Flow motion in confinement is highly associated with characteristic dimensions. At the micro- and nanoscale, flows show diverse phenomena that have never been found at the macroscale. These diverse phenomena are owing to the surface force and electrokinetic effects as well as complex molecular interactions at a reduced characteristic length. Despite a great number of new phenomena and applications emerging at the nano/microscale, controversial results have been reported and many mechanisms still remain unclear, such as ionic concentration dependence in carbon nanotubes (CNTs). Therefore, extensive work is required to solve the existing problems and explore new phenomena. The objective of this thesis is to investigate the fluid flow in confined space at the micro- and nanoscale.

First, f...[ Read more ]

Flow motion in confinement is highly associated with characteristic dimensions. At the micro- and nanoscale, flows show diverse phenomena that have never been found at the macroscale. These diverse phenomena are owing to the surface force and electrokinetic effects as well as complex molecular interactions at a reduced characteristic length. Despite a great number of new phenomena and applications emerging at the nano/microscale, controversial results have been reported and many mechanisms still remain unclear, such as ionic concentration dependence in carbon nanotubes (CNTs). Therefore, extensive work is required to solve the existing problems and explore new phenomena. The objective of this thesis is to investigate the fluid flow in confined space at the micro- and nanoscale.

First, fluid flows in converging-diverging microchannels (CDMCs) are studied. A new dimensionless number Gm is proposed to describe the geometry information of CDMCs. The product of the Gm and Reynolds number (Re) forms another dimensionless number, denoted by Re_G ( Re_G = Re•Gm ). Re_G contains both the geometric influence and the competition between viscous and inertial effects, which is found to be more suitable for flow characterization in CDMCs. Flows stay laminar when Re_G ﹤40 regardless of the geometry of the CDMCs. For laminar flows, the flow resistance model developed in the literature works well, but for transitional and turbulent flows, a scaling law is proposed in this thesis to estimate the flow resistance, which suggests a polynomial relationship between the pressure drop and flow rate.

Second, a microfluidic rectifier for Newtonian fluids is fabricated by employing asymmetric converging-diverging microchannels (ACDMCs). Because of the asymmetric structure of the microchannel, the flow resistance depends on flow directions. The highest diodicity for this rectifier is 1.77, which is superior to previous microfluidic rectifiers for Newtonian fluids. An expression for the diodicity is developed based on two scaling laws for the flow resistances in the forward and backward directions.

Third, experiments are conducted to study ion transport in carbon nanotubes. Various nonlinear relationships between the ionic conductance and the ion concentration are observed. Due to their small size and the chemical functionalization in the fabrication process, it is hypothesized that the distinct conductance-concentration (G-C) dependences are caused by the carboxylic acid groups at the CNT entrance, which could affect the energy barrier for ion transport and change the ionic conductance. Molecular dynamics simulations are performed to validate the hypothesis. The diverse G-C relationships are also predicted by the electrokinetic theory when we considered the potential generated by the functional groups at the CNT entrance. Practically, the number of functional groups at the CNT entrance is influenced by several factors, including both intrinsic and external effects, which make it difficult to regulate the ionic conductance. This poses a challenge for CNT-based nanofluidic systems in practical applications.