(230ac) Gravitational Collapse of Colloidal Gels: Structure, Dynamics, and Rheology

Reconfigurable soft solids such as viscoelastic gels have emerged in the past decade as a promising material in numerous applications ranging from engineered tissue to drug delivery to injectable sensors. These include colloidal gels, which microscopically comprise a scaffoldlike network of interconnected particles embedded in a solvent. Network bonds can be permanent or reversible, depending on the nature and strength of interparticle attractions. When attractions are on the order of just a few kT, bonds easily rupture and reform. On a macroscopic scale, bond reversibility allows a gel to transition from solidlike behavior during storage, to liquidlike behavior during flow (e.g., injection or shear), and back to solidlike behavior in situ. On a microscopic scale, thermal fluctuations of the solvent are occasionally strong enough to break colloidal bonds, temporarily allowing particles to migrate and exchange neighbors before rebonding to the network, leading to structural evolution over time. In experiments, gel aging continues and the gel may remain stable for several hours, days, or weeks, but then the gel may suddenly undergo a catastrophic collapse, sedimenting to the bottom of the container and eliminating any intended functionality of the network scaffold. Although this phenomenon has been studied extensively in the experimental literature, the microscopic mechanism underlying the collapse transition is not well understood. To study this behavior, we conduct large-scale dynamic simulation to model the structural and rheological evolution of colloidal gels subjected to various gravitational stresses, examining the detailed micromechanics in three temporal regimes: slow sedimentation prior to collapse; rapid sedimentation after the onset of rapid collapse; and the final slow compaction of the material as it approaches its final bed height. We also examine in detail the effects of gel age (structural lengthscales), interparticle attraction strength, and wall effects on the collapse behavior.