MSE Seminar Series: Mike Roland
Friday, March 16, 2012
1:00 p.m.-2:00 p.m.
Room 2110 Chemical and Nuclear Engineering Bldg.
301 405 5240
Dynamics of Vitrifying Liquids - The Difficult Problem Albert Einstein Avoided
Naval Research Laboratory
When liquids and polymers are sufficiently cooled or densified, mutual interactions begin to dominate the behavior neighboring species must make small adjustments in position in order for a given molecule to change its configuration. This intermolecular cooperativity greatly complicates the dynamics, giving rise to striking behavior. For example, small changes in temperature or pressure of a supercooled liquid can alter the time scale for molecular motions from nanoseconds to a duration exceeding the human lifespan. Despite a century of research, a fundamental understanding, much less a predictive theory, of the dynamic properties of dense liquids and polymers is lacking.
In this talk I review our progress in quantifying the role of the thermodynamic variables temperature, T, and density, ρ, on the dynamics. An important aspect of the work was the discovery that relaxation times and viscosities of molecular liquids and polymers superpose when plotted against the scaling variable T/ργ, with the scaling exponent γ a material constant. Moreover, this scaling exponent can be related to the nature of the intermolecular repulsive potential; thus, dynamic spectroscopy measurements can be used to quantify the forces between molecules. Other properties derive from the scaling behavior, including the Boyer-Spencer rule and the correlation of fluctuations in the potential energy with fluctuations in the virial pressure.