A study of orientational dynamics in supercooled liquids

The question of what occurs when liquids super cool and form glasses rather than crystallizing is one of the most important problems in condensed matter physics. However, due to the nature of the glass transition it is one of the most difficult problems to study. The fundamental difficulty to studying supercooled liquids is that the dynamics cover an enormous range of time scales ranging from sub-picosecond librations to virtually infinite times at or below the glass transition. The orientational dynamics of three supercooled liquids: salol, dibutyl plithalate (DBP) and ortho-terphenyl (OTP) are probed using multiple techniques to extract complementary data over a wide range of time and temperature scales. During the studies a new time domain method for studying orientational relaxation, Density Induced Heterodyne Amplified Rotational Dynamics (DIHARD) was discovered. The results of the measurements for OTP and DBP were compared to the predictions of Mode Coupling Theory (MCT). The time-temperature superposition principle was shown to hold for both DBP and OTP above the MCT T_C. The data also indicated that the orientational dynamics were dominated by hydrodynamic behavior. The MCT predictions of the exponents a, b, γ, and λ were only partially verified. The discrepancy arises because the minimum of the susceptibility spectrum appears to show a more complex structure then the ideal MCT predicts. In the time domain there is no clear plateau region between the alpha and beta relaxation indicating that idealized MCT is inappropriate for describing the beta dynamics of the system, although the predictions about the alpha dynamics are satisfied.