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

Rheological and mechanical properties of aqueous mono-disperse silica suspensions (LudoxA (R) HS40) are investigated as a function of particle volume fraction (I center dot (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 a parts per thousand currency signaEuro parts per thousand I center dot (p) a parts per thousand currency signaEuro parts per thousand 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 a parts per thousand currency signaEuro parts per thousand I center dot (p) a parts per thousand currency signaEuro parts per thousand 0.35, the suspensions become turbid and exhibit viscoelastic (VE) shear thinning behaviour (II), as they progressively flocculate. For 0.35 a parts per thousand currency signaEuro parts per thousand I center dot (p) a parts per thousand currency signaEuro parts per thousand 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, I center dot (p) = 0.51).