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Type: Thesis
Title: Entrance effects on solution transport through nanoporous membranes
Author: Rankin, Daniel Justin
Issue Date: 2019
School/Discipline: School of Physical Sciences
Abstract: Solution transport across nanoporous membranes occurs in many different biologically and industrially relevant processes such as filtration of waste by the kidneys and desalination of seawater. The same theoretical framework can be used to understand both of these processes, as well as many others. In general, a flux of solution is driven across a porous membrane due to an externally applied force. This external force can be a gradient in pressure, temperature, concentration, or electrical potential. At the entrance and exit of a pore the fluid streamlines and electric field lines experience a significant constriction in going from the bulk reservoirs to the narrow pores. This effect can become significant for short pores and pores with low friction and thus must be appropriately taken into account to correctly predict solution fluxes. In the first study, continuum mechanics is used to investigate the entrance effects on charge flux of electrolytes across porous membranes. The access electrical resistance, which is the electrical resistance associated with the electric field lines bending into and out of the pores, has previously been shown to make up a significant fraction of the total electrical resistance when the fluid–pore friction is low.1 Although several papers have studied the access electrical resistance,2–5 none has explicitly considered the effect of surface charge on the surfaces of the membrane facing the bulk solution even though this charge has been shown to have a significant effect on the access electrical resistance.6 In this thesis, finite element method (FEM) calculations are carried out in order to systematically study the access electrical resistance of charged pores in charged and uncharged membranes. The results are compared with predictions from two existing continuum-based theories and a new theory derived in this thesis. It is found that the FEM results agree with different theories depending on whether or not the outer-membrane surface is charged. In the second study an existing molecular dynamics (MD) algorithm is used to simulate concentration differences across pores connected to bulk reservoirs. The algorithm is found to require a modification at high solute concentrations, which had not previously been considered. In the third study the modified MD algorithm is used to investigate possible non-continuum and non-ideal effects on concentration-gradient-driven flows at high solute concentrations. Entrance effects are considered in the context of diffusio-osmotic flows, which are flows driven by forces acting on the inhomogeneous fluid layer near the membrane pore surfaces as a result of an applied concentration gradient. The access diffusio-osmotic resistance, which is the resistance to the diffusio-osmotic flux associated with the fluid streamlines bending into and out of the pores, is calculated and compared with a new theory that is derived in this thesis. The assumptions made in deriving the new theory include, amongst others a dilute solution and continuum theory. Despite these assumptions, the theory predicts the correct scaling of the MD results at two different high solute concentrations. It is found that both electrical and diffusio-osmotic access resistances can be separated from their respective total (access and pore) resistances. Depending on whether the length scales of interest, such as the pore radius, are comparable with the pore length, the access resistance can be a significant factor in determining the total resistance of the system. This is explored in this thesis in the context of both electrical and diffusio-omsotic resistance, which affect a wide range of different systems.
Advisor: Huang, David M.
Dissertation Note: Thesis (MPhil) -- University of Adelaide, School of Physical Sciences, 2019
Keywords: entrance effects
access resistance
solution transport
reverse electrodialysis
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