Hexagonal Defect Array Formed under Steady Shear Flow in Water-in-Cholesteric Liquid Crystal Emulsions

The relation between the liquid crystalline textures and the steady shear viscosity has been investigated in cholesteric emulsions, composed of water, surfactants, and cholesteric liquid crystals. Both at the substrate surface and at the surfactant-coated droplet surface, a homeotropic anchoring is enforced to the adjacent liquid crystal. Under a steady shear flow with the shear rate below 100 s^–1, we observed that point defects spontaneously appear, and tend to adopt a regular hexagonal arrangement in the low shear-rate range of 1–20 s^–1. In viscosity measurements, the shear-thinning behavior was found, showing a clear correspondence to the texture change. We estimated the height of point defects in the hexagonal array from the viscosity increase in the shear-thinning, assuming that the shear-thinning behavior is caused by the reduction of the effective gap between cone and plate, owing to the presence of the hexagonal array anchored on the plate. The relation between the estimated height and the measured lateral size of point defects agreed with that of the quadrilateral array, which is formed after termination of the shear. This agreement informs that the shape of a point defect is preserved in both defect arrays, independently of shearing conditions.     

A novel geometry for rheological characterization of viscoelastic materials

A novel geometry for generating a viscometric flow presents advantages of both cone and plate and parallel plate geometries, regarding uniform shear field and adjustable range of measurement. Kinematics and dynamics of the generated flow have been described mathematically utilizing an orthogonal curvilinear coordinate system based on the shapes of the shearing surfaces which are similar to the surface that generates the flow. Simple equations that allow the calculation of quantities of experimental interest in the rheological characterization of liquid materials, namely, shear rate, shear stress and two normal stress differences, have also been derived.The geometry, called pseudosphere, was tested with two types of fluids (Newtonian and pseudoplastic). Results show that the geometry can be used with low viscosity liquids (Newtonian liquids) by only adjusting the gapH. The behavior of pseudoplastic fluids for both low and moderately high viscosity could also be studied with this geometry. Very reproducible results were obtained when compared with those obtained with cone and plate geometry. Regions of lower shear rate could be studied using only the pseudosphere geometry.     

Einsatz rheologischer Messungen zur Beschreibung der Beschaffenheit der Korninhaltsstoffe des Getreides

Zusammenfassung In der vorliegenden Arbeit wird über die Anwendung eines Universal-Rotationsviskosimeters für Forschungszwecke in einem Getreidelaboratorium berichtet und einige dabei gewonnenen Ergebnisse diskutiert. Die Messungen wurden sowohl an niedrigviskosen Wasserextrakten und Suspensionen als auch an hochviskosen Teigen durchgeführt. Als Meßart wurden Fließ- und Viskositätskurven zur Beschreibung eines Zustandes und kontinuierliche Messungen über die Zeit und bei steigenden Temperaturen zur Erfassung der Viskositätsveränderungen durchgeführt. Mit den dabei gewonnenen Ergebnissen wird versucht, die Struktur und Beschaffenheit der Korninhaltsstoffe zu beschreiben und die Wechselwirkung dieser Merkmale und der Enzymolyse zu verdeutlichen.Schlüsselwörter Rotationsviskosimeter, Fließkurve, Viskositätskurve, kontinuierliche Viskositätsmessung, Getreidequalität

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Summary In the present paper the possibilities for using a universal rotational viscometer for research purposes in cereal laboratories have been described and discussed. This kind of instrument is suitable as well for viscosity measurements in low viscous water-extracts and water-fluor slures as in high viscous doughs of such a consistency, as used in the practical breadmaking. Drawing up flow and viscosity curves for describing the condition of chemical matters in cereal grain, continuous measurements over the time at constant or defined increasing temperatures have been choosen to record the changes of the viscosity. The obtained results have been utilised to describe the structure and the state of the main constituents of cereal grains (starch, protein, pentosan) and to make evident the alternating effect between those characteristics and the enzymolysis.

