Browsing by Subject "Free-surface flows"

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    Electrostatic Assist of Liquid Transfer in Printing Processes
    (2018-07) Huang, Chung-Hsuan
    Printing processes are being explored for the large-scale manufacture of electronic de- vices. Transfer of liquid from one surface to another plays a key role in most printing processes. During liquid transfer, a liquid bridge is formed and then undergoes sig- nificant extensional motion. Incomplete liquid transfer can produce defects that can be detrimental to device operation. One important printing process is gravure, which involves transfer of liquid from micron-scale cavities at high speeds. Electric fields are sometimes used to enhance liquid transfer, a technique known as electrostatic assist (ESA). However, its underlying physical mechanisms remain a mystery. This thesis uses a combination of theory and experiment to understand the fundamental mechanisms by which electrostatic forces influence liquid transfer. Liquid transfer without electric fields and cavities must be understood before study- ing the mechanism of ESA. We develop one-dimensional (1D) slender-jet and two- dimensional (2D) axisymmetric models of this phenomenon and compare the resulting predictions with previously published experimental data. At relatively low stretching speeds, predictions from both models of the amount of liquid transferred agree well with the experimental data. When the stretching speed is high enough, the models predict that each surface receives half the liquid, in agreement with experimental observations. For intermediate values of the stretching speed, predictions from each model can deviate substantially from the experimental data, which we speculate is due to the influence of surface defects that are not included in the models. The 1D and 2D model are modified to include electrostatic effects. The liquid be- haves like a perfect (non-conducting), or leaky dielectric (poorly conducting) material. The liquid is confined between two plates, with the top plate having a constant electro- static potential while the bottom plate is grounded. For perfect dielectrics, application of an electric field enhances liquid transfer to the more wettable surface because it slows the surface-tension-driven breakup of the bridge, thereby allowing more time for the con- tact line to retract on the less-wettable surface. For leaky dielectrics, application of an electric field can augment or oppose the influence of wettability differences, depending on the direction of the electric field and the sign of the interfacial charge. Experimental results confirm the enhancement of the amount of liquid transferred when the electric field is present, and the measured values are in good agreement with the predictions of the 1D perfect dielectric model. When one of the plate is replaced by a cavity, the presence of the cavity causes the contact line on the cavity wall to effectively pin and inhibits the liquid transfer. For perfect dielectrics, application of an electric field unpins the contact line on the cavity and leads to improvement of cavity emptying. For the leaky dielectrics, the presence of the surface charge does not further improve liquid transfer because of nearly zero electric tangential stress near the contact line on each surface.
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    Onset of dynamic wetting failure: the mechanics of high-speed fluid displacement
    (2013-07) Vandre, Eric Allen
    Dynamic wetting is crucial to processes where a liquid displaces another fluid along a solid surface, such as the deposition of a coating liquid onto a moving substrate. Numerous studies report the failure of dynamic wetting when process speed exceeds some critical value. Typically, wetting failure is a precursor to air entrainment, which produces catastrophic defects in coatings. However, the hydrodynamic factors that influence the transition to wetting failure remain poorly understood from empirical and theoretical perspectives. This work investigates the fundamentals of wetting failure in a variety of systems that are relevant to industrial coating flows. A hydrodynamic model is developed for planar and axisymmetric geometries where an advancing fluid displaces a receding fluid along a smooth, moving substrate. Numerical solutions predict the onset of wetting failure at a critical substrate speed, which coincides with a turning point in the steady-state solution path for a given set of system parameters. Flow-field analysis reveals a physical mechanism where wetting failure results when capillary forces can no longer support the pressure gradients necessary to steadily displace the receding fluid.Novel experimental systems are used to measure the substrate speeds and meniscus shapes associated with the onset of air entrainment during wetting failure. Using high-speed visualization techniques, air entrainment is identified by the elongation of triangular air films with system-dependent size. Air films become unstable to thickness perturbations and ultimately rupture, leading to the entrainment of air bubbles. Meniscus confinement in a narrow gap between the substrate and a stationary plate is shown to delay air entrainment to higher speeds for a variety of water/glycerol solutions. In addition, liquid pressurization (relative to ambient air) further postpones air entrainment when the meniscus is located near a sharp corner along the plate. Recorded critical speeds compare well to predictions from the model, supporting the hydrodynamic mechanism for the onset of wetting failure. Lastly, the common practice of curtain coating is investigated using the hydrodynamic model. Due to the complexity of this system, a new hybrid method is developed to reduce computational cost associated with the numerical analysis. Results show that the onset of wetting failure varies strongly with the operating conditions of this system. In addition, stresses from the air flow dramatically affect the steady wetting behavior of curtain coating. Ultimately, these findings emphasize the important role of two-fluid displacement mechanics during high-speed wetting. Although this work was motivated by coating flows, it is also relevant to a number of other applications such as microfluidic devices, oil-recovery systems, and splashing droplets.
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    Stretching and slipping liquid bridges: liquid transfer in industrial printing.
    (2011-08) Dodds, Shawn
    Liquid bridges with moving contact lines are found in a variety of settings, such as capillary feeders and high-speed printing processes. Despite this relevance, studies on liquid bridges often assume that the contact lines remain pinned in place during stretching. While this may be the case for some applications, contact line motion is \emph{desirable} in printing processes so that the amount of liquid transferred can be maximized. In this thesis we study several model problems to improve our understanding of how moving contact lines alter the dynamics of liquid bridges. We use the finite element method to study the stretching of a liquid bridge between either two flat plates or a flat plate and a cavity. For axisymmetric bridges we find that while the wettability of the two surfaces is a key factor in controlling liquid transfer between two flat plates, the presence of a cavity leads to fundamentally different bridge dynamics. This is due to the pinning of the contact line on the cavity wall, which leads to a decrease in the amount of liquid transferred to the flat plate. We find that the presence of inertia aids in cavity emptying by forcing the interface further into the cavity. However, this increase in emptying can be offset by an increased tendency for the production of satellite drops as the flat plate is made more wettable. To study non-axisymmetric flows we solve the Navier-Stokes equations in three dimensions. We find that when the stretching motion is asymmetric the liquid remains evenly distributed after breakup, so long as the two plates are not accelerating relative to each other. If the bridge shape is not initially cylindrical we find that the ability of the bridge to maintain its initial shape after breakup depends on the friction between the contact line and the solid. Finally, we use flow visualization to observe the stretching of liquid bridges both with and without small air bubbles. We find that while the breakup of wetting fluids between two identical surfaces is symmetric about the bridge midpoint, contact line pinning breaks this symmetry at slow stretching speeds for nonwetting fluids. We exploit this observation to force the bubbles selectively toward the least hydrophillic plate confining the bridge.