Viscoelastic Behavior

When a force is applied to a viscous fluid, it will start to deform and this deformation is proportional with the magnitude of force applied. It deforms continuously until the force is removed so that it cannot return to its original position. Viscous fluids generally exhibit viscosity while solids exhibit elasticity. Some foods show both viscous and elastic properties which are known as viscoelastic materials. The typical food example for viscoelastic fluids is wheat flour dough. Dairy cream, ice cream mix, marshmallow cream, cheese, and most gelled products are also viscoelastic foods. There is no simple constant for viscoelastic materials such as modulus because the modulus will change with respect to time.

When a viscous fluid is agitated, the circular motion causes a vortex. If a viscoelastic fluid is stirred by a rotating rod it tends to climb the rod, which is known as the Weissenberg effect (Fig. 2.14). You might have observed this effect while mixing a cake batter or bread dough at home. This is due to the production of a normal force acting at right angles to the rotational forces, which in turn acts in a horizontal plane. The rotation tends to straighten out the polymer molecules in the direction of rotation but the molecules attempt to return to their original position.

When a Newtonian fluid emerges from a long, round tube into the air, the emerging jet will normally contract. It may expand to a diameter of 10% to 15% larger than the tube diameter at low Reynolds numbers. Normal stress differences present in a viscoelastic fluid, however, may cause jet expansion (called die swell) which are two or more times the diameter of the tube. In addition, highly elastic fluids may exhibit a tubeless siphon effect (Steffe, 1996).

Another phenomenon observed in the viscoelastic material is called the recoil phenomenon. When the flow of viscoelastic material is stopped, tensile forces in the fluid cause particles to move back. However, viscous fluids stay where they are when their motion is stopped (Steffe, 1996). This phenomenon is illustrated in Fig. 2.15.

There are three different methods to study viscoelastic materials: stress relaxation test, creep test, and dynamic test.

Viscoelastic Flow

Figure 2.15 Recoil phenomenon in viscous and viscoelastic fluids. [From Steffe, J.F. (1996). Rheological Methods in Food Process Engineering, 2nd ed. East Lansing, MI: Freeman Press (available at www.egr.msu.edu/~steffe/freebook/offer.html).]

Figure 2.15 Recoil phenomenon in viscous and viscoelastic fluids. [From Steffe, J.F. (1996). Rheological Methods in Food Process Engineering, 2nd ed. East Lansing, MI: Freeman Press (available at www.egr.msu.edu/~steffe/freebook/offer.html).]

2.5.1 Stress Relaxation Test

If food materials are deformed to a fixed strain and the strain is held constant, the stress required to maintain this strain decreases with time. This is called stress relaxation. In this test, stress is measured as a function of time as the material is subjected to a constant strain. This test can be conducted in shear, uniaxial tension, or uniaxial compression. Figure 2.16 shows stress relaxation curves for elastic, viscous, and viscoelastic materials.

As can be seen in Fig. 2.16, ideal viscous substances relax instantaneously but no relaxation is observed in ideal elastic materials. Viscoelastic materials relax gradually and stop depending on the

Viscoelastic Behavior
Figure 2.16 Stress relaxation curves for elastic, viscous and viscoelastic materials [From Steffe, J.F. (1996). Rheological Methods in Food Process Engineering, 2nd ed. East Lansing, MI: Freeman Press (available at www. egr.msu.edu/~steffe/freebook/offer.html).]

molecular structure of the material. Stress in viscoelastic solids will decay to an equilibrium stress ae, which is greater than zero but the residual stress in viscoelastic liquids is zero.

The stresses in liquids can relax more quickly than those in solids because of the higher mobility of liquid molecules. Relaxation time is very short for liquids, which is 1b-13 s for water while it is very long for elastic solids. For viscoelastic material, relaxation time is 1b-1-1b6 s (van Vliet, 1999).

In recent years, the stress relaxation test has been performed to study viscoelastic behavior of sago starch, wheat flour, and sago-wheat flour mixture (Zaidul, Karim, Manan, Azlan, Norulaini, & Omar, 2bb3), potato tuber (Blahovec, 2bb3), cooked potatoes (Kaur, Singh, Sodhi, & Gujral, 2bb2), wheat dough (Li, Dobraszczyk, & Schofield, 2bb3; Safari-Ardi & Phan-Thien, 1998), and osmotically dehydrated apples and bananas (Krokida, Karathanos, & Maroulis, 2bbb).

