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 ED: Drop Impact |
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Chair: Siggi Thoroddsen, National University of Singapore Room: Hilton Chicago Continental A |
Sunday, November 20, 2005 4:10PM - 4:23PM |
ED.00001: Contact angle dynamics in droplet impact on flat surfaces: Effect of surface wettability Ilker Bayer, Constantine Megaridis Contact angle dynamics is examined experimentally during spreading/recoiling of mm-sized water droplets impacting orthogonally on various flat surfaces with $We = O(0.1)-O(10)$, $Ca = O(0.001)- O(0.01)$, $Oh = O(0.001)$ and $Bo = O(0.1)$. In this impact regime, inertial, viscous, and capillary phenomena act in unison to influence contact angle dynamics. The wetting properties of the target surfaces range from wettable to non-wettable. The objective of the work is to provide insight into the dynamic behavior of the apparent (macroscopic) contact angle $\theta$ and its dependence on contact line velocity $V_{CL}$ at various degrees of surface wetting for droplets impacting with low to moderate Weber numbers. The hydrodynamic wetting theory of Cox (1998) is implemented to relate the microscopic wetting parameters to the observed $\theta$ vs. $V_{CL}$ data. It is concluded that Cox's model works well in the fast spreading regime, but proves inadequate for slow spreading where solid/liquid interactions are dominant. In addition, the molecular-kinetic theory of wetting by Blake and Haynes (1969) is tested with good results. This study offers guidance for numerical or analytical studies, which require special attention to the boundary conditions at the contact line, and more specifically the functional dependence of contact angle on contact line speed. [Preview Abstract] |
Sunday, November 20, 2005 4:23PM - 4:36PM |
ED.00002: WITHDRAWN: Splitting of drops impacting a fiber Elise Lorenceau, Michele Adler We are interested in the way a filter (a grid for example) can stop the liquid drops of an aerosol passing through it. To study this complex problem, we focus on the impact of a drop onto a horizontal fiber. If the drop impacts the fiber with a velocity smaller than a critical velocity, it is entirely captured by the solid substrate. Both viscosity and capillarity are able to slow down the drop and the critical velocity for capture was recently determined when capillary is dominant.\footnote{E. Lorenceau., C. Clanet., D. Qu\'{e}r\'{e} Capturing drops with a thin fiber, JCIS 279, 192 (2004)} We focus on the determination of this critical velocity in the viscous regime and emphasize in particular the role of the drop shape. Then, we consider the situation when drops impact the fiber with a velocity higher than the critical velocity. In that case, the liquid phase is fragmented; part of the liquid phase remains stuck on the fiber while another part is expelled from the solid substrate. We show that depending on the properties of the liquid, different fragmentation patterns can be observed; the droplet can either be split up or explode into a myriad of dispersed droplets. [Preview Abstract] |
Sunday, November 20, 2005 4:36PM - 4:49PM |
ED.00003: Splashing on smooth and rough dry surfaces Lei Xu, Sidney Nagel In previous experiments, we studied the splash created when a drop of fluid hits a smooth dry surface and discovered that the pressure of the surrounding gas determines whether or not splashing will occur. We have now extended our studies to the case of a drop hitting a rough substrate. We systematically varied both the surface roughness and the pressure of the surrounding gas and found two distinct contributions to a splash. One is caused by air and has the same characteristics as the ``coronal'' splash observed on smooth substrates. A second, ``prompt'' splash, contribution is caused by surface roughness. We have also measured the size distribution of the droplets emitted from a splash. A broad distribution of droplet sizes is found at high gas pressures. As the gas pressure is lowered towards the splash/no-splash transition the distribution changes. At the threshold pressure, the distribution is strongly peaked at an average size. [Preview Abstract] |
Sunday, November 20, 2005 4:49PM - 5:02PM |
ED.00004: The bubble caught under a drop impacting onto a solid surface K. Takehara, S.T. Thoroddsen, T.G. Etoh We report ultra-high-speed images of the small bubble caught under a drop impacting onto a dry acrylic plate, over a range of impact Reynolds numbers between 5-30,000. The imaging is through the acrylic plate at frame rates up to 200,000 frames/s. We observe a thin disc of air caught under the center of the drop, which quickly contracts into a central bubble. The radius of the air disc reduces exponentially in time, contracting in as little as 60 $\mu $s. The initial thickness of the air disc is in the range 1-2 $\mu $m. The initial diameter of the disc is related to the bottom radius of curvature of the drop, as it impacts the plate. The initial diameter of the air disc is often marked by a ring of micro-bubbles which are left behind. The dynamics of the contraction does in some cases leave a small droplet at the center of the air bubble. [Preview Abstract] |
