Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G4: Drops IV: Breakup |
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Chair: Ellen Longmire, University of Minnesota Room: 307 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G4.00001: Breakup of liquid filaments over a wide range of Ohnesorge number Alfonso A. Castrejon-Pita, Jose R. Castrejon-Pita, Ian M. Hutchings This work presents a study of the breakup of liquid filaments over a wide range of Ohnesorge numbers (0.001 to 10) using a simple large-scale jet generator. The experimental arrangement features a variable-size nozzle, whose radius can be adjusted from 100 $\mu$m to 3 mm and which is capable of jetting liquids with viscosities between 1 mPa s and 1.4 Pa s. The actuator of this generator consists of an electromagnetic vibrator that can be driven by arbitrary waveforms in order to control the jet formation process. The instrumentation also includes a fast pressure transducer to monitor the true pressure drive produced by the actuator. The filament breakup behavior and distribution (regime diagram), in terms of aspect ratio and Ohnesorge number, is compared with the predictions from previously published models. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G4.00002: ABSTRACT WITHDRAWN |
Monday, November 21, 2011 8:26AM - 8:39AM |
G4.00003: Instability of the crown formed by a splashing droplet Xinjun Guo, Shreyas Mandre Splashing from impacts between liquid drops and a surface is of importance for many natural and industrial processes. The mechanism which dominates the crown breakup has mostly been proposed to be the capillary instability of the rim that forms and grows at the boundary of the crown. But no such theory has so far captured the main properties of the crown. Rayleigh's theory of capillary thread breakup predicts the size of secondary drops to be comparable with the thickness of the unperturbed rim, but the rim is constantly growing. Motivated by this puzzle, we study the instability of a cylindrical thread of liquid, which is constantly growing due to a source of liquid along its centerline. This is our idealized model for the breakup of a liquid lamella rim into droplets. We find that the most unstable wavelength grows with time proportionally to the radius of the thread and the amplitude of the perturbations do not grow like $e^{\lambda t}$ but like $e^{\lambda t^{1/4}}$. Also, the stability theory we use on this time-dependent non-autonomous system generalizes the traditional modal stability theory and the non-modal approaches that have developed more recently. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G4.00004: Pinch-off and reconnection of axi-symmetric drops in Stokes flow Shadi Naderi, Monika Nitsche Under certain conditions, a drop in Stokes flow subject to a non-linear strain field collapses at two points on the axis, leading to a finite time pinch-off singularity. The evolution towards pinch-off is computed using a high order method with local mesh refinement, and found to satisfy a linear self-similar scaling law, as expected. A numerical method is presented to simulate the breaking of the drop at pinch-off and its reconnection into three pieces. We verify that the drop evolution after reconnection is not affected by details of the numerical method. The post reconnection recoiling process is found to satisfy a linear scaling law as well. The computed evolution of the middle drop is furthermore observed to display pinch-off at least two more times. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G4.00005: Gas-Liquid Droplet Microfluidics Carlos Hidrovo, Brian Carroll The possibility of replacing the highly viscous continuous phase oil for a less viscous gaseous phase offers opportunities for higher flow rates, reduced pumping power, and increased droplet inertia for high speed mixing applications. Liquid droplet generation in a gaseous microflow, however, is often characterized as unwieldy, difficult, and less forgiving compared to aqueous droplet generation in silicon oils. Creating droplets in common microchannel geometries, such as T-junctions and flow focusing arrangements, is undoubtedly possible but the metamorphosis of topology, such as spherical droplets, asymmetrical slugs, trailing pears, and liquid threads is significantly different than liquid-liquid flows. This presentation addresses the fundamental operation of liquid droplet generation in a confined gaseous microflow. Droplet volume, shape, and generation rates are experimentally characterized for common and not-so common microchannel geometries. Transitions from slugs to pools to films for T-junctions and dripping to jetting to threading for flow focusing are identified. High speed images are used to quantify the discrete phase characteristics and qualify the generation and detachment process. