S. O. Ilyin

Viscoplasticity and stratified flow of colloid suspensions

Investigation of the rheology of concentrated colloid suspensions and direct observation of their flow allowed us to find several effects inherent to these media as typical soft matter. So, at low stress amplitudes, these colloids behave as mild gels with frequency independent elastic modulus and low mechanical losses. Meanwhile, suspensions demonstrate dualism of properties: at a given shear rate, they behave as viscoplastic media with clearly expressed yielding, while at a given low stress the pronounced Newtonian plateau is detected. The increase in shear rates and stresses leads to the sharp drop of the apparent viscosity, which usually is treated as the yielding effect. Transition through the yield stress is of a dynamic nature because the threshold stress depends on time and suspensions are thixotropic yielding materials. In the transient shear rate range, an unstable regime of deformation appears. It manifests itself either as deformation thickening up to jamming, or as the excitation of self-oscillations. The measuring of rheological properties in varying volume-to-surface ratio of a sample proves that flow of a suspension with high velocity at constant shear stress actually proceeds in a narrow layer inside the instrument gap. This conclusion has been confirmed by direct visual observations demonstrating that a flux is separated into three layers. A wide almost motionless layer is seen near a stationary surface. Near a moving surface, a narrow band with linear velocity profile is detected. Between them, a rather wide transient layer is observed and shear rate in this layer exceeds the average (global) shear rate by several times. Approximately, only a half of the total volume of a suspension is involved in flow. So, we observed a three-band flux of a suspension not described before. Shearing leads to an anisotropic structure of a solid phase.

A modern look on yield stress fluids

A concept of viscoplasticity advanced exactly one century ago by Bingham appears very fruitful because there are many natural and artificial materials that demonstrate viscoplastic behavior, i.e., they are able to pass from a solid to a liquid state under the influence of applied stress. However, although this transition was originally considered as a jump-like phenomenon occurring at a certain stress—the yield stress—numerous subsequent studies have shown that the real situation is more complicated. A long-term discussion about the possibility of flow at low stresses less than the yield stress came to today’s conclusion denying this possibility as being opposite to the existence of the maximal Newtonian viscosity in viscoelastic polymeric fluids. So, there is a contradiction between the central dogma of rheology which says that “everything flows” and the alleged impossibility for flow at a solid-like state of viscoplastic fluids. Then, the concept of the fragile destruction of an inner structure responsible for a solid-like state at the definite (yield) stress was replaced by an understanding of the yielding as a transition extending over some stress range and occurring in time. So, instead of the yield stress, yielding is characterized by the dependence of durability (or time-to-break) on the applied stress. In this review, experimental facts and the new understanding of yielding as a kinetic process are discussed. Besides, some other alternative methods for measuring the yield stress are considered.

Unusual rheological effects observed in polyacrylonitrile solutions

Unusual rheological effects have been revealed during the deformation of polyacrylonitrile (PAN) solutions in DMSO. The effects are observed during the study of rheological properties in a wide range of PAN concentrations and are explained by the structuring occurring at low polymer concentrations. At concentrations of at most 0.1%, the solutions exhibit the behavior of soft gels, which are characterized by yield stresses and frequency-independent storage moduli. As concentration is increased, both effects gradually vanish and the solutions are almost transformed into Newtonian liquids. The results have been explained by the formation of a supramolecular spatial structure at low polymer concentrations. As concentration is increased, the role of structuring is suppressed by the formation of a network of intermacromolecular entanglements. The ability of dilute PAN solutions to exist in two states, i.e., with destroyed structuring and in the form of a physical polymer gel, leads to stress self-oscillations and thixotropic effects. The addition of a precipitant (water) to the PAN-DMSO solutions leads to the formation of a gel throughout the concentration range.

Rheology of aqueous poly(ethylene oxide) solutions reinforced with bentonite clay

The rheological properties of bentonite clay-filled aqueous solutions of high-molecular-mass poly(ethylene oxide) (PEO) have been studied. The PEO solution is a typical polymer solution characterized by the highest Newtonian viscosity and the range of non-Newtonian flow. The addition of small amounts of bentonite to the PEO solution causes passage to a viscoplastic behavior that manifests itself as the appearance of the yield stress. Therewith, the flow at the highest Newtonian viscosity in the region of low shear stresses (rather than rates) remains possible. After passing through the yield stress, the effect of antithixotropy, i.e., an increase in the viscosity with the deformation rate in a certain shear rate region, has been observed for the multicomponent systems. The data obtained have been interpreted assuming that the addition of the solid filler to the polymer solution destroys the random network of entanglements between macromolecules, while the presence of the polymer in the clay suspension reduces the strength of the coagulation structure of bentonite.

