Capillary Forces Drive Deformation and Break-up of Soft Elastic Beams

Concentrated packings of highly swollen granular hydrogels, commonly referred to as microgels, undergo a jamming-like transition and exhibit solid-like behavior under small strains. The rheological properties of these packed microgels, including the yield stress and elastic shear modulus, can be tuned over several orders of magnitude through small changes in the overall polymer concentration. Recently, these packed microgels have been utilized in the 3D-printing community as a sacrificial support bath to provide mechanical stability to fluid phases as they are structured into complex geometries and solidify into soft elastic solids. However, capillary forces acting at the interface between immiscible pairs of printed material and support bath can lead to destabilizing instabilities, resulting in the break-up of the fluid phase prior to solidification or deformations of the soft solids after solidification. Understanding and controlling these instabilities is crucial for guiding the manufacturing of soft materials with low moduli and yield stresses. Here, we use packed granular microgels swollen in aqueous and organic solvents to explore the interfacial instabilities of 3D-printed fluid and soft elastic microbeams within immiscible support baths. By leveraging the highly tunable rheological properties of these packed microgel systems and the control offered by 3D-printing, we systematically explore the stability of fluid and soft elastic microbeams across a range of yield stresses and beam diameters. For fluid beams within immiscible support materials, we find the stability can be predicted through the interfacial tension and the feature size of the beam. For soft elastic beams printed within viscous fluid support baths we observe the emergence of new instabilities when the surface stresses are comparable to the yield stress of the microbeam. We observe the microbeams axially contracting under capillary forces resulting in the coiling of the ends and spontaneous buckling of the center of the beam.