Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session Q10: Flow Instability: Interfacial and Thin Film I |
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Chair: Vishrut Garg, Air Products and Chemicals Room: North 124 B |
Tuesday, November 23, 2021 8:00AM - 8:13AM |
Q10.00001: Spatio-temporal evolution of evaporating liquid films sheared by a gas Omair A Mohamed, Michael C Dallaston, Luca Biancofiore Evaporating liquid films under shear are utilized across a wide range of systems, |
Tuesday, November 23, 2021 8:13AM - 8:26AM |
Q10.00002: Experimental Characterization of Optically-Actuated Surface Waves on a Parametrically-Forced Fluid Interface Daniel Borrero, Jeremy T Falk, Teddy C Brewer If a free fluid interface is subjected to sufficiently strong vertical oscillations, it spontaneously becomes unstable and gives way to so-called Faraday waves. Below this threshold, the interace remains flat and is stable to perturbations. However, if the strength of the vertical forcing is close to but below the Faraday threshold, perturbations of the interface can lead to relatively long-lived, if transient, surface waves. In this talk, we present a preliminary experimental characterization of surface waves generated when an infrared laser is pulsed on the surface of a parametrically-forced silicone oil bath. The wave topography is characterized using free-surface synthetic schlieren and compared to wave fields generated when a millimetric drop bounces on the interface. The effect of the phase between the laser pulses and the vertical driving on the actuation and subsequent evolution of surface waves is discussed. |
Tuesday, November 23, 2021 8:26AM - 8:39AM |
Q10.00003: Too fast to grow: Dynamics of pendant drops sliding on a thin film Pierre-Thomas Brun, Etienne Jambon-Puillet, Francois Gallaire, Pier Giuseppe Ledda Pendant drops suspended on the underside of a wet substrate are known to accumulate fluid form the surrounding thin liquid film, a process that often results in dripping. The growth of such drops is hastened by their ability to translate over an otherwise uniform horizontal film. Here we show that this scenario is surprisingly reversed when the substrate is slightly tilted ($\approx 2$ deg) ; drops become too fast to grow and shrink over the course of their motion. Combining experiments and numerical simulations, we rationalize the transition between the conventional growth regime and the previously unknown decay regime we report. Using an analytical treatment of the Landau-Levich meniscus that connects the drop to the film, we quantitatively predict the drop dynamics in the two flow regimes and the value of the critical inclination angle where the transition between them occurs. |
Tuesday, November 23, 2021 8:39AM - 8:52AM |
Q10.00004: Experimental investigation of the electrostatically forced Faraday instability Sebastian Dehe, Maximilian Hartmann, Aditya Bandopadhyay, Steffen Hardt We study the instability of an interface between a dielectric and a conducting liquid subjected to a spatially homogeneous harmonically oscillating electric field, and show the analogies of the resulting wave structures to the mechanically induced Faraday instability. We use high-speed imaging in combination with an algorithm to evaluate light refraction at the interface to extract the instability wavelengths and modes, and investigate the influence of the liquid viscosities on both the critical instability voltage and the emerging interfacial wavelength. We demonstrate agreement between the experimentally obtained interfacial wavelengths and theoretical predictions accounting for viscosity effects. In addition, we show that depending on the influence of the boundary of the circular domain, the instability can exhibit either discrete modes of the surface harmonics, or boundary-independent patterns. Validating the theoretical predictions of the instability modes and demonstrating the analogies between mechanical and electrostatic forcing is important for a better control of electrostatically driven instabilities. |
Tuesday, November 23, 2021 8:52AM - 9:05AM Not Participating |
Q10.00005: Whirling instability of an eccentric coated fiber Shahab Eghbali, Ludovic Keiser, Edouard Boujo, Francois Gallaire The destabilization of the gravity-driven flow of a viscous liquid thread coating a vertical cylindrical fiber into a downward moving train of beads has been linked to the conjunction of the Rayleigh-Plateau and Kapitza instabilities when the surface tension dominates over gravity (small Bond numbers, $Bo$). We focus on the limit of large $Bo$ by means of experiments with highly viscous silicone oils flowing down fibers, forming centimeter-diameter threads, and linear stability analyses (LSA) of quasi-inertialess flows (large Ohnesorge number, $Oh$). Relaxing the concentricity of the fiber and the liquid thread, we show the existence of two unstable modes: pearl (P) and whirl (W) modes. The P mode depicts asymmetric beads, whereas a helical interface forms around the fiber in the W mode instability. Detailed LSA of a unidirectional flow along a rigid eccentric fiber is conducted to determine the geometric and hydrodynamic thresholds of the W mode instability. Additionally, energy analysis is carried out to elucidate the whirl formation mechanism. Despite its capillary cost, the asymmetric shear distribution around a small eccentric fiber has the potential to sustain the interface whirl, at sufficiently large $Bo$. We compare the predictions of our model to experimental results. |
Tuesday, November 23, 2021 9:05AM - 9:18AM |
