Structural change and rheology of colloidal gel under flow

Abstract

Colloidal particles are important in many fields of science and industry, because the properties of materials such as elasticity or electric conductivity can be dramatically tuned by colloidal particles. Mostly, the change of the material properties is attributed to the distinctive internal structure, which is formed by colloidal particles. A main example is a colloidal gel system. At sufficiently high concentration, colloidal particles that interact through attractive forces organize a space-filling percolated network structure, which is called colloidal gel. The gelation occurs in a wide range of colloidal particle system. Colloidal gel is characterized by solid-like behavior. The solid-like elastic property arises from the disordered solid structure of the particles that are in the dynamically arrested state. Due to the formation of a microscopic network structure, colloidal gel shows a variety of complex properties that have not been fully understood yet. When large deformation or stress is applied, colloidal gel undergoes a rupture of the network structure and flows in a strongly non-Newtonian manner, displaying many non-linear rheological behaviors. The rupture of the network structure is known as yielding. Yielding takes forms of various structural changes such as rearrangement, bond rupture, cage breakage, and structural anisotropy. Many experimental and theoretical approaches, ranging from light scattering to computer simulation, have been conducted to figure out the yielding behavior of colloidal gel. However, in spite of all these efforts, yielding of colloidal gel still leaves a lot of questions to be answered, especially about its fundamental mechanism. The yielding behavior of colloidal gel which is represented by complex structural changes, especially the rupture of stress-bearing network structure, has an effect on the stress response of colloidal gel. The complex structural changes are manifested as nonlinear rheological behaviors, for example yield stress and non-linear creep compliances. Considering the close relationship between the microstructure and the rheological behavior, characterization of the structural change of colloidal gel is crucial in understanding the non-linear rheological behaviors. Therefore, the fundamental origins of non-linear rheological phenomenon have been commonly studied by investigating the coupling between the microstructural change and the rheological property. The coupling between the microstructural change and the rheological property changes has been main issue in rheology. Typically, the coupling between the microstructure and the rheology has been studied under the dynamic oscillatory shear flow and the start-up of shear flow. In experimental studies with these flow conditions, stress responses which are measured through rheometry have been correlated to the structural evolution that is analyzed through direct visualization and scattering methods. However, the analysis of the structural evolution is restricted to some specific conditions. In addition, there are many experimental difficulties, which make it hard to observe the structural changes. As an alternative to the limit of experimental study, theoretical study using particle simulation method can be suggested. Particle simulation has advantages over experiments to some extent, because all the information on the position of the particles and their interactions (such as forces and torques) can be correctly accessible. Among many simulation methods, the Brownian dynamics simulation has been widely used as a powerful tool to study colloidal particles system. The Brownian dynamics simulation has successfully depicted many important features of colloidal systems, such as colloidal gel and glass, which are dominated by particle interaction. This work aims to study the coupling between the microstructural change and the rheological behavior of the colloidal gel by using the Brownian dynamics simulation method. The microstructural evolution and the dynamics of the colloidal gel will be studied under the start-up shear and the dynamic oscillatory shear flow. Taking the advantage of the particle simulation method, which enables the investigation of all physical information on the particle position and dynamics, the structural change of colloidal gel will be investigated from various perspectives. The structural change and dynamics under various flow conditions will be analyzed through statistical mechanics and correlated to the rheological behavior. The result offers an insight into the yielding behavior of the colloidal gel and an interpretation on the relevant non-linear rheological behavior.