Rheology Control of Colloidal Suspensions

该研究旨在提供一种新的流变控制技术，该技术可以独立地改变悬浮液的粘度水平和流动模式。普通絮凝悬浮液通常在较宽的剪切速率范围内发生剪切变薄，因为絮凝结构随着剪切速率的增加而逐渐被破坏。剪切变薄是由于胶体相互作用的性质，这导致了坚硬和脆弱的颗粒-颗粒键。为了控制流变性，必须开发一种新型的颗粒间相互作用，因此引入了聚合物桥接机制。由于聚合物链可以附着在表面的几个点上，并且不能同时从所有的位点脱附，因此聚合物桥接本质上是不可逆的。当聚合物链对颗粒表面的亲和力不很强时，通过可逆吸附在颗粒之间形成柔性桥接。由于聚合物桥是在静止状态下形成和断裂的，因此可逆桥絮凝的悬浮液在低剪切速率下是牛顿型的。高分子线圈絮凝后的悬浮液在高剪切场中表现出剪切增稠流动。桥梁在寿命期内快速拉伸所产生的力是造成剪切增厚的主要原因。提出了具有单一松弛时间的非线性粘弹性模型来定量描述剪切增稠流动。模型预测结果与实验数据吻合较好。将剪切增厚流动归因于柔性桥梁的熵弹性。通过改变聚合物的吸附亲和性来控制聚合物桥接絮凝悬浮液的流动。可逆桥接在控制胶体悬浮液的流变学方面具有很大的潜力。

The study is designed to provide a new technique of rheology control, in which the viscosity level and flow pattern of suspensions can be independently modified. Ordinary flocculated suspensions are generally shear-thinning over a wide range of shear rates, because the flocculated structure is progressively broken down with increasing shear rate. The shear- thinning profile is due to the nature of colloidal interactions, which lead to hard and fragile particle-particle bonds. For rheology control, a new type of interparticle interactions must be developed and hence the polymer bridging mechanism is introduced. Since a polymer chain may attach to the surface at several points and not be able to desorb simultaneously from all site, the polymer bridging is essentially irreversible, When the polymer chains do not have very strong affinity for the particle surface, a flexible bridge between particles is made by reversible adsorption. Since the polymer bridges are forming and breaking in a quiescent state, the suspensions flocculated by reversible bridging are Newtonian at low shear rates. The suspensions which are flocculated by polymer coils with comparable size to particles show shear-thickening flow in high shear fields. The force generated by rapid extension of bridges within the lifetime is responsible for the shear thickening. The nonlinear viscoelastic model with a single relaxation time is proposed to quantitatively describe the shear-thickening flow. The model prediction reasonably agrees with the experimental data. The shear-thickening flow is attributed to entropy elasticity of flexible bridges. The flow profiles of suspensions flocculated by polymer bridging are controlled by changing the adsorption affinity of polymers. The reversible bridging has great potential in controlling the rheology of colloidal suspensions.