Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D27: Flow Instability: Interfacial and Thin Film I |
Hide Abstracts |
Chair: Ranganathan Narayanan, University of Florida Room: Georgia World Congress Center B315 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D27.00001: The importance of inertia in vertical film boiling stability Eskil Aursand, Stephen H Davis In film boiling a continuous vapor thin-film forms between a liquid and a heated surface. For practical purposes it is of great interest to model the transient dynamics of this vapor film in order to predict the heat transfer coefficient. Such problems are typically handled in the framework of long-wave theory, where the disturbance aspect ratio (ε) is assumed to be small. Typically the Reynolds number (Re) is also assumed to be small, and this allows the removal of both ε^{2} and εRe terms, which greatly simplifies the Navier-Stokes equations. While this is appropriate for horizontal cases, in vertical film boiling the Reynolds number quickly increases, giving rise to inertial effects that the classical approach fails to capture. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D27.00002: Influence of thermal fluctuations on draining thin liquid films Maulik S. Shah, Volkert van Steijn, Chris R. Kleijn, Michiel T. Kreutzer Thermal fluctuations have been shown to influence the evolution of planar, non-draining thin liquid films, bringing predicted rupture times closer to experimental values. This work explores how thermal fluctuations, characterized by the dimensionless noise strength, θ, alter rupture times of films subjected to drainage. This drainage is induced by adding a curved part to a planar film, which is characterized by the dimensionless curvature, κ. For strong drainage (κ≥1), we find that the film ruptures due to the formation of a dimple that appears at the same location, irrespective of θ. The resulting mean rupture times are insensitive to thermal noise. By contrast, for weak drainage (κ<<1), the film ruptures at a random location on the flat portion of the film with the rupture time, T_{r}, insensitive to κ and governed by the noise strength, as T_{r} ∼(√(2θ))^{-4.2},where the exponent 4.2 is nearly 1/ω_{max}, ω_{max }, being the growth rate of the dominant wavelength based on linear stability theory. These insights, together with the transition between the drainage-dominated and the noise-dominated regime further improves our understanding on the relevance of thermal fluctuations in determining lifetime of the non-planar draining thin films found in emulsions and foams. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D27.00003: Stability analysis of evaporating falling liquid films Hammam Mohamed, Metin Muradoglu, Luca Biancofiore In order to improve the understanding of the wavy dynamics of evaporating falling liquid films, we have performed a linear stability analysis by solving the classical Orr-Sommerfeld eigenvalue problem. The key parameters of the analysis are: (i) Marangoni number $(Ma)$, describing the relation between the thermocapillary stress to the viscous stress, and (ii) the evaporation parameter $(E)$, representing the ratio between the viscous and evaporative time scales. For small $Ma$, a new unstable mode (S-mode) is observed, resulting from the change of surface tension due to the temperature gradient along the free surface. More interestingly, for a specific Ma, the S-mode and H-mode (hydrodynamic mode, due to surface inclination) combine into one unstable region, thus reinforcing each other, leading to a possible formation of dry spots. When the liquid is volatile, a new instability is detected due to vapor recoil which destabilizes the flow in a similar fashion as thermocapillarity. Vapour recoil is also accompanied by a significant thinning of the film, accelerating the film's rupture. Finally, direct numerical simulations are performed to verify the effects of $Ma$ and $E$ on the stability of the liquid films and the results are found to be in agreement with the theory. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D27.00004: The stability of evaporating binary liquid film heated from below Robson Nazareth, George Karapetsas, Sivasankaran Harish, Daniel Orejon, Khellil Sefiane, Prashant Valluri In this work we consider the evaporation of a thin liquid layer which consists of a binary mixture of volatile liquids. The mixture is on top of a heated substrate and in contact with the gas phase that consists of the same vapour of the binary mixtures. The effect of vapour recoil, thermo- and soluto-capillarity and the van der Waals interactions are considered. We derive the long-wave evolution equations for the free interface and the concentration that govern the two-dimensional stability of the layer subject to the above coupled mechanisms and perform a linear stability analysis. The developed linear theory describes two modes of instabilities, a monotonic instability mode and an oscillatory instability mode. By means of transient simulations we analyse how these instabilities depend on the destabilising effects considered. More precisely we discuss how the solutal Marangoni effect defines the mode of instability that develops during the evaporation of the liquid layer due to preferential evaporation of one of the components. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D27.00005: Stability of fluid films of nanoscale thickness involving contact lines Lou Kondic, Michael A Lam, Linda J Cummings We discuss instabilities of fluid films of nanoscale thickness, with a particular focus on films where the destabilizing mechanism allows for linear instability, metastability, and absolute stability. Our study is motivated by nematic liquid crystal films; however similar instability mechanisms, appear in other contexts, such as the well- studied problem of polymeric films on two-layered substrates. Within the long wave formulation, the nematic character of the film leads to an additional contribution to the disjoining pressure, changing its functional form. This effective disjoining pressure is characterized by the presence of a local maximum. