Rheological behaviour of vitreous humour

The vitreous humour (VH) is a complex biofluid that occupies a large portion of the eyeball between the lens and the retina, and exhibits non-Newtonian rheological properties that are key for its function in the eye. It is often possible to distinguish two different phases in VH, known as liquid and gel phases (Sebag J Eye 1: 254–262, 1987). In this work, we present a detailed rheological characterisation of the two phases of the VH under shear and extensional flow conditions. Healthy New Zealand rabbit eyes were used to measure the surface tension and the shear and extensional rheological properties of VH in different phase conformations and at different times after dissection. The results show that VH liquid phase exhibits a surface tension of 47.8 mN/m, a shear thinning behaviour reaching a viscosity plateau around 10^?3 Pa s for shear rates above ~1000 s^?1, and an average relaxation time of 9.7 ms in extensional flow. Interestingly, both VH phases present higher storage modulus than loss modulus, and the measurements performed with VH gel phase 4?±?1 h after dissection exhibit the highest moduli values. The compliance measurements for the gel phase show a viscoelastic gel behaviour and that compliance values decrease substantially with time after dissection. Our results show that the two VH phases exhibit viscoelastic behaviour, but with distinct rheological characteristics, consistent with a gel phase mostly composed of collagen entangled by hyaluronan and a second phase mainly composed of hyaluronan in aqueous solution.     

The use of rheology in the study of the composition effects on the fresh behaviour of hydraulic lime grouts for injection of masonry walls

The injection of grouts inside multi-leaf stone masonries is a technique widely used for structural consolidation. To ensure an adequate flow of the grout inside the masonry, it is crucial to assure good fresh grout properties, such as good rheological behaviour. The scope of this paper is to provide preliminary indications and valuable data about the effects of specific hydraulic lime grout composition on their rheological behaviour with the purpose of a successful injection process. Through the use of rotational rheometer together with the Taguchi method, it was possible to study the influence of water/binder ratio, the type and dosage of superplasticizer and the partial replacement of hydraulic lime by silica fume, upon the grout rheological properties. The study leads to the conclusion that polycarboxylate-based superplasticizers present better performance from a rheological point of view than the naphthalene-based superplasticizer and that the water/binder ratio and superplasticizer dosage are the most determinant factors in the fresh grout rheological behaviour. On the other hand, silica fume dosage turned out to be the factor with the least contribution to improve the grout rheological behaviour compared to the other two factors reported in this study. The results summarised in this paper are part of a larger study and precede the analysis of the performance of those grouts when injected into different porous media that simulate old masonries.     

The influence of surface friction on the calculation of stress relaxation parameters for processed cheese

The stress relaxation-time behaviour of processed cheese has been studied after compression by 5–20%. Data were corrected for surface friction effects, since it is only that part of the registered force which actually compresses the samples that relaxes exponentially. The method of successive residuals and an empirical normalization linearization procedures were used to analyse the data. In both cases it was found that several of the calculated rheological parameters, at any selected compression, were significantly higher after correcting for surface friction. The initial compression also influenced the magnitude of the rheological parameters.     

The phenomenological description of the thixotropic behaviour of fresh cement pastes

The present work reports an investigation on the rheological behaviour of fresh cement pastes. Three types of Portland cement were used. The water/cement ratio was varied in the range 0.35 ÷ 0.40. The rheological tests were performed using the coaxial-cylinder viscometer Rotovisko-Haake RV 11. The material to be tested was subjected immediately after mixing to a constant shear rate until a steady value of shear stress (equilibrium value) was attained. All the pastes tested exhibited a flow behaviour of the partially thixotropic type. A rheological model consisting of a sole constitutive equation \(\tau = \tau (\dot \gamma ,t)\) was defined according to the Cheng and Evans approach. The parameters of the constitutive equation were correlated with the cement specific surface and the water/cement ratio.     

