Printing low-viscosity filaments in microgel suspensions

Embedded three-dimensional (3D) printing is a direct-ink-writing technique that enables fabrication of complex 3D structures by the deposition of ink in a bath of yield stress fluid. Due to its solid-like resistance to flow under small stresses, the yield stress fluid retains the structure and the position of the printed filaments, thus allowing for the use of runny ink materials. We investigate the printability of weakly viscous, water-based Newtonian inks in baths of dense aqueous microgel suspensions. Far less viscous than typical polymer- or colloid-based ink materials, these Newtonian inks readily penetrate the fluidized region of the yield stress fluid behind the nozzle, making it challenging to control the shape of printed structures. We find that a higher concentration of the microgel suspension results in more unstable deposition of the inks that often leads to the formation of finger-like patterns, despite the stronger resistance of the bath fluid to flow. We explore how this instability may be correlated with the transient rheological responses of the bath fluid.