UC Irvine

The work presented in this thesis represents two research projects which, simply put, both explore the hydrodynamics of fluids flowing along a solid boundary. In practice, the two experiments take place under the extremes of their very different conditions: one taking place at room temperature, but on a nanoscopic scale at many times ambient pressure and the other visible to the naked eye, but over a range of low pressures and at temperatures colder than outer space. The first project probes the no-slip boundary condition with the direct measurements of the smallest pressure-driven water flow through single nanotubes. The no-slip boundary condition is tested by performing room-temperature water flow experiments in both bare silica hydrophilic nanotubes and polydimethylsiloxane (PDMS) coated hydrophobic nanotubes, in order to resolve conflict in current literature. Flow rates of water through hydrophilic and hydrophobic single pipes with diameters ranging from 10 $\mu$m to 200 nm have been measured. A method of coating the pipes with a hydrophobic polymer roughly 2 nm thick was developed and the hydrophobic nature of the pipes after treatment was verified. The exact diameters of the tubes were measured using a gaseous flow impedance test, imaged directly via scanning electron microscopy (SEM), or measured through atomic force microscopy (AFM). The flow rates through both the hydrophobic and hydrophilic pipes agree with theory for viscous Poiseuille flow and are effectively indistinguishable from each other.

The second research project presents the first observation of the spreading of normal and superfluid helium drops from 1.0-5.2$^\circ$K. Optical and interferometric measurements were taken from both side and below points of view within a custom designed and fabricated cryostat. Images were taken at both high-speed and low-speed to observe different spreading regimes for the drops. High speed measurements showed initial spreading speeds that follow the $r\propto t^{1/2}$ power law determined by inertia-capillary balance and show a weak viscosity dependence. The long-term measurements showed anomalously flat drop profiles and a defined contact line that persist for up to 15 minutes despite the completely wetting nature of liquid helium and the existence of a $\approx$40 nm standing film. The observed long-term spreading power law of $r\propto t^{1/7}$ was greater than those predicted by scaling solutions in lubrication theory for normal drops, and demonstrate the first experimental evidence of spreading controlled by contact line dissipation mechanisms. Superfluid drops had lifetimes too short to observe long-term spreading. The short supeerfluid drop lifetime is thought to be caused by superfluid outflow through the liquid film which covers the substrate. Several other phenomena were observed, but not rigorously studied, including apparent Leidenfrost at low $\Delta T$ and critical opalescence.