Rheological Study of Particle-Laden Fluid Interfaces with a Custom-Built Rheometer

Abstract

Understanding the dynamics and mechanical properties of complex fluid-fluid interfaces is of great importance in various industrial applications such as food processing, drug delivery, and coating. In this thesis, my research efforts focus on studying the interfacial rheology of particle-laden fluid interfaces with a custom-built device. Despite the high demand for the characterization of interfacial rheology in academic and industrial research, the study of interfacial rheology is still scarce compared to its counterpart of bulk rheometry and is limited only to specialized laboratories. One of biggest hurdles impeding the broad application of interfacial rheometry is the delicate design and the high cost of interfacial rheometers. In the first part of the thesis, a novel apparatus of the in-situ interfacial rheometer would be introduced. The simple design substantially reduces its dimension, making the new device highly portable and cost-effective for any laboratories that have access to optical microscopes for a wide range of interfacial rheology studies. Using the newly-developed interfacial rheometer, we study the rheology and microstructure of particle-laden fluid interfaces. In particular, the possibility of using Janus particles to tune the rheological response of particle-laden fluid interfaces is explored. Janus particles are heterogeneous colloids composed of two or more distinct regions with different surface properties. We find that the addition of a small amount of Janus particles can lead to a significant increase in surface moduli with enhanced elasticity, which greatly improves the stability of the interface. This drastic change in interfacial rheology is associated with the formation of local particle clusters surrounding each Janus particle. The origin of particle clusters is explained by considering the interparticle interactions at the interface. These experiments demonstrate a new way to tune the microstructure and mechanical properties of fluid-fluid interfaces. We then relate the microscopic dynamics of those particle clusters to their profound effects on the macroscopic rheology of particle-laden interfaces. By analyzing the local affine deformation of particles, we show that particles in those localized clusters experience substantially lower shear-induced stretching than their neighbors outside clusters. Such heterogeneous dynamics increase the effective surface coverage of particles, which in turn enhance the moduli of the interface, consistent with direct interfacial rheological measurements. Our findings open up an avenue for designing interfacial materials with improved mechanical properties via the control of the formation of localized particle clusters.