Non-steady Shear Research Articles

Note on the start-up of Couette flow for viscoelastic fluids

This paper is concerned with the numerical modeling of viscoelastic fluids in non-steady shear motions. Time-dependent solutions for three-constant differential models are obtained at the start-up of the planar Couette flows. The influences of (i) the Reynolds number, (ii) the value of κ− material parameter (the ratio between the retardation time and relaxation time), and (iii) the initial condition for the normal stress on the velocity and stresses distributions in the gap are investigated using the numerical solutions obtained with Mathematica software. The focus of the study is the analysis of the Jaumann model (characterized by the corotational derivative) in transitory simple shear rheological tests, as a function of initial conditions for stresses. The steady solutions, corroborated with the non-monotonicity of the steady flow curve, confirm the kink presence in the steady velocity distributions and the formation of shear bandings at Re ≥ 1. The analyses of the strain- and stress-controlled simulations performed at different initial and boundary conditions offer possible explanations of some spurious data recorded in shear measurements of complex viscoelastic fluids. The findings have important consequences for performing transient shear experiments; specifically, it is demonstrated that reproducibility and correlations between the tests require the control of initial normal stresses in the sample.

Recoil experiments determine the eigenmodes of viscoelastic fluids

We experimentally investigate the transient recoil dynamics of a colloidal probe particle in a viscoelastic fluid after the driving force acting on the probe is suddenly removed. The corresponding recoil displays two distinct timescales which are in excellent agreement with a microscopic model which considers the probe particle to be coupled to two bath particles via harmonic springs. Notably, this model exhibits two sets of eigenmodes which correspond to reciprocal and non-reciprocal force conditions and which can be experimentally confirmed in our experiments. We expect our findings to be relevant under conditions where particles are exposed to non-steady shear forces as this is encountered e.g. in microfluidic sorting devices or the intermittent motion of motile bacteria within their natural viscoelastic surrounding.

The Mesoscopic Constitutive Equations for Polymeric Fluids and Some Examples of Viscometric Flows

Constitutive equations for melts and concentrated solutions of linear polymers are derived as consequences of dynamics of a separate macromolecule. The model is investigated for viscometric flows. It was shown that the model gives a good description of non-linear effects of simple shear polymer flows: viscosity anomalies, first and second normal stresses, non-steady shear stresses.

Experimental procedure for the evaluation of the friction energy of lubricating greases

The paper is concerned with the study of a tribological system characterised by fluid friction. The main parameter of the system is the density of the friction energy, a physical quantity which is put in a direct relation with the viscous dissipation function of the lubricant. The lubricant under study is grease, a viscoelastic material which exhibits a yield stress at low strain rates. For this type of material, the shear stress function has a relevant plateau behaviour previous to the beginning of the flow. The rheological characteristics of the lubricant are determined in simple shear motions, both in creeping and stressing modes. The specific energy is graphically computed for these corresponding non-steady shear motions. Below the yield shear stress, an important time variation of the specific energy has been observed. This result is considered to be useful information on the non-steady tribological regime, implicit in the relative intensity of the associated wear process.

A ?zero-parameter? constitutive relation for simple shear viscoelasticity

Based on the Cox-Merz rule and Eyring's expression for the nonlinear shear viscosity, a Wagner-type constitutive relation with no nontrivial adjustable parameters is proposed for simple shear viscoelasticity. The predictions for a number of non-steady shear flows are worked out analytically. It is shown that most features of shear viscoelasticity are reproduced by the model.

Nonsteady shear flow of micropolar fluids

An application of Eringen's theory [1–3] of micropolar fluids has been made to the problem of nonsteady shear flow. An exact solution to the system of governing equations is given. Two nondimensional parameters, i.e. the coupling coefficient and the length ratio are introduced, and when the length ratio vanishes, the solution reduces to its classical form. Graphs representing fluid velocity for different values of those parameters are drawn at the end.

Comparison of viscometer geometries for non-steady shear flows

Suitability of cone- and-plate, parallel disk and concentric cylinder viscometric geometries for nonsteady shear flow of non-Newtonian fluids was analyzed. Both the cone- and plate and the parallel disk viscometers possess the undesirable property that the equations of motion do not permit “simple” solutions, i. e., solutions for which only one pair of components of the shear stress tensor is non-zero. No such difficulty appears in the analysis of the concentric cylinder viscometer, and therefore this instrument is to be preferred for this type of study.