Understanding Rheology Of Structured Fluids
Understanding Rheology of Structured FluidsKeywords: structured fluids, sol gel transition, solution, yield stress, thixotropy, viscosity, mechanical stability,shelf life, flow curve, inks, cosmetics, dispersions, foodAAN016GENERAL CONSIDERATIONSFluids: Simple or StructuredFluid materials, by definition, are systems which flow whensubjected to stress. How they respond to an input stress is theheart of rheological testing and may be a complex issue.There are many types of fluids: pure substances, mixtures,dispersions and solutions, falling into the categories of eithersimple or structured fluids. Each has its own unique behaviorwhen subjected to stress. In general, when a material hasa uniform phase, such as a solution or pure substance, it isreferred to as a simple fluid. Materials which contain morethan one phase, such as solid particles dispersed in a liquid,gas particles in foam or an emulsion of immiscible liquidsare considered structured fluids since their rheologicalbehavior is in general dominated by the interactions of theconstituents.Figure 1: Viscosity of a structured fluid as a function of shear rate andparticle concentration1Structured fluidsMany of the materials we use each day are structured fluids.Most foods, cosmetics, pharmaceuticals and paints containparticles or droplets of an immiscible fluid suspended in acarrier liquid. A number of soft semisolid materials also fallunder the category of structured fluids since they have amultiphase structure and exhibit complex flow behavior.Some examples would be cheeses, lipstick, caulk, and breaddough.Many factors affect the stability of structured fluids. Theviscosity of the liquid phase in dispersions usually playsan important role on the flow properties of the material.Dispersions have wide variations in performance dependingon particle size, shape, concentration, and any attractionwith the continuous phase in which they are suspended.When there is a repulsive electrostatic or steric force betweenparticles they end not to settle rapidly, instead forminga network structure which will stabilize the suspension ifundisturbed. Shearing or even Brownian motion can destroythis delicate structure and break down the fluids viscosity.Structured fluids do not obey a simple linear relationshipbetween applied stress and flow (Newtonian fluid behavior)as shown in Figure 1 for suspensions of latex particles withincreasing volume fraction in water.Nearly all these materials have a viscosity that drops at higherrates of shear velocity resp, stress. This is the phenomenon ofshear thinning which becomes progressively larger as thevolume concentration of solid particles increases. At highconcentration of solid content, the low shear rate viscosityregion disappears completely, the material is yielding.Some materials show after the shear thinning region withincreasing rate or stress, an increase of the viscosity, usuallydue to structure rearrangements as a result of the appliedshear. This is referred to as flow induced shear thickening.Characteristic flow parameters and functionsBingham FlowEugene Bingham, a colloid chemist, first coined the term“Rheology.” He also showed that for many real fluids acritical level of stress must be attained in order to initiateflow. Below this critical stress, τ y, the material behaves as asolid, absorbing the stress energy without flowing. Once thethreshold of critical stress has been reached, the materialyields to flow, hence the term, yield stress.The yield stress is thereason, why you need to shake or tap a bottle to make theketchup flow. Materials which exhibit Newtonian flow beyondthe yield bear the name Bingham Fluids.Plastic FlowMost materials do not exhibit Newtonian flow after the yield,but have a viscosity that decreases (shear thinning) until aplateau is reached. Lipsticks, drilling muds and toothpaste aregood examples of shear thinning non-Newtonian materials1 AAN016
with a yield stress.PseudoplasticitySome materials do not have a yield stress, nevertheless theybehave nonlinear. These are considered pseudoplastic. Theyflow instantaneously upon application of stress but alsodisplay shear thinning behavior. Polymer solutions exhibitpseudoplastic flow as does bread dough and many paintsand cosmetics. A plot of viscosity versus shear rate for differenttypes of materials is shown in Figure 2.Figure 3: Structure build up after previous shear monitored with smallstrain amplitude oscillatory testingFigure 2: Viscosity versus shear rate for different types of materialThe viscosity of thixotropic materials does not follow the samepath on structure breakdown and recovery. In most cases,when the shear rate is slowed, the stress path lags forming ahysteresis loop, which then returns to a point lower than theinitial critical shear stress. The area within the hysteresis looprepresents the energy consumed in structure breakdown(Figure 4).DilatancyDilatancy, also known as shear thickening, is an unusualphenomenon whereby materials actually increase theirviscosity upon stirring or shearing. In some cases these aredense suspensions of solid particles in a fluid medium, whichdevelop greater spacing between particles during agitation.This behavior is infamous in quicksand, moist beach sand andcertain pharmaceuticals