Studies on weak and strong wheat flour dough showed that stress relaxation tests at high strain values could differentiate dough from high-protein and low-protein wheat cultivars (Safari-Ardi & Phan-Thien, 1998). Stress relaxation measurements on wheat flour dough and gluten in shear showed that relaxation behavior of dough could be explained by two relaxation processes: a rapid relaxation over b.1 to 1b s and a slower relaxation occurring over 1b to 1b,bbb s (Bohlin & Carlson, 198b). The rapid relaxation process is related to small polymers that relax rapidly and longer relaxation time is associated with high molecular weight polymers found within gluten. Similarly, stress relaxation behavior of wheat dough, gluten, and gluten protein fraction obtained from biscuit flour showed two relaxation processes: a major peak at short times and a second peak at times longer than 1b s (Li et al., 2bb3). Many researchers showed that a slower relaxation time is associated with good baking quality (Bloksma, 199b; Wang & Sun, 2bb2).

2.5.2 Creep Test

If a constant load is applied to biological materials and if stresses are relatively large, the material will continue to deform with time. This is known as creep. In a creep test, an instantaneous constant stress is applied to the material and the resulting strain is measured as a function of time. There is a possibility of some recovery of the material when the stress is released as the material tries to return to its original shape.

Creep test can be performed in uniaxial tension or compression. The creep curves for elastic, viscous, and viscoelastic materials are shown in Fig. 2.17. For liquids, strain increases with time in a steady manner and the observed stress will be constant with time. Ideal viscous material shows no recovery since it is affected linearly with stress. Viscoelastic material such as bread dough shows partial recovery. They show a nonlinear response to strain since they have the ability to recover some structure by storing energy.

Results of a creep test are expressed as creep compliance (J = y/t). For ideally elastic materials, creep compliance is constant while it changes as a function of time for viscoelastic materials.

Recently, viscoelastic properties of wheat dough having different strengths were determined using a creep test (Edwards, Peressini, Dexter, & Mulvaney, 2bb1). A creep time of 1b,bbb s was enough to reach the steady-state flow for all of the dough with different strengths. When wheat flour dough was analyzed by a creep-recovery test, a maximum recovery strain of wheat dough was correlated to some of the parameters provided by using a mixograph, farinograph, and texture analyzer (Wang & Sun, 2bb2). When a creep recovery test was applied to biscuit dough, there was an increase in percentage of recovery as aging time was increased (Pederson, Kaack, Bergs0e, & Adler-Nissen, 2bb4). This shows that dough is becoming less extensible but more recoverable as aging increases. Maximum strain and recovery were strongly affected by wheat cultivar. The rheological measurements from creep test and

Viscoelatic Behaviour

Figure 2.17 Creep and recovery curves for elastic, viscous and viscoelastic materials. [From Steffe, J.F. (1996). Rheological Methods in Food Process Engineering, 2nd ed. East Lansing, MI: Freeman Press (available at www.egr.msu.edu/~steffe/freebook/offer.html).]

Figure 2.17 Creep and recovery curves for elastic, viscous and viscoelastic materials. [From Steffe, J.F. (1996). Rheological Methods in Food Process Engineering, 2nd ed. East Lansing, MI: Freeman Press (available at www.egr.msu.edu/~steffe/freebook/offer.html).]

oscillation tests showed that rice dough with 1.5% and 3.0% HPMC had similar rheological properties to that of wheat flour dough (Sivaramakrishnan, Senge, & Chattopadhyay, 2004).

2.5.3 Dynamic Test (Oscillatory Test)

In dynamic tests, either rate is controlled (stress is measured at a constant strain) or stress is controlled (deformation is measured at a constant stress amplitude). That is, materials are subjected to deformation or stress which varies harmonically with time. Usually, a sinusoidal strain is applied to the sample, causing some level of stress to be transmitted through the material. Then, the transmitted shear stress in the sample is measured (Fig. 2.18).

Concentric cylinder, cone, and plate or parallel viscometers are suitable for this purpose. This test is suitable for undisturbed viscoelastic materials as a function of time. Both elastic and viscous components can be obtained over a wide range of time. The main disadvantage of this test is that it can be used in the region in which stress is proportional with strain. Otherwise interpretation of data is hard. Moreover, the breakdown of structure of material may occur during the experiment (van Vliet, 1999).

The magnitude and the time lag (phase shift) of the transmission depend on the viscoelastic nature of the material. Much of the stress is transmitted in highly elastic materials while it is dissipated in frictional losses in highly viscous ones. The time lag is large for highly viscous materials but small for highly elastic materials.

A storage modulus (G) that is high for elastic materials and loss modulus (G) that is high for viscous materials are defined as follows: , T0 cos 0

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  • STEPHANIE
    Is cream viscoelastic fluid?
    1 year ago

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