The rheological properties of solder and solar pastes and the effect on stencil printing

Solar and solder pastes are widely used in the electronics industry. Solder paste is the principal joining medium in the assembly of surface mount components, whilst solar paste is used in the manufacture of semiconductor solar cells in the photo-voltaic industry. The stencil printing of both solder and solar pastes is a very important and critical stage in the assembly process. With miniaturisation of components, this is likely to continue. The challenge in stencil printing at such dimensions is in achieving repeatable deposition of both solar and solder pastes from print to print. To meet this challenge requires an understanding of the flow behaviour of both solar and solder pastes. The rheological properties of solar and solder pastes have been evaluated through three different types of experiments. Existing models were applied to compare their rheological behaviour under these schemes. One striking difference was that solar paste showed a higher viscosity than solder paste. Both solar and solder pastes were found to be non-Newtonian materials, showing a decrease in viscosity with increasing shear rates. In this paper we investigate the rheological properties of both solder and solar paste under steady shear and creep-recovery tests. Received: 12 October 1999 Accepted: 11 July 2000     

Simulation of curing-induced viscoplastic deformation: a new approach considering chemo-thermomechanical coupling

A modelling and simulation approach for plastic deformation effects in curing resins is presented. For this purpose known rheological models of viscoelasticity and viscoplasticity are combined and a thermochemical element is added to account for chemical shrinkage and thermal expansion. The degree of cure of the resin has a major influence on the behaviour of the curing material, and therefore, the material model is formulated depending on the degree of cure. It affects the viscoelastic behaviour as well as the chemical shrinkage and the yield function of the viscoplastic part of the model. For the yield function a von Mises approach with isotropic hardening is chosen, where the initial yield stress as well as the yield surface depends on the degree of cure and the temperature.     

Phenomenological description of the thixotropic behaviour of gypsum plaster pastes

Gypsum plaster pastes are concentrated suspensions of calcium sulfate hemihydrate in water. The knowledge of their rheological behaviour plays an important role in the manufacture of gypsum products. In the present work, the approach suggested by Cheng and Evans has been taken into consideration in order to derive a constitutive equation suitable for characterizing completely the flow behaviour of these materials in the first stages after preparation. To this end, gypsum plaster pastes were produced with solid volume fractions in a range of practical interest. The rheological tests were performed using a coaxial-cylinder viscometer.     

Behaviour of double emulsions in shear flows

The stability behaviour of heterogeneous drops indicates that, for liquid-liquid dispersions, we may have to modify the stability criterion to incorporate either the rheological characteristics of the dispersion or its cohesion or both.     

Influence of thermal history on rheological properties of various bitumen

This paper focuses on the influence of thermal history on the rheological properties of unmodified and polymer modified bitumen (PMB), measured at elevated service temperatures, and contributes to the development of test methods for measuring binder properties, which can be used as indicators for asphalt rutting. It was found that the storing and preparation conditions prior to the rheological measurement can have a large influence, especially in the range of long loading times or low frequencies. For elastomer modification, the homogenization and sample pouring temperature and the corresponding change in microstructure, as revealed by fluorescence microscopy, have a large impact on the rheological measurements. For binders with semi-crystalline modifiers, the storage conditions between sample preparation and testing have the largest impact on the rheological behaviour. This can be related to variations in crystallinity, as shown by calorimetry. The main conclusion from this study is that sample preparation and handling is extremely important for the rheological properties of PMBs. Reproducibility can only be achieved when these conditions are controlled more accurately, especially in binder specification tests for rutting susceptibility.     

The Effect of Hydration on the Mechanical Behaviour of Hair

Human hair is a biological material constantly exposed to different external factors,such as humidity, sunrays, temperature, chemical treatments, etc. All these treatments influence and modify its physical behaviour. Studying the biophysical properties of human hair is very important in both dermatology to provide useful markers for the diagnosis of hair disorders and in cosmetics to develop better hair-care products. Water is one of the external factors whose action on the mechanical behaviour of hair is the most visible. To understand the role of water in the biophysical behaviour of hair, it is essential to study its influence on alpha-keratin, which composes the major part of the structure of hair. The influence of water on the biomechanical behaviour of hair has been studied using relaxation tests. The generalised Maxwell model was used to analyse rheological results. The results indicate a modification of the rheological behaviour of hair before and after immersion in water and during ambient air-drying. Finally, a correlation between the rheological results and the chemical bond structure of hair is discussed.     