such as a suspension of penicillin.Shear thickening often result from material instability andstructure rearrangements or phase separation.ThixotropyFor many fluid materials, viscosity is mostly independent oftime, and is only a function of the shear rate and temperature.For concentrated dispersions their viscosity does not reacha steady value for some time upon application of stress, orshear rate. This steady state is dependant on the stabilizationof internal network structures that can be broken down byshearing, and require time to rebuild. A steady state plateauin viscosity is reached if an equilibrium has been establishedbetween structure breakdown and rebuilding. Upon ceasingthe shear rate which caused the breakdown, the materialreforms its internal network, and the viscosity recovers (Figure3). The term used to describe this phenomenon is Thixotropy.In studying such materials it can be beneficial to destroy thenetwork structure entirely by shearing the material, giving aclean-slate for examination of the path by which the viscosityrebuilds.1Laun, M. Private communication 1964Figure 4: Hysteresis loop of a thixotropic materialRheopexyWhereas a thixotropic fluid’s viscosity decreases over timeunder an imposed constant shear rate, a rheopectic fluid’sviscosity increases under an imposed shearing action.A rheopectic fluid such as a dense suspension of latexparticals or plastisols will gel when agitated. If allowed torest, a rheopectic fluid will return to its original lower viscosity.The viscosity-shear rate curve forms a hysteresis loop andthe hysteresis can be repeated indefinitely. This is a way todistinguish between true and apparent rheopectic behavior- fluids that change physically or chemically (gelling, solventevaporation) while a shear is imposed also experiencea viscosity increase. These changes, however, will not be2 AAN016
reversible and therefore do not represent true rheopexy.Time Dependency- Creep and Creep RecoveryThe stress and strain rate dependent behavior of a materialmay be only part of the picture. In many cases timedependency has to be considered also. Materials are alsotime dependent. Hookean and Newtonian materials respondimmediately upon an input stress or strain rate. When a stressis imposed on a so-called viscoelastic” material, it does notimmediately respond with constant flow, even though thestress may be sufficiently above the critical stress or yield point.Upon removal of the stress, these types of materials recoverto their original state, but slowly, and usually incompletely. Thisbehavior is referred to as creep. Creep studies can also beused to determine the yield stress of materials (see Figure 5).A series of creep and recovery (application of a constantstress followed by a period of zero stress) can be performedin incrementally higher and higher stress levels.rest behaves viscoelastic and the time dependent stress orviscosity build-up competes with the viscosity decrease dueto structure break-down with increasing stress.Figure 6: Yield stress measurement of a cosmetic cream based on theviscosity maximum method in a stress rampCritical strain and strain sweepFigure 5: Series of creep tests to determine the yield pointUsually the rheological properties of a viscoelastic materialare independent of strain up to a critical strain level gc.Beyond this critical strain level, the material’s behavior isnon-linear and the storage modulus declines. So, measuringthe strain amplitude dependence of the storage and lossmoduli (G’, G”) is a good first step taken in characterizingvisco-elastic behavior: A strain sweep will establish the extentof the material’s linearity. Figure 7 shows a strain sweep fora water-base acrylic coating. In this case, the critical strainγc is 6%. Below 6% strain, the structure is intact, the materialbehaves solid-like, and G’ G”, indicating that the materialis highly structured. Increasing the strain above the criticalstrain disrupts the network structure.Below the yield stress the material behaves as a solid,with complete recovery. When the material fails to recovercompletely, it has reached its yield stress.EXAMPLES OF RHEOLOGICAL TEST METHODSYield stress and stress rampStructured fluids often will not flow unless they have reacheda critical stress level called the yield stress, below which amaterial is “fully” elastic and above which the structure of thematerial breaks and it flows. The Yield stress is an importantparameter in product delivery and use such as the easewith which a shampoo can be dispensed from a bottle, orthe consistency of sour cream. In production, the yield stressdetermines the force needed to start pumping through apipeline or fill a container with the product. The stress ramp, inFigure 6, is the most frequently used technique to measure theyield stress today. The stress at the viscosity maximum, whichis readily measurable for most structured fluids, provides areproducible and representative value for the yield stress.The viscosity maximum is pronounced, if the material atFigure 7: Strain sweep for a water-based acrylic coatingThe material becomes progressively more fluid-like, themoduli decline, and G” exceeds G’ eventually. The strengthof the colloidal forces is reflected by tan δ (G”/G’). A tan δ3 AAN016
