Rheology Control of Colloidal Suspensions

该研究旨在提供一种新的流变控制技术，其中悬浮液的粘度水平和流动模式可以独立地修改。普通絮凝悬浮液通常在宽范围的剪切速率下剪切稀化，因为絮凝结构随着剪切速率的增加而逐渐分解。剪切稀化曲线是由于胶体相互作用的性质，其导致硬的和脆弱的颗粒-颗粒键。对于流变控制，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。由于聚合物链可以在几个点处附着到表面并且不能从所有位点同时解吸，因此聚合物桥接基本上是不可逆的。当聚合物链对颗粒表面不具有非常强的亲和力时，通过可逆吸附在颗粒之间形成柔性桥。由于聚合物桥在静止状态下形成和断裂，因此通过可逆桥连絮凝的悬浮液在低剪切速率下是牛顿的。与颗粒尺寸相当的聚合物线团絮凝的悬浮液在高剪切场中表现出剪切增稠流动。桥梁在使用寿命内的快速延伸所产生的力是剪切增稠的原因。提出了一个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。剪切增稠流动归因于柔性桥梁的熵弹性。通过改变聚合物的吸附亲合力来控制聚合物架桥絮凝悬浮液的流动剖面。可逆桥连在控制胶体悬浮液的流变性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.