Solid-like rheology

Rheology is an important technique for looking at polymer properties and association characteristics. Gels are characterised by their solid-like rheology even though they comprise ca. 99 % immobilised liquid material. 

Solid-like rheological response arises due to frictional interactions between clay layers. 

Although aH these models provide a description of the rheological behavior of very dry foams, they do not adequately describe the behavior of foams that have more fluid in them. The shear modulus of wet foams must ultimately go to zero as the volume fraction of the bubbles decreases. The foam only attains a solid-like behavior when the bubbles are packed at a sufficiently large volume fraction that they begin to deform. In fact, it is the additional energy of the bubbles caused by their deformation that must lead to the development of a shear modulus. However, exactly how this modulus develops, and its dependence on the volume fraction of gas, is not fuHy understood. 

The mechanisms of static friction and stick-slip motion, as discussed in the last section, are supposed to be a good description of dry friction. Another case, perhaps more general in engineering practices, to be addressed in this section is lubricated sliding where liquid lubricant, consisting of a few molecule layers, is confined between two solid walls. Both experimental and theoretical studies indicate, as we have discussed in Chapter 5, that there are substantial changes in rheology of the confined lubricant, and the liquid may transit practically to a solid-like state when film thickness becomes molecularly thin [32,33]. 

Galgali and his colleagues [46] have also shown that the typical rheological response in nanocomposites arises from frictional interactions between the silicate layers and not from the immobilization of confined polymer chains between the silicate layers. They have also shown a dramatic decrease in the creep compliance for the PP-based nanocomposite with 9 wt% MMT. They showed a dramatic three orders of magnitude drop in the zero shear viscosity beyond the apparent yield stress, suggesting that the solid-like behavior in the quiescent state is a result of the percolated structure of the layered silicate. 

Semisolids dosage forms, as a class, are plastic in behavior, i.e., they retain their shape until acted upon by an outside force, in which case they deform and the deformations are permanent. The common denominator to all semisolid systems which gives them their special rheological character is that they all have a permanent three-dimensional structure. This structure is sufficient when undisturbed to impart solid-like properties but which is easily broken down and realigned under some strain or applied force (4). The semisolid systems used pharmaceutically include... 

In the definition of rheology there are two processes, deformation and flow. Deformation suggests the presence of solid-like behavior and flow suggests the presence of fluid-like behavior. Many foods have both solid and fluid properties. The objective of this section is to provide methods for the evaluation of the rheological properties of foods. In Chapter HI, flow properties of foods are the focus. In Chapter H2, deformation properties are the focus. 

The solid-like properties may be measured by non-destructive dynamic rheology analysis or by destructive methods using a Penetrometer, Jelly Tester, Instron instruments, or other types of texture analyzers. The latter methods are the most useful due to their simplicity and speed. Texture analysis of whippable emulsion must always be compared with the amount of air incorporated into the foam, which is known as percentage overrun and is calculated as follows ... 

Electrorheological (ER) fluids are materials whose rheological properties (viscosity, yield stress, shear modulus, etc.) can be readily controlled using an external electric field. For example, in some cases, they can switch from a liquid-like material to a solid-like material within a millisecond with the aid of an electric field, by means of the so-called ER effect.1617 The unique feature of the ER effect is that ER fluids can reversibly and continuously change from a liquid state to a solid state. ER fluid research is focused mainly on the automotive and robotics industry as electrical and mechanical interfaces for applications such as clutches, brakes, damping devices, fuel injection, and hydraulic valves. However, more recently, there is growing... 

It is easy to understand that these solutions must exhibit viscoelastic properties. Under shear flow the vesicles have to pass each other and, hence, they have to be deformed. On deformation, the distance of the lamellae is changed against the electrostatic forces between them and the lamellae leave their natural curvature. The macroscopic consequence is an elastic restoring force. If a small shear stress below the yield stress ery is applied, the vesicles cannot pass each other at all. The solution is only deformed elastically and behaves like Bingham s solid. This rheological behaviour is shown in Figure 3.35. which clearly reveals the yield stress value, beyond which the sample shows a quite low viscosity. 

For a viscoelastic solid (like an organogel), any rheological description should give a constant finite elastic modulus and infinite viscosity at zero frequency or long times. The situation is somewhat comparable to that of a cross-linked network [2. The equilibrium shear modulus for small deformations is proportional... 

These rheological parameters have been successfully correlated to textural attributes of hardness and spreadabUity and provide information pertaining to the fat crystal network (69). The value of G is useful in assessing the solid-like stmcture of the fat crystal network. Increases in the value of G typically correspond to a stronger network and a harder fat (66). Alternatively, G" represents the fluid-like behavior of the fat system. This value can be related to the spreadability of a fat system, because increases in G" indicate more fluid-like behavior under an applied shear stress. The tan 8 is the ratio of these two values. As the value of 5 approaches 0° (stress wave in phase with stress wave), the G" value approaches zero, and therefore, the sample behaves like an ideal solid and is referred to as perfectly elastic (68). As 8 approaches 90° (stress is completely out of phase relative to the strain). 

The data from small deformation rheology indicate that at higher cooling rates, the fat samples are more solid-like in nature. Conversely, the low cooling rate of 0.1°C/min results in a softer, less elastic system. 

Figure 5-7 shows the frequency dependences of the storage and loss moduli at various times during the reaction, from 6 minutes before G to 6 minutes after it. Note that at tc (labeled Gel Point in Fig. 5-7), G and G" follow power laws over the entire frequency range For times less than this (labeled —2 and —6 in Fig. 5-7), the curves slope downward at low frequencies, which is indicative of fluid-like behavior, while at times after the gel point (labeled -t-2 and - -6), G flattens at low frequency—a characteristic of solidlike behavior. Thus, the intermediate state with a power-law frequency dependence over the whole frequency range is the transitional state between liquid-like and solid-like behavior, and therefore it defines the gel point. This rheologically determined gel point coincides with the conventional value, namely the maximum degree of cure at which...

Suspensions or dispersions of particles in a liquid medium are ubiquitous. Blood, paint, ink, and cement are examples that hint at the diversity and technological importance of suspensions. Suspensions include drilling muds, foodstuffs, pharmaceuticals, ointments and cremes, and abrasive cleansers and are precursors of many manufactured goods, such as composites and ceramics. Control of the structure and flow properties of such suspensions is often vital to the commercial success of the product or of its manufacture. For example, in consumer products, such as toothpaste, the rheology of the suspension can often determine consumer satisfaction. In ceramic processing, dense suspensions are sometimes molded (Lange 1989) and then dried and sintered or fired into optical components, porcelin insulators, turbine blades, fuel cells, and bricks (Rice 1990 Simon 1993). Crucial to the success of the processing is the ability to transform a liquid, moldable suspension into a solid-like one that retains its shape when removed from the mold. These examples could be multiplied many times over. 

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