less than 1 suggests that the particles are highly associateddue to the colloidal forces and sedimentation could occur: ahigh tan δ at given concentration suggests that the particlesare largely unassociated. For a stable system, an intermediatetan δ is desired. Critical strains for electrostatically stabilizedsystems are about 0,01% to 0,5% for sterically stabilizedsystems, about 1% to 5%.elastic recoil, which is important to keep the shape of thedough after extruding to avoid variations in product size(Figure 9).The product of critical strain γc and complex modulus G*below γc is a good indication of the materials yield stressand correlates well with the yield stress determined from theviscosity maximum obtained in a stress ramp.Structure and frequency sweepAfter the fluid’s linear viscoelastic region has been definedby a strain sweep, its structure can be further characterizedusing a frequency sweep at a strain below the criticalstrain γc. This provides more information about the effect ofcolloidal forces, the interactions among particles or droplets.In a frequency sweep, measurements are made over arange of oscillation frequencies at a constant oscillationamplitude and temperature. Below the critical strain, theelastic modulus G’ is often nearly independent of frequency,as would be expected from a structured or solid-like material.The more frequency dependent the elastic modulus is, themore fluid-like is the material. Figure 8 illustrates the transitionsolid-fluid with frequency sweep data measured on a slurryof a simulated solid rocket propellant at both a low (0,5%)and a high strain amplitude (5%).Figure 9: Creep recovery of cookie doughStructure changes and thixotropic loopThixotropic material will lose structure during shear, andrebuild it on standing. This behavior is a key factor in theability of a paint or cosmetic to be easily applied to asurface (through structure breakdown in spreading) andthen rebuild its structure and viscosity so that it does notdrip and run. Latex paints need to flow smoothly as they aresheared by a roller or brush, so the viscosity must break down.A time lag for leveling of the paint (to conceal brush strokes)is necessary, after which the viscosity must rebuild to avoiddrips and sagging. Food products also display this importantbehavior. Mayonnaise being spread on a sandwich or mixedin a salad must break down its structure to be distributed andthen rebuild to the right mouth feel so that it doesn’t seepinto the bread or taste thin and runny in a salad. In figure 10,the thixotropy is shown in the difference of the up and downstress ramps for 3 hand lotions. The area between the curvesis an indication of the extent of the thixotropy.Figure 8: Frequency sweep on a simulated rocket propellant material:shows a more fluid-like behavior at high strain amplitudes (G” G’),more solid-like at low strains (G’ G”)Yield stress and Creep recoveryThe creep test probes the time-dependent nature of asample. A characteristic creep experiment provides criticalparameters such as zero shear viscosity (ηo) and equilibriumcompliance (Jeo), which measures the elastic recoil of amaterial. After a sample is allowed to creep under load, thematerial’s elastic behavior can be obtained by abruptlyrelieving the imposed stress and measuring the extent thesample recovers. Cookie doughs which had nearly the sameviscosities showed significant differences in compliance orFigure 10: Thixotropy of hand lotions - Stress ramps at 25 C4 AAN016
Flow curve and step shear rateStress relaxationThe viscosity of a material according to the rate at which itis sheared, provides important information about processingand performance.This can be important in production wherestirring, dispensing and pumping of the product will subjectit to a variety of shear rates. Low shear rate behavior can berelated to storage conditions of materials: sedimentation,phase separation, and structure retention. Single pointviscosity information does not profile the material across aspectrum of shear rates. Materials that may behave the sameat one end of the flow curve may show dramatic differenceat the other, which relates to structural differences in thesematerials as shown in Figure 11 for two adhesive dispersions.Stress relaxation experiments apply a step strain deformationto create an instantaneous strain and to monitor the stressdecay as the specimen is held over time in the sameconstrained state. The profile of stress relaxation is importantin materials that will be subjected to repetitive strain, todetermine if the stress can be dissipated with the timescale of typical use. Overloading of stress in a material cancause damage and failure in later use. The stress relaxationof human cartilage is shown in Figure 13 after the materialhas been subjected to a 1% strain at 23 C. Besides greases,soft solids such as caulks, bread dough, dairy products, andpharmaceutical creams have been studied extensivelyusing stress relaxation tests.Figure 11: Flow curve of 2 adhesive dispersions. The products differsignificantly at high shear ratesFigure 13: Stress relaxation behavior of human cartilageTemperature dependence in oscillatory temperature rampA dynamic temperature ramp study does not alwaysmean heating. In the accompanying example (Figure 12),Carrageenan was cooled at 1.5 C/min from 70 to 20 C andheld at