Jan Mewis

Yield stress and thixotropy: on the difficulty of measuring yield stresses in practice

The yield stress of many yield stress fluids has turned out to be difficult to determine experimentally. This has led to various discussions in the literature about those experimental difficulties, and the usefulness and pertinence of the concept of yield stress fluids. We argue here that most of the difficulties disappear when taking the thixotropy of yield stress fluids into account, and will demonstrate an experimental protocol that allows reproducible data to be obtained for the critical stress necessary for flow of these fluids. As a bonus, we will show that the interplay of yield stress and thixotropy allows one to account for the ubiquitous shear localization observed in these materials. However, due to the thixotropy the yield stress is no longer a material property, since it depends on the (shear) history of the sample.

Shear thickening in model suspensions of sterically stabilized particles

The onset of shear thickening (dilatancy) has been studied in submicron model suspensions of sterically stabilized spherical particles. Stress controlled rheometers have been used for this purpose, so that measurements can be performed beyond the onset of sudden shear thickening. Systematic data are presented for the effect of particle size, particle concentration, and the nature of the suspending medium on the onset of shear thickening. As a first approximation, the critical shear rate changes inversely proportional with the medium viscosity. The changing solvency of the medium for the stabilizer polymer introduces additional changes through effects on the thickness and stiffness of the steric barrier. Thinner, stiffer barriers cause lower critical shear rates. In the softer systems the critical shear stress becomes independent of particle concentration in dense suspensions. This does not seem to be the case for the harder systems. The effect of particle radius a could shed some light on the underlying m...

Thixotropy: Build-up and breakdown curves during flow

Thixotropic systems are usually characterized by measuring the stress transients resulting from sudden changes in shear rate. By using a procedure that takes into account the transfer function of the transducer on a strain-controlled rheometer, the initial part of the transients could be accessed. In this manner complete transient stress curves and accurate constant-structure curves could be recorded on a model system consisting of fumed silica particles dispersed in a Newtonian medium. In agreement with previously obtained stress jump data [Dullaert and Mewis (2005b)], an initial viscoelastic response is detected that precedes the structural time effects in either build-up or breakdown experiments. The effect of flow history on the characteristic times and on the shape of the transient curves is investigated. It is shown that common assumptions in thixotropic models, such as a structure that can be characterized by a single parameter or the existence of exponential stress transients, do not generally hol...

General nonlinear rheological behavior of associative polymers

Shear thickening is very common in telechelic associative polymers. Yet, it is not a universal characteristic of the whole class of associative polymers. Here, more general features of associative behavior are investigated. In particular, nonlinear rheological properties have been studied for both associative hydrophobically modified ethoxylated urethanes and hydrophobically modified alkali-soluble emulsions (HASE) polymers. A known common characteristic is the failure of the Cox-Merz rule for the viscosities. The equivalent relation for the first normal stress is examined here, a parallelism with the results for the viscosities has been observed. More pronounced similarities between telechelics and the more complex HASE systems can be seen in some other nonlinear properties. The dynamic moduli display strain hardening for both moduli at intermediate strains at all but the lowest frequencies, even when no shear thickening can be detected in steady state shear flow. The linear relaxation functions differ quite strongly among associative polymers. However, they all display a specific non-linear relaxation. It includes short time strain hardening followed by strain softening starting at long relaxation times. The results indicate that the same microstructural mechanisms are responsible for the rheological behavior of both classes of associative polymer.

Dynamic mechanical-properties of polymerically stabilized dispersions

Abstract The rheological properties of suspensions, containing polymerically stabilized, monodisperse PMMA particles in nonaqueous media, have been measured in oscillatory flow. The contribution of the soft stabilizer shell has been investigated by changing the particle radius and keeping the shell thickness constant. The scaling laws for hard spheres can be used to reduce data taken at different temperatures, with a correction for the difference in thermal expansion between particle and suspending medium. At relatively low frequencies the suspensions relax approximately as Maxwell fluids. Comparison of the plateau storage moduli with results for hard spheres indicates a systematic deviation due to the softness of the stabilizer layer. This could provide a means for deducing the interparticle potential from a comparison of the experimental data with simulation results. For the smallest particles calculation of the relaxation times provides a direct measurement of particle mobility. In concentrated systems the concentration dependence of the relaxation times closely follows that of the zero shear viscosity but the Cox-Merz analogy between steady-state and oscillatory data does not hold.

