Direct numerical simulation of stochastically forced laminar plane couette flow: peculiarities of hydrodynamic fluctuations

Probing nonlinear rheology layer-by-layer in interfacial hydration water

Viscoelastic fluids exhibit rheological nonlinearity at a high shear rate. Although typical nonlinear effects, shear thinning and shear thickening, have been usually understood by variation of intrinsic quantities such as viscosity, one still requires a better understanding of the microscopic origins, currently under debate, especially on the shear-thickening mechanism. We present accurate measurements of shear stress in the bound hydration water layer using noncontact dynamic force microscopy. We find shear thickening occurs above ∼ 10(6) s(-1) shear rate beyond 0.3-nm layer thickness, which is attributed to the nonviscous, elasticity-associated fluidic instability via fluctuation correlation. Such a nonlinear fluidic transition is observed due to the long relaxation time (∼ 10(-6) s) of water available in the nanoconfined hydration layer, which indicates the onset of elastic turbulence at nanoscale, elucidating the interplay between relaxation and shear motion, which also indicates the onset of elastic turbulence at nanoscale above a universal shear velocity of ∼ 1 mm/s. This extensive layer-by-layer control paves the way for fundamental studies of nonlinear nanorheology and nanoscale hydrodynamics, as well as provides novel insights on viscoelastic dynamics of interfacial water.

Dynamics of two trapped Brownian particles: Shear-induced cross-correlations

The dynamics of two Brownian particles trapped by two neighboring harmonic potentials in a linear shear flow is investigated. The positional correlation functions in this system are calculated analytically and analyzed as a function of the shear rate and the trap distance. Shear-induced cross-correlations between particle fluctuations along orthogonal directions in the shear plane are found. They are linear in the shear rate, asymmetric in time, and occur for one particle as well as between both particles. Moreover, the shear rate enters as a quadratic correction to the well-known correlations of random displacements along parallel spatial directions. The correlation functions depend on the orientation of the connection vector between the potential minima with respect to the flow direction. As a consequence, the inter-particle cross-correlations between orthogonal fluctuations can have zero, one or two local extrema as a function of time. Possible experiments for detecting these predicted correlations are described.

Dynamics of a trapped Brownian particle in shear flows

The Brownian motion of a particle in a harmonic potential, which is simultaneously exposed either to a linear shear flow or to a plane Poiseuille flow is investigated. In the shear plane of both flows the probability distribution of the particle becomes anisotropic and the dynamics is changed in a characteristic manner compared to a trapped particle in a quiescent fluid. The particle distribution takes either an elliptical or a parachute shape or a superposition of both depending on the mean particle position in the shear plane. Simultaneously, shear-induced cross-correlations between particle fluctuations along orthogonal directions in the shear plane are found. They are asymmetric in time. In Poiseuille flow thermal particle fluctuations perpendicular to the flow direction in the shear plane induce a shift of the particle's mean position away from the potential minimum. Two complementary methods are suggested to measure shear-induced cross-correlations between particle fluctuations along orthogonal directions.

Direct measurement of shear-induced cross-correlations of Brownian motion

Shear-induced cross-correlations of particle fluctuations perpendicular and along streamlines are investigated experimentally and theoretically. Direct measurements of the Brownian motion of micron-sized beads, held by optical tweezers in a shear-flow cell, show a strong time asymmetry in the cross-correlation, which is caused by the non-normal amplification of fluctuations. Complementary measurements on the single particle probability distribution substantiate this behavior and both results are consistent with a Langevin model. In addition, a shear-induced anticorrelation between orthogonal random displacements of two trapped and hydrodynamically interacting particles is detected, having one or two extrema in time, depending on the positions of the particles.

Simulation model of concentrated colloidal nanoparticulate flows

This paper presents a simulation model of concentrated colloidal nanoparticulate flows to investigate self-organization of the nanoparticles and rheology of the colloid. The motion of solid nanoparticles is treated by an off-lattice Newtonian dynamics. The flow of solvent is treated by an on-lattice fluctuating Navier-Stokes equation. A fictitious domain method is employed to couple the motion of nanoparticles with the flow of solvent. The surface of nanoparticles is expressed by discontinuous solid-liquid boundary to calculate accurately contact interaction and Derjaguin-Landau-Verwey-Overbeek interaction between the nanoparticles. At the same time, the surface is expressed by continuous solid-liquid boundary to calculate efficiently hydrodynamic interaction between the nanoparticles and the solvent. Unlike other simulation models that focus on the hydrodynamic interaction, the present model includes all crucial interactions, such as contact force and torque, van der Waals force, electrostatic force, hydrodynamic force, and torque including thermal fluctuation of the solvent that causes translational and rotational Brownian motions of the nanoparticles. Especially the present model contains the frictional force that plays a significant role on nanoparticles in contact with one another. A fascinating novelty of the present model is that computational cost is constant regardless of the concentration of nanoparticles. The capability of the present simulation model is demonstrated by two-dimensional simulations of concentrated colloidal nanoparticles in simple shear flows between flat plates. The self-organization of concentrated colloidal nanoparticles and the viscosity of colloid are investigated in a wide range of Péclet numbers.