On the Mechanics of Extremely Soft Solids: from Rheological Characteristics to Morphological Instabilities

Soft networks consisting of polymers with irreversible crosslinks produce solid-like gels whose real-world applications include tissue bandages, membranes for water purification, and soft actuators. Recently, there has been a growing interest in polymeric soft solids whose equilibrium shear moduli lie in the range of O(0.1-100) Pa. These extremely soft solids bear a natural connection to biological polymer networks and exhibit viscoelastic stress relaxation due to sparse crosslinking between the constituent polymers. To best harness the capabilities of these soft gels, a deeper understanding of the materials’ viscoelastic properties is required. However, uncovering the microstructural origin of viscoelasticity in networked gels is hindered by the stochastic and probabilistic nature of the network formation.



In our work, we prepared viscoelastic soft solids by crosslinking the end groups of polydimethylsiloxanes at stoichiometry approaching the critical gel point. With this material, we investigated two pattern-forming instability behaviors - one where gravity deforms a slab of soft solid [1] and the other where a soft solid is `extruded’ into a thin gap during which we observed a new kind of surface instability, which we termed the `furrowing instability’ [2]. We explain how the extreme deformability of the material and the rate dependence of the instability are connected to the rheology of the material. Finally, we will touch on the microstructural origin of viscoelasticity in sparsely cross-linked solids by considering the time-dependent relaxation of topological defects.