| AVS 61st International Symposium & Exhibition | |
| Nanometer-scale Science and Technology | Monday Sessions |
| Session NS+SE-MoM |
| Session: | Delivering Energy and Mass at the Nanoscale |
| Presenter: | Matthew Pelton, University of Maryland, Baltimore County |
| Authors: | M. Pelton, University of Maryland, Baltimore County D. Chakraborty, University of Melbourne, Australia E. Malachosky, University of Chicago P. Guyot-Sionnest, University of Chicago J.E. Sader, University of Melbourne, Australia |
| Correspondent: | Click to Email |
Studies of acoustic vibrations in nanometer-scale particles can provide fundamental insights into the nanomechanical properties of nanoscale materials, and into the mechanical coupling between the nanoparticles and their environment. Metal nanoparticles allow for all-optical, non-contact measurements, using ultrafast laser pulses to generate and probe high-frequency acoustic vibrations. In early studies, the decay of the signal due to nanoparticle vibrations was dominated due to vibrations in nanoparticle size. By using highly uniform bipryamidal gold nanoparticles, we were able to overcome the effects of inhomogeneous damping and measure the rate at which the acoustic oscillations dissipate energy. Measurements in low-viscosity liquids such as water showed a strong “intrinsic” damping occurring within the nanoparticles themselves, and an environmental damping due to viscous coupling to the surrounding liquid. This fluid damping was described quantitatively using a parameter-free model.
In higher-viscosity liquids, however, the measured oscillation frequencies and damping rates deviate strongly and qualitatively from the predictions of this model. The deviations are explained quantitatively as arising from non-Newtonian effects in the liquid. The nanoparticles vibrate at very high frequencies (20 GHz), so that their vibration periods are comparable to the intrinsic relaxation times of the liquid. The structure-fluid interaction is thus dominated by viscoelastic effects. The observed viscoelasticity is not due to molecular confinement, but is a bulk continuum effect arising from the short time scale of vibration. This represents the first direct mechanical measurement of the intrinsic viscoelastic properties of simple bulk liquids, and opens a new paradigm for understanding extremely high frequency fluid mechanics, nanoscale sensing technologies, and biophysical processes.