Laponite gels - visco-elasto-plastic analogues for geological laboratory modelling

Abstract

Laponite® is a synthetic clay that, depending on concentration, temperature and curing time, forms a clear, transparent thixotropic fluid or brittle visco-elasto-plastic gel when mixed with water. Here we present the results of rheological and mechanical testing of gel-forming Laponite RD (LRD) to evaluate its suitability as a rock analogue in laboratory analogue experiments. Rheological tests of 2–4 wt% concentrations of LRD in deionised water were carried out at temperatures between 20 and 50 °C, and after curing times of 3 to 14 days. Our results show that LRD gels change from a brittle, elastic-dominant, linear viscoelastic material to a plastic material as shear strain increases. The linear viscoelastic region occurs at shear strains, γ < 10% after which the material yields and then undergoes strain hardening before a peak stress occurs at γ = 15–20%. LRD then strain softens up to γ < 26.2%, beyond which it behaves as a plastic material. Empirical equations are provided that predict increases in the Young's and complex shear moduli of LRD with increasing concentration and ageing time. LRD can be used to model elastic deformation when γ < 10% at a shear strain rate of 0.1 s−1 and plastic deformation when γ > 26.2%. LRD is an ideal material for modelling the behaviour of rocks during the emplacement of magma and the propagation of brittle fractures in the upper crust. Its ease of preparation, low surface tension, full transparency, chemical and biological stability and photoelastic properties provide further advantages for analogue laboratory modelling compared to other frequently used visco-elastic gels, such as pig skin gelatine.