Particulate gels

A particulate gel breaker for acid fracturing for gels crosslinked with titanium or zirconium compounds is composed of complexing materials such as fluoride, phosphate, sulfate anions, and multicarboxylated compounds. The particles are coated with a water-insoluble resin coating, which reduces the rate of release of the breaker materials of the particles so that the viscosity of the gel is reduced at a retarded rate [205]. 

The dynamic viscoelasticity of particulate gels of silicone gel and lightly doped poly-p-phenylene (PPP) particles has been studied under ac excitation [55]. The influence of the dielectric constant of the PPP particles has been investigated in detail. It is well known that the dielectric constant varies with the frequency of the applied field, the content of doping, or the measured temperature. In Fig. 11 is displayed the relationship between an increase in shear modulus induced by ac excitation of 0.4kV/mm and the dielectric constant of PPP particles, which was varied by changing the frequency of the applied field. AG increases with s2 and then reaches a constant value. Although the composite gel of PPP particles has dc conductivity, the viscoelastic behavior of the gel in an electric field is qualitatively explained by the model in Sect. 4.2.1, in which the effect of dc conductivity is neglected. 

Figure 2.1 Scanning electron micrographs of Fe -induced cold-set gels of P-lactoglobulin filamentous gel (top) and particulate gel (bottom). The gel microstructure depends on the iron/protein ratio. At low iron/protein ratios, a homogeneous filamentous network is obtained, whereas at high iron/protein ratios an aggregated particle gel is produced. Reproduced from Chen et al (2006a) with permission.

Eiser et al. (2009) have recently reported on the pH-induced transformation of a hard-boiled egg from a white, brittle particulate gel to a transparent, elastic polymer gel (see Figure 6.20). Eggs were incubated in their hard protective shells for up to 26 days in a strong alkaline solution (0.9 M NaOH + 0.5 M NaCl, pH 12) at room temperature. These harsh experimental conditions are apparently rather similar to those used in a traditional Chinese method developed over two thousand years ago as a way of preserving eggs so that they would remain safely edible for many months. 

Cold-set whey protein gels obtained by addition of calcium ions to preheated whey proteins have been used to deliver iron (Remondetto et al. 2002). By modulating the conditions of formation, gels with different microstructures (particulate or filamentous) were formed with different encapsulating properties. Filamentous whey protein gels were more efficient than particulate gels in delivering bioavailable iron to the intestine, as less iron was released at acidic but more at alkaline pH (Remondetto et al. 2004). 

This chapter is devoted to the properties of polymeric gel-forming liquids. Particulate gels are discussed in Chapter 7. The structure of a polymeric gel is sketched in Fig. 5-1. Since this book is devoted to materials that are in some sense liquid, or at least liquefiable, we shall not say much about hard, irreversible, chemical gels such as cured epoxies or vulcanized rubber, but shall focus instead on chemical pre-gels and thermally reversible physical gels, both of which can be considered borderline fluids. This chapter is confined to a brief overview. Much more detail can be found in Winter and Mours (1997), and volume 101 of the Faraday Discussions. ... 

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