Creep and slow flow in athermal soft hydrogel suspensions

The slow deformation observed when a material is exposed to a constant force is called "creep". 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 displays interesting creep and slow flow behavior with easily quantifyable features. We will highlight two examples. We probed the creep properties of packings of athermal soft spheres with a sinking ball viscometer and the well-known split bottom geometry. 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. The split-bottom constant flow studies reveal a similar confinement effect, suppressing the unnaturally large shear bands that emerge in hydrogel suspensions. The observed scaling laws and flow dynamics provide a clear benchmark for new theory that explains slow flow and creep, and our work provides the tantalizing prospect that creep can be controlled by a boundary stress.