Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G33: Drops: Bouncing, Impact and Dynamic Surface Interactions I |
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Chair: David Chappell, Claremont Graduate University Room: Ballroom A |
Monday, November 23, 2015 8:00AM - 8:13AM |
G33.00001: Enhanced Downward Acceleration of a Bouncing Droplet Due to the Lubrication Force David Chappell, Matthew Cessna, Ali Nadim We explore the dynamics of moderately viscous (50-100 cSt) silicone oil drops bouncing on a vertically vibrated oil bath. When the driving acceleration of the bath is larger than a threshold value, drops can bounce indefinitely due to the presence of a thin air layer separating the drop from the bath. We present experimental evidence that the drop can temporarily ``stick'' to the oil bath during the rebound process causing it to be pulled downward briefly with the downward-accelerating bath. Thus, for a small time interval during each bounce, the drop's downward acceleration can exceed that of gravitational free-fall. A simple model incorporating the lubrication force between the drop and the bath, allowing for the deformation of the latter, is developed and found to match the observed dynamics closely. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G33.00002: Drop impact on thin liquid films using TIRM Min Pack Drop impact on thin liquid films is relevant to a number of industrial processes such as pesticide spraying and repellent surface research such as self-cleaning applications. In this study, we systematically investigate the drop impact dynamics on thin liquid films on plain glass substrates by varying the film thickness, viscosity and impact velocity. High speed imaging is used to track the droplet morphology and trajectory over time as well as observing instability developments at high Weber number impacts. Moreover, the air layer between the drop and thin film upon drop impact is probed by total internal reflection microscopy (TIRM) where the grayscale intensity is used to measure the air layer thickness and spreading radius over time. For low We impact on thick films (We $\sim$ 10), the effect of the air entrainment is pronounced where the adhesion of the droplet to the wall is delayed by the air depletion and liquid film drainage, whereas for high We impact (We \textgreater 100) the air layer is no longer formed and instead, the drop contact with the wall is limited only to the film drainage for all film thicknesses. In addition, the maximum spreading radius of the droplet is analyzed for varying thin film thickness and viscosity. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G33.00003: Surface topography measurements for pilot-wave hydrodynamics Adam Damiano, Daniel Harris, Pierre-Thomas Brun, John Bush We present the results of our attempt to refine the surface Schlieren technique originally developed by Moisy et al. (2009, 2012) to resolve the surface topography associated with capillary wave fields. Our technique is applied to infer the wave field that accompanies millimetric droplets self-propeling on the surface of a vibrating fluid bath. Apart from a shadow region on the order of the drop’s cross-sectional area, the waves are resolved to a micron scale, allowing for quantitative comparison with existing theoretical models of the wave field. The technique is used to yield insight into the interaction between walking droplets and submerged barriers. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G33.00004: Uncertainty Quantification on Entrapped Air in Droplet Impact Events Seyedshahabaddin Mirjalili, Gianluca Iaccarino, Ali Mani Recent investigations have revealed that entrapment of air films under liquid-liquid impacts can lead to subsequent breakup processes forming many microbubbles per impact. In this work we consider a canonical setting in which individual liquid drops impact a deep flat pool as a model representative of this phenomena. We present an investigation of the uncertainty in the entrapped air associated with the angle of impact relative to the interface-normal direction. In practice, this uncertainty can be induced by surface waves or measurement errors; understanding this sensitivity might help in incorporating impact models as subgrid scale models in large-scale calculations. We have employed the direct numerical simulations of the Navier-Stokes equations in conjunction with a diffuse interface method to track the phase interface. For UQ analysis a quadrature-based and a regression-based non-intrusive polynomial chaos approach are compared. Using the same set of simulations, quadrature-based NIPC showed better convergence than regression-based NIPC. Our results indicate that even order 10 degree variability in the incident angle can lead to significant variability in the entrapped air film. Impact on various measures such as total entrapped volume and film thickness is discussed. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G33.00005: Settling of a sphere through a fluid-fluid interface: influence of the Reynolds number Jean-Lou Pierson, Jacques Magnaudet When a particle sediments through a horizontal fluid-fluid interface (a situation frequently encountered in oceanography as well as in coating processes), it often tows a tail of the upper fluid into the lower one. This feature is observed in both inertia- and viscosity-dominated regimes. Nevertheless the tail evolution and the particle motion are found to highly depend on the ratio of the two effects, i.e. on the Reynolds number. In this work we study numerically the settling of a sphere through a horizontal fluid-fluid interface using an Immersed Boundary Method combined with a Volume of Fluid approach. To get some more insight into the underlying physical mechanisms, we combine this computational approach with a semi-analytical description based on the concept of Darwin "drift" which allows us to predict the interface evolution, hence the thickness of the film encapsulating the sphere, in the two limits of Stokes flow and potential flow. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G33.00006: High Velocity Droplet Rebound On Liquid Pools William Doak, Danielle Laiacona, Paul Chiarot, Guy German Rebound of high velocity, periodic droplet streams off viscous liquid pools is studied experimentally. Droplets, approximately 60 micrometers in diameter, impact the oil surface at velocities up to 13 m/s and at angles between 2--25 degrees. The oil surface does not degrade or lose its ability to provide rebound even after millions of droplet impacts. The oil was varied to examine the effect that surface tension and viscosity had on droplet rebound. Stable rebound is achievable on oils varying in dynamic viscosity in the range 13--970 Pa.s and surface tensions in the range 19--28 mN/m. When rebound occurs, a consistent 29{\%} loss of droplet kinetic energy is observed. This is a surprising relationship due to the fact that it holds true for all cases of stable rebound regardless of the oil used. We further observe an upper inertial limit where droplets no longer provide stable rebound and instead become fully entrained in the oil pool. This limit is governed by the Rayleigh-Plateau instability and can be characterized and predicted using a modified version of the Weber number. The droplet rebound presented in this study is unique due to the size, velocity, and frequency of the droplets used. Another unique feature is that the rebound manifests itself as an effectively static phenomenon. No motion of the interface -- oscillations, waves, or otherwise -- was observed during rebound. The quasi-static nature of rebound enabled distinctions to be made regarding energy dissipation and the transition from droplet rebound to entrainment. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G33.00007: Leave the seat down: The physics of droplet streams impacting a free surface Nathan Speirs, Randy Hurd, Jesse Belden, Tadd Truscott Even though toilets are designed for sitting, men often stand to urinate. This behavior can be unintentionally messy and encouragement to sit is often disregarded. In an effort to improve communication, we present a physics-based examination of the overwhelming benefits of sitting to pee. Single droplet impact onto a water surface has been studied intensely for over a century; however, very little is known about the effect of multiple droplet impacts. A single droplet impact predominantly forms a hemispherical subsurface cavity. In contrast, a multi-droplet impact creates a deep conical cavity, often 15 times deeper than the single droplet counterpart. A competition between the cavity formation time and droplet frequency maximize the cavity length and duration, implying that cavity size and shape is dependent on droplet diameter, speed and frequency. Upon collapse, larger subsurface cavities result in the formation of larger jets capable of projecting droplets significantly further than single droplet cavities, emphasizing why even a sharp shooter can create a mess. We utilize high-speed imaging and controlled droplet experiments to unravel the key frequencies and parameters at play, offering several suggestions for those wishing to make the transition. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G33.00008: A model for bouncing droplets: effects of obstacles and confinements Aslan Kasimov, Luiz Faria We propose a simple model for particle-wave interactions that captures many aspects of experiments with droplets bouncing on a vibrating bath. The model results from shallow water, small viscosity, and linear approximation to free surface hydrodynamics. The droplet motion is governed by an equation with a force that depends on the wave slope. We study a droplet motion in a wave guide, a droplet passing through a single/double slits, and other interactions that are known experimentally to exhibit quantum effects. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G33.00009: Nonmonotonic Response of Drop Impacting Liquid Film Xiaoyu Tang, Abhishek Saha, Delin Zhu, Chao Sun, Chung K. Law Drop impact on liquid film is ubiquitous in both natural phenomena and industrial applications. The dynamics of the gas layer trapped between the drop and the deformed liquid surface play a crucial role in determining the impact outcomes. However, a quantitative measurement of this gas layer dynamics is extremely challenging because it is hidden behind the deformed liquid film. In this study, high-speed white light interferometry enables the measurement of the gas layer dynamics during the drop impact with high resolutions and is complemented by side view shadowgraphy to observe the penetration process below the liquid surface. Drop impacting with different inertia onto liquid film with various thicknesses is systematically studied to obtain a phase diagram of different outcomes in the h/R-\textit{We} space, where h/R is the liquid thickness normalized by drop radius, and \textit{We} is the drop Weber number. It is observed that there exists a critical \textit{We}$_{C}$ beyond which the drop always merges with the liquid film. However, for \textit{`subcritical'} conditions, there exists a merging peninsula in otherwise globally bouncing region. Across this peninsula, as the liquid film thickness increases, the impact outcome transits from bouncing to merging and to bouncing again. The merging time within this peninsula is longer compared to its \textit{`supercritical'} counterpart, indicating different merging mechanisms. Based on scaling analysis, the boundaries between different zones are identified and compared with experiments. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G33.00010: Deceleration of noncoalescing droplets Jacob Hale Free liquid droplets are observed to glide, or skirt along the surface of a bath of the same fluid, slowing at an exponential rate. The droplet deforms the bath surface creating a dimple which travels along the surface as a wave pulse. Viscous coupling of the droplet and bath surfaces through a thin air film causes a no-slip condition that leads to viscous drag on the bath and perturbs the wave motion of the otherwise free surface. Assuming a linear velocity profile in the bath near the no-slip zone, we show that viscous dissipation in the bath alone accounts for the loss in kinetic energy of the droplet. [Preview Abstract] |
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