Designing rheology protocols to quantify the 'printability' of soft-materials

Colloidal processing has demonstrated its potential in producing complex ceramics and composites with tailored microstructures. Through careful formulation and in conjunction with additive manufacturing (AM), colloidal processing can create dense or porous structures with precisely controlled geometries for applications that range from personalised medical implants and electrochemical energy storage systems to environmental clean-up. Additive manufacturing (AM) techniques have undergone enormous growth in the past couple of decades and have already revolutionised the process of rapid prototyping and manufacture of multifunctional, complex architectures. Robocasting (an AM technique also known as direct ink writing or 3D printing) involves continuous extrusion of colloidal pastes or gels through a fine nozzle to create 3D structures layer-by-layer. Precise formulation of the inks is critical in order to ensure "printability." There are several key properties that colloidal slurries should obtain through careful formulation and control of rheology. They must be shear thinning in order to easily flow through the nozzle at modest shear rates and then immediately set into a non-flowing structure once printed. They must also be able to support multiple layers on top and retain the shape across spans without deformation. "Printability" is still a broad concept and different fields have different sets of criteria as to what they define as printable. By combing extensional, shear and oscillatory rheology this work aims to create a quantifiable metric that will help predict the printability of soft materials.
2D colloids of graphene oxide - (GO, 2D flakes with amphiphilic nature) - behave as multifunctional additives and aid the printing of a wide range of materials. GO colloids exhibit a fascinating rheology and can aid the processing of different materials to develop 'printable' formulations. GO colloids in water form printable networks at relatively low concentrations and can behave as multifunctional additives and aid the printing of a wide range of materials. Due to unique surface chemistry GO, can be used to act as a rheology modifier that imparts a shear thinning behaviour and shear yield stress to suspensions of other materials even when is used in small concentrations. This work provides an in-depth rheological study of GO suspensions with a wide range of behaviours from Newtonian-like to viscoelastic 'printable' soft solids. The combination of extensional and shear rheology reveals the network formation process as GO concentration increases from <0.1 vol% to 3 vol%. Results from the extensional tests showed how the GO transitions from a Newtonian-like liquid (<0.1vol%) to weakly (<0.8vol%) and then strongly-established structural network as concentration is increased. Initial results demonstrate that the quantification of 'printability' can be based on three rheology parameters: the stiffness of the network via the storage modulus (G'), the solid-to-liquid transition or flow stress (of), and the flow transition index, which relates the flow and yield stresses (FTI=of/oy).