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Water with particulates flowing through various confinements is ubiquitous, both in natural and synthetic systems, such as biological cell membranes, porous media, and micro-/nanofluidic devices. Understanding the detailed transport mechanism of both, water and particulates in such systems, is the essential first step to understanding complicated biological phenomena, as well as to developing effective technology, including filtration/desalination membranes, drug delivery systems, and various biomedical devices. At a micro-/nanoscale, the viscous force plays a more dominant role over the inertia force. In such a low-Reynolds-number regime, the flow becomes linear and time reversible. While the features of low-Reynolds-number hydrodynamics are simple and intuitive, exploring beyond this regime would allow for the development of more sophisticated transport technologies, as well as shedding a light on overlooked phenomena. In this thesis, we demonstrate unusual dynamics of colloidal particles driven by electrokinetic, Brownian, or magnetic forces under confinement, such as in channels and next to walls. Electrophoresis is a widely used separation method for particulates and molecules, and it is known to have a linear response to an applied electric field. However, above a sufficiently high field, the response becomes nonlinear due to the non-negligible perturbation of double layers caused by the external electric field and advection. We systematically study nonlinear electrophoresis by developing our customised microfluidic setup. The competition between electrophoresis and electroosmosis results in an unexpected voltage-tunable trapping of particles inside channels. This is the first kind of particle trapping using the nonlinear electrokinetic effect and can be potentially used for particle separation. Using the same microfluidic setup, we also discover that passive Brownian particles diffuse differently depending on the length of the channels. The underlying physics of this change in diffusivity is identified as the leaking fluid flow from the open ends of the channel. We investigate how the boundary condition of the reservoirs affects the flow field inside a channel by taking into account the minute compressibility of water. Lastly, we also investigate how steric effects play a role in particle transport near a solid boundary. We show that externally driven active colloids can trap and transport passive particles by steric interactions with a wall, which irreversibly changes the trajectories of particles across streamlines. Such an irreversible steric interaction is expected to play an important role in channel transport as well. The simplicity of our model experimental and theoretical systems makes these results transferable to a wide range of other systems. |
