Browsing by Subject "superhydrophobic"

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    A numerical and theoretical study of drag reduction using superhydrophobic surfaces
    (2018-09) Li, Yixuan
    Motivated by the potential drag reduction benefits of superhydrophobic surfaces (SHS), direct numerical simulation (DNS) and theoretical analyses are used to explore the interaction between SHS and turbulent channel flow. First, DNS is used to study the drag reduction by SHS in laminar channel flow. Resolved multi-phase simulations using the volume of fluid (VOF) methodology are performed to study the effects of groove geometry, interface shear rate and meniscus penetration independently. An analytical solution for the flow in a laminar channel with grooved surface with gas-pocket within is obtained. The solution accounts for both the groove geometry and the trapped fluid properties, and shows good agreement with simulation results. The solution is used to propose a scaling law that collapses data across fully wetted to fully gas-filled regimes. The trapped gas is simulated as both flat and meniscal interfaces. The drag reduction initially increases with interface deflection into the groove and then decreases for large deflections as the interface velocity approaches zero due to the proximity to the bottom of the groove. Next, the geometric effect of SHS in turbulent flow is studied by performing DNS at friction Reynolds number $\Rey_\tau = 400$ over longitudinal grooves whose size is comparable to the viscous sublayer thickness. It is found that despite the bulk flow being close to that of a flat-wall channel, the slip effect of the grooves causes some differences within the viscous sublayer. Spectral analysis of the velocity transfer function between the interior and the exterior regions of the grooves shows that the grooves suppress the energy at low frequencies. The DNS reveals negligible Reynolds shear stress near the grooves, which motivates an unsteady Stokes flow model. It is assumed that the flow in the vicinity of the grooves is governed by the unsteady Stokes equations, forced by an oscillating external flow. The effects of streamwise, spanwise and vertical velocity, freestream wavenumber and the height of freestream perturbation above the groove are studied. The non-dimensional parameter $\omega L^2/\nu$ obtained from this model problem ($L$ is half of the groove span, $\omega$ is the frequency of the external turbulent signal and $\nu$ is the kinematic viscosity) is used to relate the model to the current DNS. Good agreement is seen with DNS at low frequencies. It is suggested that higher frequency disturbances are produced by smaller spanwise structures near the wall, and when this effect is accounted for, good agreement is also observed at higher frequencies. Finally, we study multiphase flow within grooved textures exposed to external unsteadiness. We derive analytical expressions for multiphase unsteady Stokes flow within periodic grooves driven by oscillating streamwise/spanwise freestream velocity. Good agreement is obtained between the analytical solution and DNS performed with the VOF method. The effect of oscillation frequency, Reynolds number, and the multiphase interface location on the transfer function between the input signal external to the groove and output near the interface, is examined. Also, the effective slip length and the shear stress over the grooved plane are studied.
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    Producing Melt Blown Nano-/Micro-fibers with Unique Surface Wetting Properties
    (2015-09) Wang, Zaifei
    Melt blowing is a one-step process producing microfibers with an average fiber diameter (dav) of 1 – 5 μm. One application of melt blown fibers is filter media of water-in-diesel engine fuel filtration. In order to meet the increasingly higher requirements on fuel cleanliness, enhancing the filter efficiency has been desired. One attractive strategy is applying nanofibers (dav < 1 μm). The other is modifying the fiber surface wetting properties, such as superhydrophobic. The goal of this work is to produce melt blown nano-/micro-fibers with superhydrophilic and/or superhydrophobic surfaces for water-in-diesel filtration. In this thesis, we advanced the new technique of producing nanofibers from melt blown fiber-in-fiber polymer blends by understanding the nanofiber formation and developing water-extractable nanofibers. Firstly, poly(styrene) (PS)/poly(butylene terephthalate) (PBT) and PS/poly(ethylene-co-chlorotrifluoroethylene) (ECTFE) binary blends were melt blown followed by tetrahydrofuran (THF) extraction. The control extrusions demonstrated that the core nanofibers were primarily generated during the fiber blowing process. We speculate that both aerodynamic and fiber-fiber drag influence the nanofiber formation. Varying the minor phase volume fraction revealed the nanofiber breakup. We proposed that both the single-droplet-elongation and dumbbell-formation contribute to the formation of nanofibers. In addition, we hypothesize that the coalescence leads to the formation of non-uniform nano-/micro-fibers. Following the discussion on nanofiber formation, we showed nanofiber fabrication from water-extractable melt blown fiber-in-fiber polymer blends containing a water-dispersible sulfopolyester (SP). This new method eliminates issues associated with organic solvents. Also, it provides another route to prepare multilayer nano-/micro-fiber composites. Surface wetting modifications of melt blown PBT fibers were achieved by alkaline hydrolysis and subsequent fluorination. Application of alkaline hydrolysis generates sponge-like PBT fibers decorated with hydroxyl and carboxyl functional groups, resulting in a superhydrophilic fiber mat surface. The subsequent fluorination creates a sticky-superhydrophobic surface. An alternative achieving enhanced or even superhydrophobic PBT fiber surfaces is incorporation of a random perfluorinated multiblock copolyester (PFCE). Adding only 5 wt% of PFCE into PBT leads to over 20 wt% of PFCE on PBT fiber surfaces. The blooming of fluorine and the overall mat roughness increased the water contact angle from about 128° to above 150° and lowered the fiber mat surface adhesion energy by 25%. Finally, the appendices show some preliminary works on: 1) synthesis and characterizations of poly(L-lactide) and perfluoropolyether block copolymer; 2) the effects of alkaline hydrolysis on surface topography and mechanical properties of melt blown PBT fiber mats; 3) compatibilized polyamide 6 and poly(ethene-co-tetrafluoroethene) (PETFE) blends, which, however, were not suitable for melt blowing due to high viscosity; 4) collection of individual melt blown nanofibers.