Sunday, November 20, 2005 5:02PM - 5:15PM |
ED.00005: Experimental Analysis and Predictive Modeling of Droplet Impingement and Spreading onto a Solid, Smooth Surface Brett Bathel, Michael Huisenga, Lukas Johnson, Neena Stephen, Albert Ratner An experimental study of the impact and spreading process of water droplets on a smooth glass surface is presented. Impact droplet velocity, diameter, Reynolds number, and Weber number varied from 1.3 to 3.1 m/s, 2.5 to 4.2 mm, Re 3610 to 12100, and We 60 to 515, respectively. The impact surface was smoothed to $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} \quad \lambda $ to minimize solid-surface induced flow instabilities during expansion. A high speed digital camera was used to capture the droplet motion at 2200 Hz with an image plane spatial resolution of $\sim $16 $\mu $m. The size of the data set allowed for calculation of uncertainty related to characteristic spread time, \textit{$\tau $}$_{max}$, and maximum spread ratio, \textit{$\beta $}$_{max}$. Applying previous literature values for \textit{$\beta $}$_{max}$ and a newly formulated model for \textit{$\tau $}$_{max}$ as boundary functions, a new function predicting radial expansion from impact to \textit{$\beta $}$_{max}$ was defined. A statistical comparison with existing literature models for the entire expansion process was made based upon the experimental results obtained. [Preview Abstract] |
Sunday, November 20, 2005 5:15PM - 5:28PM |
ED.00006: Lattice Boltzmann Simulations of Drop Impingement on Walls Shiladitya Mukherjee, John Abraham The lattice Boltzmann method (LBM) (Chen and Doolen, 1998; Succi 2001) [1, 2] is employed to simulate drop impingement. The Weber number range considered is 1 to 200 and Ohnesorge number range is 0.01 to 0.001. Different stages of evolution of the impinged drop, namely, spreading, recoil and oscillation are reproduced. A model is proposed and applied to simulate wettability. The prediction of the transient evolution of the impinged drop and wettability effects are the new contributions of this work. \newline \newline \textbf{References} \newline [1] Chen, S. and Doolen, G.D., \textit{Ann. Rev. Fluid Mech.} 30: 329-364 (1998). \newline [2]Succi, S. \textit{The Lattice Boltzmann Equation for Fluid Dynamics and Beyond}, Oxford University Press, 2001. [Preview Abstract] |
Sunday, November 20, 2005 5:28PM - 5:41PM |
ED.00007: Resolving Droplet Splashing with Adaptive Mesh Refinement and Accurate Curvature Estimation Samuel Johnson, Jean-Pierre Delplanque The numerical simulation of droplet splashing is a particularly difficult task. This is due in part to the necessity of accurately representing the liquid-gas interface's geometry and physics. However, a possibly more significant obstacle is resolving important droplet features with disparate length scales, while maintaining a reasonable level of computational efficiency. Presented here is an advanced parallel implementation of the volume-of-fluid method with accurate interface reconstruction and curvature estimation. Adaptive mesh refinement is used to resolve small length-scale features, without the unnecessary computational cost of over-resolving trivial regions of the computational domain. This implementation of the volume-of-fluid method is well suited to the task of simulating droplet splashing and is applied to that problem here. [Preview Abstract] |
Sunday, November 20, 2005 5:41PM - 5:54PM |
ED.00008: Applying Contact Angles to 3D VOF Simulations of Drop Impact Onto an Inclined Surface Markus Bussmann, Shahriar Afkhami Volume-of-fluid, or VOF, methods have been successfully applied in many contexts to the study of normal drop impact onto a solid surface. In such simulations, the application of a contact angle is relatively straightforward, because in axisymmetric coordinates the contact line appears as a triple point. But when the impact is asymmetric, as when a drop impacts an inclined surface, the contact angle boundary condition must be applied along the entire contact line, which is difficult given that the VOF methodology does not explicitly track interfaces. With that as background, we present results of a 3D VOF model applied to the problem of a drop impacting an inclined surface, and focus on the issue of contact angle specification. Several options are presented for applying the boundary condition, inspired by several recent papers that present different approaches for the axisymmetric problem. Results are then presented that quantify the influence of the choice of approach, and results are also compared to known behaviours obtained experimentally. [Preview Abstract] |
