Date on Master's Thesis/Doctoral Dissertation
Mechanical Engineering, PhD
Committee Co-Chair (if applicable)
colloids; hydrodynamic interactions; self assembly; pair-wise interaction; long-term suspension; microgravity
Within a colloidal suspension gravity may compromise the observation of governing physical interactions, especially those that are weak and/or take significant time to develop. Conducting the experiment in a long-term microgravity environment is a viable option to negate gravitational effects, though significant resources are required to do so. While it may not be possible to simulate long-term microgravity terrestrially, particles can resist quick sedimentation in a confined suspension system rotating vertically with appropriate rotation speed. The goal of the investigation is to demonstrate the existence of long-term particle suspension regime for a certain colloidal suspension while characterizing colloidal behavior due to hydrodynamic interactions. First, to understand the colloidal suspension in a rotational system, I studied the colloidal behavior in such a system where colloidal particles and underlying surfaces interact to each other hydrodynamically. Therefore, I studied the collective behavior of colloidal particles (4.0 µm PMMA), located near the solid surface in a fluid medium confined in a cylindrical cell (3.0 mm diameter, 0.25 mm height) which was rotated vertically at a low rotational speed (20 rpm). The observed colloidal behavior was then validated through a Stokesian dynamics simulation where the concept of hydrodynamic contact force or lubrication interactions were avoided which is not physically intuitive and mathematically cumbersome. Rather, I adopted hard-sphere like colloidal collision or mobility model. I found that colloidal agglomeration is a function of the applied rotation scheme, either forming colloidal clusters or lanes. While evolving into dynamic structures, colloids also laterally migrate away from the underlying surface. While forming colloidal structures due to hydrodynamic interactions among particles and nearby solid surface, particles migrate away from the surface and eventually redistribute throughout the sample cell. After redistribution, I demonstrated long term colloidal stability within the sample cell. When particles are redistributed with relatively equal spacing and not concentrated near a solid surface, structure formation is minimized and does not evolve any further which can be considered as long-term suspension.
Rahman, Md Mahmudur, "Towards long term colloid suspension in a vertically rotated system." (2021). Electronic Theses and Dissertations. Paper 3740.