Rheological peculiarities of concentrated suspensions

The rheological properties of concentrated suspensions of metal oxides dispersed in transformer oil, which are used as electrorheological fluids, are systematically studied. Colloidal particles have intermediate sizes between nano- and microsized scales. Low-amplitude dynamic measurements show that the storage moduli of the examined suspensions are independent of frequency and these materials should be considered as solidlike elastic media. The storage modulus is proportional to the five-powdered particle volume concentration. At the same time, a transition through an apparent yield stress with a reduction in the viscosity by approximately six orders of magnitude is distinctly seen upon shear deformation. The character of the rheological behavior depends on the regime of suspension deformation. At very low shear rates, a steady flow is possible; however, upon an increase in the rate, an unsteady regime is realized with development of self-oscillations. When constant shear stresses are preset, in some range of stresses, thickening of the medium takes place, which can also be accompanied by self-oscillations. In order to gain insight into the nature of this effect, measurements are performed for samples with different volume/surface ratios, which show that, in some deformation regimes, suspension is separated into layers and slipping occurs along a low-viscosity layer with a thickness of several dozen microns. Direct observations show a distinct structural inhomogeneity of the flow. The separation and motion of layers with different compositions explain the transition to the flow with the lowest apparent Newtonian viscosity. Thus, the deformation of concentrated suspensions is associated with self-oscillations of stresses and slipping along a low-viscosity interlayer.

Application of large amplitude oscillatory shear for the analysis of polymer material properties in the nonlinear mechanical behavior

The results of studying the nonlinear viscoelastic properties obtained through the generation of large strain amplitudes are interpreted via plotting of Lissajous-Bowditch figures in two different systems of coordinates: (i) stress versus strain and (ii) the derivative of stress with respect to the phase angle versus strain. The former system yields the integral characteristic of dissipative loss in the deformation cycle, and the latter yields a measure of elasticity of a material. The generality of such approach to analyze large deformations, even in the case of an extremely complex shape of the nonlinear response, is due to the fact that it is not related to the a priori choice of any rheological constitutive equation. The developed method was tested on supramolecular structures and polymer solutions and melts. Novel results allow estimation of the character of nonlinearity development, i.e., the dependences of pseudoplasticity, dilatancy, and stiffening or softening of the medium on the shear. The comparison between the proposed measures of nonlinearity and large strain nonlinearity characteristics described in the literature shows that the integral characteristics are in good qualitative agreement with other measures of nonlinearity. However, in some cases, the proposed approach gives a more objective and consistent estimation than other measures of nonlinearity give.

Rheology and adhesive properties of filled PIB-based pressure-sensitive adhesives. I. Rheology and shear resistance

Two nanosized fillers (fumed silica Rosil-175 up to 20 wt% and halloysite nanotubes up to 40 wt%) were used to suppress cold flow of polyisobutylene (PIB)-based adhesives. Rheological measurements revealed the presence of the yield stress in the PIB-halloysite system which is indicative of the three-dimensional network formation in the bulk of the matrix. However, further rheology tests identified low strength of the network. In case of Rosil, no yield stress was detected, only gradual increase in the viscosity. Shear bank testing showed 4–5 times increase in the PIB-halloysite system time to failure when 40 wt% of halloysite was incorporated into the PIB matrix. Increase in the time to failure for PIB-Rosil systems was over two orders of magnitude – at the Rosil content up to 20 wt%. Such superior properties of the Rosil adhesion joints are considered to be the result of Rosil high specific area and low strength of the halloysite nanotubes network.

Rheological properties of road bitumens modified with polymer and solid nanosized additives

The study of the viscoelastic properties of composites based on road bitumens have shown that the addition of polymeric modifiers (poly(bytadiene-block-styrene) or devulcanized rubber particles) substantially increases the storage and loss moduli and decreases the intensity of reduction in the storage modulus with temperature by several orders of magnitude. However, at high polymer content, growth inadmissible from the point of view of acceptable technological parameters is observed in the apparent viscosity. The introduction of carbon nanotubes into bitumen does not substantially affect its viscoelastic properties. Filling with meta-kaolin promotes an increase in the storage modulus at elevated temperatures. It has been shown that a direct correlation may be established between the objective characteristics of bitumen-based composites and standard specification parameters, such as penetration depth and heat resistance.

Viscosity of polyacrylonitrile solutions: The effect of the molecular weight

The copolymer of acrylonitrile with methyl acrylate and itaconic acid (93: 5.7: 1.3) is synthesized via free-radical copolymerization. For solutions of the initial copolymer and its 12 fractions in dimethyl sulfoxide, viscosities are measured in a wide shear-stress range. The viscosity behavior of 10% solutions is examined in more detail, and the rheological studies of several fractions are performed in the concentration interval from 5 to 20%. All solutions exhibit weak non-Newtonian behavior. This makes it possible to determine the zero-shear viscosity. The dependences of this viscosity on the number-average and weight-average molecular masses for the equiconcentrated solutions obey an exponential law with the same exponent, equal to 2.3. The macromolecules of copolymers are inclined toward association. For low-molecular-mass fractions, this effect is the most pronounced.

Sol–gel transition and rheological properties of silica nanoparticle dispersions

Possible variants of the rheological behavior of silica model dispersions have been analyzed. Different types of interaction between the particles and a dispersion medium make it possible to obtain different systems from low-viscosity sols to gels. Proton-donor (water) and aprotic (dimethyl sulfoxide) media have been used for comparison. Dispersions in the aprotic medium behave as non-Newtonian viscous fluids exhibiting shear thinning or shear thickening depending on deformation rate. Aqueous dispersions are viscoelastic and viscoplastic objects that exhibit the shear thickening at stresses higher than the yield stress. The introduction of small amounts of poly(ethylene oxide) into the organic dispersion medium initiates gelation. An increase in the polymer content in the dispersion medium above the concentration corresponding to the formation of a macromolecular network promotes an increase in stiffness and strength of the gels. The rheological behavior of gels is influenced by the polymer molecular mass and its affinity for a solvent.