Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session L8: Drops IX |
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Chair: Ying Sun, Drexel University Room: 25A |
Monday, November 19, 2012 3:35PM - 3:48PM |
L8.00001: Bag Breakup of Viscous Drops Varun Kulkarni, Daniel Guildenbecher, Stephanie Firehammer, Paul Sojka Secondary breakup of drops has been of interest since the early 1900s. The present work focuses on the drop fragmentation process in the presence of a continuous gas - jet at \textit{We} corresponding to the bag breakup regime. Its purpose is to extend current understanding of inviscid drops to the viscous case through a combination of theoretical and experimental efforts. Various aspects of the physical process, such as regime boundaries, drop fragment sizes, and initiation time, which have been hitherto mostly empirical, are studied. The theoretical formulations are based on conservation equations and hydrodynamic linear stability analysis. Techniques which involve extensive testing using PDA and high speed imaging are employed to compare model predictions with experimental data. The breakup event, as visualized through the bag expansion extent is seen to occur at a slower rate than its inviscid counterpart and is captured adequately by theory. This revealed the reasons for \textit{Oh} dependence. Also seen is the emergence of a bimodal droplet size distribution corresponding to the rim and bag fragments. Finally, the extent of bimodality was seen to be dependent on \textit{Oh}. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L8.00002: Self-similar breakup of near-inviscid liquids Alfonso A. Castrejon-Pita, J. Rafael Castrejon-Pita, John R. Lister, E. John Hinch, Ian M. Hutchings Experimental results are presented for the final stages of drop pinch-off and ligament breakup for different initial conditions. Water and ethanol were used as working fluids. High-speed imaging and image analysis were utilized in order to determine the contraction rate of the thinning neck and the shape of the liquid thread just before the breakup. Our results show that the geometry of the breakup of near-inviscid fluids is self-similar in the domain of simple dripping. We also demonstrate that, independently of the initial conditions, the necking of these liquids scales with $\tau^{2/3}$, asymptotically giving a unique breakup angle of $18.0 \pm 0.4^\circ$. Both observations are in complete agreement with previous theoretical predictions. The angle converges towards self similarity like $\tau^{1/2}$, also as predicted. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L8.00003: The Breakup of Water Cylinders Behind Normal Shocks J.C. Meng, T. Colonius We simulate the drift and breakup of a water cylinder in the flow behind a normal shock. The unsteady Euler equations, closed using the stiffened-gas equation of state, are solved with a compressible, multicomponent, shock- and interface-capturing algorithm. The effects of surface tension and viscosity are negligible at early times compared to the larger shear forces. Computed drift velocities are in good agreement with experiments. For the high- speed flow regimes considered, the breakup mode is stripping. Pressure gradients arise on the cylinder's surface causing it to deform laterally. As the cylinder is flattened, sheets of liquid are drawn off the periphery and break up further downstream. Unsteady vortex shedding is observed in the wake of the disintegrating cylinder. As the shock Mach number is increased, higher airflow velocities result in faster breakup and greater cylinder accelerations. These accelerations are subject to fluctuations that grow with shock strength. Qualitative features of the flow are compared to images from experiments on cylinders and drops. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L8.00004: Impact force of a falling drop Dan Soto, Cristophe Clanet, David Quere Controlling droplet deposition is crucial in many industrial processes such as spraying pesticides on crops, inkjet printing or spray coating. Therefore, the dynamics of drop impacts have been extensively studied for more than one century. However, few literature describe the impacting force of a drop on a solid flat surface, although it might be a way to measure the size distribution of a collection of falling drops. We investigated experimentally how the instantaneous force at impact depends on impact velocity and drop radius. We also propose a new model to understand our observations. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L8.00005: Fabrication of nano-emulsions by bursting bubble at a liquid-liquid interface Jie Feng, Matthieu Roch\'e, Daniele Vigolo, Luben Arnaudov, Simeon Stoyanov, Howard A. Stone Bubbles bursting at interfaces is a familiar everyday occurrence and plays a role in important processes of transport across interfaces. Here we demonstrate that the bursting of air bubbles at an air-oil-water interface in the presence of a surfactant and a co-surfactant leads to the dispersion of nano-droplets in water. Using high-speed imaging we investigate the mechanism for the dispersion of objects and show that small droplets detach from the boundary of the bubble towards the bulk water during collapse of the bubble. We also characterize the size and stability of the dispersed objects with dynamic light scattering and microscopy techniques. The observations indicate that a well-defined population of few-hundred-nm-sized droplets is produced by bubble bursting, along with a broad range of sizes above 1 $\mu $m. We propose that the dispersed objects are formed because of the rapid motion of the bubble interface during collapse. By varying experimental conditions, we show that the size of the droplets is influenced mainly by the amount of surfactant in the oil phase. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L8.00006: Drop impact on a hydrophobic elastic beam Sean Gart, Katie Norris, Daniel Chique, Sunghwan Jung Plant surfaces found in nature often exhibit hydrophobic wetting properties; a particular example is the surface of leaves. Most leaves are compliant~enough~to survive while being impacted by rain droplets. Here, we investigate this leaf-drop system exhibiting a unique system of coupled elasticity and drop dynamics. By replacing the leaf with a thin piezoelectric cantilever beam, we further measure and harvest this drop kinetic energy as a workable model for an energy-harvester from rain drops. