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 L10: Drops: Instability, Break-up and Splashing IDrops
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Chair: Lydia Bourouiba, Massachusetts Institute of Technology Room: 503 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L10.00001: Splash Dynamics of Falling Surfactant-Laden Droplets Nur Sulaiman, Lewis Buitrago, Eduardo Pereyra Splashing dynamics is a common issue in oil and gas separation technology. In this study, droplet impact of various surfactant concentrations onto solid and liquid surfaces is studied experimentally using a high-speed imaging analysis. Although this area has been widely studied in the past, there is still not a good understanding of the role of surfactant over droplet impact and characterization of resulting splash dynamics. The experiments are conducted using tap water laden with anionic surfactant. The effects of system parameters on a single droplet impingement such as surfactant concentration (no surfactant, below, at and above critical micelle concentration), parent drop diameter (2-5mm), impact velocity and type of impact surface (thin and deep pool) are investigated. Image analysis technique is shown to be an effective technique for identification of coalescence to splashing transition. In addition, daughter droplets size distributions are analyzed qualitatively in the events of splashing. As expected, it is observed that the formation of secondary droplets is affected by the surfactant concentration. A summary of findings will be discussed. [Preview Abstract] |
Monday, November 20, 2017 4:18PM - 4:31PM |
L10.00002: Drop splashing: the role of surface wettability and liquid viscosity. Hamed Almohammadi, Alidad Amirfazli There are seemingly contradictory results in the literature about the role of surface wettability and drop viscosity for the splashing behavior of a drop impacting onto a surface. Motivated by such issues, we conducted a systematic experimental study where splashing behavior for a wide range of the liquid viscosity (1-100 cSt) and surface wettability (hydrophilic to hydrophobic) are examined. The experiments were performed for the liquids with both low and high surface tensions (\textasciitilde 20 and 72 mN/m). We found that the wettability affects the splashing threshold at high or low contact angle values. At the same drop velocity, an increase of the viscosity (up to 4 cSt) promotes the splashing; while, beyond such value, any increase in viscosity shows the opposite effect. It is also found that at a particular combination of liquid surface tension and viscosity (e.g. silicone oil, 10 cSt), an increase in the drop velocity changes the splashing to spreading. We relate such behaviors to the thickness, shape, and the velocity of the drop's lamella. Finally, to predict the splashing, we developed an empirical correlation which covers all of the previous reported data, hence clarifying the ostensible existing contradictions. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L10.00003: The Primary Break-up Instabilities in a gas-liquid coaxial atomizer combined with electro-spray Rodrigo Osuna, Nathanael Machicoane, Alberto Aliseda We present an experimental study of a canonical coaxial gas-liquid atomizer, balancing the physics of gas-assisted atomization and electro-sprays. The laminar liquid stream is injected through a long straight metallic pipe at the center of the turbulent gas jet. The liquid needle is used as the anode, while the cathode is formed by a ring located on the streamwise face of the coaxial gas chamber. The gas Reynolds number ranges from 10$^{\mathrm{4}}$-10$^{\mathrm{6}}$, while keeping the liquid Reynolds number constant at 10$^{\mathrm{3}}$. The electrospray voltage applied is varied from 100 to 5000 V and the resulting negative charge transferred to the liquid jet spans from O(10$^{\mathrm{-3}}$ -- 10$^{\mathrm{-1}})$ Coulomb per cubic meter. The relative influence of the high speed gas to the liquid electric charge on the primary instability and jet break-up is studied. The effect of the electric field on the atomization process is characterized by high speed visualization at the nozzle exit, complemented with the resulting droplet size distribution in the mid field after break-up has ended. The quantitative visualization captures the fast dynamics of the interface de-stabilization and clearly shows the changes in the liquid stream instabilities caused by the electric field. These instabilities control the liquid droplet sizes and their spatio-temporal distribution in the spray, as measured from light interferometry. [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L10.00004: Edge-effect fragmentation in the context of foliar disease transmission S. Lejeune, T. Gilet, L. Bourouiba Rain-induced foliar pathogen propagation is inherently linked to raindrop fragmentation upon impact on infected leaves. Close to leaf edges, the outcome of a drop impact is complex and asymmetric. Despite the ubiquitous nature of impacts close to edges, little is known on the role of edges in shaping drop fragmentation (edge-effect fragmentation). To address this gap, we