The relationship between the compressional and shear strengths of poroelastic colloidal gels

Strong particulate gels are widely believed to behave poroelastically in compression, e.g. in sedimentation, even though they consolidate irreversibly because of the stickiness of the particles. Particulate gels are usually adhesive as well as cohesive and so wall effects are to be expected in general [Michaels & Bolger (1962)]. These are rarely manifest on process engineering scales, although they can be important in the laboratory and with formulated products in small containers. When the assumption of non-linear poroelasticity is combined with the idea that adhesive failure (yield) is brittle, relatively speaking, a prescription emerges for the ratio of shear to compressive strength S and how this varies with density. S is predicted to be of order unity at the gel-point and then to increase rapidly thereafter. The predictions are consistent with the experimental data available for both dilute and concentrated gels, although more data would be welcome. A critique of a recent paper by Condre et al. (2007) on wall effects in very dilute gels is given in which it is argued, amongst other things, that it is not necessary to invoke granular in order to account for their results, simple adhesion suffices.