Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L11: Drops: Pinch-off and CoalescenceDrops
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Chair: Claas Willem Visser, Harvard University Room: 504 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L11.00001: On the universality of Marangoni-driven spreading Claas Visser, Bram Van Capelleveen, Robin Koldeweij, Detlef Lohse When two liquids of different surface tensions come into contact, the liquid with lower surface tension spreads over the other. Here we measure the dynamics of this Marangoni-driven spreading in the drop-drop geometry, revealing universal behavior with respect to the control parameters as well as other geometries (such as spreading over a flat interface). The distance $L$ over which the low-surface-tension liquid has covered the high-surface-tension droplet is measured as a function of time $t$, surface tension difference between the liquids $\Delta\sigma$, and viscosity $\eta$, revealing power-law behavior $L(t)\sim t^{\alpha}$. The exponent $\alpha$ is discussed for the early and late spreading regimes. Spreading inhibition is observed at high viscosity, for which the threshold is discussed. Finally, we show that our results collapse onto a single curve of dimensionless $L(t)$ as a function of dimensionless time, which also captures previous results for different geometries, surface tension modifiers, and miscibility. As this curve spans 7 orders of magnitude, Marangoni-induced spreading can be considered a universal phenomenon for many practically encountered liquid-liquid systems. [Preview Abstract] |
Monday, November 20, 2017 4:18PM - 4:31PM |
L11.00002: Pinch-off dynamics of a liquid column pulled by a sphere settling through an interface Jacques Magnaudet, Jean-Lou Pierson We study the dynamics of the liquid column that forms past a settling sphere as it crosses the horizontal interface separating two immiscible liquids. Depending on the viscosity contrast between the two fluids, on the fluid and sphere-to-fluid density contrasts, and on the Bond number, the primary pinch-off is found to be either shallow or deep, i.e. taking place either close to the initial interface or close to the sphere. We rationalize these observations through simple scaling laws predicting that, if the sphere velocity stays constant during the breakthrough, the transition from shallow to deep pinch-off takes place when the Bond number exceeds a critical value depending on the sphere-to-fluid density contrast. However, the viscosity contrast frequently results in large sphere accelerations or decelerations. We show that their influence is similar to a (negative or positive) buoyancy effect and may be accounted for via a modified expression of the critical Bond number; this approach allows us to predict correctly the observed tendencies. We finally compare scaling laws and mechanisms controlling pinch-off dynamics in the present situation, where inertia is comparable in the two fluids, with those involved in the case of air cavities created by an an impact at a free surface. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L11.00003: Delayed coalescence of surfactant drops Myrthe Bruning, Maxime Costalonga, Stefan Karpitschka, Jacco Snoeijer When two drops of the same liquid meet, they merge into one drop to minimize their surface. However, the coalescence behavior can be different if the drops have different compositions, hence different surface tensions. Instead of merging, frequently a non-coalescence behavior is observed, and one drop may chase the other. Here we study the coalescence of surfactant solutions on a substrate. By varying both their contact angle $\theta$ and their difference in surface tension $\Delta\gamma$, a phase diagram is constructed, displaying several regimes with distinct timescales of non-coalescence. In all regimes the drop with lower surfactant concentration is pushed forward by the other drop. We show that the behavior not only depends on $\theta$ and $\Delta\gamma$, but also on the surfactant concentrations of the two drops. The lifetime of the non-coalescence state of a pure water and a surfactant drop is much larger than for two surfactant drops, even if $\theta$ and $\Delta\gamma$ are identical in both cases. We explain this from a competition between surfactant advection and desorption. In the case of two surfactant drops, surfactants accumulate at the surface of the downstream drop, reducing the surface tension difference and thereby the duration of the non-coalescence state. [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L11.00004: A PIV Study of Drop-interface Coalescence with Surfactants Weheliye Hashi Weheliye, Teng Dong, Panagiota Angeli In this work, the coalescence of a drop with an aqueous-organic interface was studied by Particle Image Velocimetry (PIV). The effect of surfactants on the drop surface evolution, the vorticity field and the kinetic energy distribution in the drop during coalescence were investigated. The coalescence took place in an acrylic rectangular box with 79{\%} glycerol solution at the bottom and Exxsol D80 oil above. The glycerol solution drop was generated through a nozzle fixed at 2cm above the aqueous/oil interface and was seeded with Rhodamine particles. The whole process was captured by a high-speed camera. Different mass ratios of non-ionic surfactant Span80 to oil were studied. The increase of surfactant concentration promoted deformation of the interface before the rupture of the trapped oil