Rheo-optical investigation of immiscible polymer blends

Abstract Linear conservative dichroism has been investigated as a potential tool for tracking (in situ and in real time) the flow-induced microstructure of immiscible polymer blends. Dilute mixtures of poly(dimethylsiloxane) in poly(isobutene) and vice versa have been used as model systems. It is demonstrated that different mechanisms for structural changes can be identified with this technique: droplet deformation, droplet retraction, fibril break-up and coalescence. Time-resolved in-situ SALS measurements support the structural interpretation of the dichroism data. Breaking up of stretched filaments by Rayleigh instabilities can be deduced from dichroism measurements. The resulting strings of droplets can be detected with SALS. The time scales for the structural changes can be determined by dichroism, which makes it possible to study in situ the effect of the various process and material parameters. As an illustration, the effect of shear rate on fibril break-up and on droplet coalescence after sudden changes in shear rate has been considered. Break-up times decreased only slightly with increasing shear rate; coalescence times scaled with the square of shear rate rather than with strain under the conditions of the present experiments.

Constitutive equations based on the transient network concept

Abstract A consitutive theory for polymetric liquids based on the transient network concept is developed, following Wiegel and Jongschaap. The Phan-Thien-Tanner equation is shown to follow from the general theory with two critical assumption, one of these is quasi-equilibrium of the internal structure, which preludes consistency of application in ”fast flows“. The Marrucci model can be made consistent with the general format with a small change in the kinetic equation that can be deduced from asymptotic behaviour and leaves the steady viscometric behavior unchanged. The simplest genaral formulation requires the linear viscoelastic spectrum and two parameters; the latter cannot be determined uniquely from steady viscometric flow data.

Shear thickening in polymer stabilized colloidal dispersions

A micromechanical model for the onset of shear thickening in polymer stabilized colloidal dispersions is derived. The model incorporates both the stabilizing forces of the polymer brush or “coat” and the associated modification of the hydrodynamic interactions. Comparison is made to simulations and experiments on well-characterized dispersions. The model is shown to provide a quantitative prediction for the onset of shear thickening in polymer coated colloidal dispersions as compared to simulations. For comparison with experiments, where independent characterization of some parameters is not available, the onset of shear thickening can be predicted with realistic assumptions for the model parameters. The model quantifies the sensitivity of the shear thickening transition to both the interaction potential and the hydrodynamic permeability of the polymer coat.

Colloidal Suspension Rheology: Frontmatter

Colloidal suspensions are encountered in a multitude of natural, biological, and industrially relevant products and processes. Understanding what affects the flow behavior, or rheology, of colloid suspensions, and how this flow behavior can be manipulated, is important for successful formulation of products such as paint, polymers, foods, and pharmaceuticals. This book is the first devoted to the study of colloidal rheology in all its aspects. With material presented in an introductory manner, and complex mathematical derivations kept to a minimum, the reader will gain a strong grasp of the basic principles of colloid science and rheology. Beginning with purely hydrodynamic effects, the contributions of Brownian motion and interparticle forces are covered, before the reader is guided through specific problem areas such as thixotropy and shear thickening; special classes of colloid suspensions are also treated. The techniques necessary for measuring colloidal suspension rheology are presented along with methods to correlate and interpret the results. An essential guide for academic and industrial researchers, this book is also ideal for graduate course use.

Colloidal Suspension Rheology: Contents

Colloidal suspensions are encountered in a multitude of natural, biological, and industrially relevant products and processes. Understanding what affects the flow behavior, or rheology, of colloid suspensions, and how this flow behavior can be manipulated, is important for successful formulation of products such as paint, polymers, foods, and pharmaceuticals. This book is the first devoted to the study of colloidal rheology in all its aspects. With material presented in an introductory manner, and complex mathematical derivations kept to a minimum, the reader will gain a strong grasp of the basic principles of colloid science and rheology. Beginning with purely hydrodynamic effects, the contributions of Brownian motion and interparticle forces are covered, before the reader is guided through specific problem areas such as thixotropy and shear thickening; special classes of colloid suspensions are also treated. The techniques necessary for measuring colloidal suspension rheology are presented along with methods to correlate and interpret the results. An essential guide for academic and industrial researchers, this book is also ideal for graduate course use.