Optical measurements of acoustic oscillations in metal nanoparticles provide a sensitive probe into the mechanical properties of materials at GHz frequencies and nanometer length scales. In these experiments, an incident pump laser heats the nanoparticles, leading to their expansion and the excitation of mechanical vibrations. The vibrations produce oscillations in the plasmon resonance frequency of the nanoparticles, which are monitored by measuring the change in transmission through the sample of a second, probe laser pulse. By making these measurements on a highly monodisperse sample of bipyramidal gold nanoparticles, we were able to determine both the frequency and the decay rate of the vibrations. Measurements on nanoparticles in different solvents made it possible to determine the portion of damping and the vibrational frequency shift that are due to coupling to the surrounding liquid environment. Viscous damping could account for results at low viscosities, but significant discrepancies were observed for higher viscosities. The discrepancies were ultimately resolved by accounting for the viscoelastic nature of the surrounding liquids. For more viscous liquids, relaxation times are higher, and thus more of the vibrational energy is stored as elastic energy in the surrounding liquid. This reduces damping, and the restoring force provided by the stored energy increases the vibrational frequency, the opposite of what would occur for an ordinary Newtonian fluid. These measurements demonstrate that metal nanoparticles can serve as nanoscale rheometers, with the picosecond response times required to reveal viscoelastic effects in conventional liquids.
Matthew A. Pelton, "Optical measurements of nanoparticle vibrations for fluid mechanics on the nanoscale (Conference Presentation)," Proc. SPIE 10347, Optical Trapping and Optical Micromanipulation XIV, 103471T (Presented at SPIE Nanoscience + Engineering: August 09, 2017; Published: 21 September 2017); https://doi.org/10.1117/12.2275529.5583365927001.
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