductivity we have observed is for diam Au particles suspended in ethanol.
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The thermal conductivity , thermo-optic coefficient , and effective beam waist of the laser were determined in independent experiments with the fused silica (FS) heater substrate in air. In this analysis, we used a heat capacity of . The metal-line heater dimensions (separation— and width—) were verified with an optical microscope.
