Figure 2 Non-Newtonian fluids are fluids that donot obey the Newton law of viscosity. In these fluids the viscosity varies with shear rate and one singlemeasurement is not sufficient to know the properties of the fluid. Also thereare other factors affect flow properties like temperature and pressure. Non-Newtonian fluids change theirviscosity or flow behaviour under stress. If you apply a force to these fluids(say you hit, shake or jump on them), the sudden action can cause them to getthicker and act like a solid, or in some cases it results in the oppositebehaviour and they may get runnier than they were before. Remove the stress(let them sit still or only move them slowly) and they will return to theirearlier state.
Figure 3 oobleck One ofthe most non-Newtonian fluids that we may not know is the mixture of cornflourand water which called oobleck. This fluid is a runny fluid, but if you apply astress on it suddenly acts a solid and its particles will hold on to each otherin a strong way. You can also make from it a solid part in your hand, but whenyou stop moving your hand it will return in its liquid phase again. In thiscase, oobleck’s viscosity increases with the applied stresses. Types ofnon-Newtonian fluids. Notall the non-Newtonian fluids behave in the same way when a stress applied. Someof them become more fluid and the other become more solid.
Also some of thenon-Newtonian fluids’ behavior differs due to the amount of stress applied andothers their behavior differs due to the length of time the stress applied onit. Non-Newtonian fluids are divided into two main categories: time dependentwhich includes shear thinning, shear thickening, Bingham, and Herschel Bulkey.And time independent includes thixotropic and rheopectic. · Shear thickening fluids or dilatant. In this typethe viscosity will changes dependent on the force applied, it becomes moreharder when the level of applied stress increases ( like oobleck) .
ü Figure 4 behavior when stress applied common example of shearthickening fluids is a mixture of cornstarch and water called oobleck . where people can run over this kind ofsolutions and yet, they will sink if they stand still ü Quicksandü Sillyputtyü printinginksü vinyl resin pastes ü suspensions at high solid content such as wetbeach sand which shows its dilatancy through the fact it stiffens when troddenon Applications : Shock absorption Systems Automotive Suspension – Magnetic particles suspended Impact Stress Cushioning – Sport / Athletics Accident damage and injury mitigation – Transport Impulse Distribution Systems Smart Body Armour · Shear thinning fluidsor Pseudoplastic. Unlike the shearthickening fluids, this type gets runnier when the stress or the force appliedincreases.
The graph above shows how both dilatant and pseudoplasticnon-Newtonian fluids behave as a force is applied. The key thing here is thatit doesn’t matter how long the force is applied for, changes in viscosity onlydepend on the size of the force.examples include ketchup, motor oil, paints and blood.When modern paints are applied the shear created by thebrush or roller will allow them to thin and wet out the surface evenly. Onceapplied the paints regain their higher viscosity which avoids drips and runs. Ketchup is a shear-thinning fluid, caused by the additionof a relatively small amount of Xanthan gum – usually 0.5%.
applicationShear thinning proves useful in many applications, fromlubricating fast-moving engine parts to making an otherwise stiff biocompatiblehydrogel injectable · Rheopectic fluids. It is a type offluid that gets more viscous when they are stressed over time. It will not getmore viscous when applying an instantaneous force. It requires sustainableforce to increase the viscosity. Example Gypsumpaste ,Cream A real life example of a rheopectic fluid is cream. If youstir cream once it won’t have any effect. But if you continually add a force ofstirring it will increase its viscosity and become thicker.
ApplicationThere is ongoing research into new ways to make and userheopectic materials. There is great interest in possible military uses of thistechnology. Moreover, the high end of the sports market has also begun torespond to it. Body armor and combat vehicle armor are key areas where effortsare being made to use rheopectic materials. Work is also being done to usethese materials in other kinds of protective equipment, which is seen aspotentially useful to reduce apparent impact stress in athletics, motor sports,transportation accidents, and all forms of parachuting. In particular, footwearwith rheopectic shock absorption is being pursued as a dual-use technology thatcan provide better support to those who must frequently run, leap, climb, ordescend. · Thixotropic fluids.
