Development of a Horizontal Temperature Gradient Cell for the Optical Microscopic Observation and Its Application for Research Works on Non-Equilibrium Transient Phenomena

Abstract

We have developed a sophisticated sample cell with which one can impose horizontal temperature gradient on the sample under an optical microscope. The temperatures of the hotter and the cooler sides of the sample are individually maintained respectively by electric heaters and circulating water of which temperature is controlled accurately in a huge reservoir. Thus, with this cell we can attain the temperature difference (ΔT) up to 30.0°C for the sample which is embedded in the horizontal gap with 1 mm. The accuracy of the temperature control is ± 0.1°C. Furthermore, temperature jump (T-jump) experiments can be conducted by sudden switching of the water circulation paths from two individual reservoirs maintained at different temperatures T1 and T2 (corresponding to the T-jump from T1 to T2). Even by this primitive method, a satisfactorily quick T-jump can be accomplished (for instance, within 90 sec for 30.0°C T-jump from 50.0°C to 20.0°C). Both the linearity of the horizontal temperature gradient (ΔT) across the sample and homogeneity of the temperature normal to ΔT (namely, parallel to hotter and cooler side walls) were rigorously examined by employing a liquid crystal sample of which nematic-isotropic phase boundary was visually observed under the optical microscope with the hotter and the cooler temperatures being set respectively above and below the nematic-isotropic transition temperature. As a result, we found that the nematic-isotropic boundary is firmly parallel to the side walls, ensuring homogeneity of the temperature normal to ΔT. To examine the linearity of ΔT, both of the hotter and the cooler temperatures were step-wisely changed with keeping ΔT constant. At each step the horizontal position of the nematic-isotropic boundary was measured and was plotted as a function of the cooler side temperature. As a result, the plots exhibited good linearity for ΔT below 30.0°C. We show some interesting results as examples of application of our horizontal temperature gradient cell to research works on the non-equilibrium transient phenomena ; the first observation of overshoot crystallization in a mixture of n-alkane (paraffin) and easy determination of the equilibrium melting temperature of polymer.