Now showing 1 - 5 of 5
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    Thickness regimes of power law liquids dip coated onto permeable substrates
    (01-04-2021)
    Sathyanath, Rahul
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    Coating thin liquid films with complex rheological behaviour on permeable substrates is often an important requirement in several applications such as contact lenses, textiles, and paper-based electronics. Here, we extend the classical Landau-Levich problem of dip coating of Newtonian liquids on rigid substrates to liquids of power-law rheology on permeable substrates. Our results suggest distinct deviation from the classical Landau-Levich relation through exhibition of different regimes of varying dependence of coating film thickness on withdrawal speed. A process map is presented depicting these coating thickness regimes for a wide range of operating parameters such as the substrate permeability factor, power-law exponent of the liquid, and a rescaled capillary number.
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    Liquid film entrainment during dip coating on a saturated porous substrate
    (08-06-2020)
    Sathyanath, Rahul
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    Aarthi, A.
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    The present work examines the effect of substrate surface porosity on the coating thickness and meniscus profile during dip coating under saturated porous media conditions. The classical Landau-Levich formulation is modified by encoding the influence of porosity in an effective Navier slip boundary condition at the porous substrate surface. It is shown that simplified Navier slip-based model works well for creeping flow through the porous medium. The film height profile equation is derived as a function of a rescaled capillary number (Ca‾) and a substrate permeability factor, with inertial effects neglected. Numerical solutions show that the classical 2/3rd power dependence of film thickness on capillary number is recovered only at sufficiently high Ca‾ values. As Ca‾ is decreased, a marked deviation is seen. The shrinking of the entrainment meniscus and the change in meniscus curvature are analyzed in detail. The theoretical results are also validated with a suitable experimental system.
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    Emergent patterns and stable interfaces during radial displacement of a viscoelastic fluid
    (20-11-2021)
    Palak,
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    Sathyanath, Rahul
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    Bandyopadhyay, Ranjini
    The displacement of a more viscous fluid by a less viscous fluid renders the fluid-fluid interface unstable and leads to intricate patterns called viscous fingers. Since the fluids experience shear during displacement, it should be possible to influence the emergence of patterns and instability dynamics through control of rheological parameters, such as elasticity or relaxation time in case of a viscoelastic fluid. In this article, we record and analyze the interfacial fingering patterns that emerge when a Newtonian fluid (glycerol-water mixtures of different viscosities) displaces a shear-thinning viscoelastic fluid (aqueous cornstarch suspensions of varying concentrations) in a radial Hele-Shaw cell geometry. While Newtonian-non-Newtonian fluid pair displacements have drawn attention of researchers in the past, the current work showcases the various regimes of emergent patterns over a wide range of viscosity ratios of the two fluids, and the effect of fluid elasticity on the rate of its displacement. As the ratio of viscosities of the inner and outer fluids is increased, radial branched patterns are replaced by more stable interfaces that display finger coalescence. Increasing the viscosity of the displacing fluid and the concentration-dependent elasticity of the outer viscoelastic fluid both lead to significant suppression of interfacial instabilities. A linear stability analysis of the interface, using viscosity ratio as the only control parameter, is employed to predict the dominant wavelength of interfacial perturbation. The perturbation wavelength computed numerically is found to match closely with the spacing between fingers measured experimentally at the onset of interfacial instability. It is suggested that control of instabilities during miscible displacement of a viscoelastic fluid (mud slurries, for example) by a Newtonian fluid has implications in material processing, such as in ensuring minimal mixing of phases while maximizing sweep efficiency during material recovery.
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    Morphological features and dewetting behaviour of thin polymer films coated on porous substrates
    (01-01-2021)
    Sathyanath, Rahul
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    Polymer thin films coated onto non-wetting substrates tend to become unstable and dewet when heated above the glass transition temperatures. Such dynamics are relevant to applications like organic microelectronics and lithography that require non-continuous and arrayed thin polymer films. In several situations, the substrate of interest has finite permeability due to its porous surface, as in paper, fabric, or sandstones, which may modify the hydrodynamics of a liquid film. In the present work, we report model experiments of thin polystyrene films dewetting nanoporous alumina membranes of different pore sizes. Polystyrene films are prepared by spin coating a toluene-based solution, and heated above the glass transition temperature to initiate dewetting. The effect of annealing time on morphologies of the film is studied. During annealing, the film dewets the porous substrate. However, as pores become saturated, excess polymer self-organizes into a uniform coating. The dynamics of liquid film dewetting near permeable substrates are explained from a fluid mechanical perspective through numerical simulations.
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    Features of colloidal silica deposits dip coated onto porous alumina membranes from aqueous suspensions
    (01-11-2021)
    Sathyanath, Rahul
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    Aarthi, A.
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    The present work is directed towards understanding the development of coating morphologies and the accompanying hydrodynamics of suspension flows during dip coating on porous substrates. Colloidal silica particles are deposited from aqueous suspension onto nanoporous alumina membranes. The choice of colloidal silica as the model system is motivated by its popularity in fabrication of antireflection coatings, and as nanostructured material templates for electrochemical and catalytic applications. Nanoporous alumina as substrate is notable for its chemical and thermal stability, and its ability to form self-organized pore structures. The objective here is to characterize the dependence of coating morphologies on membrane pore size, wettability, dip coating speed, and size of colloidal particles in suspension. A coating regime map is formulated based on different morphologies obtained for coatings deposited under different operating parameter spaces. The map indicates existence of two perceptibly distinct regimes which emanate from particle-induced interfacial deformation of the meniscus: Scarcely populated particulate deposition regime and densely packed coating regime having significant amount of particle deposition. The experimental results are analysed and explained using numerical results from earlier mathematical models in literature which cater to modified Landau-Levich formulation for porous substrates, as well as the hydrodynamics of colloidal assembly on moving substrates.