Particles in Gradients: Combining Diffusiophoresis and Diffusio-osmosis
Burak Akdeniz is a PhD student in the department Soft matter, Fluidics and Interfaces. (Co)Promotors are prof.dr.ir. R.G.H. Lammertink and dr. J.A. Wood from the faculty Science & Technology.
Diffusio-osmosis is a type of surface flow induced by a concentration gradient. Similarly, diffusiophoresis is the movement of particles under a concentration gradient due to induced surface flow. The aim of this work is to gain further insight into the behaviour of diffusiophoresis and diffusio-osmosis at different gradient types and to analyze the movement of coated particles and ion exchange particles in gradients. The knowledge gained provides a comprehensive understanding of diffusiophoresis in electrolyte gradients as well as less studied gradient types such as non-electrolyte or nanoparticle concentration gradients. The scope of each chapter is highlighted below.
Chapter 1, the introductory chapter of the thesis, highlights the diffusiophoretic and diffusio-osmotic phenomena. It considers particle behaviour under nonelectrolyte and electrolyte gradients by providing analytical equations. This is followed by a review of available microfluidic structures used to observe diffusiophoresis and diffusio-osmosis. The basic experiments are shown and the importance of absolute and relative gradients for nonelectrolyte and electrolyte solutes is explained. Additionally, the chapter provides background information on polymers, polyelectrolytes and ion exchange particles.
Chapter 2 emphasizes that the zeta potential variation of the particle and microfluidic surface due to the variation in salt concentration must be included in the simulation for diffusiophoretic or diffusio-osmotic studies. The theoretical rate of diffusiophoresis and diffusio-osmosis depends on the zeta potential value in the electrolyte-solute gradients and the electrolyte concentration changes in the system, which affect the zeta potential value. The simulations were in agreement with the experimental observations when the zeta potential changes were included. Therefore, the zeta potential of the particle and/or wall must be measured over the experimental range of electrolyte concentration and included in the simulations to accurately describe the system.
By observing the importance of the variation of the zeta potential value with salt concentration, Chapter 3 highlights a way to reduce the concentration dependence of the zeta potential of particles by coating them with polyelectrolytes. Coating a bilayer of polyelectrolytes reduces the electrolyte concentration dependence of the zeta potential values. This allows a constant zeta potential to be assumed in simulations. In addition, the averaged zeta potential values showed a dependence on the salt concentration during coating. Overall, the coated polyelectrolyte layer not only provides a constant zeta potential for diffusiophoresis experiments, but also make it possible to prepare different averaged zeta potential value particles by changing the background salt concentration during the polyelectrolyte coating process, which can be used in diffusiophoresis experiments.
Chapter 4 extends the types of solutes used in diffusiophoretic and diffusio-osmotic studies to include polyelectrolytes, neutral polymers and nanoparticle gradients. The main feature of these gradients is that the molecules are excluded from the tracer particle surface. Under these conditions, the tracer particles move towards the lower concentration of the gradients. Furthermore, the diffusiophoretic behaviour of the tracer particles in each solute type was evaluated and the main differences/similarities between the gradient types were highlighted. It is found that the addition of salt in the charged solute gradients (polyelectrolytes) reduces the movement of the tracer particles. This change is due to the reduction in electrostatic interaction. On the other hand, the addition of salt to the neutral polymer gradients does not influence the movement of tracer particles. In addition, the behaviour of the tracer particles in the nanoparticle gradient was experimentally analyzed and could also be simulated by considering the interaction between the tracer particles and the nanoparticles.
Chapter 5 focuses on the behaviour of ion exchange particles under the electrolyte gradient. The main function of ion exchange resins is to exchange ions from the environment. The main difference in the presence of an electrolyte gradient is highlighted between the ion exchange particle with and without exchange, and different behaviours were observed. In the ion exchange experiments, the resins were first excluded from the surface (similar to the without ion exchange case), then the ion exchange resins approached each other in the ion exchange case. A series of experiments and simulations were carried out to understand this behaviour. The interplay between diffusiophoretic and diffusio-osmotic motions explains the observed behaviour.
The main conclusions of the thesis are underlined in Chapter 6. In addition, further recommendations for the study of diffusiophoresis and diffusio-osmosis are presented. By providing recommendations in a broader context, the aim is to provide a fundamental understanding of diffusiophoresis and enable further potential applications.
