Browsing by Subject "capillary flow"

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    Capillary Flow and Evaporation in Open Microchannels
    (2021-05) Kolliopoulos, Panayiotis
    Capillary flow is the spontaneous wicking of liquids in narrow spaces without the assistance of external forces. Examples of capillary flow can be found in numerous applications ranging from lab-on-a-chip devices to printed electronics manufacturing. Open rectangular microchannels often appear in these applications, with the lack of top resulting in a complex free-surface morphology and evaporation. While prior work has demonstrated that evaporation hinders capillary flow, the underlying fundamentals that are vital to the design and optimization of applications such as printed electronics manufacturing are still lacking. In this thesis we investigate the fundamentals of capillary flow and evaporation in open microchannels using theory and experiment. We initially consider flow of nonvolatile liquids to elucidate the capillary-flow dynamics. We develop a novel self-similar lubrication-theory-based (LTB) model accounting for the complex free-surface morphology and compare model predictions to those from the widely used modified Lucas-Washburn (MLW) model, as well as experimental observations over a wide range of channel aspect ratios and equilibrium contact angles. We identify the limitations of the MLW and LTB models and demonstrate the importance of accounting for the effects of the complex free-surface morphology on capillary flow. We also show that the LTB model accurately captures the dynamics of fingers that extend ahead of the front meniscus which are not accounted for by the MLW model. Capillary flow of evaporating liquid solutions are examined using two theoretical models. We first develop a Lucas-Washburn-type one-dimensional (1D) model, which accounts for concentration-dependent viscosity and uniform evaporation. The second model is a lubrication-theory-based model, which accounts for the complex free-surface morphology, non-uniform solvent evaporation, Marangoni flows due to gradients in solute concentration and temperature, and finite-size reservoir effects. Both models are compared to prior capillary-flow experiments of aqueous poly(vinyl alcohol) solutions in the presence of evaporation. While the 1D model qualitatively captures evaporation effects on the flow dynamics, it underestimates their magnitude. The lubrication-theory-based model predictions are in good agreement with experimental observations, and predicted evaporation rates are comparable with experimental estimates. Numerical results also reveal significant qualitative differences in capillary flow of evaporating pure solvents and liquid solutions. Additionally, Marangoni flows are found to promote more uniform solute deposition patterns after solvent evaporation. Ultimately, these findings advance the fundamental physical understanding of capillary flow with evaporation and provide guidelines for the design and optimization of numerous applications.
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    New Approaches for Printed Electronics Manufacturing
    (2015-09) Mahajan, Ankit
    In printed electronics, electronic inks are patterned onto flexible substrates using roll-to-roll (R2R) compatible graphic printing methods. For applications where large-area, conformal electronics are necessary, printed electronics holds a competitive advantage over rigid, semiconductor circuitry, which does not scale efficiently to large areas. However, in order to fully realize the true potential of printed electronics, several manufacturing hurdles need to be overcome. Firstly, minimum feature sizes produced by graphic printing methods are typically greater than 25 µm, which is at least an order of magnitude higher for dense, high performing electronics. In this thesis, conductive features down to 1.5 µm are demonstrated using a novel inkjet printing-based process. Secondly, high-resolution printed conductors usually have poor current-carrying capacity, especially for longer wires in large-area applications. This thesis explores the fundamentals of aerosol-jet printing and reveals the regime for printing high-resolution lines with excellent current carrying capacity. Additionally, a novel manufacturing process is demonstrated, which can process 2.5 µm wide conductive wires with linear resistances as small as 5 Ω mm-1. Another challenge for printed electronics manufacturing is to deal with topography produced on the substrate surface by printed features. Besides complicating the subsequent use of contact-printing methods, surface topography is a source of poor device yields as well. This thesis describes two novel methodologies of creating topography-free printed surfaces. In the first method, nanometer-level smooth, planarized silver lines are obtained using a transfer printing approach. In the second method, open microchannels, imprinted in plastic substrates, are filled with a controlled amount of metal using liquid-based additive processes, to obtain conductive wires flush with the substrate surface. Finally, this thesis addresses the issue of overlay alignment, which is the most significant challenge of printed electronics manufacturing. Multi-layered electronic devices require alignment of multiple layers of different materials with micron-level tolerances, which is a daunting task to accomplish on deformable, moving substrates in R2R production formats. This thesis describes a novel, self-aligned manufacturing strategy for printed electronics that relies on capillary flow of inkjet-printed inks within open micro-channels. Multi-level trench networks, pre-engineered on the substrate surface, are sequentially filled with different inks which, upon drying, form stacked layers of electronic materials. Using this approach, fully self-aligned fabrication of all the major building blocks of an integrated circuit is demonstrated. Overall, this thesis presents several new manufacturing avenues for realizing high-performing and dense electronics on plastic by R2R processing.