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

胶体加工在生产具有定制微结构的复杂陶瓷和复合材料方面已显示出其潜力。通过精心配方并结合增材制造（AM），胶体加工可以创建具有精确控制几何形状的致密或多孔结构，适用于从个性化医疗植入物和电化学储能系统到环境清理的各种应用。增材制造（AM）技术在过去的几十年里经历了巨大的发展，并且已经彻底改变了多功能复杂架构的快速原型制作和制造过程。机器铸造（一种增材制造技术，也称为直接墨水书写或3D打印）涉及通过精细喷嘴连续挤出胶体糊状物或凝胶，以逐层创建3D结构。油墨的精确配方对于确保“印刷适性”至关重要。有几个关键的性质，胶体浆料应通过精心配方和流变性控制获得。它们必须剪切变薄，以便以适度的剪切速率轻松流过喷嘴，然后在打印后立即设置为非流动结构。它们还必须能够支撑顶部的多层结构，并在不变形的情况下保持跨跨的形状。“可印刷性”仍然是一个广泛的概念，不同的领域有不同的标准来定义什么是可印刷性。通过结合拉伸、剪切和振荡流变学，这项工作旨在创建一个可量化的度量，这将有助于预测软材料的可印刷性。氧化石墨烯的二维胶体（GO，具有两亲性的二维薄片）表现为多功能添加剂，有助于打印各种材料。氧化石墨烯胶体表现出迷人的流变性，可以帮助加工不同的材料，以开发“可打印”的配方。氧化石墨烯胶体在水中形成相对低浓度的可打印网络，可以作为多功能添加剂，并有助于打印各种材料。由于氧化石墨烯独特的表面化学性质，它可以作为一种流变改性剂，即使在小浓度使用时，也可以赋予其他材料的悬浮液剪切变薄行为和剪切屈服应力。这项工作为氧化石墨烯悬浮液提供了深入的流变学研究，该悬浮液具有从类牛顿到粘弹性“可打印”软固体的广泛行为。拉伸和剪切流变学的结合揭示了氧化石墨烯浓度从<0.1 vol%增加到3 vol%时的网络形成过程。拉伸试验的结果表明，随着浓度的增加，氧化石墨烯如何从类似牛顿的液体（<0.1vol%）转变为微弱的（<0.8vol%），然后是牢固建立的结构网络。初步结果表明，“可打印性”的量化可以基于三个流变参数：通过存储模量（G'）计算的网络刚度，固-液过渡或流动应力（of），以及流动过渡指数（FTI=of/oy），该指数与流动和屈服应力有关。

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).