Yield stress and wall slip phenomena in colloidal silica gels

Abstract

Evidence of wall slip and magnitude of yield stress are examined for colloidal gels consisting of hydrophobic silica, polyether, and lithium salts using geometries with serrated, smooth, hydrophilic and hydrophobic surfaces. Serrated plates, which provide minimal wall slip, are used to compare different methods of measuring yield stress: conventional extrapolation of shear stress in steady shear experiments and dynamic experiments at large strain amplitudes. In the latter, the yield stress is denoted by the maximum in the elastic stress, the product of the elastic modulus and strain (G Ј ␥), when plotted as a function of strain amplitude. Although excellent agreement is observed in the yield stress values using both these techniques, the dynamic method seems preferable considering its experimental ease, accuracy, and lack of extrapolation. In the presence of smooth geometries, the silica gels show evidence of wall slip with a concomitant decrease in yield stress. Using underestimation of yield stress as a measure of wall slip, we find slip to be unaffected by changes in the gel modulus obtained through incorporation of additional silica or salts. The use of smooth surfaces compared to serrated surfaces leads to approximately a 60% reduction in yield stress for all such samples. Finally, control of wall slip is attempted using plates modified to have different surface energies. Hydrophobic plates reduce slip significantly and produce data comparable to those with the serrated plates. In contrast, hydrophilic plates have minimal effect on slip and produce data analogous to those obtained using smooth plates. These results can be explained based on the fact that the particle-lean layer, responsible for slip, remains so with hydrophilic plates as it repels the hydrophobic silica particles in favor of the polar solvent. In contrast, the hydrophobic silica interacts with the hydrophobic plates, thus reducing slip.

Rheology of Concentrated Tomato-Derived Suspensions: Effects of Particle Characteristics

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Rheological Characterization of Hydrogen Peroxide Gel Propellant

“…However, in the shear rate range of 1 to 1000 s-1, the gel apparent viscosity decreases as the shear rate increases, but no significant effect of varying the ambient temperature on the apparent viscosity was observed. Figure 2 and 3 show that as soon as the H 2 O 2 -SiO 2 gel is disturbed at a very low shear rate (less than 2 s -1 ), the flow is initiated with some yield stress [9] (in the case of the 1 to 20 s-1 shear rate range (Table 3)), and a further increase in shear rate leads to thinning of gel. The initial apparent viscosity of gel with some yield stress for H 2 O 2 -SiO 2 gel decreases with increase in test temperature (the curve shifts downwards), which is possibly due to rapid breaking or weakening of gel network, at elevated temperatures from 283.15 to 303.15 K (Fig.…”

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“…This is especially relevant when dealing with nanoparticle formulations on a technical scale where high particle loading is required [Willenbacher et al (2011)]. When these suspensions are forced to flow through fine geometries, the presence of nearby walls and surfaces may induce additional interactions or wall slip [Walls et al (2003); Kalyon (2005); Ballesta et al (2012)], which can lead to nonuniform flows and clogging [Wyss et al (2006)]. …”