the lower temperature for 1 hour. The strain during thecooling period was high: 10%, but during the isothermal perioda very low strain, 0.1%, was maintained to avoid disturbingthe structure being formed at that low temperature. DynamicTemperature Ramp studies can simulate production cycles,storage and use conditions or evaluate long term stabilityof, for example, cosmetic creams. Rheological testing canpredict behavior without large costly batch studies.Figure 12: Carrageenan temperature ramp is used to reproduceproduction cycles, storage or use conditionsFLUIDS MATERIALS–APPLICATIONSDecorative and Protective CoatingsRheological tests are used widely to evaluate functionalcoatings in terms of their properties and performance. Duringmanufacturing as they are mixed and transferred, andduring application by spraying, brushing, coating, or dipping,coatings are subjected repeatedly to shear and extensionover a range of magnitudes, rates and durations. Afterapplication the coating may distort, and, inevitably, it ages.Rheological testing provides a convenient way to measureperformance- critical rheological changes occurring duringthe life cycle of decorative and protective coatings. Amongthe chief performance aspects of coatings influencedby rheology are leveling, sagging, spatter resistance, andbrushability.Leveling refers to the ability of a coating to flow laterally anddiminish differences in thickness of adjacent areas of thecoating. This is an important property affecting smoothness,gloss, color, and mechanical behavior. Leveling involveschanges in surface tension due to solvent loss or reaction andis influenced by the material’s yield stress and viscosity. Datafrom strain sweeps have proven to be an effective predictorof flow and leveling. Table 1 compares the subjective rankingof leveling behavior of six latex paint versus their complex5 AAN016
viscosity at 25% strain.and development of thick film resistor, capacitor, electrode,solder, and thermoset conductor pastes applied by highspeed screen printing onto electronic circuit boards andcomponents.Newspaper inks encounter high shear rates and abruptshear rate and shear direction changes as they pass throughthe nips of the printing press. These deformations generatehigh shear, normal, and extensional stresses. Accordingly,they require a battery of tests to properly characterizethem, including steady and transient shear studies andmeasurement of normal stresses and elongational viscosity.Table 1: Comparison of leveling behavior versus complex viscosity ofLatex paintSagging is undesirable flow of a coating down a verticalsurface. Whether a coating will sag or not depends on itsthickness and its viscosity at low shear rates. For a coatingto resist sagging, the product of its density, the gravitationalconstant, and its thickness must not exceed its yield stress.Figure 14 shows viscosity versus shear rate and shear stressversus shear rate for three model inks(3). Yield stresses wereobtained by extrapolating the steady state shear stress tozero shear rate.Spatter resistance in spraying is related to the elasticity ofthe coating and depends strongly on the elongationalviscosity of the fluid. Spatter resistance can be predictedfrom measurements of the elastic modulus G’ at high strains.To achieve the needed combination of low viscosity topass through small sprayer orifices and yet resist sagging,the coating must be highly pseudoplastic and have rapidviscosity recovery.Brushability is a matter of the effect of shear rate on viscosity.Newtonian fluids are difficult to brush: highly pseudoplasticcoatings brush easily.Printing InksTwo major classes of inks are those used for screen printingin the graphics arts and electronics industry, and those usedfor printing newspaper. While their compositions differ, theyshare a characteristic: rheological complexity.Screen printing is possible because inks can be madethixotropic. Initially highly viscous, these inks shear-thin rapidlyunder the high shear rates generated as they are squeezedthrough t
Understanding Rheology of Structured Fluids Keywords: structured fluids, sol gel transition, solution, yield stress, thixotropy, viscosity, mechanical stability, shelf life, flow curve, inks, cosmetics, dispersions, food 1 AAN016 Figure 1: Viscosity of a structured fluid as a function of shear rate and particle concentration1
Rheology with Application to Polyolefins Teresa Karjala, Ph.D. and Dr. Ir. Sylvie Vervoort . to study structured materials without disturbing the structure . F. A. Morrison, Understanding Rheology, Oxford (2001). R.G. Larson, The Structure and Rheology of Complex Fluids, Oxford (1998).
Rheology of structured fluids. TA Instruments - a brief introduction TA Instruments (a subsidiary of Waters ltd.) is the worldwide leader in Thermal Analysis, Rheology and Microcalorimetry equipment TA Instruments is a manufacturer based in New Castle (Delaware) but
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Additionally, understanding the rheological behavior of such fluids helps advance the fundamental understanding of CNT systems, as the intrinsic behavior of the CNTs is less likely to be masked by the rheology of the suspending medium. This chapter is structured in the following way:
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