Nanoparticle-Fluid Interactions at Ultrahigh Acoustic Vibration Frequencies Studied by Femtosecond Time-Resolved Microscopy.

Liquid viscous and viscoelastic properties are very important parameters in determining rheological phenomena. Mechanical resonators with extremely high vibrational frequencies interacting with simple liquids present a wide range of applications from mass sensing to biomechanics. However, a lack of understanding of fluid viscoelasticity greatly hinders the utilization of mechanical resonators. In this paper, the high frequency acoustic vibrations of Au nanoplates with large quality factors were used to probe fluid properties (water, glycerol, and their mixtures) through time-resolved pump-probe microscopy experiments. For water, viscous damping was clearly observed, where an inviscid effect was only detected previously. Adding glycerol to the water increases the fluid viscosity and leads to a bulk viscoelastic response in the system. The experimental results are in excellent agreement with a continuum mechanics model for the damping of nanoplate breathing modes in liquids, confirming the experimental observation of viscoelastic effects. In addition to the breathing modes of the nanoplates, Brillouin oscillations are observed in the experiments. Analysis of the frequency of the Brillouin oscillations also shows the presence of viscoelastic effects in the high-viscosity solvents. The detection and analysis of viscous damping in liquids is important not only for understanding the energy dissipation mechanisms and providing the mechanical relaxation times of the liquids but also for developing applications of nanomechanical resonators for fluid environments.