Fayer Lab: Dynamics in Complex Liquids

Dynamics in Complex Liquids
Jie Li

Some liquids remains in the liquid state even below their melting points although they should crystallize. Liquids below their melting points are called supercooled liquid. As a supercooled liquid is cooled further it passes through the glass transition and becomes a glassy solid. A glass is a "liquid that has lost its ability to flow."

Near the glass transition, the viscosity can increase 13 orders of magnitude as the temperature is lowered a few tens of degree (see Fig. 1). Although glasses have been widely used for hundreds of years, we still do not have a detailed understanding of the glass transition. What is the glass transition? How can a liquid exist in a thermodynamically non-equilibrium state, a supercooled liquid?
Liquid crystals are materials that show a rich phase behavior. At high temperature, nematogens are in the isotropic phase. As the temperature is lowered, the isotropic nematogen liquid undergoes the isotropic to nematic phase transition (see Fig. 2). In the nematic phase, molecules are aligned in a manner that has a net projection along a particular direction called the director. Above but close to the N-I phase transition temperature, TNI, orientational relaxation dynamics are strongly influenced by the local structures (pseudo-nematic domains) that exist in the isotropic phase.

The long time scale relaxation, dominated by the randomization of the pseudo-nematic domains, is described by Landau-de-Gennes (LdG) theory. The orientation relaxation time diverges at nematic-isotropic phase transition. However a detailed understanding of the intradomain dynamics at short times is still unclear.
Optical heterodyne detected optical Kerr effect (OHD-OKE) experiment provides us with a powerful tool to study these problems. In OHD-OKE experiments, a pump pulse induces a transient birefringence in the liquid by inducing a small orientational anisotropy along the electric field direction of the pump pulse. A second pulse, the probe pulse, measures the decay of this birefringence over a range of times from ~100 fs to tens of µs. The OKE signal is the time derivative of the orientation correlation function.

The measurements give a direct view of how the relaxation dynamics over 8 orders of magnitude in time. slows down during phase transition. The experimental system is unique because over the very broad range of times spanned that covers both microscopic and macroscopic time scales. Figure 3 shows a typical example of the supercooled liquid data on log plot. The amplitude changes ~5 orders of magnitude and the time window spans almost 5 orders of magnitude and is typically wider for liquid crystals.

Figure 4 shows data for nematogens in both the isotropic phase (red) and the nematic phase (purple). The nature of the dynamics changes abruptly and dramatically at the isotropic to nematic phase transition. Figure 5 shows additional supercooled liquid data as the glass transition is approached. An oscillatory feature appears at short time.
The striking similarities and significant difference between nematogen dynamics in the isotropic phase and supercooled liquids give us a hint for understanding the phase transitions in these two classes of liquids.

Are there similar domain structures in supercooled liquids? Are these domains the cause of the glass transition and dynamical slowing down in supercooled liquid? What determines the value of the intermediate power exponent? What causes the new features in nematic phase? We are continuing experiments on these and other systems, and we are performing theoretical calculations in collaboration with Professor Hans C. Andersen at Stanford University.

“The Boson Peak in Supercooled Liquids: Time Domain Observations and Mode Coupling Theory,” Hu Cang, Jie Li, Hans C. Andersen, and M. D. Fayer, J. Chem. Phys. 123, 064508(7) (2005). pdf

“Ultrafast to Slow Orientational Dynamics of a Homeotropically Aligned Nematic Liquid Crystal,” Jie Li, Irene Wang, and M. D. Fayer, J. Phys. Chem. B 109, 6514-6519 (2005). pdf

“Orientational Dynamics of the Ionic Organic Liquid 1-Ethyl-3-Methylimidazolium Nitrate,” Hu Cang, Jie Li, and M. D. Fayer, J. Chem. Phys. 119, 13017-13023 (2003). pdf

“Dynamics in Supercooled Liquids and in the Isotropic Phase of Liquid Crystals: A Comparison,” Hu Cang, Jie Li, V.N. Novikov, and M. D. Fayer, J. Chem. Phys., 118, 9303-9311 (2003). pdf