Slow flow and creep in soft deformable hydrogel particle packings
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Soft, low friction hydrogel packings display a range of interesting phenomena that make them an ideal model system to study the micro-macro link in particulate media. One feature observed in soft hydroel packings is “creep”, which is the slow deformation observed when a material is exposed to a constant force. Creep is commonly observed for many amorphous materials such as metals, colloidal systems and polymers. The slow motion of creep is usually thermally driven and challenging to understand, probe and control. We will describe how athermal, soft hydrogel suspensions display interesting creep behavior with easily quantifyable features, as probed with a sinking ball viscometer. We found that in the sinking ball viscometer tests, creep persists over large deformations and has a power law form, with diffusive dynamics. The creep amplitude is exponentially dependent on both applied stress and the concentration of hydrogel inside the solvent, suggesting that a competition between driving and confinement determines the dynamics [1]. Slow flows can also be mechanically imposed, and such flows induce creep as well, for example in the well-known split bottom geometry. The split-bottom constant flow studies reveal a similar confinement effect, suppressing the unnaturally large shear bands that emerge in hydrogel suspensions [2]. The observed scaling laws and pressure sensitivity provide a clear benchmark for new theory that explains creep and slow flows. We will connect these flow observations to particle-scale contact force measurements to suggest further work on building a microscopy-inspired micro-macro link for the common constitutive modelling approaches for slow granular flows.
