Decoupling dynamics from the rheology of surface-anisotropic colloids

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Colloids consist of micron-sized particles dispersed in a fluid. Due to the small size of the particles, their motion is affected by collisions with the fluid molecules. Colloids are found in a broad range of engineered products, such as pharmaceuticals, dairy foods, and oil slurries. Despite their ubiquity, traditional engineering approaches have not accounted for the fact that these particles possess rough surfaces. This difference in shape from a perfect, idealized, smooth sphere does not cause problems when the particle loading is low. However, when such particles are packed more closely, large disagreements between experimental data and theories are found. These deviations have hampered the engineering of processing flow conditions for suspensions containing rough particles that generate frictional interactions due to solid-solid contact. The goal of this project is to investigate the effect of colloidal surface roughness on a variety of flow phenomena. Connections between dynamics, microstructure, and properties of the suspensions will be made using a specially constructed apparatus mounted on a confocal microscope. The results from this work will lead to design rules for industry for the use of colloid particles with rough or irregular surfaces in their products. In addition, knowledge gained from this project will be used in outreach activities that utilize social media for STEM education. A series of Reddit interactive outreach activities that present complex scientific concepts about the flow and texture of materials in an interesting and digestible way will be made available to the general public. The goal is to capture the imagination of the nation's younger generation and to guide them towards the wonders and discoveries present in today's science and engineering careers.This award aims to understand the microscopic dynamics of model polymeric colloids with well-controlled surface anisotropy. The central hypothesis is that the translational and rotational diffusion of colloids with various surface features becomes independently decoupled at high particle loadings. The decoupling of dynamics leads to significant departures in the linear and nonlinear rheology from current theoretical predictions. This work will generate new physical understanding of how particle diffusivity affects the linear and nonlinear rheology of concentrated suspensions. In this regime, fluid suspensions become increasingly glassy in their mechanical properties. The fundamental rationale behind these phenomena is still not well understood, particularly when particles make solid-solid contacts with one another. This work will investigate the microscopic mechanisms by imaging smooth and rough colloids flowing within a fast confocal rheometer setup designed to concurrently capture rotational dynamics during shear flow. Brownian Dynamics simulations with short-range hydrodynamic interactions will be incorporated to support experimental observations of dynamics and connections to glassy rheology.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.