Mechanistic understanding of thermally induced fluid flow and chemical transport in nano-channels is critical for describing thermally coupled phenomena in natural nanomaterials (e.g. clays) and advancing the nanomaterials applications in engineering such as those proposed for drug delivery, water desalination, low-grade thermal energy recovery, and lab-on-a-chip devices. The aim of the research presented in this thesis has been to advance the understanding of the physics behind thermally induced fluid flow (thermo-osmosis), thermally induced chemical transport (thermal diffusion), and thermally coupled hydro-mechanics (microstructure swelling) in nanochannels. In the course of the work, these processes have been studied at the nanoscale via molecular dynamics (MD) simulations and linked with thermodynamics and interfacial fluid properties. In this research, thermo-osmosis and thermo-diffusion phenomena in electrically neutral and negatively charged silica nanochannels have been investigated by thermo-osmotic and mechano-caloric systems, which involved the flow of pure water and ionic solutions of NaCl due to thermal gradient. The research on thermal diffusion has been focused on revealing how thermally induced fluid flow and chemical transport of ions in charged silica nanochannel overlap. For these two problems, the effects of thermodynamic conditions, nanoconfinement, and surface charge density on interfacial water have been quantified. In addition, thermally induced mass transport of pure water and subsequent swelling of Na montmorillonite (Na-MMT) have been studied at temperatures between 298 and 500 K and for a range of nanochannel sizes to reveal the elevated temperature effects on swelling pressure, electric double-layer structure, and the interlayer counterion solvation structure. The results reported in the thesis show and highlight the validity of the Onsager reciprocal relation, and Poiseuille's law for thermo-osmosis at the nanoscale/sub-nanoscale. For thermo-osmosis, it was found that a thermal gradient induces tangential stress gradient and drives the thermo-osmotic flow in the boundary layers adjacent to solid surfaces. This fluid flow was found to be enhanced by decreasing the surface charge or increasing the ionic strength, due to the structural modifications of the aqueous electrical double layer. For the case of thermo-diffusion analysis, it was found that both nano-confinement and surface charges caused the interfacial chemicals to act more thermophilic by modifying the hydrogen bond network and ion solvation structures of the interfacial water. Inhomogeneous thermal diffusive behaviour of chemicals in pore fluid was also observed. For microstructure swelling, a reduction of clay swelling pressure with increasing temperature was observed, which is caused by weakening the hydration repulsion induced by surface and counterion hydration. This can be observed by the deterioration of the layering structure of interfacial liquid and the shrinkage and weakening of the electric double layer. The applicability and breakdown of the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory at elevated temperatures have been examined, and the causes for the breakdown were identified. The findings reported in the thesis advance the fundamental understanding of nanoscale thermally coupled transport and hydro-mechanical processes by providing microscopic quantifications. The developed theoretical and numerical tools offer a base for investigating a broader range of coupled heat and mass transfer phenomena and deformation behaviour in nanoscale space, in more complex thermodynamic conditions and physicochemical environments.
|Date of Award||31 Dec 2023|
- The University of Manchester
|Supervisor||Andrey Jivkov (Supervisor) & Majid Sedighi (Supervisor)|