Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q21: Instability in Films and InterfacesInstabilities Interfacial
|
Hide Abstracts |
Chair: Pierre-Thomas Brun, Princeton University Room: 706 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q21.00001: Dynamics of a radially expanding liquid sheet: Experiments Nayanika Majumdar, Mahesh Tirumkudulu A recent theory predicts that sinuous waves generated at the center of a radially expanding liquid sheet grow spatially even in absence of a surrounding gas phase. Unlike flat liquid sheets, the thickness of a radially expanding liquid sheet varies inversely with distance from the center of the sheet. To test the predictions of the theory, experiments were carried out on a horizontal, radially expanding liquid sheet formed by collision of a single jet on a solid impactor. The latter was placed on a speaker-vibrator with controlled amplitude and frequency. The growth of sinuous waves was determined by measuring the wave surface inclination angle using reflected laser light under both atmospheric and sub-atmospheric pressure conditions. It is shown that the measured growth rate matches with the predictions of the theory over a large range of Weber numbers for both pressure conditions suggesting that the thinning of the liquid sheet plays a dominant role in setting the growth rate of sinuous waves with minimal influence of the surrounding gas phase on its dynamics. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q21.00002: Anisotropic instability of a stretching film Bingrui XU, Minhao Li, Daosheng Deng Instability of a thin liquid film, such as dewetting arising from Van der Waals force, has been well studied, and is typically characterized by formation of many droplets. Interestingly, a thin liquid film subjected to an applied stretching during a process of thermal drawing is evolved into an array of filaments, i.e., continuity is preserved along the direction of stretching while breakup occurs exclusively in the plane of cross section. Here, to understand this anisotropic instability, we build a physical model by considering both Van der Waals force and the effect of stretching. By using the linear instability analysis method and then performing a numerical calculation, we find that the growth rate of perturbations at the cross section is larger than that along the direction of stretching, resulting in the anisotropic instability of the stretching film. These results may provide theoretical guidance to achieve more diverse structures for nanotechnology. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q21.00003: Faraday Instability in a Conjugated-Double-Liquid-Layer System Sicheng Zhao, Steffen Hardt, Mathias Dietzel The Faraday instability of two conjugated liquid layers of disparate thickness is investigated theoretically. Owing to the coupling effect, a unique instability mode is shared between the two layers. At the same time, caused by the disparity of the layer thicknesses, the top thick layer destabilizes with wavelengths at the same order of its own thickness, i.e. the short-wavelength mode (SW), while the bottom thin film exposes a long-wavelength (LW) mode. Different from cases where the two layers are both sufficiently thick, the deformation of the liquid-liquid interface in the present system is mainly induced by the viscous shearing effect. Because the inertial force is negligible in the bottom film, the potential Rayleigh-Taylor instability is rather weak even when the top layer has a higher density. The influence of the bottom film on the top layer can be characterized as a generalization of the classical Navier-slip boundary condition which lowers the instability threshold. Therefore, as the thickness of the bottom film becomes very small, the present system serves as a testing ground for the influence of the wall boundary condition on the onset of instabilities in liquid films. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q21.00004: Parametric excitation of large-scale Marangoni convection in a liquid layer with insoluble surfactant Alexander Mikishev, Alexander Nepomnyashchy The large-scale Marangoni convection in a liquid layer heated from below in the presence of an insoluble surfactant on its deformable free surface is considered. The layer is subject to vertical vibrations with the frequency 2ω. Nonlinear amplitude equations governing the evolution of large-scale disturbances of temperature, surfactant concentration and surface deformation are derived in the limit of small Biot number and large capillary number. The system of linearized equations is investigated by means of the Floquet approach. The thresholds of subharmonic, harmonic and quasi-periodic types of instabilities are determined. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q21.00005: Confinement effects in premelting dynamics Satyajit Pramanik, John Wettlaufer We examine the effects of confinement on the dynamics of premelted films driven by thermomolecular pressure gradients. Our approach is to modify a well-studied setting in which the thermomolecular pressure gradient is driven by a temperature gradient parallel to an interfacially premelted elastic wall. The modification treats the increase in viscosity associated with the thinning of films studied in a wide variety of materials using a power law and we examine the consequent evolution of the elastic wall. We treat (i) a range of interactions that are known to underlie interfacial premelting and (ii) a constant temperature gradient wherein the thermomolecular pressure gradient is a constant. The difference between the cases with and without the proximity effect arises in the volume flux of premelted liquid. The proximity effect increases the viscosity as the film thickness decreases thereby requiring the thermomolecular pressure driven flux to be accommodated at larger temperatures where the premelted film thickness is the largest. Implications for experiment and observations of frost heave are discussed. [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q21.00006: Modelling of hydrothermal instabilities in a capillary bridge Dipin Pillai, Alex WRay, Ranga Narayanan We examine the behaviour of a capillary bridge/boat suspended between two heated plates. Such systems are common in many physical situations such as crystal growth processes. However, as shown experimentally by Messmer et al. [1], the system exhibits a complex array of behaviours driven by a Marangoni instability. While qualitative arguments have been advanced for these behaviours in the past, we develop a complete low-order model to elucidate the mechanisms at work. The model takes into account viscosity, surface tension, Marangoni stress and inertia as well as a full convection-diffusion equation for the thermal effects. Detailed comparisons of flow fields and thermal distributions are made with experiments.\\ {}{[1]} B. Messmer, T. Lemee, K. Ikebukuro, I. Ueno, and R. Narayanan, "Confined thermo-capillary flows in a double free-surface film with small Marangoni numbers." International Journal of Heat and Mass Transfer 78 (2014): 1060-1067. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q21.00007: Designing functional materials with interfacial instabilities in thin films Pierre-Thomas Brun, Joel Marthelot, Liz Strong, Pedro Reis We harness interfacial instabilities in thin liquid films to spontaneously fabricate lens-shaped solid structures. A liquid elastomeric polymer is coated on a flat substrate, which is then turned upside-down, leading to the formation of a pattern of drops emerging through the Rayleigh-Taylor instability. As the polymer cures, this array of liquid drops solidifies, thereby permanently structuring the geometry of the originally fluid system. Upon curing, the structure can be used as a network or the solid drops may be peeled off from the substrate and used individually as lenses, making this method an inherently scalable fabrication pathway. Carefully designing a layout of surface defects, etched on the substrate onto which the thin film is initially deposited, is used as seed perturbations that trigger the instability in a controlled manner. This perturbation field significantly augments the monodispersity of the final solid structures, thereby making the fabrication process both robust and scalable. Timing and initial conditions allow us to control the drops amplitude. While centimeter drops are the norm, we demonstrate that much smaller scales are attainable, so that this method could be used in the micro-fabrication of soft materials. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q21.00008: Modelling of cracking in evaporating, particle-laden sessile drops Arandeep S. Uppal, Richard V. Craster, Omar K. Matar Biological fluids can exhibit a variety of complex phenomenon. For example, during evaporation complex particle-laden droplets exhibit a gel transition at the contact line. This gel subsequently invades into the fluid phase and cracks. The resultant crack pattern can give an insight into the initial composition of the fluid. The cracking process itself is not well understood, with experimental observations giving multiple potential reasons. To further understand why and how these gels crack, we study the fracture of solid annuli within the framework of the Griffith theory. Under a quasi-static assumption, we consider hypotheses presented within the literature for two-dimensional annuli. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q21.00009: Influence of viscoelasticity on bubble coalescence in wormlike micellar solutions Vineeth Chandran Suja, Aadithya Kannan, Bruce Andrew Kubicka, Gerald Fuller The stability of bubbles against coalescence in wormlike micellar (WLM) solutions is of importance for many industries. The viscoelasticity of the WLM solutions plays a key role in the bubble stability and can be tuned to change the various stages in the drainage of the thin film between bubbles leading to coalescence. Using a single bubble thin film interferometric technique, we have studied the influence of WLM solution (CTAB /Sodium Salicylate) viscoelasticity on the three stages of thin film drainage-- the fluid entrainment, dimple formation and its stability, and the transition to a black film prior to coalescence. The fluid entrained decreased with a decrease in the viscoelastic moduli. The bubble also deformed resulting in the formation of a dimple. The dimple formation and its washout depends on the viscoelastic properties of the solution. For lower surfactant concentration, the dimples were seen to become unstable and have larger washout velocities. Further, depending on the relaxation time of the WLM solution, complex washout dynamics involving rapid dimple deceleration, recoil and oscillatory washouts are observed. As the film thins further, we observe stark step-wise transition to a black film, as opposed to pure surfactant systems that exhibit multiple steps. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q21.00010: On the stability of surfactant-laden interfaces in thin-film shear flows Anna Kalogirou, Mark Blyth In this study, we investigate the stability of a two-fluid shear flow with a surfactant-populated interface. The two fluids have in general different densities, viscosities and depths, but here we consider the case with one of the layers being very thin compared to the other. We therefore derive an asymptotic model in the thin-layer approximation, consisting of a set of nonlinear PDEs to describe the evolution of the film and interfacial surfactant disturbances. A novel feature is the presence of a nonlocal term due to multiphase coupling. Interfacial instabilities are induced due to the acting forces of gravity and inertia, as well as the action of Marangoni forces generated as a result of the dependence of surface tension on the local surfactant concentration. We find that in the inertialess limit, a stably stratified flow can become unstable if an insoluble surfactant is present at the interface. Inertial flows are known to be unstable in the absence of surfactant (due to density and viscosity stratification); yet, we identify regions in parameter space where stability is supported due to the existence of the surfactant monolayer at the interface. The destabilising mechanism related to the Marangoni forces will also be discussed. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q21.00011: Thin film flow along a periodically-stretched elastic beam Chris Boamah Mensah, Greg Chini, Oliver Jensen Motivated by an application to pulmonary alveolar micro-mechanics, a system of partial differential equations is derived that governs the motion of a thin liquid film lining both sides of an inertia-less elastic substrate. The evolution of the film mass distribution is described by invoking the usual lubrication approximation while the displacement of the substrate is determined by employing a kinematically nonlinear Euler--Bernoulli beam formulation. In the parameter regime of interest, the axial strain can be readily shown to be a linear function of arc-length specified completely by the motion of ends of the substrate. In contrast, the normal force balance on the beam yields an equation for the substrate curvature that is fully coupled to the time-dependent lubrication equation. Linear analyses of both a stationary and periodically-stretched flat substrate confirm the potential for buckling instabilities and reveal an upper bound on the dimensionless axial stiffness for which the coupled thin-film/inertial-less-beam model is well-posed. Numerical simulations of the coupled system are used to explore the nonlinear development of the buckling instabilities. [Preview Abstract] |
Tuesday, November 21, 2017 3:13PM - 3:26PM |
Q21.00012: Induced motion of a sphere due to a flexible elastic sheet Bhargav Rallabandi, Naomi Oppenheimer, Thomas Salez, Howard A. Stone A sphere translating parallel to a rigid wall in Stokes flow experiences an increased drag but no normal force. In contrast, a sphere translating along the surface of a soft elastic substrate experiences an induced normal force due to the coupling between hydrodynamic stresses and elastic deformation. Here, we use theory and experiments to show that an analogous effect occurs for a particle moving near a flexible elastic membrane with bending and stretching resistances. Applying the Lorentz reciprocal theorem in the lubrication limit, we find that the induced force on the particle is repulsive, scaling with the square of its translational speed and inversely with the bending modulus and tension of the membrane. The theoretical predictions are validated by experiments of a sphere driven by gravity down a vertically suspended elastic sheet, where we observe a spontaneous motion of the sphere away from the sheet. The general theoretical approach and the specific results are pertinent to the dynamics of objects near biological membranes and other deformable interfaces. [Preview Abstract] |
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. |
© 2024 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
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700