film. At the early stages after film rupture, two counter-rotating vortices appeared at the bottom of the drop which then travelled to the upper part. The propagation rates, as well as the intensities of the vortices decreased at high surfactant concentrations. At early stages, the kinetic energy was mainly distributed near the bottom part of the droplet, while at later stages it was distributed near the upper part of the droplet. [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L11.00005: Drop Coalescence with a Liquid/liquid Moving Interface Teng Dong, Weheliye Hashi Weheliye, Panagiota Angeli In this work, the coalescence of a single drop with a moving liquid/liquid interface was investigated. The experiments were conducted in a rectangular flow channel with 5cm x 5cm square section and 1m length. The channel had two openings at the inlet and the outlet with 5 mm height to allow the flow of an aqueous phase at the channel bottom. An organic phase filled the channel on top of the flowing aqueous phase film. A drop of the aqueous phase was created through a nozzle fixed near the inlet of the channel. The drop moved along the channel and eventually coalesced with the moving aqueous phase film. The velocity profiles of both phases were studied first. The rest times of the drops from the moment they contacted the interface to the rupture of the trapped oil film under various flow conditions were further investigated. [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L11.00006: Scaling during capillary thinning of particle-laden drops Sumeet Thete, Brayden Wagoner, Osman Basaran A fundamental understanding of drop formation is crucial in many applications such as ink-jet printing, microfluidic devices, and atomization. During drop formation, the about-to-form drop is connected to the fluid hanging from the nozzle via a thinning filament. Therefore, the physics of capillary thinning of filaments is key to understanding drop formation and has been thoroughly studied for pure Newtonian fluids using theory, simulations, and experiments. In some of the applications however, the forming drop and hence the thinning filament may contain solid particles. The thinning dynamics of such particle-laden filaments differs radically from that of particle-free filaments. Moreover, our understanding of filament thinning in the former case is poor compared to that in the latter case despite the growing interest in pinch-off of particle-laden filaments. In this work, we go beyond similar studies and experimentally explore the impact of solid particles on filament thinning by measuring both the radial and axial scalings in the neck region. The results are summarized in terms of a phase diagram of capillary thinning of particle-laden filaments. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L11.00007: Scaling during drop formation of Newtonian and complex fluids Brayden Wagoner, Sumeet Thete, Michael Harris, Osman Basaran Free surface flows such as drop formation and filament breakup are ubiquitous in applications as diverse as ink jet printing, crop spraying, and spray coating. In these flows, a finite time singularity arises at the instant the filament pinches off. The interplay between the dominant forces in the vicinity of the pinch-off singularity determines the temporal variation of the radial and axial scales during capillary thinning of filaments and is key to understanding the underlying dynamics. In experiments, the radial scale is readily determined by measuring the variation of the minimum filament radius with time remaining until pinch-off. Inferring the temporal variation of the axial scale is, however, more challenging. Here, we present a new method for inferring the axial scale during capillary pinch-off. The accuracy of the experimental measurements of the radial and axial scales is validated by demonstrating that they accord with well-known predictions from theory and simulations of capillary pinching of Newtonian filaments. Furthermore, we show that the novel method for determining the axial scale can be used to gain insights into pinch-off dynamics of complex fluids where theoretical and computational understanding is lacking. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L11.00008: On the origins of surfactant-driven microthread cascades in jet breakup Pritish Kamat, Osman Basaran A startlingly beautiful phenomenon arises in capillary thinning of surfactant-laden liquid threads: just prior to pinch-off, a series of progressively thinner, microscopic threads telescope out from the rupture location. Similar microthread cascades have been observed in a myriad of other interface rupture problems including during breakup of highly viscous Newtonian fluid threads in air or another viscous fluid, in electrospinning, and in breakup of 2D liquid lenses on a free-surface. Despite their prevalence, the mechanistic understanding of how microthread cascades originate remains poor. Conventional wisdom claims that microthread cascades might originate from minuscule interfacial perturbations produced by noise of either molecular or ambient origin. However a plausible noise source has not yet been identified. Here, we present novel insights on the formation of surfactant-driven microthread cascades in rupturing axisymmetric liquid threads. Using experimentally benchmarked simulations, devoid of any artificial inputs, we demonstrate that surfactants are capable of generating spontaneous perturbations via a fully deterministic process. The mechanics of the process is elucidated and the critical roles played by inertia and Marangoni stress are identified. