Newtonian fluid mechanics, in which the shear stress is proportional to the strain rate, is synonymous with the flow of simple liquids such as water. We report the measurement and theoretical verification of non-Newtonian, viscoelastic flow phenomena produced by the high-frequency (20 GHz) vibration of gold nanoparticles immersed in water-glycerol mixtures. 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. DOI: 10.1103/PhysRevLett.111.244502 PACS numbers: 47.50.Àd, 47.61.Àk, 62.25.Fg, 87.15.ht A fluid is said to be Newtonian if it exhibits a simple linear relationship between shear stress and strain rate. This description, which underlies conventional fluid dynamics, provides an excellent approximation to the behavior of real fluids, provided the time scale for measurement is long compared to the time required for stresses to propagate in the fluid. In simple liquids, including water and glycerol, these ''relaxation times'' are on the order of 1 ps-1 ns [1][2][3]. These time scales are short compared to the time scales associated with the motion of macroscopic objects in the fluids, which allows interactions between solid structures and simple fluids to be described by classical Navier-Stokes treatments [4,5]. These treatments hold even for micrometer-scale objects, such as the cantilevers found in atomic force microscopes and microelectromechanical devices, because they have characteristic vibrational frequencies in the kHz to MHz range [6,7]. Non-Newtonian fluid mechanics is therefore conventionally associated only with complex fluids that have long relaxation times, such as polymer solutions and melts, dense colloidal suspensions such as corn starch in water, and fluids near their phase transitions [8,9]. Scaling objects down to nanometer size scales increases their characteristic vibrational frequencies up to the GHz or THz range [10]. Fluid-structure interactions on these length scales thus have the potential to show significant deviations from Newtonian behavior, even for simple liquids.Departures from Newtonian behavior have been reported for simple liquids under extreme confinement, due to structural reorganization and surface effects on the molecular scale [11][12][13]. For bulk fluids, by contrast, direct mechanical observation of non-Newtonian behavior has been limited to solid structures interacting with dilute gases [14]. In this case, the effects can be predicted rigorously by the Boltzmann equation [15]. For simple bulk liquids, however, rigorous theoretical description of, and experimental access to, the non-Newtonian regime remain outstanding problems in the physical sciences.We access this regime directly for the first time by exciting a...

To fabricate robust metallic nanostructures with top-down patterning methods such as electron-beam lithography, an initial nanometer-scale layer of a second metal is deposited to promote adhesion of the metal of interest. However, how this nanoscale layer affects the mechanical properties of the nanostructure and how adhesion layer thickness controls the binding strength to the substrate are still open questions. Here we use ultrafast laser pulses to impulsively launch acoustic phonons in single gold nanodisks with variable titanium layer thicknesses, and observe an increase in phonon frequencies as a thicker adhesion layer facilitates stronger binding to the glass substrate. In addition to an all-optical interrogation of nanoscale mechanical properties, our results show that the adhesion layer can be used to controllably modify the acoustic phonon modes of a gold nanodisk. This direct coupling between optically excited plasmon modes and phonon modes can be exploited for a variety of emerging optomechanical applications.

Reversible exchange of photons between a material and an optical cavity can lead to the formation of hybrid light-matter states where material properties such as the work function [ Hutchison et al. Adv. Mater. 2013 , 25 , 2481 - 2485 ], chemical reactivity [ Hutchison et al. Angew. Chem., Int. Ed. 2012 , 51 , 1592 - 1596 ], ultrafast energy relaxation [ Salomon et al. Angew. Chem., Int. Ed. 2009 , 48 , 8748 - 8751 ; Gomez et al. J. Phys. Chem. B 2013 , 117 , 4340 - 4346 ], and electrical conductivity [ Orgiu et al. Nat. Mater. 2015 , 14 , 1123 - 1129 ] of matter differ significantly to those of the same material in the absence of strong interactions with the electromagnetic fields. Here we show that strong light-matter coupling between confined photons on a semiconductor waveguide and localized plasmon resonances on metal nanowires modifies the efficiency of the photoinduced charge-transfer rate of plasmonic derived (hot) electrons into accepting states in the semiconductor material. Ultrafast spectroscopy measurements reveal a strong correlation between the amplitude of the transient signals, attributed to electrons residing in the semiconductor and the hybridization of waveguide and plasmon excitations.