present a series of drop impact experimental results with impact point close to the surface edge. We focus on the liquid sheet expansion in the air and the role of the edge in introducing the asymmetry in such expansion. We link the edge-induced asymmetry of the sheet to the emergence of different families of droplet-producing fragmentation processes. We discuss how our results can help shed light on foliar disease transmission. [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L10.00005: A simple discrete-time model for describing droplet generation in a periodic energy landscape Georgios Katsikis, Anatoly Rinberg, Manu Prakash The generation of droplets at low Reynolds numbers is driven by non-linear dynamics that give rise to complex patterns concerning both the droplet-to-droplet spacing and the individual droplet sizes. Here we demonstrate an experimental system in which a time-varying energy landscape provides a periodic magnetic force that generates an array of droplets from an immiscible mixture of ferrofluid and silicone oil. The resulting droplet patterns are periodic, owing to the nature of the magnetic force, yet the droplet spacing and size can vary greatly by tuning a single bias pressure applied on the ferrofluid phase; for a given cycle period of the magnetic force, droplets can be generated either at integer multiples (1, 2, etc.), or at rational fractions (3/2, 5/3, 5/2, etc.) of this period with mono- or multidisperse droplet sizes. We develop a discrete-time dynamical systems model not only to reproduce the phenotypes of the observed patterns but also provide a framework for understanding systems driven by such periodic energy landscapes [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L10.00006: Contact lines are unstable even under non-splashing droplets Min Pack, Paul Kaneelil, Ying Sun Drop impact is fundamental to natural and industrial processes such as rain-induced soil erosion and spray coating technologies. In this study, we elucidate the interfacial instabilities formed by air entrainment at the wetting front of impacting droplets on atomically smooth, viscous silicone oil films of constant thickness with varying droplet velocity, viscosity, surface tension, and ambient pressures. A high-speed total internal reflection microscopy technique accounting for the Fresnel relations at the droplet interface allowed for in-situ measurements of an entrained air rim at the wetting front. The growth of the air rim is a prerequisite to the instability which is formed when the gas pressure balances the capillary pressure near the wetting front. A critical capillary number, which inversely scales as the ambient pressure, is predicted and the result agrees well with the experiments. The wavenumber in the instability is shown to increase with viscosity and velocity but decrease with surface tension of the impacting drop. We thus conclude that the instability mechanism is in qualitative agreement with the Saffman-Taylor instability -- where the low viscosity air is displacing the higher viscosity droplet. The low We contact line instabilities observed in this study provide a paradigm shift in the conventional understanding of hydrodynamic instabilities under drop impact which usually require We \textgreater \textgreater 10. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L10.00007: Experiments on the breakup of drop-impact crowns by Marangoni holes Abdulrahman Aljedaani, Aditya Jetly, chunliang wang, Sigurdur Thoroddsen We investigate experimentally the breakup of the Edgerton crown due to Marangoni instability when a highly viscous drop impacts on a thin film of lower-viscosity liquid, which also has different surface tension than the drop liquid. The presence of this low-viscosity film modifies the boundary condition, giving effective slip to the drop along the solid substrate. This allows the drop to form a regular bowl-shaped crown, which rises vertically away from the solid and subsequently breaks up through the formation of a multitude of Marangoni holes. Previous experiments have proposed that the breakup of the crown results from a spray of fine droplets ejected from the thin film These droplets can hit the inner side of the crown forming spots with lower surface tension, which drive the hole formation. We test the validity of this assumption with close-up imaging to identify individual spray droplets, to show how they hit the crown and their lower surface tension drive the hole formation. Surprisingly, in experiments with pools of higher surface tension, we also see hole formation. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L10.00008: Surface acoustic wave driven oscillatory instability of sessile droplets nicolas chastrette, Michael Baudoin, Olivier Bou-Matar, Laurent Royon, Philippe Brunet, Regis Wunenburger Surface acoustic waves (SAW) of frequency larger than say 1 MHz constitute an efficient and reconfigurable technique for transporting sessile droplets. It has been observed that the excitation of sessile droplets with SAWs of frequency of the order of 20 MHz leads to some low frequency ($\sim 100 \rm Hz$) inertial-capillary vibrations, which dramatically improve the mobility of the drop. The nonlinear mechanism responsible for the transfer of energy from the acoustic wave to these low frequency inertial-capillary waves remains unexplained in the literature. In order to determine its origin, we have studied a simplified system consisting in a larger sessile drop ($10~\rm \mu l$) excited at lower frequency (800 kHz to 2 MHz) by a plane SAW. By combining high speed imaging of the oscillating free surface of the droplet and pressure measurements inside the acoustic cavity using a needle hydrophone, we correlate the structure of the excited oscillation eigenmodes, their instability threshold, and the frequency and amplitude of the intracavity acoustic standing wave. We propose an instability mechanism combining the phase modulation of the acoustic wave by the oscillating free surface and the acoustic radiation pressure exerted by the acoustic wave on the free surface as feedback. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L10.00009: Piercing a Liquid Droplet by a Gas Jet Nasser Ashgriz, Shayan Sadeghpour, Amirreza Amighi Impingement of a gas jet on a suspended water droplet is studies experimentally. The jet penetration process depends on the jet velocity and jet to droplet diameter ratio. At small jet to diameter ratios and large enough jet velocities, the jet penetrated through the droplet. The fragmentation process of the droplet depends on the jet velocity. The gas tunnel inside the droplet has a peculiar shape, being smaller on the windward side and larger on the leeward side of the droplet. A large bubble forms on the leeside of the droplet, which burst forming small droplets. At larger jet to droplet diameter, the droplet fragmentation becomes more significant, with large droplet deformation. Different types of breakup process are identified and categorized according to jet to droplet diameter ratio, as well as droplet Weber number. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L10.00010: Breakup Behavior of a Capillary Bridge on a Hydrophobic Stripe Separating Two Hydrophilic Stripes Maximilian Hartmann, Steffen Hardt The breakup dynamics of a capillary bridge on a hydrophobic area between two liquid filaments occupying two parallel hydrophilic stripes is studied experimentally. In addition calculations with the finite-element software \textit{Surface Evolver} are performed to obtain the corresponding stable minimal surfaces. Droplets of de-ionized water are placed on substrates with alternating hydrophilic and hydrophobic stripes of different width. Their volume decreases by evaporation. This results in a droplet shaped as the letter ``H'' covering two hydrophilic stripes separated by one hydrophobic stripe. The width of the capillary bridge $d(t)$ on the hydrophobic stripe during the breakup process is observed using a high-speed camera mounted on a bright-field microscope. The results of the experiments and the numerical studies show that the critical width $d_{\mathrm{crit}}$, indicating the point where the capillary bridge becomes unstable, mainly depends on the width ratio of the hydrophilic and hydrophobic stripes. It is found that the time derivative of $d(t)$ first decreases after $d_{\mathrm{crit}}$ has been reached. The final breakup dynamics then follows a $t^{\mathrm{2/3}}$ scaling. [Preview Abstract] |
Monday, November 20, 2017 6:15PM - 6:28PM |
L10.00011: Development of three-dimensionality in the wake of a nitrobenzene drop falling in water. Louis L. Steytler, Rekha R. Rao, Arne J. Pearlstein The development of three-dimensionality in the wake of a nitrobenzene drop falling in water is investigated numerically using an Arbitrary Lagrangian-Eulerian method, with a moving mesh and an interface-fitted grid. Computations covering flow regimes from steady to unsteady laminar flow for Reynolds numbers (based on the continuous phase properties) up to approximately 800 (i.e., over a range of drop sizes) are reported. The vortical structures in the wake are compared to available experimental results. The flow structures internal to the drops and the drop shapes and trajectories are reported. Some implications for mass and heat transfer are discussed. [Preview Abstract] |
Monday, November 20, 2017 6:28PM - 6:41PM |
L10.00012: Faraday instability on a sphere: Platonic solids and drift Laurette Tuckerman, Ali-higo Ebo Adou, Damir Juric, Jalel Chergui, Seungwon Shin A liquid drop subjected to an oscillatory radial force comprises a spherical version of the Faraday instability. A linear stability analysis of this problem is carried out by applying the Kumar and Tuckerman Floquet method for a spherical geometry. The time-dependent shape of the drop and the velocity field in and around it are calculated using BLUE, a code based on a hybrid Front-Tracking/Level-set algorithm for Lagrangian tracking of arbitrarily deformable phase interfaces. For frequencies corresponding to spherical harmonics of low degree, simulations show Platonic solids which alternate with their duals on a short time scale while drifting on a long time scale. [Preview Abstract] |
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