Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session KD: Liquid Breakup and Coalescence III |
Hide Abstracts |
Chair: Francois Blanchette, University of Chicago Room: Hilton Chicago Continental A |
Monday, November 21, 2005 4:10PM - 4:23PM |
KD.00001: Unexpected Breakup Dynamics of Compound Jets Pankaj Doshi, Ronald Suryo, Robert Collins, Michael Harris, Osman Basaran Understanding the breakup dynamics of a compound or a two-fluid jet is of great importance in applications such as micro-/nano-encapsulation and emulsion formation. Breakup dynamics of compound jets are studied computationally using finite element analysis. For single fluid jets, it has been known for over a century that a jet is unstable (stable) to small amplitude perturbations if the wavelength of the perturbation $\lambda $ is larger (smaller) than the unperturbed circumference of the jet. Response of compound jets is quite similar to that of single-fluid jets if $\lambda $ is larger (smaller) than the unperturbed circumference of the outer surface of the jet. However, it is shown that an unexpected oscillatory instability results if $\lambda $ is larger than the unperturbed circumference of the inner surface of the jet but smaller than that of the outer surface of the jet when the ratio of the interfacial tension of the outer interface to that of the inner interface is much larger than unity. Pressure fields, streamlines, and energies are interrogated to elucidate the physics of the instability. [Preview Abstract] |
Monday, November 21, 2005 4:23PM - 4:36PM |
KD.00002: Multiple coalescence and pinch off at a fluid interface Francois Blanchette, Terry P. Bigioni, Eric I. Corwin We investigate numerically and experimentally the dynamics of the coalescence of a drop coming in contact with a horizontal interface. We focus on cases where the drop repeatedly coalesces and pinches off, forming a sequence of progressively smaller drops. We determine the regime in which such a cascade can occur and describe for the first time the details of the mechanism behind multiple coalescence. Viscous damping of capillary waves is found to be crucial in determining whether pinch off will occur or not, despite the fact that only a small fraction of the available energy is dissipated by viscous effects. When pinch off does occur, we also characterize the following bouncing of the residual drop on the oscillating interface. [Preview Abstract] |
Monday, November 21, 2005 4:36PM - 4:49PM |
KD.00003: Coalescence of Spreading Droplets on a Wettable Substrate W.D. Ristenpart, P.N. McCalla, H.A. Stone We investigate experimentally the coalescence dynamics of two slowly spreading droplets on a highly wettable substrate. Upon contact, surface tension drives a rapid motion perpendicular to the line of centers that joins the drops and lowers the total surface area. The coalescence behavior is characterized by the time-dependent width $w_m$ of the growing meniscus bridge between the two drops. We find that the growth rate of $w_m$ is always viscously dominated and at early times exhibits power-law-like behavior wherein $w_m\sim t^{0.7\pm0.1}$. Moreover, the growth rate is highly sensitive to both the radii and heights of the droplets at contact. This feature differs significantly from the behavior of freely suspended droplets, in which the coalescence growth rate depends only weakly on the droplet size. We present scaling arguments that accord with the experimental observations. [Preview Abstract] |
Monday, November 21, 2005 4:49PM - 5:02PM |
KD.00004: Cascade of coalescences for droplets near liquid interfaces St\'{e}phane Dorbolo, Karen Mulleners, Tristan Gilet, Nicolas Vandewalle An experimental study on the coalescence of liquid droplets in a system composed of two immiscible liquids is carried out. When a liquid droplet is deposited on the interface separating both liquids it floats momentarily before coalescing with the bottom layer. In some cases the droplet does not coalesce entirely. A smaller satellite droplet is pinched off and partial coalescence may start again. This results in a cascade of successive coalescence. The coalescence cascade is found to be characterized by the liquid viscosities, the liquid densities and the initial radius of the released droplet. The fine study of these parameters yields some interesting information about multiple coalescence. The evolution of the radii of successive droplets is studied and found to be independent of the liquid viscosities. On the other hand the number of steps in a coalescence cascade is shown to be a function of the viscosities and the initial droplet radius. Several arguments are provided that give rise to the existence of a critical as well as a maximum droplet radius. [Preview Abstract] |
