Optical detection of photothermally induced temperature changes in glass-forming liquids

This PhD research made use of advanced all optical methods to measure the response of the glassforming materials glycerol and salol to photothermal excitation, with the aim of extracting information on the frequency and temperature dependence of the heat capacity and the thermal expansion coefficient. Since the characteristic relaxation times of glassformers can vary over several orders of magnitude, the main challenge of the work was to develop techniques that are able to monitor the temperature and density response to photothermal excitation in a large bandwidth. Two approaches were explored: fluorescence-based thermometry and thermal lens detection.The first part of the manuscript reports on the development of an all-optical thermometry based on fluorescence. The temperature-sensitive nature of molecular fluorescence of RhB dissolved in the glycerol sample provided the basis for designing optical detection systems whereby analysis of changes in quantum yield, the peak wavelengths, the width of the emission peaks, and other spectral attributes can provide a local measurement of temperature. This scheme resulted in remote and ultrafast (sub-microsecond level) temperature detection with a bandwidth of 5 kHz and sensitivity of ~16 mK·Hz-1/2. Monitoring of the transient thermal response caused by pulsed lased irradiation and the periodic temperature change caused by periodically modulated laser heating by fluorescence thermometry is shown to be feasible. The approach was exploited for the investigation of the thermal relaxation of supercooled glycerol. Results obtained in two photothermal excitation schemes, one involving illumination of the sample by sinusoidally modulated laser light and one using a laser pulse to heat the sample, showed temperature responses of which the shape and dependence on the DC temperature qualitatively correspond with expectations and simulations for a relaxing material.The second part reports on the simultaneous spectroscopy of the specific heat capacity and thermal expansivity of glycerol and salol by making use of a wideband time-resolved thermal lens technique. An analytical model is presented that describes TL transients in a relaxing system subjected to impulsive laser heating. A set of TL waveforms, with a temporal resolution of the order of nanosecond and durations up to 20 ms, have been analyzed for glycerol and salol upon supercooling, from 300 K to 200 K, and 255 K to 228 K, respectively. The challenge of this method is to disentangle the mixed influences of the frequency dependent heat capacity and thermal expansion coefficient from the signals. Satisfactory fitting of the TL signals to the model allowed the assessment of relaxation strength and relaxation frequency of the two quantities up to sub-100 MHz of glycerol and sub-10 MHz of salol, further extending the bandwidth of specific heat and thermal expansion spectroscopy compared to previous work reported in literature. The obtained VFT plots of specific heat capacity and thermal expansion are found to be parallel in the probed temperature range. The respective fragilities are comparable to those of dielectric and mechanical susceptibilities, confirming the universal relaxation behavior between the different response functions. Interestingly, the respective relaxation strengths are quite different.

Publication year:2022

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