Gels, weak

In practice, sedimentation is an important property of colloidal suspensions. In fonnulated products, sedimentation tends to be a problem and some products are shipped in the fonn of weak gels, to prevent settling. On the other hand, in applications such as water clarification, a rapid sedimentation of impurities is desirable. 

Sihcone polymers when cured into elastomers by themselves ate weak, gel-like materials. For this reason, fillers must be used to provide reinforcement. The type of fillers (qv) used in sihcone sealants varies widely two of the most common fillers are fumed sihca and calcium carbonate. 

Entanglement and weak gel formation are characteristic of some oilfield polysaccharides such as guar and starch, but are present only weakly, if at all, in both xanthan and succinoglycan solutions. Solutions of xanthan and succinoglycan are thus able to pass through porous media such as rock, while guar and starch cannot because of their gel-like nature. Hence the different uses of these polymers in the oilfield. 

Mixing different cross-linkers (19b and 19c) yielded systems with a strong gel-weak gel transition, rather than a distinct sol-gel shift. When the concentration of each of the cross-linkers was above the critical percolation threshold, the kinetically slower cross-linker (19c) dictated the gel properties. Upon addition of enough of a competitive binding additive, such as DMAP, to drop the concentration of the active cross-linking units below their individual percolation thresholds but still allowing the total amount of both active cross-linkers (19b and 19c) to be above the percolation threshold results in a gel whose properties are now controlled by the kinetically faster cross-linker (19c). 

Fernandez et al. (2007) have characterized the rheological behavior of the mashed potatoes with added biopolymers using steady shear measurements. Fresh and frozen/thawed mashed potatoes present shear thinning with yield stress (Canet et al., 2005a), and dynamic shear data reveal weak gel-like behavior in potato purees (Alvarez et al., 2004). The effects are strongly... 

Figure 6.6 shows that helices of intact K-carrageenan form weak gels at sufficiently high concentrations (> 1%) even in a good solvent such as 0.1 M Nal (Chronakis et al., 1996 Borgstrom et al., 1998). 

Although the gelation properties of whey proteins are of great importance in many foods (Mulvihill, 1992) and it is possible to form a weak gel in creams by the formation of a continuous network of fat globules, most important milk gels are those involving casein micelles which can be made to form a gel matrix either by isoelectric precipitation (acid-induced gel) or by the action of a proteolytic enzyme (rennet-induced gel). Both gel types... 

Micronization is an optional process that helps to reproduce the fatty sensation afforded by polysaccharides. The 0.2-nm particles in Avicel (FMC, 1993) and the 1-5-nm particles in the rice mimetic (Pszczola, 1991) simulate lipid emulsion rheology as well as lipid oral sensations. The simulation mechanism implicates a weak gel structure and an expansive surface where a large volume of water is immobilized. 

It has been pointed out that the alkyl silicones of high R/Si ratio are oily liquids or weak gels when polymerized by heat alone, and that in order to get resinous and adhesive properties the R/Si ratio must be brought down to about 1.5 for a methyl silicone and about 1.0 for a butyl or benzyl silicone. 

Fig. 5. Schematic illustration of the fluid-to-gel transition observed for colloidal silica inks. The bottom graph is a plot of zeta potential as a function of pH for PEl-coated silica and bare silica microspheres suspended in water. The upper graph is a log-log plot of shear elastic modulus as a function of shear stress for concentrated silica gels of varying strength (o) denotes weak gel pH = 9.5 and ( ) denotes strong gel pH = 9.75 (Ref. 36).

Schenz, T.W. (1997). Using rheology of weak gels to improve fluid foods. Food Technol. 51(3), 83-85. 

All films prepared for contact angle studies were cast from an excess of dry polymer swollen and fluidized by a small quantity of added liquid (usually triple-distilled water, the last two distillations being in an allquartz apparatus). The pH of these weak gels at the initiation of film formation was always 5. All our surface films were gel-dried rather than sol-dried. Although swelling may not have reached equilibrium prior to film drying, the films were only about 1 mm thick and their surface properties, as determined by contact angle measurements, were always reproducible within 5°. 

Figure 3-47 Plunger-Cup Geometry for Determination of Modulus of Weak Gels (from Oakenflill etal., 1989).

Mleko, S. and Foegeding, E. A. 2000. pH induced aggregation and weak gel formation of whey protein polymers../. Food Sci. 65 139-143. 

The rheological properties of the complex network structure of tomato pastes may be assumed to be made up of two contributions (1) one network structure contributed by the solids phase, in proportion to 0s, and (2) another network structure contributed by the liquid (continuous phase), in proportion to 0i = 1 - 0s- However, the effective continuous phase is not the low-viscosity serum itself, but a highly viscous liquid that is an integral part of the tomato paste. Also implicit is that the solids fraction plays a major role in the structure of the TP samples, that is, it can be considered to be the structuring component. These assumptions are also in line with the weak gel behavior indicated by the dynamic shear data. 

Because of the wide variability in tomato pastes due to different cultivars and processing methods, the constants in the structure-based correlations would not be universally applicable. However, the correlations can be used to obtain order of magnitude estimates of the rheological properties of tomato pastes without extensive and expensive experimentation. The various constants are valid within the ranges of the variables from which they were derived. More importantly, it is our hope that studies will be conducted to make use of the concepts presented here wi th respect to the weak gel nature of tomato pastes, shift factors for Cox-Merz rule, and structure based (d>s and = 1 — s) relationships for correlating the rheological parameters. 

Oakenftill (1984) developed an extension of Equation 6.1 for estimating the size of junction zones in noncovalently cross-linked gels subject to the assumptions (Oakenfull, 1987) (1) The shear modulus can be obtained for very weak gels whose polymer concentration is very low and close to the gel threshold, that is, the polymer chains are at or near to maximum Gaussian behavior. (2) The formation of junction zones is an equilibrium process that is subject to the law of mass action. Oakenfull s expression for the modulus is (Oakenfull, 1984) ... 

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