Monday, November 21, 2005 5:02PM - 5:15PM |
KD.00005: Important Parameters in Drop Coalescence at Planar Surfaces Pirouz Kavehpour An experimental study has been performed to establish the principal elements that govern drop coalescence. The study consisted of placing drops of various sizes and physical properties on a planar interface with the aid of a high speed digital camera. The experimental portion of the project was aimed at capturing the time of coalescence and the size of the secondary drop that formed after coalescence had finished. Results of the experiments showed clear patterns with respect to inertial and viscous terms. Dimensional analysis indicated that \textit{Oh} had a strong influence on the behavior of drop coalescence. The ratio of secondary drop radius to primary drop radius was calculated to be approximately constant when Oh was much smaller than unity. However, as Oh approached unity from the lower bound, the value of r$_{i}$ decayed. No secondary drop was observed when Oh was greater than unity. Normalized coalescence times confirmed this trend by being properly scaled with inertial time scales for small Oh and preferring viscous time scales when Oh was greater than unity. During the coalescence of a drop with a planar interface, a hole is generated in a microscopic film that separates the drop from the interface. The experiment captured two separate processes, film rupture and the closing of the hole. During the film rupture, the hole radius demonstrated a power law time dependence. The dimensionless drop rupture radii and times fit onto a single master curve and were independent of their physical properties during the opening. [Preview Abstract] |
Monday, November 21, 2005 5:15PM - 5:28PM |
KD.00006: Binary Drop Coalescence in Liquid/Liquid Systems Jungyong Kim, Ellen Longmire Drop pairs of water/glycerin mixture were injected horizontally into silicone oil and, due to gravitational effects, traveled on downward trajectories before colliding. Flow visualization and PIV measurements were obtained in index-matched fluids to characterize coalescence and rebounding behavior. Planar PIV was used to examine large-scale drop motion. In a dual field measurement, stereo PIV and planar long distance microscope PIV were used for resolving larger and smaller scale motion respectively. Experiments were performed for Weber numbers [We] in the range of 1-50. Higher We caused stronger drop deformation and enhanced interface instability, leading to film rupture. By adjusting the initial separation distance and drop volume, trajectory angles could be controlled somewhat. Steeper collision angles encouraged rebounding as opposed to coalescence. Velocity and vorticity fields of the impact zone will be discussed in relation to coalescence and rebounding behavior for several cases [Preview Abstract] |
Monday, November 21, 2005 5:28PM - 5:41PM |
KD.00007: Drop coalescence and film rupture with van der Waals forces Ashley J. James, Xueli Jiang During the collision of two drops the thin film that forms between the drops as they approach each other plays an important role. Viscous forces limit the rate at which the film thins, so the film pressure rises, which tends to cause the drops to rebound. For coalescence to occur the film must become thin enough for the small-scale, attractive van der Waals forces to overcome the high pressure. Two methods to compute van der Waals forces during drop coalescence have been developed. In one method the van der Waals potential between the two drops is computed directly to obtain the force. In the other method a disjoining pressure is applied to the film between the drops. A comparison of the results using the two methods is presented. For validation numerical simulations of the rupture of a fluid film using the disjoining pressure method are compared to the lubrication theory of Vaynblat {\it et al.} ({\it Phys. Fluids}, 2001). The effect of van der Waals forces on drop collision and film rupture is described. [Preview Abstract] |
Monday, November 21, 2005 5:41PM - 5:54PM |