Effects of surface properties on the impact process of a yield stress fluid drop

The impact of a yield stress fluid drop onto a solid surface with diversified interface properties has been experimentally investigated. Two smooth substrates with distinct surface energies and three similar substrates with different roughnesses have been used. The bulk shear rheological behaviour of Carbopol gels, concentrated suspensions of swollen micro-gels, has been measured. Wall friction has also been characterized on each substrate. Slip effects of gels proved to be greater on a more hydrophobic substrate. They decreased with an increase in roughness. The drop hydrodynamics during the impact was correlated with the wall friction of the gels on all substrates and with the ratio of surface roughness to size of the swollen micro-gels. At very low impact velocities, the gravitational subsidence amplitude depends greatly on surface properties. At higher impact velocities, no significant difference is observed during the spreading phase. The drop behaviour differs during the retraction depending on the substrate. Interface effects during the retraction stage proved to diminish when the yield stress value increases.     

Linear rheology of diluted linear, star and model long chain branched polymer melts

We present experiments and theory on the diluted melt dynamics of monodisperse entangled polymers of linear, star and H-shaped architecture. Frequency-dependent rheological data on a series of progressively diluted linear, star and H-polymers are in good agreement with a refined tube-model theory that, for H-polymers, combines star polymer melt behaviour at high frequency, with linear polymer reptation behaviour at low frequencies. Taking into account the effect of dilution via some simple scaling relations, mild polydispersity and by incorporating the high frequency Rouse modes, we are able to model quantitatively the entire frequency range. This work suggests a novel rheological route to analysing long chain branching in polymer melts. Received: 6 April 2000/Accepted: 21 December 2000     

Rheological behaviour of low moisture molten maize starch

The rheological behaviour of a molten maize starch in low hydrated phase was investigated using a pre-shearing rheometer. This apparatus enables one to impose well-characterized thermomechanical treatment on the product before viscosity measurement. The influence of temperature, moisture content and intensity of the treatment were studied. The rheological behaviour of the maize starch may be described by a power law with exponential dependences on temperature, water content and mechanical energy provided to the product before measurement.     

Rheological behaviour of dilute suspensions of platelet particles

The rheological behaviour of Newtonian suspensions of platelets with an aspect ratio ranging from 0.006 to 0.2 is studied in this work. Using model particles to correlate the aspect ratios square-shaped particles with their sedimentation behaviour, the results obtained were used to estimate the average aspect ratio of mica particles. A comparison is made between the aspect ratio measured by sedimentation experiments and that determined from the rheological behaviour of dilute suspensions. Their good agreement shows that the average aspect ratio of a polydispersed suspension of platelets estimated using packing experiments correlates well with the apparent aspect ratio given by the intrinsic viscosity of the suspension.     

