title: Electrokinetic Phenomena in Colloidal Systems: Experimental and Numerical Investigations of Electrophoresis and Diffusiophoresis
Creators: name: Rashidi, Mansoureh
Publisher: name: Graduate Studies
Year: 2024
Subjects: subject: Electrokinetic phenomena
subject: droplet electrophoresis
subject: particle diffusiophoresis
subject: Education--Sciences

description: Transport and manipulation of particles and droplets are of significant interest in various fields, including biology, separation systems, and reaction engineering. In this regard, electrokinetic phenomena such as electrophoresis (EP) and diffusiophoresis (DP) have been widely adopted. EP involves the motion of particles or droplets under the influence of an external electric field, whereas DP refers to the spontaneous motion of colloids driven by a bulk concentration gradient of dissolved solute in the liquid phase. Improving the flexibility and controllability of colloidal EP and DP is crucial for a more widespread application in analytical, biomedical and other applications due to their inherent complexity. The objective of this thesis is to explore the effect of EP and DP on manipulation of droplets and particles in microscale systems. A combination of experimental and numerical techniques are employed to investigate these phenomena. The first system examines how amphoteric surfactants, having a specific structure carrying both positive and negative groups, affect the electrokinetic velocity of droplets in an external electric field. This type of surfactant alter their charge under the influence of changing pH, which allows them to impact the surface charge of droplets, which can be applied to gain control over the droplet movement through pH switching. The second system involves a numerical study of the EP of droplets containing mobile ions, considering both variable and constant permittivity within the EDL surrounding the droplets, which is relevant for systems of oil droplets that contain free charges. The third system examines the DP of porous and non-porous particles in electrolytes with different compositions, laying the groundwork for increased control of particle movement with applications in catalysis and load-carrying colloids. The first experimental study demonstrated that the electrokinetic velocity of oil-in water emulsion droplets stabilized by an amphoteric surfactant can be effectively controlled by changing the pH from acidic to alka- line under an external electric field. At the isoelectric pH of the surfactant monomers, varying the surfactant concentration altered the direction of droplet motion due to the competing effects of EP and electroosmotic flow (EOF) along the channel wall. Numerical simulations for EP of droplets containing mobile ions showed a significant reduction in the droplet EP velocity when variable permittivity was considered within the outer EDL. Additionally, decreasing the viscosity ratio of oil to electrolyte resulted in reduced droplet velocity due to the formation of a vortex around the droplet. It was also found that with constant permittivity in the outer EDL, variations in κaouter had a more significant effect on droplet EP velocity than changes in κainner. The third study, focusing on the experimental DP of porous and non-porous particles, demonstrated that electrolyte composition has a substantial impact on the particle DP behavior. The findings indicated that particle motion direction was governed by the interaction between double layer polarization (DLP), EOF and EP. Overall, this thesis advances the understanding of particle and droplet manipulation using electrokinetic phenomena, specifically through electric fields and concentration gradients. The study offers new insights into droplet EP, demonstrating its potential for designing separation strategies based on pH variations, which is particularly relevant in systems with natural pH gradients like the human body. It also has practical applications in soil remediation, where controlling droplet movement through pH gradients can be beneficial. The research on EP of droplets with mobile ions is helpful for improving water and waste treatment processes, such as lubricant removal from processing streams. These findings are also significant for manipulating biological colloids, especially for nanocarriers like liposomes or nanocapsules that encapsulate ions. Additionally, the investigation into the DP behavior of porous particles provides knowledge for advancing separation technologies and targeted drug delivery systems. By elucidating these mechanisms, the research paves the way for optimizing particle and droplet transport efficiencies, thereby enhancing the effectiveness and precision of various biomedical and environmental processes.