Dynamic correlation length scales under isochronal conditions.

Area of Science:

• Condensed matter physics
• Physical chemistry

Background:

• The dramatic changes in liquid behavior approaching the vitreous state, including massive increases in dynamic timescales and transport properties, remain a significant unsolved problem.
• These dynamics are associated with increased dynamic heterogeneity, characterized by spatial variations and correlations in molecular mobilities.

Purpose of the Study:

• To investigate the coupling between changing liquid dynamics and dynamic heterogeneity.
• To determine if dynamic heterogeneity is the cause of the observed changes in liquid dynamics.

Main Methods:

• The study employed the first nonlinear dielectric experiments under elevated hydrostatic pressures on two liquids.
• Measurements focused on the third-order harmonic component of susceptibilities.
• Molecular dynamic simulations were used to support experimental findings.

Main Results:

• The number of dynamically correlated molecules was extracted for various state points.
• For non-associated liquids, the dynamic correlation volume was found to depend primarily on relaxation time, largely independent of temperature and pressure.
• Molecular dynamics simulations confirmed that the maximum in four-point dynamic susceptibility is invariant along isochrones for non-hydrogen-bonding liquids.

Conclusions:

• Dynamic cooperativity appears to be the principal control parameter governing the slowing of molecular motions in supercooled materials.
• The findings suggest that dynamic heterogeneity is coupled to, and likely causes, the dramatic changes in liquid dynamics near the vitreous state.