Acoustic fields in a liquid medium can trap and suspend small particles at their pressure nodes. Recent measurements demonstrate that nanorods immersed in these fields generate autonomous propulsion, with their direction and speed controlled by both the particle’s shape and density distribution. Specifically, slender nanorods with an asymmetric density distribution about their geometric centre are observed to move steadily with their low density end leading the motion; particle geometry exerts an equally significant and potentially opposing effect. In this article, we investigate the physical mechanisms underlying this combined density/shape induced phenomenon by developing a simple yet rigorous mathematical framework for axisymmetric particles. This only requires solution of the (linear) unsteady Stokes equations, which can be performed numerically or analytically. The theory holds for all particle shapes, particle aspect ratios (length/width) and acoustic frequencies. It is applied to slender dumbbell-shaped particles and asymmetric nanorods – these provide model systems to investigate the competing effects governing propulsion. This shows that geometric and density asymmetries in the particle generate axial jets that can produce motion in either direction, depending on the relative strengths of these asymmetries and the acoustic Reynolds number (dimensionless frequency). Strikingly, the propulsion direction is found to reverse with increasing frequency, an effect that is yet to be reported experimentally. The general theory and mechanism described here enable the a priori design and fabrication of nano-motors in fluid for transport of small-scale payloads and robotic applications.

The study of acoustic vibrations in nanoparticles provides unique and unparalleled insight into their mechanical properties. Electron-beam lithography of nanostructures allows precise manipulation of their acoustic vibration frequencies through control of nanoscale morphology. However, the dissipation of acoustic vibrations in this important class of nanostructures has not yet been examined. Here we report, using single-particle ultrafast transient extinction spectroscopy, the intrinsic damping dynamics in lithographically fabricated plasmonic nanostructures. We find that in stark contrast to chemically synthesized, monocrystalline nanoparticles, acoustic energy dissipation in lithographically fabricated nanostructures is solely dominated by intrinsic damping. A quality factor of Q = 11.3 ± 2.5 is observed for all 147 nanostructures, regardless of size, geometry, frequency, surface adhesion, and mode. This result indicates that the complex Young's modulus of this material is independent of frequency with its imaginary component being approximately 11 times smaller than its real part. Substrate-mediated acoustic vibration damping is strongly suppressed, despite strong binding between the glass substrate and Au nanostructures. We anticipate that these results, characterizing the optomechanical properties of lithographically fabricated metal nanostructures, will help inform their design for applications such as photoacoustic imaging agents, high-frequency resonators, and ultrafast optical switches.

Transient absorption microscopy is used to examine the breathing modes of single gold nanowires in highly viscous liquids. By performing measurements on the same wire in air and liquid, the damping contribution from the liquid can be separated from the intrinsic damping of the nanowire. The results show that viscous liquids strongly reduce the vibrational lifetimes but not to the extent predicted by standard models for nanomaterial-liquid interactions. To explain these results a general theory for compressible viscoelastic fluid-structure interactions is developed. The theory results are in good agreement with experiment, which confirms that compressible non-Newtonian flow phenomena are important for vibrating nanostructures. This is the first theoretical study and experimental measurement of the compressible viscoelastic properties of simple liquids.

The dynamics of mechanical structures can be strongly affected by the fluid in which they are immersed. Ultrafast laser spectroscopy has recently provided fundamental insight into this fluid-structure interaction for nanoparticles immersed in a range of viscous fluids. In this article, we present results of a rigorous finite-element analysis and commensurate scaling theory that enable interpretation and analysis of these experiments, for the extensional vibrational modes of axisymmetric nanoparticles immersed in viscous fluids. Right circular, conical, and bipyramidal axisymmetric cylinder geometries are considered. We also develop an approximate analytical model that accounts for finite viscous penetration depth, which displays excellent agreement with finite-element results for particles of large aspect ratio. The finite-element results agree well with available measurements for particles in low-viscosity fluids such as water, but significant discrepancies exist at higher viscosities. Possible mechanisms for these differences are discussed.

Plasmon hybridization theory, inspired by molecular orbital theory, has been extremely successful in describing the near-field coupling in clusters of plasmonic nanoparticles, also known as plasmonic molecules. However, the vibrational modes of plasmonic molecules have been virtually unexplored. By designing precisely configured plasmonic molecules of varying complexity and probing them at the individual plasmonic molecule level, intramolecular coupling of acoustic modes, mediated by the underlying substrate, is observed. The strength of this coupling can be manipulated through the configuration of the plasmonic molecules. Surprisingly, classical continuum elastic theory fails to account for the experimental trends, which are well described by a simple coupled oscillator picture that assumes the vibrational coupling is mediated by coherent phonons with low energies. These findings provide a route to the systematic optical control of the gigahertz response of metallic nanostructures, opening the door to new optomechanical device strategies.