Unlike the rheopectic fluids, thixotropicfluids get runnier when applied a sustainable stress on it. Also it does notget runnier when applying an instantaneous force.Thisgraph shows how both rheopectic and thixotropic non-Newtonian fluids behave asa force is applied. The key thing here is that the force has to be sustained -the longer the force is applied the more the viscosity changes Figure 6 Behavior when stress is applying Example Paint ,Cosmetics ,Asphalt ,Glue ApplicationsMany kinds ofpaints and inks—e.g. plastisols used in silkscreen textile printing—exhibitthixotropic qualities. In many cases it is desirable for the fluid to flowsufficiently to form a uniform layer, then to resist further flow, therebypreventing sagging on a vertical surface. Some other inks, such as those usedin CMYK-type process printing, are designed to regain viscosity even faster,once they are applied, in order to protect the structure of the dots foraccurate color reproduction.
Solder pastes usedin electronics manufacturing printing processes are thixotropic. Thread-lockingfluid is a thixotropic adhesive that cures anaerobically. Thixotropy has beenproposed as a scientific explanation of blood liquefaction miracles such asthat of Saint Januarius in Naples. Semi-solid castingprocesses such as thixomoulding use the thixotropic property of some alloys(mostly light metals) (bismuth). Within certain temperature ranges, withappropriate preparation, an alloy can be put into a semi-solid state, which canbe injected with less shrinkage and better overall properties than by normalinjection molding. Fumed silica is commonly used as a rheologyagent to make otherwise low-viscous fluids thixotropic. Examples range fromfoods to epoxy resin in structural bonding applications like fillet joints.
· Rheological mathematical modelsThere are several rheologicalmathematical models applied on rheograms in order totransform them to informationon fluid rheological behaviour. For non-Newtonian fluids thethree models presented beloware mostly applied (Seyssiecq & Ferasse, 2003).· Herschel Bulkleymodel The Herschel Bulkley modelis applied on fluids with a non linear behaviour and yield stress. It isconsidered as a precise model since its equation has three adjustableparameters, providing data (Pevere & Guibaud, 2006). The Herschel Bulkleymodel is expressed in equation 5, where t0 represents the yield stress.
? = t0 + K * g n (5) The consistency index parameter (K) gives an idea of the viscosity of the fluid. However, to be ableto compare K-values for different fluids they should have similar flow behaviourindex (n). When the flow behaviour index is close to 1 the fluid´s behaviourtends to pass from a shear thinning to a shear thickening fluid. When n is above1, the fluid acts as a shear thickening fluid. According to Seyssiecq andFerasse (2003) equation 5 gives fluid behaviour information as follows: t0 = 0 & n = 1 Þ Newtonian behaviour t0 > 0 & n = 1 Þ Bingham plastic behaviour t0 = 0 & n< 1 Þ Pseudoplastic behaviour t0 = 0 & n> 1 Þ Dilatant behaviour Herschel-Bulkley fluids include both shear thinning and shearthickening materials. The practical examples of such materials are greases,colloidal suspensions, starch pastes, tooth pastes, paints, and blood flow inan artery.
· Ostwald model The Ostwald model (Eq. 6),also known as the Power Law model, is applied to shear thinning fluids which donot present a yield stress (Pevere et al., 2006). The n-value in equation 6gives fluid behaviour information according to: ? = K * g (n-1) (6) n < 1 Þ Pseudoplastic behaviour n = 1 Þ Newtonian behaviour n > 1 Þ Dilatant behaviour · Bingham model The Bingham model (Eq. 7)describes the flow curve of a material with a yield stress and a constantviscosity at stresses above the yield stress (i.e.
a pseudo-Newtonian fluidbehaviour; Seyssiecq & Ferasse, 2003). The yield stress (t0) is the shear stress (t) at shear rate(g) zero and the viscosity (h) is the slopeof the curve at stresses above the yield stress.t = t0 + h * g (7)t0 = 0 Þ Newtonian behaviour t0 > 1 Þ Bingham plastic behaviour