We present a thorough investigation of the collective dynamics of liquid Ag combining neutron Brillouin scattering and ab initio molecular dynamics (AIMD) determinations of the dynamic structure factor $S(Q,\omega )$. The main scope of this work is not only to provide experimental results for some important dynamical properties of this liquid metal in the wave-vector range $4<Q<16{\mathrm{nm}}^{-1}$, but also to inquire about the scarce detectability of shear waves apparently characterizing two elements of group IB, differently from other metals. In fact, as in the case of Au, a transverse-like dynamics is not deducible from the experimental $S(Q,\omega )$ of Ag, despite the indisputable quality of the neutron data collected on the BRISP spectrometer at the Institut Laue Langevin in Grenoble. However, the significant agreement between experiment and AIMD calculations allowed for an in-depth study of the simulated $S(Q,\omega )$ in a $Q$ range overlapping and extending the experimental one. A multimode analysis, already proven very successful in the description of various dynamical properties of fluid systems, is shown to be extremely effective also to analyze the intermediate scattering function predicted by AIMD at the various $Q$ values, and eventually enables a reliable determination of both longitudinal and transverse branches in the dispersion curve of this liquid. Throughout the paper we highlight the importance of referring to theoretically well-founded models for $S(Q,\omega )$ and of imposing physical constraints in a fit-based analysis: These ensure that the used models obey fundamental properties of the dynamic structure factor.

The dynamic structure factor $S(k,\omega )$ of a $77\%$ He and $23\%$ Ne gaseous mixture at $T=39.3\mathrm{K}$ and total number density $n=15.8{\mathrm{nm}}^{-3\mathrm{}}$ has been measured by inelastic neutron scattering at small angles. In the range of wave vectors studied, $0.7<k/{\mathrm{nm}}^{-1\mathrm{}}<1.8\mathrm{}$, we find clear side peaks at frequencies following the hydrodynamic sound dispersion $ck$ of the binary mixture (with $c$ the adiabatic sound speed in the mixture). Surprisingly, neither fast modes nor a transition from normal to fast sound are observed in the above low- $k$ region, indicating that hydrodynamic behavior persists up to $k$ values which are larger than those predicted by theories and computer simulations.

We present an experimental and simulation study of the collective dynamics of liquid ${\mathrm{D}}_2$ in the range of exchanged wave vectors $3\le Q\le 14{\mathrm{nm}}^{-1}$. Neutron Brillouin scattering results for the center-of-mass dynamic structure factor of this moderately quantum fluid are compared, on an absolute scale, with those obtained by three different quantum simulation methods such as ring polymer molecular dynamics and two versions of the Feynman-Kleinert approach. The experimental data can be well described by dynamical models typically used for liquids. Some discrepancies show up both among simulations, and between simulations and experimental data. Such discrepancies are found to mainly concern the damping of the propagating sound modes.

The response of Newtonian liquids to small perturbations is usually considered to be fully described by homogeneous transport coefficients like shear and dilatational viscosity. However, the presence of strong density gradients at the liquid/vapor boundary of fluids hints at the possible existence of an inhomogeneous viscosity. Here, we show that a surface viscosity emerges from the collective dynamics of interfacial layers in molecular simulations of simple liquids. We estimate the surface viscosity to be 8–16 times smaller than that of the bulk fluid at the thermodynamic point considered. This result can have important implications for reactions at liquid surfaces in atmospheric chemistry and catalysis.

A thorough analysis of the transverse current autocorrelation function obtained by molecular dynamics simulations of a dense Lennard-Jones fluid reveals that even such a simple system is characterized by a varied dynamical behavior with changing length scale. By using the exponential expansion theory, we provide a full account of the time correlation at wavevectors $Q$ between the upper boundary of the hydrodynamic region and $Q_{\mathrm{p}}/2$, with $Q_{\mathrm{p}}$ being the position of the main peak of the static structure factor. In the $Q$ range studied, we identify and accurately locate the wavevector at which shear wave propagation starts to take place, and show clearly how this phenomenon may be represented by a damped harmonic oscillator changing, in a continuous way, from an overdamped to an underdamped condition. The decomposition into exponential modes allows one to convincingly establish not only the crossover related to the onset of transverse waves but, surprisingly, also the existence of a second pair of modes equivalent to another oscillator that undergoes, at higher $Q$ values, a similarly smooth over to underdamped transition.

We used the high-resolution Inelastic X-ray Scattering beamline of the Advanced Photon Source at Argonne National Laboratory to measure the terahertz spectrum of pure water and a dilute aqueous suspension of 15 nm diameter spherical Au nanoparticles (Au-NPs). We observe that, despite their sparse volume concentration of about 0.5%, the immersed NPs strongly influence the collective molecular dynamics of the hosting liquid. We investigate this effect through a Bayesian inference analysis of the spectral lineshape, which elucidates how terahertz transport properties of water change upon Au-NP immersion. In particular, we observe a nearly complete disappearance of the longitudinal acoustic mode and a mildly decreased ability to support shear wave propagation.