Q10.00006: Electrohydrodynamic interfacial instability at a stagnation point Mohammadhossein Firouznia, Michael J Miksis, Petia M Vlahovska, David Saintillan A wide variety of physical systems and engineering applications involve the deformation of fluid interfaces under the combined effects of an applied electric field and an external fluid flow. Here, we present analytic and numerical approaches to study the stability of a planar interface separating two immiscible fluids subject to an imposed stagnation point flow and a tangential electric field. The interfacial charge dynamics is modeled by accounting for Ohmic conduction, advection by the flow and finite charge relaxation. Using this model, we perform a local linear stability analysis in the vicinity of the stagnation point to study the behavior of the system in terms of the relevant dimensionless groups of the system. Further, we present a numerical normal-mode linear stability analysis based on the full system of equations and boundary conditions using the boundary element method. This allows us to uncover the most unstable eigenmodes directly. Our analysis demonstrates how the interplay between charge convection and conduction in the dominant mode of instability leads to a stabilizing effect. Finally, using numerical simulations of the full nonlinear problem, we demonstrate how the coupling of flow and interfacial charge dynamics gives rise to nonlinear phenomena such as tip formation and the development of charge density shocks. |
Tuesday, November 23, 2021 9:18AM - 9:31AM |
Q10.00007: Asymmetric instability in shear thinning flow down a fiber Chase T Gabbard, Joshua B Bostwick Thin film flow down a fiber is common in many industrial applications including fiber coating and heat/mass transfer processes. Such flows are subject to a number of instabilities including Plateau-Rayleigh breakup, isolated bead formation, and convective instabilities, which typically result in symmetric beading patterns. Recently, an asymmetric instability was observed in Newtonian fluids that is dependent upon surface tension and fiber diameter, but independent of viscosity. Here, we perform an experimental study of thin film flow down fibers with non-Newtonian polymer solutions and reveal how shear thinning behavior can also give rise to an asymmetric instability. We prepare several xanthan gum solutions, tuning their rheology by adding sodium chloride (NaCl) and surfactant (Tween 20). Experimental results show that increasing the intensity of shear thinning, with all other fluid properties held constant, leads to asymmetric beading patterns. We conclude by proposing a physical mechanism for these experimental observations. |
Tuesday, November 23, 2021 9:31AM - 9:44AM |
Q10.00008: Two-dimensional absolute/convective instability analysis through the Riesz transform and application to draperies structures in limestone caves Francois Gallaire, Pier Giuseppe Ledda, Gioele Balestra, Gaetan Lerisson, Benoit Scheid, Matthieu Wyart We study the role of hydrodynamic instabilities in the morphogenesis of typical karst draperies structures encountered in limestone caves. This problem is analyzed using the long wave approximation for the fluid film that flows under an inclined substrate, in the presence of substrate variations that grow according to a deposition law. We numerically study the linear evolution of a localized initial perturbation both in the fluid film and on the substrate, i.e. the Green function. A novel approach for the spatio-temporal analysis of two-dimensional signals resulting from these linear simulations is introduced, based on the concepts of the Riesz transform and the monogenic signal, the multidimensional complex continuation of a real signal. This transform constitutes the two-dimensional analogue of the Hilbert transform. The deposition linearly selects substrate structures aligned along the streamwise direction, as the spatio-temporal response is advected away. We suggest that these linear selection mechanisms contribute to the formation of draperies under inclined cave ceilings. |
Tuesday, November 23, 2021 9:44AM - 9:57AM |
Q10.00009: Local dynamics during spontaneous thinning of thin films on a substrate Vishrut Garg, Sumeet S Thete, Christopher R Anthony, Pritish M Kamat, Osman A Basaran Applications in bio-sensing, energy-efficient materials and photonics have led to an ever-increasing demand for functional patterns on surfaces at micro and nano scales. A promising method for pattern generation is through the spontaneous rupture and dewetting of thin liquid films on a substrate due to intermolecular forces. The liquid film is known to exhibit power-law (deformation-rate-thinning) rheology in many cases, where 0 < n ≤1 is the power-law exponent (n = 1 for a Newtonian fluid). Here, we study the spontaneous thinning and rupture of a thin film of a power-law fluid on a substrate under the influence of van der Waals forces of attraction. We demonstrate that in contrast to thinning of Newtonian films, fluid inertia can become significant during thinning of films of power-law fluids which leads to a breakdown of lubrication theory. The resulting flow transitions are investigated by employing a first of its kind two-dimensional model for film thinning that is able to accurately resolve the highly disparate length scales that are typical in thin film problems. |
Tuesday, November 23, 2021 9:57AM - 10:10AM |
Q10.00010: Spontaneous Formation of Micro-Well During Stretching of Liquid Bridge With Hole Sachin D Kanhurkar, Prasanna Gandhi, Amitabh Bhattacharya Stretching of liquid bridges has a vast number of applications in engineering and biology. High-viscosity liquid bridges, stretched between two plates in the presence of a hole on the top plate, may be used to fabricate micro-well structures quickly and repetitively. To understand the flow dynamics in this system, we perform numerical simulations of the setup using diffuse-interface dual-grid level-set method. In the simulations, we introduce a hole in the bottom plate and then lift the top plate with constant velocity. We find that the air bubble originating from the hole shrinks in size at a low Capillary number (i.e., viscous to surface tension force). On the other hand, at a high Capillary number, the height of the air bubble increases during plate separation. The growth rate of the air bubble depends mainly on the size of the hole at the bottom plate. Once the hole diameter crosses a threshold, the air bubble grows faster than the separation rate of the plates, leading to spontaneous formation of a micro-well. |
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