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L8.00007: Impact, Rebound, and Deflection of High-Velocity Continuous Droplet Streams Paul Chiarot, John Donovan, William Doak Continuous ink jet (CIJ) systems generate streams of droplets $\sim $30 $\mu $m in diameter at rates of up to 350 kHz and velocities in excess of 20 m/s. Diverse applications can benefit from this technology; however, reliable manipulation of the jet, including droplet steering and basic on/off control, remains difficult to achieve. We report a novel strategy to manipulate the trajectory of high-velocity CIJ droplet streams using the dielectrophoretic (DEP) force. Droplet rebound at shallow angles is a key feature of this strategy. Therefore, high-velocity droplet impact with hydrophobic and superhydrophobic surfaces was investigated. Rebound from the hydrophobic surface was governed by the Weber number and impact angle. For the superhydrophobic surface, two distinct operating regimes describe the response of the reflected droplet stream. In the first regime, the droplets remain discrete and uniform after the impact, but exhibit significant deformation and oscillation. In the second regime, the droplets coalesce into a puddle at the surface. The droplet spacing of the incoming stream determines which regime rules; with the critical value a function of the Weber number. In the first regime, an accounting of the kinetic and potential energies reveals that neither droplet oscillation nor rotation can fully account for the loss of translational kinetic energy. This suggests that significant internal circulation must occur in the droplets at impact. A simple dynamic model predicts the trajectory of the droplet streams modified by the DEP force. This work is in collaboration with Dr. Thomas Jones at the University of Rochester. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L8.00008: Oscillation and recoil of single and consecutively printed droplets Xin Yang, Viral Chhasatia, Ying Sun Drops are often used as building blocks for line and pattern printing where their interactions play an important role in determining the morphology and properties of deposited functional materials. In this study, the impact, spreading and oscillation of single and consecutively printed drops on substrates of different wettabilities are examined using a high speed camera. The results show that, for a single droplet impacting at a low Weber number, both the inertia and surface tension play important roles in the initial spreading stage before the droplet starts to oscillate. On a substrate of higher wettability, drop oscillation is damped down faster due to stronger viscous dissipation resulted from a longer liquid oscillation path. It is also found that when a drop impacting on an evaporating sessile drop sitting on a hydrophobic substrate, recoil of the combined drop is observed, in contrast to no recoil for the impact of a single drop under the same condition. Furthermore, a single-degree-of-freedom vibration model for the height of oscillating single and combined drops on a hydrophobic substrate is established. The results show that as viscosity of liquid increases, damping of drop oscillation becomes faster, and the combined drop oscillates longer compared to a single drop. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L8.00009: Breakup of impulsively actuated jets from thin films C. Frederik Brasz, Matthew Brown, Yiannis Ventikos, Craig Arnold Understanding the breakup of liquid jets into droplets is important for printing applications, and much progress has been made on slender jets emanating from nozzles (as in inkjet printing). A more recent alternative to inkjet printing is blister-actuated laser-induced forward transfer (BA-LIFT), in which a laser pulse is absorbed in a polymer layer adjacent to a thin film of ink, forming an expanding blister that ejects ink as a jet. The lack of a nozzle allows for a wider range of inks, and the formation of the jet from an expanding blister in a thin film significantly changes the fluid physics. We study the breakup of these jets computationally by forcing an ink film with a boundary that deforms according to experimental time-resolved measurements of expanding blisters. Computational results are compared with experimental images, and parametric studies explore the effects of varying properties like laser energy, ink viscosity, surface tension, and film thickness on jet breakup. Scaling arguments are presented to justify the observed power laws for these parametric studies. [Preview Abstract] |
Monday, November 19, 2012 5:32PM - 5:45PM |
L8.00010: Coalescence, evaporation and particle deposition of consecutively printed colloidal drops Viral Chhasatia, Xin Yang, Jaymeen Shah, Ying Sun In applications such as inkjet printing and spray deposition, colloid drops are often used as building blocks for line and pattern printing where their interactions play important roles in determining the deposition morphology and properties. In this study, the particle deposition dynamics of two consecutively printed evaporating colloidal drops is examined using a fluorescence microscope and a synchronized side-view camera. The results show that the relaxation time of the water--air interface of the merged drop is shorter than that of a single drop impacting on a dry surface. It is also found that both morphology and particle distribution uniformity of the deposit change significantly with varying jetting delay and spatial spacing between two drops. As the drop spacing increases while keeping jetting delay constant, the circularity of the coalesced drop reduces. For the regime where the time scale for drop evaporation is comparable with the relaxation time scale for two drops to completely coalesce, the capillary flow induced by the local curvature variation of the air--water interface redistributes particles inside a merged drop, causing suppression of the coffee-ring effect for the case of a high jetting frequency while resulting in a region of particle accumulation in the middle of the merged drop at a low jetting frequency. By tuning the interplay of wetting, evaporation, capillary relaxation, and particle assembly, the deposition morphology of consecutively printed colloidal drops can be controlled. [Preview Abstract] |
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