Real space analysis of colloidal gels: triumphs, challenges and future directions

Abstract

Colloidal gels constitute an important class of materials found in many contexts and with a wide range of applications. Yet as matter far from equilibrium, gels exhibit a variety of time-dependent behaviours, which can be perplexing, such as an increase in strength prior to catastrophic failure. Remarkably, such complex phenomena are faithfully captured by an extremely simple model—‘sticky spheres’. Here we review progress in our understanding of colloidal gels made through the use of real space analysis and particle resolved studies. We consider the challenges of obtaining a suitable experimental system where the refractive index and density of the colloidal particles is matched to that of the solvent. We review work to obtain a particle-level mechanism for rigidity in gels and the evolution of our understanding of time-dependent behaviour, from early-time aggregation to ageing, before considering the response of colloidal gels to deformation and then move on to more complex systems of anisotropic particles and mixtures. Finally we note some more exotic materials with similar properties.

Protein-Polymer Mixtures in the Colloid Limit: Aggregation, Sedimentation and Crystallization

“…Note that in depletion systems, aggregation and gelation are identified with the liquid-gas phase boundary. 40,41,77 Thus while these are non-equilibrium states, comparison with equilibrium phase behaviour is nevertheless highly informative. For lower protein volume fraction below we tested, a dotted line is drawn based on the intuition from literature.…”

“…27,67,[69][70][71][72][73] In colloidal systems, an effective attraction between the particles can be induced by adding non-adsorbing polymers. 74,75 This can lead to gelation [75][76][77][78] and enhanced crystallization rates. 48,49 Although they are often smaller than colloids, polymers are typically rather larger than proteins, 33,79 leading to the concept of the protein limit, [80][81][82] where the polymers are so much larger than the proteins that the relevant lengthscale is the intra-polymer persistence length, rather than the polymer radius of gyration that is typically considered in the case of colloid-polymer mixtures.…”

Direct Imaging of Contacts and Forces in Colloidal Gels

“…An important class of soft solids is colloidal gels 8 , which are encountered in numerous foods 9 , cosmetics, coatings, crop protection suspensions and pharmaceutical formulations. In addition to colloidal systems, a wide range of materials also exhibit gelation including proteins 10,11 phase-demixing oxides 12 , and metallic glassformers 13 .…”

“…A promising way to address phenomena such as gel failure is to use particle-resolved studies where the coordinates of individual particles are tracked 8,16 . In soft amorphous solids such as colloidal glasses, this technique has been used to image local re-arrangements of colloidal particles which may be precursors to large-scale failure 17,18 .…”

“…In colloidal gels, particle resolved studies have revealed the rich nature of their local structure [19][20][21][22][23] . So far, while investigations of gel failure have related yielding to local crystallization 24 and ingenious combinations of rheological methods and simulation and scattering have revealed the role of local plasticity 25 and bulk two-point structure 26 , direct imaging of particle rearrangements have largely focussed on colloidal glasses 8,17,18 rather than gels 27 .…”