Stress-structure coupling and nonlinear rheology of Lennard-Jones liquid

Structural effects of water clusters on viscosity at high shear rates

In this study, we use molecular dynamics simulations of liquid water to investigate how shear thinning affects the viscosity of liquid water by structural changes of the hydrogen bond network. The effect of shear on viscosity can be divided into two parts: shear-induced destruction of the hydrogen bond network and the influence of the water structure on shear viscosity. First, strong shear destroys tetrahedral structures and thus reduces the connectivity of the hydrogen bond network. It is mainly because shear deformation, characterized by compression and expansion axes, respectively, triggers the destruction and formation of hydrogen bonds, resulting in anisotropic effects on water structures. At the same time, shear destroys large clusters and enhances the formation of small ones, resulting in a decrease in average cluster sizes. Second, the change of viscosity obeys a power law relationship with the change of hydrogen bond structures, highlighting a one-to-one correspondence between structure and property. Meanwhile, in order to explain why the structure affects viscosity, we define hydrogen-bond viscosity and find that the cooperative motion of the water structures can promote momentum transfer in the form of aggregations. Hydrogen-bond viscosity accounts for 5%-50% of the total viscosity. Our results elucidate that water structures are the important structural units to explain the change of water properties.

Viscoelasticity of Low-Molecular-Weight Polyelectrolytes

Shear-thickening fluids that absorb the impact energy of high-velocity projectiles are of great interest for aerospace and body-armor applications. In such a frame, we investigate transient states of neat and aqueous polyelectrolytes (PE) having low molecular weights and containing poly([2-(methacryloyloxy)ethyl]trimethylammonium) as polycations and poly(acrylamide-co-acrylic acid) as polyanions. We compare results with those of bulk water. We employ nonequilibrium molecular dynamics to simulate oscillatory shear, mainly in the linear viscoelastic regime. We find that neat PE exhibits properties of a viscoelastic solid, whereas water and the aqueous mixture of PE conform to viscoelastic liquids with Maxwellian behavior at low angular frequencies. Terminal relaxation times are ∼0.499 and ∼1.385 ps for water and the aqueous mixture of PE, respectively. At high angular frequencies, storage moduli show anomalous behaviors that correspond to transitions between shear thinning and shear thickening in complex shear viscosities. The change in potential energy with the increase of the angular frequency is mainly driven by intramolecular interactions for neat PE, whereas short-range Coulomb interactions are the major contributions for water and the aqueous mixture of PE. Upon observation of the molecular configurations, only the local polyionic structure in the aqueous mixture of PE shows improvement when increasing the angular frequency, whereas the rest remains barely affected. Thus, the water structure in the aqueous mixture of PE allows the storage of energy elastically through the hydrogen-bond network at large angular frequencies, whereas the mechanical contribution of polyions weakens and fully vanishes at the beginning of shear thinning, explaining the superimposed data with data of bulk water. Our method and findings set the path for future molecular simulations in the nonlinear viscoelastic regime with more complex underlying molecular mechanisms.

Coupling between Translational Diffusion of a Solute and Dynamics of the Heterogeneous Structure: Higher Alcohols and Ionic Liquids

Translational diffusion of nonpolar monoatomic solutes in a room-temperature ionic liquid and 1-octanol was studied by molecular dynamics simulation. The diffusion coefficient was evaluated in two different ways: (1) from the mean-square displacement of a freely diffusing solute and (2) from the time correlation function of force acting on a fixed solute. The diffusion of the free solute is much greater than the prediction of the Stokes-Einstein (SE) relation when the size of the solute is small, as has been reported by many experimental works. In contrast, the friction on fixed small solutes follows the SE relation. The mechanism of the solute diffusion in both solvents was then analyzed based on the coupling between the translational motion of the solute and the collective dynamics of the heterogeneous intermediate-range structure characteristic to these solvents. Analysis revealed that the coupling is present in all systems, but the relaxation is fast in the cases of free and small solutes. This suggests that the coupling can relax through the motion of the solute when the solute is free and small, while the relaxation of the heterogeneous structure itself is required for large or fixed solutes. The difference in the relaxation dynamics of the friction on the solute and the shear viscosity is explained as the coupling with different dynamic modes of the solvent. Therefore, the validity of the SE relation may not be a good criterion to judge whether the mechanisms of the diffusion and the viscosity are the same or not.

Shear Thinning and Nonlinear Structural Deformation of Ionic Liquids with Long Alkyl Chains Studied by Molecular Dynamics Simulation

Nonequilibrium molecular dynamics (MD) simulation was performed on imidazolium-based ionic liquids of two different alkyl chain lengths, and shear-rate-dependent viscosity was evaluated. Compared with the frequency-dependent linear shear viscosity determined by equilibrium MD simulation, shear thinning occurs at the shear rate several times lower than that predicted by the Cox-Merz rule. The deformation of the structure factor was also evaluated as the function of shear rate. The onset of shear thinning corresponds to that of the nonlinearity in the deformation of the charge-alternation mode in both ionic liquids, which is in harmony with the result of our previous work that the shear relaxation of ionic liquids is mainly coupled to the structural relaxation of the charge-alternation mode. In the presence of the polar-nonpolar domain structure characteristic to ionic liquids with a long alkyl chain, in particular, the nonlinearity in the domain mode begins within the Newtonian regime of shear viscosity.

Translation-orientation coupling and Cox-Merz rule of liquid hexane

Equilibrium and non-equilibrium molecular dynamics simulations are performed on liquid hexane in order to clarify the origin of the Cox-Merz rule of liquids composed of chain-like molecules. The relation between the frequency-dependent complex shear viscosity and the shear-rate dependent nonlinear viscosity follows the Cox-Merz rule as expected. The slowest viscoelastic relaxation mode is explained by the translation-orientation coupling mechanism, and the saturation of the shear-induced orientational order is observed in the non-equilibrium simulation at the onset of the shear thinning. The origin of the Cox-Merz rule is discussed in terms of the translation-orientation coupling.

Coupling between the mesoscopic dynamics and shear stress of a room-temperature ionic liquid

Shear viscosity of an ionic liquid is governed by the dynamics of the charge-alternation mode irrespective of the presence of the domain structure.