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D27.00006: Including thermal effects in computing dynamics of thin films on thermally conductive substrates Ryan H. Allaire, Lou Kondic, Linda J. Cummings Thin film dynamics, particularly on nanoscale, is a topic of extensive interest. The process by which thin liquids evolve is far from trivial and can lead to dewetting and drop formation. Not only does it involve resolving fluid mechanical aspects of the problem, but also requires the coupling of other physical processes, including liquid-solid interactions and heat transfer. In this talk, we focus on multiscale aspects of the problem. Separation of length scales (in-plane length scales are larger than those in the out-of-plane direction) allows for formulation of asymptotic theory that reduces the complicated problem of Navier-Stokes equations in evolving domains to a fourth-order nonlinear partial differential equation for fluid thickness. To include thermal effects, in the form of surface tension gradients, the local temperature profile must be calculated on a temporally evolving domain, presenting numerical challenges. In this talk, we present a thermal transport model, based on asymptotic theory, which reduces the computational complexity and produces consistent results with that of a full heat conduction model. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D27.00007: Thermocapillary instabilities in additive manufacturing environments Katarzyna N Kowal, Stephen H Davis, Peter W Voorhees In additive manufacturing, or three-dimensional printing, material is deposited under rapidly moving heat sources and subsequently solidifies under the ambient thermal field. The substrate on which the melt is deposited ranges from solid to liquid. A model is proposed in which the deposited melt feels effective slip and heat transfer at its base. We determine the effect of various substrates on the hydrothermal instabilities of the liquid melt pool. In particular, we investigate the onset of three-dimensional thermocapillary instabilities in a low-capillary-number liquid layer of arbitrary depth and find that the preferred mode of instability consists of a selection of two- and three-dimensional hydrothermal waves, longitudinal rolls and longitudinal travelling waves. Which of these appear depends intricately on the properties of the substrate involved. As the slip increases, two-dimensional hydrothermal waves and longitudinal travelling waves exchange stability with longitudinal rolls and oblique hydrothermal waves. The details of the instability affect the microstructure of the solidified material, which is crucial as it determines the properties of the resulting product. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D27.00008: Asymptotics of selfsimilar blowup profiles of the thin film equation Michael Dallaston We consider asymptotically self-similar blow-up profiles of the thin film equation. It has previously been shown that blow up is only possible when the exponents in the thin film equation are above a certain criticality threshold. We show that in the limit that the criticality threshold is approached from above, the similarity profiles exhibit a well-defined structure consisting of a peak near the origin, and a thin algebraically decaying tail; these are connected by an inner region, which is mathematically (to leading order) equivalent to the problem near an apparent contact line in lubrication flow. Matching between the regions ultimately gives the asymptotic relationship between the height of the peak and the distance from the criticality threshold, from which all other properties of the profile may be deduced. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D27.00009: Nonlinear Dynamics of Electrostatic Faraday Instability in Thin Films Dipin Pillai, Ranganathan Narayanan A nonlinear inertial lubrication model is developed for the evolution of an interface between a perfect conducting liquid and a perfect dielectric gas subject to periodic electrostatic forcing. Inertial thin films are shown to become unstable to finite-wavelength Faraday modes at the onset, prior to the long-wave pillaring instability reported in the full lubrication limit. The pillaring-mode instability is subcritical with the interface approaching either the top or bottom wall, depending on the liquid-gas holdup. On the other hand, the Faraday modes exhibit subharmonic or harmonic oscillations that saturate to standing waves at low-forcing amplitudes. Unlike the pillaring mode, wherein the interface approaches the wall, Faraday modes may exhibit saturated standing waves when the instability is subcritical. At higher forcing amplitudes the interface may approach either wall, again depending on the liquid-gas holdup. Also, a gravitationally unstable configuration of such thin films cannot be stabilized by periodic electrostatic forcing, unlike mechanical Faraday forcing. In this case, it is observed that the interface exhibits oscillatory sliding behavior, approaching the wall in an “earthworm-like" motion. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D27.00010: Dewetting and pattern formation in ultra-thin films of nematic liquid crystal: effects of variable substrate anchoring Linda Cummings, Michael A Lam, Lou Kondic We consider free surface flow of ultra-thin films of nematic liquid crystal (NLC) on a solid planar substrate. NLCs typically consist of rod-like molecules, which align with their neighbors, imparting effective elasticity and anisotropy. NLC molecules also have a preferred alignment at boundaries, a phenomenon known as anchoring. We study flows where the free surface anchoring is perpendicular to the surface (homeotropic anchoring), and parallel to the substrate (planar anchoring), and we focus particularly on cases where the planar substrate anchoring is spatially inhomogeneous. We use lubrication theory to formulate a long-wave model for the 3D flow of NLCs under these conditions. Our model accounts for the anisotropic stress tensor of NLCs (Leslie-Ericksen theory); for van der Waals' interactions between the NLC molecules and the substrate; and for the inhomogeneous strong planar substrate anchoring, as well as weak homeotropic anchoring at the free surface. We present simulations showing how the details of the substrate anchoring can strongly influence the dewetting patterns observed. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2019 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
1 Research Road, Ridge, NY 11961-2701
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700