Mit 11 Abbildungen     

An innovative extensional viscometer for low-viscosity and low-elasticity liquids

A spinline-type extensional viscometer is described in which an innovative method of tensile stress measurement is employed. A limited amount of liquid flows through a vertical capillary at a constant flow rate under the influence of a constant pressure head. The drainage time decreases when the liquid stream leaving the capillary is stretched by the application of vacuum. These drainage times are measured in a manner similar to that used for intrinsic viscosity measurements. The measured difference in drainage times, with and without stretching, is trivially related to the extensional stress at the capillary exit, and this provides a very simple method of accurately determining fluid stretching forces having a magnitude as low as 10^-4 N; stresses at other axial locations in the stretched liquid jet are obtained by means of a force balance in the usual manner. The validity of the proposed technique is demonstrated by obtaining the expected results for a Newtonian oil having a shear viscosity of 56.2 mPa-s. Also presented are preliminary data on polyethylene oxide-in-water solutions having an even lower shear viscosity.     

The measurement of normal-stress differences using a cone- and-plate total thrust apparatus

The proposal ofJackson andKaye (1966) for evaluating both differences of normal stress in a viscometric flow by using only total thrust measurements in a cone- and-plate viscometer is extended. An analytic relation valid for all values of the separation between cone and plate is obtained, which is shown to include as special cases the well-known cone- and-plate formula, the parallel-plate formula ofKotaka et al. (1959) andJackson andKaye's.Examples of the method's application to experimental results are given.On leave from: Department of Chemical Engineering, Pembroke Street, Cambridge.     

Simulation of non-Newtonian ink transfer between two separating plates for gravure-offset printing

The inks used in gravure-offset printing are non-Newtonian fluids with higher viscosities and lower surface tensions compared to Newtonian fluids. This paper examines the transfer of a non-Newtonian ink between two parallel plates when the top plate is moved upward with a constant velocity while the bottom plate is held fixed. Numerical simulations were carried out using the Carreau model to explore the behavior of a non-Newtonian ink in gravure-offset printing. The volume of fluid (VOF) model was adopted to demonstrate the stretching and break-up behaviors of the ink. The results indicate that the ink transfer ratio is greatly influenced by the contact angle, especially the contact angle at the upper plate (α). For lower values of α, oscillatory or unstable behavior of the position of minimum thickness of the ink between the two parallel plates during the stretching period is observed. This oscillation gradually diminishes as the contact angle at the upper plate is increased. Moreover, the number of satellite droplets increases as the velocity of the upper plate is increased. The surface tension of the conductive ink shows a positive impact on the ink transfer ratio to the upper plate. Indeed, the velocity of the upper plate has a significant influence on the ink transfer in gravure-offset printing when the Capillary number (Ca) is greater than 1 and the surface tension dominates over the ink transfer process when Ca is less than 1.     

Effect of droplet size on the rheological properties of highly-concentrated w/o emulsions

Rheological properties of highly concentrated emulsions of the water-in-oil type were studied. Water phase (concentration approximately 91%) consists of a supersaturated aqueous solution of nitrate salts; water comprises less than 20% by mass. The average size of droplets, D, in the emulsions was varied. It was found that the emulsions are non-Newtonian liquids and flow curves measured in a sweep regime of shearing have clear low-shear-rate Newtonian domain. The complete flow curves are fitted by the Cross equation. The elastic modulus is practically constant in a very wide frequency range. Hence the viscoelastic relaxation processes might be expected at times >>100 s and in the short-term side of the curve at approximately 0.01 s. The elastic modulus (measured in oscillating testing and in elastic recovery as well) is proportional to D^-2 while the Newtonian viscosity is proportional to D^–1.The time effects were observed: it was found that the emulsions behave as rheopectic materials because prolonged shearing results in an increase of viscosity in the low shear rate domain of several orders of magnitude.Presented in part at the First Annual European Rheological Conference, Guimarães, Portugal, 11–13 September 2003     

Moving boundary in a non-Newtonian fluid

Moving boundary value problem in non-Newtonian fluid is considered. Exact analytical solution for the flow of second-grade fluid for a rigid moving plate oscillating in its own plane, is obtained. The Doppler effect has been observed due to the motion of the plate. The shearing stress on the plate is also calculated. It is concluded that the solutions for stationary porous boundaries can be obtained from the solutions of moving rigid boundaries.     

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     

A rheological study of shear induced crystallization

The isothermal crystallization of three isotactic polypropylene (iPP) types, with different molar mass (distributions), was studied after a well defined shear treatment of the melt at an elevated temperature and a subsequent quench to the crystallization temperature. For these experiments a standard rheometer of the cone and plate configuration was used. The development of the crystallization was monitored by dynamic oscillatory measurements. Shearing in the melt was shown to enhance subsequent crystallization at lower temperatures. Not only the total shear at constant rate is of importance, but also the chosen combination of rate and shearing time. Moreover, a pronounced influence of molar mass was detected. The exploration of the melting temperatures and times which are necessary for an erasion of the memory effects showed that the effect of shearing could not completely be erased, possibly as a consequence of mechanical degradation.     

