More Like this

1. Molecular origins of bulk viscosity in liquid water

   https://doi.org/10.1039/d0cp01560a  

   Yahya, Ahmad; Tan, Luoxi; Perticaroli, Stefania; Mamontov, Eugene; Pajerowski, Daniel; Neuefeind, Joerg; Ehlers, Georg; Nickels, Jonathan D. (May 2020, Physical Chemistry Chemical Physics)

   null (Ed.)

   The rapid equilibrium fluctuations of water molecules are intimately connected to the rheological response; molecular motions resetting the local structure and stresses seen as flow and volume changes. In the case of water or hydrogen bonding liquids generally, the relationship is a non-trivial consideration due to strong directional interactions complicating theoretical models and necessitating clear observation of the timescale and nautre of the associated equilibrium motions. Recent work has illustrated a coincidence of timescales for short range sub-picosecond motions and the implied timescale for the shear viscosity response in liquid water. Here, neutron and light scattering methods are used to experimentally illustrate the timescale of bulk viscosity and provide a description of the associated molecular relaxation. Brillouin scattering has been used to establish the timescale of bulk viscosity; and borrowing the Maxwell approach, the ratio of the bulk viscosity, ζ , to the bulk modulus, K , yields a relaxation time, τ B , which emerges on the order of 1–2 ps in the 280 K to 303 K temperature range. Inelastic neutron scattering is subsequently used to describe the motions of water and heavy water at the molecular scale, providing both coherent and incoherent scattering data. A rotational (alternatively described as localized) motion of water protons on the 1–2 ps timescale is apparent in the incoherent scattering spectra of water, while the coherent spectra from D 2 O on the length scale of the first sharp diffraction peak, describing the microscopic density fluctuations of water, confirms the relaxation of water structure at a comparable timescale of 1–2 ps. The coincidence of these three timescales provides a mechanistic description of the bulk viscous response, with the local structure resetting due to rotational/localized motions on the order of 1–2 ps, approximately three times slower than the relaxations associated with shear viscosity. In this way we show that the shear viscous response is most closely associated with changes in water network connectivity, while the bulk viscous response is associated with local density fluctuations. 

   more » « less
2. Transport signatures of plasmon fluctuations in electron hydrodynamics

   https://doi.org/10.1063/10.0022363  

   Zverevich, Dmitry; Levchenko, Alex (December 2023, Low Temperature Physics)

   In two-dimensional electron systems, plasmons are gapless and long-lived collective excitations of propagating charge density oscillations. We study the fluctuation mechanism of plasmon-assisted transport in the regime of electron hydrodynamics. We consider pristine electron liquids where charge fluctuations are thermally induced by viscous stresses and intrinsic currents, while attenuation of plasmons is determined by the Maxwell mechanism of charge relaxation. It is shown that, while the contribution of plasmons to the shear viscosity and thermal conductivity of a Fermi liquid is small, plasmon resonances in the bilayer devices enhance the drag resistance. In systems without Galilean invariance, fluctuation-driven contributions to dissipative coefficients can be described only in terms of hydrodynamic quantities: intrinsic conductivity, viscosity, and plasmon dispersion relation. 

   more » « less
3. Comprehensive Dynamics in a Polyelectrolyte Complex Coacervate

   https://doi.org/10.1021/acs.macromol.3c01540  

   Akkaoui, Khalil; Digby, Zachary A; Do, Changwoo; Schlenoff, Joseph B (January 2024, Macromolecules)

   The linear viscoelastic response, LVR, of a hydrated polyelectrolyte complex coacervate, PEC, was evaluated over a range of frequencies, temperatures, and salt concentrations. The PEC was a nearly-stoichiometric blend of a quaternary ammonium poly([3-(methacrylamido)propyl]trimethylammonium chloride), PMAPTAC, and poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt), PAMPS, an aliphatic sulfonate, selected because they remain fully charged over the conditions of use. Narrow molecular weight distribution polyelectrolytes were prepared using fractionation techniques. A partially deuterated version of PMAPTAC was incorporated to determine the coil radius of gyration, Rg, within PECs using small angle neutron scattering. Chain dimensions were determined to be Gaussian with a Kuhn length of 2.37 nm, which remained constant from 25 to 65 0C. The LVR for a series of matched molecular weight PECs, mostly above the entanglement threshold, exhibited crossovers of modulus versus frequency classically attributed to the reptation time, relaxation between entanglements, and the relaxation of a Kuhn length of units (the “monomer” time). The scaling for zero shear viscosity, η0, versus chain length N, was η0 ~ N3.1, in agreement with “sticky reptation” theory. The lifetime and activation energy, Ep, of a pair between polyanion and polycation repeat units, Pol+Pol-, were determined from diffusion coefficients of salt ions within the PEC. The activation energy for LVR of salt-free PECs was 2Ep, showing that the key mechanism limiting the dynamics of undoped PECs is pair exchange. An FTIR technique was used to distinguish whether SCN- acts as a counterion or a co-ion within PECs. Doping of PECs with NaSCN breaks Pol+Pol- pairing efficiently, which decreases effective crosslinking and decreases viscosity. An equation was derived that quantitatively predicts this effect. 

   more » « less
4. Dynamics and memory of boundary stresses in discontinuous shear thickening suspensions during oscillatory shear

   https://doi.org/10.1039/D0SM01917H  

   Rathee, Vikram; Blair, Daniel L.; Urbach, Jeffrey S. (February 2021, Soft Matter)

   null (Ed.)

   We report direct measurements of spatially resolved surface stresses of a dense suspension during large amplitude oscillatory shear (LAOS) in the discontinuous shear thickening regime using boundary stress microscopy. Consistent with previous studies, bulk rheology shows a dramatic increase in the complex viscosity above a frequency-dependent critical strain. We find that the viscosity increase is coincident with that appearance of large heterogeneous boundary stresses, indicative of the formation of transient solid-like phases (SLPs) on spatial scales large compared to the particle size. The critical strain for the appearance of SLPs is largely determined by the peak oscillatory stress, which depends on the peak shear rate and the frequency-dependent suspension viscosity. The SLPs dissipate and reform on each cycle, with a spatial pattern that is highly variable at low frequencies but remarkably persistent at the highest frequency measured ( ω = 10 rad s −1 ). 

   more » « less
5. Energy transport in glasses

   https://doi.org/10.1039/c9sm02171j  

   Flenner, Elijah; Wang, Lijin; Szamel, Grzegorz (January 2020, Soft Matter)

   The temperature dependence of the thermal conductivity is linked to the nature of the energy transport at a frequency ω , which is quantified by thermal diffusivity d ( ω ). Here we study d ( ω ) for a poorly annealed glass and a highly stable glass prepared using the swap Monte Carlo algorithm. To calculate d ( ω ), we excite wave packets and find that the energy moves diffusively for high frequencies up to a maximum frequency, beyond which the energy stays localized. At intermediate frequencies, we find a linear increase of the square of the width of the wave packet with time, which allows for a robust calculation of d ( ω ), but the wave packet is no longer well described by a Gaussian as for high frequencies. In this intermediate regime, there is a transition from a nearly frequency independent thermal diffusivity at high frequencies to d ( ω ) ∼ ω −4 at low frequencies. For low frequencies the sound waves are responsible for energy transport and the energy moves ballistically. The low frequency behavior can be predicted using sound attenuation coefficients. 

   more » « less