Non-hydrodynamic transverse collective excitations in hard-sphere fluids.

Area of Science:

• Physics
• Condensed Matter Physics
• Statistical Mechanics

Background:

• Collective excitations are fundamental to understanding fluid dynamics.
• Hard-sphere fluids serve as a model system for studying dense matter behavior.
• Hydrodynamic theories describe fluid behavior at long wavelengths and low frequencies.

Purpose of the Study:

• To investigate collective excitations in hard-sphere fluids across various wave numbers and packing fractions.
• To identify and characterize non-hydrodynamic transverse excitations.
• To evaluate the role of transverse excitations in sound dispersion and compare with existing theories.

Main Methods:

• Molecular dynamics simulations were employed.
• Simulations covered a wide range of wave numbers.
• Analysis focused on transverse current spectral functions and longitudinal excitation dispersion.

Main Results:

• Non-hydrodynamic transverse excitations were observed for packing fractions η≥0.395.
• Longitudinal excitations exhibited negative dispersion, deviating from linear hydrodynamic laws.
• The findings contradict the Frenkel line approach's assertion on transverse excitations defining sound dispersion.

Conclusions:

• Transverse excitations in hard-sphere fluids do not solely define positive sound dispersion.
• Calculated cutoff Frenkel frequencies for transverse excitations were discussed in relation to shear wave dispersion.
• The study provides insights into the complex dynamics of dense fluids.