(4ct) Yield in Colloidal Gels Under the Start-up Shear Flow: Role of Hydrodynamic Interactions and Size Polydispersity

Colloidal gels are familiar in everyday materials such as foods and personal-care stuff, and they are also important in industries, leveraging biological and pharmaceutical materials. In these applications, one key property of colloidal gels is the transition from solid-like to liquid-like behavior when subjected to external stress. Such a so-called yield behavior was traditionally viewed as a one-step process. Applied stress ruptures the bonds between the particles and tears them apart from the particle network structure. However, recent studies revealed that gels can undergo yielding in two steps, depending on the particle volume fraction, bond strength, and flow strength. Despite the promising aspects of the optimized two-step yielding, the physical origins that result in primary and secondary yield in colloidal gels remain murky. Indeed, several studies proposed that the two-step yield may result from hydrodynamics, size polydispersity, heterogeneity, or a combination of all of these effects. My postdoctoral research work with Prof. Roseanna Zia is to understand the physical origins that result in two-step yield in colloidal gels.

To examine the role of hydrodynamics and size polydispersity, my research aims at: (1) understanding the role of full many-body hydrodynamic and lubrication interactions in the two-step yield in colloidal gels, (2) developing the computational model to incorporate size polydispersity, which is then utilized to explore the role of size polydispersity and its corresponding impact on hydrodynamics in the gelation and subsequent yield of binary colloidal suspensions.

With an understanding of the hydrodynamics in colloidal suspensions and the phase behavior of colloidal gels, I performed Massively Parallelized Accelerated Stokesian dynamics (MPASD) simulations. Our model accurately describes the large-scale particle network structure of colloidal gels where the particle dynamics are coupled by many-body hydrodynamic and lubrication interactions, along with interparticle attractions. MPASD simulations enable us to propose the role of lubrication interactions that result in secondary yield: Shear flow brings many particles close together, after which strong lubrication interactions play a role to make the particles hard to separate and correspondingly result in the pronounced stress peak at the secondary yield.

In a parallel study, I have developed the MPASD to incorporate size polydispersity by reformulating the many-body hydrodynamic interactions to include the particle radius of the finite-size quadrupoles and octopoles of the force moments. My understanding of polydisperse systems has evolved from my graduate research work under the supervision of Prof. Kyung Hyun Ahn (Seoul National University). I performed the Brownian dynamics simulations to understand the formation mechanism of a stratified layer composed of only small particles in the drying of bidisperse colloidal films. I examined the stress and microstructure development during the drying process and demonstrated that the physical origin of the formation of a stratified layer is related to the normal stress gradient in the film thickness direction. This experience, combined with the MPASD model, will enable me to explore open questions of the role of size polydispersity in two-step yield.

I will synthesize the findings on both the role of hydrodynamics and size polydispersity to interrogate gelation and subsequent yield of size-disparate binary suspensions. I believe that this body of work will provide an ultimate understanding of the role of hydrodynamics, size polydispersity, heterogeneity, and a combination of all of these effects. As a faculty member, I would continue using this computational model to establish a platform for studying how hydrodynamics and size polydispersity affect aggregation, flow, viscosity, structure, and rheological properties in suspensions. Moreover, I hope to change the landscape of the study of model colloidal gels by approaching realistic gel systems, such as suspensions of proteins and sticky biomolecules in biological, pharmaceutical, and industrial applications.

Teaching Interests:

My teaching interest focuses on the fundamental courses taken by chemical engineering undergraduate students, including transport phenomena and fluid mechanics. I am also interested in rheology, mathematics, and numerical methods for engineers. In addition, I am interested in developing a course that incorporates suspension mechanics, microhydrodynamics, and particle dynamics simulation tools at the graduate level.

My teaching philosophy is 1) creating supportive environments to help increase student engagement and 2) guiding the students on how to develop solutions to solve problems. My philosophy has been shaped by my own experiences as a Teaching Assistant (TA). I was a TA for an experimental laboratory class on fluid mechanics and helped students with their lab exercises. I found that students have diverse backgrounds and different levels of understanding, and it is important to provide supportive environments to identify their differences and know their needs. I strived to interact with students, answering their questions and discussing outcomes. This allowed me to know what kinds of knowledge they have and adjust my teaching style based on their needs, which I believe provides a better learning experience for students. I was also a TA of fluid mechanics and physical chemistry classes. As a TA, I have held many office hours to answer their questions and help them solve the problems. I asked key principles of the students to probe what they know and discussed how to work toward an answer with those principles. I believe it helped students to develop their own ability to solve problems, which is important when they explore new ideas, and as a result, become independent researchers.