Viscosity of some clay-based coating colors at high shear rates

The shear viscosity of clay-based coating colors containing latex and carboxymethyl cellulose (CMC) has been measured over a relatively large shearrate region. In the shear-rate range of 50–1500 s^–1 the measurements were performed using a rotational viscometer and, at higher shear rates extending into the region 10⁵ – 10⁶ s^–1, a high pressure capillary viscometer was employed. The viscosity of the clay colors increased with increasing CMC-concentration, but the influence of the CMC-content was less pronounced at higher shear rates. The apparent shear-thinning behavior of the investigated colors could, in part, be attributed to the shear-thinning of the corresponding polymer (CMC) solution constituting the liquid phase of the color, but the influence of another factor was also indicated. At low shear rates, the interaction between the color components can produce relatively high viscosity levels, but in the high shear rate region these interactions appear to be less important for the viscosity level. It is also of interest to note that the viscosity dependence on the solids content in the high shear-rate region could be described with reasonable accuracy using an empirical equation neglecting interactions between the color components.     

Normal stress measurements

An apparatus designed to measure the dynamic viscoelastic response of polymer melts is described. Dynamic elasticity (G) and viscosity () can be measured over a frequency range 10^–2-10^–3 Hz and at temperatures up to 350 °C. The sample under test is held in a cone and plate assembly. A small strain is introduced by driving the plate with a variable speed synchronous motor and off-centre cam at low frequencies and by an electromagnetic vibrator at high frequencies. The amplitudes of the cone and plate are detected using the optical lever principle and photocell strips. The phase difference between the cone and plate is measured from a recorder trace at low frequencies and by direct reading on a meter at high frequencies.Results are described of measurements on silicone fluids, and on commercial grades of polyethylene with different molecular weight distributions and degrees of branching.A discussion is given of the correlation between the dynamic viscosity measurements and those taken under steady-flow conditions.Paper presented at the Conference on Experimental Rheology, University of Bradford, April 17–19, 1968. — Original paper published in J. Sci. Instruments Series 2,1, 1102–1112 (1968).     

Laminar length of a non-Newtonian fluid jet

The laminar length of a submerged jet of a non-Newtonian fluid is measured employing a flow visualization technique, which makes use of the birefringent property of the fluid and a circular polariscope. Fluids of two different concentrations are studied. The results indicate that the concentration, hence the non-Newtonian nature of the fluid, has no influence on the laminar length of the jet for 600 〈 Re 〈 1100. In the Reynolds number range of 50 to 200 the laminar length is affected by the viscous properties of the fluid. The results also indicate that there is not much difference in the laminar length of a Newtonian and non-Newtonian jet for 600 〈 Re 〈 1100.The experimental facility is also used as a “falling head” capillary viscometer. The laminar length data and the viscosity data are taken simultaneously. The viscosity obtained from the present test facility is found to agree with that obtained using a standard rotary viscometer in trend only.     

Predicting terrain parameters for physics-based vehicle mobility models from cone index data

To provide terrain data for the development of physics-based vehicle mobility models, such as the Next Generation NATO Reference Mobility Model, there is a desire to make use of the vast amount of cone index (CI) data available. The challenge is whether the terrain parameters for physics-based vehicle mobility models can be predicted from CI data. An improved model for cone-terrain interaction has been developed that takes into account both normal pressure and shear stress distributions on the cone-terrain interface. A methodology based on Derivative-Free Optimization Algorithms (DFOA) has been developed in combination with the improved model to make use of continuously measured CI vs. sinkage data for predicting the three Bekker pressure-sinkage parameters, k_c, k_ϕ and n, and two cone-terrain shear strength parameters, c_c and ϕ_c. The methodology has been demonstrated on two types of soil, LETE sand and Keweenaw Research Center (KRC) soils, where continuous CI vs. sinkage measurements and continuous plate pressure vs. sinkage measurements are available. The correlations between the predicted pressure-sinkage relationships based on the parameters derived from continuous CI vs. sinkage measurements using the DFOA-based methodology and that measured were generally encouraging.     