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G4.00006: A mesoscale study of shear pinch-off Marco Arienti, Xiaoyi Li The dynamic behavior of a liquid thread undergoing thinning and pinch-off under stretching is simulated using Many-body Dissipative Particle Dynamics (MDPD). The mesoscale nature of this method is first verified with the well-known capillary pinch-off dynamics, where the cascade of self-similar regimes -- inviscid (2/3 power law), inertial-viscous (linear power law), and stochastic (0.418 power law) -- can be fully captured in a single simulation. Computationally, the imposition of axial stretching requires a new boundary treatment because periodicity cannot accommodate opposing motions at the two ends of the liquid thread. A new algorithm implementing multi-component non-periodic boundary conditions (MCNPBC) is shown to enforce a prescribed liquid-gas interface at the boundary while maintaining a constant number density of both components. The first feature makes multi-component particle methods more flexible, since the computational boundaries can cut through the liquid thread; the second is crucial because in MDPD the repulsive interaction is density-dependent. With this boundary treatment, the strain rate due to an outer fluid is introduced as a parameter in the balance of capillary, viscous, and inertial forces leading to pinch-off. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G4.00007: Slender body theory for tip streaming: The formation of concentrated emulsions composed by micron-size drops Elena Castro-Hernadez, F. Campo-Cortes, J.M. Gordillo Under creeping flow conditions a liquid of viscosity $\mu_i$ is injected through a cylindrical capillary tube immersed into an outer immiscible fluid with a viscosity $\mu_o$. The outer fluid flows parallel to the axis of the tube. If the outer capillary number $Ca_o>1$ and the inner to outer flow rate ratio $Q_i/Q_o\ll 1$, the interface exhibits a cone-jet transition similar to that observed in electrosprays. The jet emanating from the tip of the cone is so small that the drops formed can be several orders of magnitude smaller than the diameter of the injector. We present a slender body theory that provides a third order ordinary differential equation for the shape of the interface. The theoretical shape, which is found by shooting with just a single parameter from far downstream towards the tube exit, faithfully reproduces the cone-jet transition observed in experiments for arbitrary values of the three dimensionless numbers that control this physical situation: $Ca_o$, $\mu_i/\mu_o$ and $Q_i/Q_o$. This theory is a powerful design method since it can be used to check the efficiency of new geometrical designs to generate monodisperse emulsions, avoiding the need to build and test them in the laboratory. Drop size is well predicted by the classical Tomotika's stability analysis. [Preview Abstract] |
Monday, November 21, 2011 9:31AM - 9:44AM |
G4.00008: Stabilizing toroidal and higher-genus droplets using viscoelastic media Alberto Fernandez-Nieves, Ekapop Pairam We inject a viscous liquid through a needle into another rotating viscous liquid to generate toroidal droplets. These droplets are unstable and undergo a transformation into spherical droplets driven by surface tension: They either break ala Rayleigh-Plateau or grow fatter to become a single spherical droplet depending on the aspect ratio of the torus. By replacing the outer phase with a viscoelastic fluid with a non-zero yield stress we can stabilize these and other non-zero genus droplets. We will examine this stabilization mechanism and present criteria to effectively prevent the break-up of these droplets. [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G4.00009: The hydrodynamics of splash-cup seed dispersal Guillermo Amador, Yasukuni Yamada, David Hu Over 20 species of plants have raindrop-sized flowers that catch raindrops opportunistically, directing the resulting splash to scatter their 0.3 mm seeds over distances of 1 meter. In this combined experimental and theoretical study, we measure the effectiveness of this drop-based projectile launching. High speed videography is used to visualize drop impacts onto the conical flowers of the plants {\it Chrysosplenium} and {\it Mazus} as well as their shape mimics fabricated using a 3D printer. We observe dispersal distance is strongly dependent on the impact parameter between the drop and flower. Off-center impacts maximize dispersal distance by amplifying fluid velocity by a factor of three in a manner similar to the oblique impact of a jet onto a horizontal plane (Kate et al 2007). Further increase in dispersal distance is accomplished through increase of the inertia of the seed by encapsulation into drops. [Preview Abstract] |
Monday, November 21, 2011 9:57AM - 10:10AM |
G4.00010: Mechanical and dynamical responses of pinned drops and polymer balloons Chun-Ti Chang, Susan Daniel, Paul Steen Surface tension is responsible for the nonlinear response of individual spherical-cap drops. This nonlinearity determines the aggregate behavior of a system of interacting drops. Potential applications for droplet systems include switchable-adhesion devices and micro-dosing of personalized pharmaceutical drugs. For a single spherical-cap drop, the pressure-volume (pV) response exhibits one maximum. With the coverage of its free surface by a soft polymer film, the drop becomes a polymer balloon and can change its stability as a result of the reinforced surface tension. The appearance of a second turning point, for example, increases considerably the complexity of the system behaviors exhibited. In this study, we contrast the static and dynamical responses of drops and polymer balloons, taking the perspective that their nonlinear behaviors fall into 1-parameter continuum of pV-responses. Behavior observed in experiment for both excited and unexcited situations will be reported. [Preview Abstract] |
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