KD.00008: Boundary Integral Simulations of Drop Coalescence L. Gary Leal, Yosang Yoon, Fabio Baldessari We report on boundary integral simulations of the flow-induced coalescence of a pair of equal size drops undergoing a head-on collision in an axisymmetric extensional flow. For the small capillary numbers that are relevant to the coalescence of drops in the 10-50 micron diameter range, a direct comparison can be made between the theory and experimental data, including both head-on and glancing collisions. In the latter case, the time-dependent force due to rotation of the drop pair can be mimicked via head-on collisions with a time-dependent velocity gradient This analogy allows us to study the mechanism for coalescence in cases where it is observed experimentally to occur during the latter half of a collisions, after the hydrodynamic force has begun to pull the drops apart. [Preview Abstract] |
Monday, November 21, 2005 5:54PM - 6:07PM |
KD.00009: A finite-element phase-field method for simulating interfacial dynamics in complex fluids Pengtao Yue, Chunfeng Zhou, James J. Feng, Carl Ollivier-Gooch, Howard H. Hu We present a novel and efficient finite-element method for treating interfacial problems involving rheologically complex fluids. Two key ingredients of the method are a phase-field representation of the interface and an adaptive meshing scheme that allows fine interfacial resolution at manageable computational cost. In the phase-field framework, the interface is seen as a thin layer across which material properties change rapidly but continuously. Thus, a set of governing equations are derived that hold for both fluids across the interface. This circumvents the cumbersome task of interface tracking. The surface tension emerges from the mixing energy at the interface, and the energy-based formalism easily incorporates complex rheology. The challenge of the method lies in resolving the interfacial layer on a fixed Eulerian grid. This is handled by adaptive meshing on a unstructured grid using the phase- field as the criterion for local refinement and coarsening. We will present several simulations on drop deformation, retraction, coalescence and breakup for Newtonian and viscoelastic liquids and nematic liquid crystals. While some of these serve as validations of our new method, the results also reveal novel physics governing the interplay between interfacial dynamics and bulk rheology. [Preview Abstract] |
Monday, November 21, 2005 6:07PM - 6:20PM |
KD.00010: Effects of inertia on the rheology of a dilute emulsion of drops in steady shear Xiaoyi Li, Kausik Sarkar Effects of inertia on the rheology of a dilute Newtonian emulsion are investigated using DNS. The drop shape and flow are computed by solving the Navier-Stokes equation in two phases using Front-tracking method at nonzero Reynolds numbers of 0.1 and 1.0. Effective stresses are computed using Batchelor's formulation, where the interfacial stress is obtained from the simulated drop shape and the perturbation stress from the velocity field. At low Reynolds number, the simulation is compared successfully with various analytical results and experimental measurements. At higher inertia deformation is enhanced and the tilt angle of the drop becomes larger than forty-five degree. The inertial morphology directly affects interfacial stresses. The first and the second interfacial normal stress differences are found to change sign due to the change in drop orientation. The interfacial shear stress is enhanced by inertia and decreases with capillary number at lower inertia but increases at higher inertia. The total excess stresses including perturbation stress contribution shows similar patterns. [Preview Abstract] |
Monday, November 21, 2005 6:20PM - 6:33PM |
KD.00011: Windswept droplets Jose Bico, Francois Besselievre, Marc Fermigier A small droplet impacting a glass window usually remains stuck on the pane. How can we expel it? One possible solution consists in coating the glass surface with a hydrophobic layer. Another solution is to blow it off. We explore this last solution (partly combined with the first one). The droplet starts moving when the wind exceeds a threshold velocity, depending essentially on the surface wettability and the drop size. Above this threshold, the drift speed of the droplet results from a balance between aerodynamic drag and viscous dissipation near the contact lines. The results for different experimental conditions collapse on a master curve, once the wind speed is rescaled as a Weber number and the droplet velocity as a capillary number. While small droplets remain almost spherical caps, larger ones are strongly deformed and take the shape of a sausage, perpendicular to the wind direction. We finally determine the conditions in which satellite droplets are left at the rear of the moving drop, an issue crucial for blow drying processes. [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