Asymptotic analysis of the finite cone-and-plate flow of a non-newtonian fluid

Consider the shearing flow of a viscoelastic fluid trapped by surface tension between a cone and a plate. An asymptotic analysis of this problem in the limit of small gap angle has been done. This limit is realized in many practical situations. It is assumed that the Deborah number De, the Reynolds number Re, and the retardation parameter β are all order unity and that the shape of the free surface is very nearly spherical. Closed form analytic expressions are obtained for the leading terms of the primary and weak secondary motion of the fluid as well as the meniscus shape. It is found that the velocity field is bounded and continuous if and only if . There is a family of curves in the De-β plane on which the velocity field has a removable singularity at the origin. The secondary flow is made up of either one or two toroidal vortices. The meniscus has a bulge near the rotating cone and a trough near the stationary plate.     

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 dependence of viscoelastic flow properties on the structure for styrene-butadiene copolymers

The flow properties for 300 kg/m³ solutions of four-arm, star-branched random and block styrene-butadiene-styrene copolymers in n-butylbenzene are presented. The viscosity, η, first and second normal stress differences, N₁ and N₂, and the steady sher compliance, J, were determined as a function of the shear rate from cone-plate shearing data obtained with a stiffened Model R17 Weissenberg Rheogoniometer. The normal stress differences were determined from total normal force and plate pressure distribution data. Four sensitive, miniature, variable-capacitance pressure transducers mounted in the 7.4-cm plate with their approximately 2.4 mm in diameter pressure-sensing diaphragms flush with the plate surface provided data for the pressure distribution on the plate. In general, the data extend from the zero shear-rate viscosity region somewhat into the shear thinning region. Based on limited data, the zero shear viscosities for the random copolymers increased with (M₃)^3.2, whereas those for the 28% block and 38% block polymers increased with (M_w)^?;6, respectively. The latter high exponents are believed to be a consequence of a network with junctions formed by dispersed phase polystyrene block domains. The sign for N₂ was opposite that of N₁ and the ratio N₂/M₁ for all of the star copolymers averaged –0.214 with a standard deviation of 1.5%. This value is within ±1% of the ratios for tetrachain polybutadienes and polystyrenes and is significantly lower than the –0.29 for linear polybutadiene and polystyrene solutions in normal butylbenzene. The N₂/N₁ ratio did not vary significantly over the shear-rate range investigated.     

Experimental and conceptual problems in the rheological characterization of wheat flour doughs

Wheat flour dough is an industrially important material and a better understanding of its rheological behavior could have long ranging impact on the agricultural and the food processing industries. However, rheological characterization of dough is proving to be difficult due to a range of testing issues and anomalies in flow behavior. In a cone-and-plate rheometer wheat flour doughs “roll-out” of the gap before steady state viscosities can be established, as discussed by Bloksma and Nieman (1975). However, the mirror image of the transient viscosity-time plot obtained using a cone-and-plate viscometer has been used to obtain an estimate of steady shear viscosity behavior (Gleissle, 1975). To check this transient methodology for doughs, a second method, in addition to cone-and-plate transient flow, for determination of the shear viscosity, was needed. For this, capillary extrusion was chosen. Both a piston-driven and pressure driven capillary rheometer were employed. End corrections were determined to provide information on both the shear viscosity and, following Binding (1988), the extensional viscosity of the doughs. There are few data available on end corrections for doughs, though published data by Kieffer indicate that the corrections are unexpectedly very high. In this present work it was found that the end correction experiments were very difficult and imprecise in part due to the time-dependent nature of the doughs and difficulties in preparing replicate batches required to compare dies of differing L/R values. Further it was unexpectedly found that the samples, though prepared by normal mixing procedures to the “optimum” level, were so heterogeneous that large fluctuations in the pressure at constant output rate (in the piston-driven rheometer) and in output rate at constant pressure (in the pressure-driven instrument) were observed. These fluctuations could be eliminated by overmixing of the doughs, but overmixed doughs are of little practical interest. Although the problems encountered in this work were significant, it was encouraging that even these preliminary studies indicate that rheological measurements are effective in differentiating between spring and winter wheats. Defining a constitutive model for dough rheology still remains a major challenge, as results from one type of testing do not corroborate the findings from a different type of testing. Received: 19 May 1998 Accepted: 27 July 1998