Deformation regimes in soft materials under large amplitude oscillatory shear

Three-dimensional printing of advanced functional materials via direct ink writing requires the design and processing of soft solids or yield stress materials with high solids concentrations. The deformation, flow, and microstructure evolution of such complex soft materials are central in additive manufacturing extrusion technologies. In this study, we investigate both bulk properties and microscopic dynamics using rheo-microscopy, particle tracking, and birefringence mapping. This work presents an integrated framework that maps bulk rheology to local spatiotemporal heterogeneities during yielding of soft materials by combining oscillatory rheology experiments with simultaneous imaging. Using a Pluronic F127 hard gel matrix with increasing concentrations of silica fillers as a model for direct ink writing formulations, we quantify the evolution of deformation pathways under large-amplitude oscillatory shear tests with increasing concentration <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mo>(</a:mo> <a:mi mathvariant="normal">Φ</a:mi> <a:mo>)</a:mo> </a:math> . Mapping local events onto bulk rheological metrics unveils a consistent trend from a matrix-dominated to a filler-dominated deformation regime. At low concentrations <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mo>(</c:mo> <c:mi mathvariant="normal">Φ</c:mi> <c:mo>≲</c:mo> <c:mn>0.31</c:mn> <c:mo>)</c:mo> </c:mrow> </c:math> , the matrix dominates the yielding mechanism, leading to an affine-banding-fracture pathway. However, at high concentrations <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mo>(</e:mo> <e:mi mathvariant="normal">Φ</e:mi> <e:mo>></e:mo> <e:mn>0.31</e:mn> <e:mo>)</e:mo> </e:mrow> </e:math> , the filler-dominated system undergoes multiple local events under small deformations, from which cracks propagate and fracture the material into yielded and solid like regions. This work reconciles bulk and local observations in a flow regime map for particle suspensions in a gel matrix, highlighting when a soft material exhibits the desired affine flow and when it undergoes catastrophic failure. The results demonstrate that the deformation of highly concentrated composites is complex, where heterogeneous yielding and fracture coexist and negatively impact printability and shape fidelity. The experimental evidence presented in this work provides a timely avenue toward understanding the heterogeneous deformation and flow of complex soft materials, which play a critical role in a wide breadth of applications beyond 3D printing.