Rheological and mechanical properties of silica colloids: from Newtonian liquid to brittle behaviour

Abstract

Rheological and mechanical properties of aqueous mono-disperse silica suspensions (Ludox® HS40) are investigated as a function of particle volume fraction (φ p ranging from 0.22 to 0.51) and water content, using shear rate tests, oscillatory methods, indentation and an ultrasonic technique. As the samples are progressively dried, four regimes are identified; they are related to the increasing particle content and the existence and behaviour of the electrical double layer (EDL) around each particle. For 0.22 ≤ φ p ≤ 0.30), the suspensions are stable due to the strong electrostatic repulsion between particles and show Newtonian behaviour (I). As water is removed, the solution pH decreases and the ionic strength increases. The EDL thickness therefore slowly decreases, and screening of the electrostatic repulsion increases. For 0.31 ≤ φ p ≤ 0.35, the suspensions become turbid and exhibit viscoelastic (VE) shear thinning behaviour (II), as they progressively flocculate. For 0.35 ≤ φ p ≤ 0.47, the suspensions turn transparent again and paste-like, with VE shear thinning behaviour and high elastic modulus (III). At higher particle concentration, the suspensions undergo a glass transition and behave as an elastic brittle solid (IV, φ p = 0.51).

Surface patterns in drying films of silica colloidal dispersions

“…Results are shown in figure 7. Our results on the HS sample are consistent with values available in the literature 37 . Note that, for all studied samples, the loss modulus (G (ω)) appears to be at least one order of magnitude lower than G 0 .…”

Drying colloidal systems: Laboratory models for a wide range of applications

“…This transition toward a gel phase can also be investigated in the case of charged particles without solvent removal but by tuning the interparticle forces through the addition of ionic species to the dispersion [75]. This solidification process is accompanied by complex changes in the rheological behaviour of the dispersions [4]. After a slow increase, the viscosity, η, exhibits a sharp increase and finally diverges ( fig.…”

“…Accounting for these effects is possible if the full dependence of the equation of state Π(φ, c s , Z) is used in eq. (4). In this case the colloid flux is not −D(φ)∇φ anymore.…”

“…In particular polystyrene particles exhibit a specific glass transition temperature, that is estimated from the particle core composition; this property governs the mechanical properties of the particles. The choice of the colloidal system provides a medium tunable and well-defined rheological and mechanical properties [4]. During the solvent evaporation, that is limited by diffusion of water in the air, the density of particles increases until the formation of a close-packed network of particles whose pores are still filled with solvent, e.g.…”

“…Aggregation or repulsive interaction result in different particles network depending on both the system and the evaporation rate. Adapted from Di Giuseppe et al [4]. nanoscopic structure of the film evolves and crosses the liquid-solid boundary as the medium follows an out-ofequilibrium route governed mainly by thermodynamics and transport phenomena ( fig.…”