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L11.00009: The initial regime of drop coalescence Christopher Anthony, Michael Harris, Osman Basaran Drop coalescence plays a key role in both industry and nature. Consequently, study of the phenomenon has been the focus of numerous experimental, computational and theoretical works to date. In coalescence, two drops come into contact and a liquid bridge forms between them. As time advances, this bridge grows from microscopic to macroscopic scales. Despite the large volume of work dedicated to this problem, currently experiment, theory, and computation are not in perfect agreement with respect to the earliest times following the initial contact of the drops. Experiments report an initial regime where the radius of the connecting bridge grows linearly in time before a transition to either a Stokes regime or an inertial regime where either viscous or inertial forces balance capillary force. In the initial linear regime, referred to as the inertially-limited viscous regime, all three forces are thought to be important. This is in contrast to theory which predicts that all coalescence events begin in the Stokes regime. We use high accuracy numerical simulation to show that the existing discrepancy in the literature can be resolved by paying careful attention to the initial conditions that set the shape and size of the bridge connecting the two drops. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L11.00010: Jumping Mechanism of Self-Propelled Droplet Yongsheng Lian, Yan Chen The self-propelled behavior of coalesced droplets can be utilized to enhance heat transfer performance of dropwise condensation. It has been recognized that the droplet self-propelling is the combined result of the conversion of surface energy to kinetic energy and the unsymmetrical boundary conditions imposed on the droplets. However, the roles of boundary conditions, which largely determine the conversion ratio of surface energy to the effective jumping kinetic energy, are not well understood. In this paper we use a numerical approach to investigate the boundary condition effect on the self-propelling behavior. A Navier-Stokes equation solver for multiphase flows is used to describe the flow field. The moment of fluid interface reconstruction technique is applied to resolute the interfaces. A direction splitting method is applied to advect the interface. And an approximate projection method is used to decouple the calculation of velocity and pressure. Comparisons are made with experimental results and show the simulation can accurately capture self-propelling behavior. Our simulation show the vertical flow velocity inside the coalesced droplet can increase the normalized jumping velocity but the contact area between droplets and substrate can decrease jumping velocity. High viscous dissipation is observed at the beginning of the coalescence which reduces jumping velocity.~ [Preview Abstract] |
Monday, November 20, 2017 6:15PM - 6:28PM |
L11.00011: Liquid stresses associated with a bubble pinch-off event Oliver McRae, Peter Walls, Venkatesh Natarajan, Chris Johnson, Chris Antoniou, James Bird The interface between two fluids can quickly change shape when subjected to various forces. For example, capillary forces can rapidly deform a liquid-air interface during bubble coalescence or pinch-off events. This process can lead to significant stresses in the nearby fluid, stresses which can be quantified and presented in terms of an energy dissipation rate (EDR). The EDR surrounding bubbles as they change shape is particularly relevant to the efficiency of bioreactors, as a large EDR can damage or kill suspended cells. Here we investigate numerically the magnitude and extent of stresses that develop around spontaneous bubble breakup, geometrically similar to bubble formation at a sparger used in aeration. We present the EDR levels experienced by a particular volume of liquid surrounding the original bubble to illustrate the potential for these bubble formation events to damage or kill surrounding cells. We also compare these results to stresses associated with bubbles bursting at a free surface, and relate our findings to experiments of bubbles breaking up surrounded by cells in a microfluidic device. We believe this work will be pertinent in sparger design with a goal of understanding and mitigating the damaging effect bubble formation can have on cells undergoing aeration. [Preview Abstract] |
Monday, November 20, 2017 6:28PM - 6:41PM |
L11.00012: Fragmentation of a viscous suspension jet Joris Ch\^ateau, Elisabeth Guazzelli, Henri Lhuissier As viscosity is increased, a liquid capillary jet accelerated by gravity stretches over increasingly large distances before it eventually breaks up. Paradoxically, adding solid particles to the liquid, which increases the effective viscosity, shortens the jet considerably. At the light of experiments with capillary bridges and jets of suspensions of non-Brownian, density-matched, spherical particles with different particle sizes and a large particle volume fraction ($=50\%$), we will rationalize this apparent contradiction for the different regimes of jet break-up from the consideration of discrete particulate effects. Consequences will also be drawn for the size of the drops following the break-up. [Preview Abstract] |
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