Onset of gravitational collapse of colloidal gels: an optical microscopy study
Colloidal gels are ubiquitous in daily life, such as paint. A colloidal gel can support its own weight for a finite delay time in gravity. This delay time sets the shelf life of many products. Hence, understanding the gravitational stability of colloidal gels could promote their industrial application. This work presents results on the mesoscopic scale (~ 102 μm) aiming to bridge macroscopic observations with the microscopic perspective in previous studies. In a model system of colloids + polymers, typical kinds of gel collapse were studied using fluorescence microscopy. Two collapse mechanisms are found for low and high colloidal volume fractions (φc), respectively. At low φc, denser debris falls through and breaks the gel structure. At high φc, solvent droplets rise to the top of a gel allowing the solvent to be expelled from the gel quickly. For gels with intermediate φc, these two mechanisms cooperate. Large-scale hydrodynamic remixing and recirculation are observed before the onset of gel collapse, emphasizing that hydrodynamics is crucial. The imaging results also suggest that the menisci of the samples play an important role of the gel collapse. By eliminating most of the curved meniscus, the collapse of gel can be postponed. Previous hypothesis for gel collapse is based on the competition between the yield stress of gel and gravitational stress. Our results on large and small particle gels show similar collapse behaviours both on macro- and meso-scopic scales. Considering that the yield stress varies sensitively with the size of the particle, this similarity suggests different mechanisms should be developed for the collapse of small and large particle gels.