Transport properties of nanoconfined fluids : from pressure-driven to electro-/thermo-osmotic flows

Abstract

Nanoconfined fluids are ubiquitous in many areas, such as water and ions in biological channels. Extensive experimental and numerical investigations have unveiled that nanoconfined fluids can behave greatly different from bulk fluids due to the confinement effects, especially their transport properties. It is anticipated that the study of water and ion transport at the nanoscale could help resolve some global challenges, including water shortages and energy crises. A substantial effort has been devoted to exploring the transport of confined water and ions in the past twenty years, but a more intensive understanding of nanoscale flows under different driving forces is highly desired. In this thesis, water and ion transport through nanochannels, driven by various external forces, is investigated by molecular dynamics simulations. First, pressure-driven water transport through graphene-coated copper nanochannels is investigated. Water flows through nanochannels are greatly enhanced due to the graphene coatings. Monolayer graphene coatings can induce a 45 times enhancement of the water flow rate, caused by the considerable water slippage on the graphene surfaces. The dynamics of interfacial water molecules is probed in detail, including the dipole relaxation time, the hydrogen-bond lifetime, and the free energy barrier, offering a molecular picture to understand the enhanced flows considered. Second, electromechanical ion transport through graphene nanochannels is studied, and a coupling between pressure-driven and electroosmotic flows is explored. It is found that ionic currents in electromechanical flows are enhanced compared with the linear superpositions of the currents generated by the individual electroosmotic and pressure-driven flows. The additional current induced by the coupling effect shows a nonlinear relation with the strength of the electric field. This nonlinear coupling is attributed to the reduction of the total potential energy barrier. Third, thermally driven transport of potassium chloride solutions confined in hydrophobic nanochannels is investigated, and the thermoelectric properties of the nanofluidic system are predicted. A strong size dependence of the thermoelectric response for small channels is reported. A remarkable thermoelectric response is demonstrated for the system with a nanochannel of 1.0 nm in height, whose Seebeck coefficient and figure of merit reach 30.6 mV/K and 4.6, respectively. It is found that the velocity slippage on the hydrophobic surfaces can amplify the thermoelectric response, while the mean excess enthalpy of the confined solution controls the size dependency. The studies in this thesis offer molecular insights into understanding the transport properties of nanoconfined solutions, which may promote applications of nanofluidic devices for energy conversion/harvesting.

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Zhigang LI (Supervisor)