Micro-mechanical insights into the stress transmission in strongly aggregating colloidal gel

Abstract

Predicting the mechanical response of the soft gel materials under external deformation is of paramount importance in many areas, such as foods, pharmaceuticals, solid-liquid separations, cosmetics, aerogels and drug delivery. Most of the understanding of the elasticity of gel materials is based on the concept of fractal scaling with very little microscopic insights. Previous experimental observations strongly suggest that the gel material loses the fractal correlations upon deformation and the range of packing fraction up to which the fractal scaling can be applied is very limited. Also, correctly implementing the fractal modeling requires identifying the elastic backbone, which is a formidable task. So far, there is no clear understanding of the gel elasticity at high packing fraction and the correct length scale that governs its mechanical response. In this work, we undertake extensive numerical simulations to elucidate the different aspects of stress transmission in the gel materials. We observe the existence of two percolating networks of compressive and tensile normal forces close to the gel point. We also find that the probability distribution for the compressive and tensile part normalized by their respective mean shows universal behavior irrespective of varied interaction potential, thermal energy and particle size distribution. Interestingly, there are also large number of contacts with zero normal force, and consequently, a peak in the normal force distribution is observed at $f_n\approx0$ even at higher pressure. We also identify the critical internal state parameters such as mean normal force, force anisotropies and the average coordination number and propose simple constitutive relations that relate the different components of the stress to the internal state parameters. The agreement between our model prediction and the simulation observation is excellent.