Tuning colloidal gels by shear

Abstract

Using a powerful combination of experiments and simulations we demonstrate how the microstructure and its time evolution are linked with mechanical properties in a frustrated, out-of-equilibrium, particle gel under shear. An intermediate volume fraction colloid-polymer gel is used as a model system, allowing quantification of the interplay between interparticle attractions and shear forces. Rheometry, confocal microscopy and Brownian dynamics reveal that high shear rates, fully breaking the structure, lead after shear cessation to more homogeneous and stronger gels, whereas preshear at low rates creates largely heterogeneous weaker gels with reduced elasticity. We find that in comparison, thermal quenching cannot produce structural inhomogeneities under shear. We argue that external shear has strong implications on routes towards metastable equilibrium, and therefore gelation scenarios. Moreover, these results have strong implications for material design and industrial applications, such as mixing, processing and transport protocols coupled to the properties of the final material.

Colloidal gels tuned by oscillatory shear

“…In previous work it has been found that BD is able to successfully capture qualitatively the fundamental microstructural changes under steady shear flow and also after flow cessation, even though hydrodynamic interactions (HI) are not included. 22 In both experiments and BD simulations ( Fig. 1) the first maxima of G'' appears at strain amplitude of about 2.5%.…”

“…Attractions were introduced with the superposition of a modified Asakura-Osawa (AO) potential 44 .The potential was calculated to be the product of the osmotic pressure and the overlap volume which changes for each pair of particles. The attraction range of ξ = 0.1 and attraction strength of U dep (2R)= -20 k B T was set as in previous work 22 . Near the point of contact and for a distance of ξ g =0.01R, we modified the classic AO potential as to introduce a constant potential of -20 k B T. The choice to implement this modification was motivated so as to approach the experimental conditions by introducing a small amount of interparticle flexibility, without modifying the basic hard sphere algorithm.…”

“…53 The reason for this is that heterogeneity in quiescent gels depends on the attraction strength and exhibiting a maximum around the gelation point and decreases with increasing attraction strength inside the gel state towards some steady state value at high attractions. 7,22 Therefore clusters in relatively more homogenous and stronger attractive gels are expected to be compactifed more when, agitated by shear, larger heterogeneities are induced as the system is driven towards the thermodynamic equilibrium i.e. phase separation.…”

“…On the other hand lower attractive strength gels, already heterogeneous in nature, do not experience significant cluster compactification (or further increase in their heterogeneity) under shear and therefore their elasticity is not affected that much after flow cessation. Moreover lower attraction gels are expected to exhibit stronger aging due to Brownian activated microstructural coarsening under quiescent conditions 7,22,54,55 than strong gels in which Brownian motion is entirely frozen; therefore in the latter shear-induced microstructural changes are more important (an effect of shear induced overaging) than in the former. This finding is in agreement with arrested phase separation mechanism as stronger gels well inside the gel boundary are essentially quenched further away from thermodynamic equilibrium and thus might be assisted by shear to move towards a more stable structure closer to a phase separated system.…”

Superposition rheology and anisotropy in rheological properties of sheared colloidal gels

“…The open symbols show the evolution of the storage moduli parallel to the previous flow direction, indicated with the subscript h. Such time evolution after cessation of flow of what are typically called "the" moduli was reported for many types of colloidal gels, with an increase of orders of magnitude in storage modulus [37,[53][54][55][56][57]. In comparison, the loss moduli are far less affected by flow.…”