高浓度水煤浆反触变性的流变模式

本文根据速率型物料的假设和实验的等结构方法,确定了水煤浆反触变性的流变状态方程及速率方程。并对正、反触变性的特征进行了比较。     

Flow effects of blood constitutive equations in 3D models of vascular anomalies

The effects of different blood rheological models are investigated numerically utilizing two three‐ dimensional (3D) models of vascular anomalies, namely a stenosis and an abdominal aortic aneurysm model. The employed CFD code incorporates the SIMPLE scheme in conjunction with the finite‐volume method with collocated arrangement of variables. The approximation of the convection terms is carried out using the QUICK differencing scheme, whereas the code enables also multi‐block computations, which are useful in order to cope with the two‐block grid structure of the current computational domain. Three non‐Newtonian models are employed, namely the Casson, Power‐Law and Quemada models, which have been introduced in the past for modelling the rheological behaviour of blood and cover both the viscous as well as the two‐phase character of blood. In view of the haemodynamical mechanisms related to abnormalities in the vascular network and the role of the wall shear stress in initiating and further developing of arterial diseases, the present study focuses on the 3D flow field and in particular on the distribution as well as on both low and high values of the wall shear stress in the vicinity of the anomaly. Finally, a comparison is made between the effects of each rheological model on the aforementioned parameters. Results show marked differences between simulating blood as Newtonian and non‐Newtonian fluid and furthermore the Power‐Law model exhibits different behaviour in all cases compared to the other models whereas Quemada and Casson models exhibit similar behaviour in the case of the stenosis but different behaviour in the case of the aneurysm. Copyright © 2005 John Wiley & Sons, Ltd.     

Rheology and flow studies of drag-reducing gravel packing fluids

Viscoelastic additives are widely used as drag reducers in the oil and gas industry, and both polymeric additives and micellar surfactants are commonly used in well gravel packing applications. While the behaviour of polymeric additives such as the polysaccharide xanthan gum is well characterized in the literature, much less is known about how the rheology of the viscoelastic surfactants affects drag reduction, despite widespread use. In this study, we performed a number of rheological tests as well as flow loop experiments on solutions of a zwitterionic surfactant to understand the structural characteristics of the fluids in order to make better process predictions. Unlike xanthan, which displays typical viscoelastic liquid characteristics, zwitterionic surfactant-based fluids display elastic gel-like behaviour. The gel-like behaviour suggests long and relatively unbreakable chain lengths of the wormlike micelles in the viscoelastic surfactant solution at room temperature leading to gelation by entanglement. Also, a shear-thickening behaviour of viscoelastic surfactant samples at higher shear rates is observed, possibly as a result of shear-induced structures. Finally, we present a novel representation scheme to depict the flow loop results for drag in the laminar and turbulent regime, and relate this data to the rheological characterization.     

Rheological properties of suspensions of the green microalga Chlorella vulgaris at various volume fractions

A systematic study of the rheological properties of solutions of non-motile microalgae (Chlorella vulgaris CCAP 211-19) in a wide range of volume fractions is presented. As the volume fraction is gradually increased, several rheological regimes are observed. At low volume fractions (but yet beyond the Einstein diluted limit), the suspensions display a Newtonian rheological behaviour and the volume fraction dependence of the viscosity can be well described by the Quemada model (Quemada, Eur Phys J Appl Phys 1:119–127, 1997). For intermediate values of the volume fraction, a shear thinning behaviour is observed and the volume fraction dependence of the viscosity can be described by the Simha model (Simha, J Appl Phys 23:1020–1024, 1952). For the largest values of the volume fraction investigated, an apparent yield stress behaviour is observed. Increasing and decreasing stress ramps within this range of volume fractions indicate a thixotropic behaviour as well. The rheological behaviour observed within the high concentration regime bears similarities with the measurements performed by Heymann and Aksel (Phys Rev E 75:021505, 2007) on polymethyl methacrylate suspensions: irreversible flow behaviour (upon increasing/decreasing stresses) and dependence of the flow curve on the characteristic time of forcing (the averaging time per stress values). All these findings indicate a behaviour of the microalgae suspensions similar to that of suspensions of rigid particles. A deeper insight into the physical mechanisms underlying the shear thinning and the apparent yield stress regime is obtained by an in situ analysis of the microscopic flow of the suspension under shear. The shear thinning regime is associated to the formation of cell aggregates (flocs). Based on the Voronoi analysis of the correlation between the cell distribution and cell sizes, we suggest that the repulsive electrostatic interactions are responsible for this microscale organisation. The apparent yield stress regime originates in the formation of large-scale cell aggregates which behave as rigid plugs leading to a maximally random jammed state.