Sunday, November 20, 2005 5:54PM - 6:07PM |
ED.00009: Ultra-High-Speed imaging of drop impacts onto liquid surfaces T.G. Etoh, S.T. Thoroddsen, K. Takehara This talk will present ultra-high-speed video images of novel drop impact phenomena. The video camera used in this work was developed by Etoh \textit{et al.} (2003)\footnote{Etoh, T. G., Poggemann, D., Kreider, G. \textit{et al.} (2003) `An image sensor which captures 100 consecutive frames at 1000000 frames/s'. \textit{IEEE Trans. Electron Devices}, \textbf{50}, No. 1, pp. 144-151.} and will be described in some detail. It can capture 100 images at up to 1,000,000 frames/sec, with 260 by 312 pixel resolution irrespective of the frame-rate used. We study drop impacts onto liquid surfaces revealing ejecta sheets, which form within the first 200 $\mu $s after contact of the drop with the pool liquid. These sheets can self-intersect as well as enclose cylinders of air, to form bubbles on the crown. These sheets are also shown to break up into droplets through the formation of tendrils and a sling-shot. Other experiments show that drops impacting onto very thin layers of liquid, break up in novel ways, through the formation of holes in the ejecta sheets. [Preview Abstract] |
Sunday, November 20, 2005 6:07PM - 6:20PM |
ED.00010: Crater Morphology and Bubble Entrainment during Drop Impact on a Liquid Pool Qiang Deng, A.V. Anilkumar, T.G. Wang We have examined in detail the morphology of the impact crater formed during the impact of a liquid drop on a liquid pool for a range of We, Fr and viscosities. In a narrow impact regime, air bubbles are entrained into the pool as a consequence of capillary wave focusing at the crater tip. At the same time, a high speed thin jet is ejected upwards, and the critical crater cone angle is observed. At other times, the bubble pinch-off process can be averted due to the dissipation of the capillary waves, and the lack of wave focusing. All of the aspects can be explained by examining the crater morphology. The results will be presented at this talk. [Preview Abstract] |
Sunday, November 20, 2005 6:20PM - 6:33PM |
ED.00011: WITHDRAWN: Axisymmetric Adaptive Drop/Interface Impacting Study Xiaoming Zheng, John Lowengrub, Vittorio Cristini The impact of a drop upon an interface is studied using an axisymmetric adaptive level-set/finite element method. Under certain conditions, the drop will rebound off the interface before breaking through. The drop fluid and the fluid below the interface are identical. We characterize the behavior in terms of the relevant nondimensional parameters: the Reynolds number, the Weber number, and the viscosity and density ratios of the fluid components. One of the primary difficulties in performing numerical simulations of such flows is the accurate resolution of the lubrication forces that arise in the near contact region between the drop and interface. To overcome this difficulty, we use a spatially and temporally adaptive mesh together with a new, stable and accurate projection method for the Navier-Stokes equations and a mass-conserving level-set algorithm for capturing the motion of the drop and interface (J. Comp. Phys., v. 208, 2005). We validate our algorithm by successfully matching the recent experimental results on drop/interface impact by Mohamed-Kassim and Longmire (Phys. Fluids, v. 15, 2003). [Preview Abstract] |
Sunday, November 20, 2005 6:33PM - 6:46PM |
ED.00012: Simulation of Droplet Impact on a Liquid-Liquid Interface Emil Coyajee, Rene Delfos, Harmen Slot, Bendiks Jan Boersma In this study, a liquid droplet is released into a container of a heavier, more viscous liquid resting underneath another layer of the same fluid as the droplet. The droplet accelerates upward due to buoyancy until it impacts on the interface between the two liquid layers. Upon impact, both the droplet and the interface undergo large deformation. The droplet is rebounded by the elasticity of the interface, resulting in a damped oscillatory motion. Finally, after all fluid has come to rest, the droplet rests in a more or less elliptic configuration against the liquid-liquid interface. Numerical simulations are performed with a combined LS/VOF method using the novel concept of multiple marker functions to represent multiple interfaces in a single computational cell. Therefore, interfaces can collide without numerical coalescence. To validate our results, simulations are compared with physical laboratory experiments. [Preview Abstract] |
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