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 G12: Drops: Electric Fields & InteractionsDrops Electro
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Chair: Chun-Ti Chang, National Taiwan University Room: 505 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G12.00001: On-demand Droplet Manipulation via Triboelectrification Wei Wang, Hamed Vahabi, Matthew Cackovic, Rui Jiang, Arun Kota Controlled manipulation of liquid droplets has attracted tremendous interest across different scientific fields over the past two decades. To date, a variety of external stimuli-mediated methods such as magnetic field, electric field, and light have been developed for manipulating droplets on surfaces. However, these methods usually have drawbacks such as complex fabrication of manipulation platform, low droplet motility, expensive actuation system and lack of precise control. In this work, we demonstrate the controlled manipulation of liquid droplet with both high (e.g., water) and low (e.g., n-hexadecane) dielectric strengths on a smooth, slippery surface via triboelectric effect. Our highly simple, facile and portable methodology enables on-demand, precise manipulation of droplets using solely the electrostatic attraction or repulsion force, which is exerted on the droplet by a simple charged actuator (e.g., Teflon film). We envision that our triboelectric effect enabled droplet manipulation methodology will open a new avenue for droplet based lab-on-a-chip systems, energy harvesting devices and biomedical applications. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G12.00002: Crater Formation on Electrodes during Charge Transfer with Aqueous Droplets or Solid Particles Eric S. Elton, Ethan R. Rosenberg, William D. Ristenpart We report that metallic electrodes are physically pitted during charge transfer events with water droplets or other conductive objects moving in strong electric fields ($>$1 kV/cm). \emph{Post situ} microscopic inspection of the electrode shows that an individual charge transfer event yields a crater approximately 1 to 3 microns wide, often with features similar to splash coronae. We interpret the crater formation in terms of localized melting of the electrode via resistive heating concurrent with dielectric breakdown through the surrounding insulating fluid. A scaling analysis indicates that the crater diameter scales as the inverse cube root of the melting point temperature $T_m$ of the metal, in accord with measurements on several metals (660$^\circ$C $\le T_m \le$ 3414$^\circ$C). The process of crater formation provides a possible explanation for the longstanding difficulty in quantitatively corroborating Maxwell's prediction for the amount of charge acquired by spheres contacting a planar electrode. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G12.00003: Critical frequency for coalescence of emulsions in an AC electric field. Zhou Liu, Faizi Hammad Ali, Ho Cheung Shum Applying an electric field to trigger the coalescence of emulsions has been applied in various applications which include crude oil recovery, emulsion stability characterization as well as pico-injection and droplet-based chemical reaction in microfluidics. In this work, we systematically investigated the responses of surfactant-stabilized emulsions to a controlled AC electric field using a customer-built chip. At a given amplitude of the AC voltage, we found a critical frequency beyond which the emulsions remain stable. When the frequency is decreased to below the critical value, emulsions coalesce immediately. Such critical frequency is found to be dependent of amplitude of the AC voltage, viscosity of the fluids, concentration and type of the surfactant as well as the electric conductivity of the droplet phase. Using a model based on the drainage of thin film, we have explored the mechanism behind and interpret this phenomenon systematically. Our work extends the understanding of the electro-coalescence of emulsions and can be beneficial for any applications involve the coalescence of droplets in an AC electric field. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G12.00004: ABSTRACT WITHDRAWN |
Monday, November 20, 2017 11:27AM - 11:40AM |
G12.00005: The Role of Drag Force in Shedding of Multiple Sessile Drops. Aysan Razzaghi, Sayyed Hossein Banitabaei, Alidad Amirfazli A sessile drop placed on a solid surface can shed, if the drag force due to a shearing airflow overcomes the drop adhesion to the surface. Sessile drop shedding is of importance due to its applications in condensation, fuel cells, icing, etc. Majority of the studies so far have considered the shedding of a single sessile droplet; however, in the applications above, multiple sessile droplets appear on a surface. Shedding of sessile drops in different arrangements, i.e. tandem, side by side, triangle, and rectangle have been investigated both experimentally and through VOF simulations. The minimum air velocity (Ucr) at which the drop(s) at the upstream dislodge from the surface was measured. Drops were placed in a wind tunnel with increasing air velocity at a rate of 1m/s2. It has been found that Ucr, deviates from its value for a single drop due to presence of the neighboring drops. The amount of the deviation is closely related to the flow pattern and interaction of drop wakes which are elucidated numerically. The interacting wakes change the drag force on the drops. Generally, the adhesion force is not affected by presence of other drops. As such, when the drops' wakes are interacting strongly, Ucr can increase by 45{\%}. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G12.00006: Emulsion droplet interactions: a front-tracking treatment Lachlan Mason, Damir Juric, Jalel Chergui, Seungwon Shin, Richard V. Craster, Omar K. Matar Emulsion coalescence influences a multitude of industrial applications including solvent extraction, oil recovery and the manufacture of fast-moving consumer goods. Droplet interaction models are vital for the design and scale-up of processing systems, however predictive modelling at the droplet-scale remains a research challenge. This study simulates industrially relevant moderate-inertia collisions for which a high degree of droplet deformation occurs. A hybrid front-tracking/level-set approach is used to automatically account for interface merging without the need for `bookkeeping' of interface connectivity. The model is implemented in Code BLUE using a parallel multi-grid solver, allowing both film and droplet-scale dynamics to be resolved efficiently. Droplet interaction simulations are validated using experimental sequences from the literature in the presence and absence of background turbulence. The framework is readily extensible for modelling the influence of surfactants and non-Newtonian fluids on droplet interaction processes. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G12.00007: Resonance of water balloons Chun-Ti Chang, Paul Steen Water balloons and water drops exhibit similar resonance behaviors. For water drops, numerous resonance modes have been reported. The dynamics of drops is the competition between inertia and surface tension. In contrast, the inertia competes the elasticity of the membrane for resonating water balloons. Despite such difference, a one-to-one correspondence is observed between the mode shapes of drops and balloons. In this talk, we showcase the families of resonance modes for drops and balloons. Additionally, we compare their dispersion relations and symmetry-breaking mechanisms. Based on these, we conclude that water balloons are essentially drops with a much higher effective surface tension. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G12.00008: Gravitational Collision Efficiencies of Small Drops with Application to Raindrop Growth John Stark, Michael Rother Historically, much work has been done to improve our understanding of raindrop formation and growth in the atmosphere. Current work by the authors has focused on calculating gravitational collision efficiencies for the interactions of small, spherical drops, including hydrodynamic and buoyant forces, the lubrication force, retarded and unretarded van der Waals forces, internal drop circulation, and Maxwell slip. Raindrops having radii less than or equal to 30 $\mu$m are considered. For such, small drops, the surrounding fluid inertia, as measured by the Reynolds number, remains negligible. However, drop inertia, as measured by the Stokes number, can be significant, particularly for drops with radii between 10 and 30 $\mu$m. At finite Stokes numbers and low Reynolds numbers, the hydrodynamic forces remain linear in the drops’ translational velocities, but the hydrodynamic forces do not balance the applied forces. The objective of the work presented here is to analyze the relative importance of effects being considered and compare our collision efficiency results with those obtained previously by several different authors. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G12.00009: Dynamics of Two Ferrofluid Drops in a Rotating Field: An Experimental Study Ching-Yao Chen, Wen-Yuan Lo, Jun-Yi Lu Interactions of two ferrofluid drops in a uniform rotational field are investigated. Distinct modes of motion are experimentally observed. The drops remain self-spin without apparent mutual attraction if the initial distance apart is farther than a critical range. If the initial distance is less than the critical range, in addition to self-spins of individual drops, strong magnetic attractive force makes drops approaching closer to contact each other. After the contact, multiple modes of motion are observed. The drops might be locked to form a separated single-magnetized object to proceed self-spin. For a few cases, these locked drops might coalesce after a few rounds. Different mode of interaction after contact is the planetary motion. Instead of drops locked, the drops are bounced away without strong mutual constraint. While proceeds self spins, the drops revolve along their center of mass. Experimental results suggest that, the key factor for these distinct modes of interaction is the approaching speed at the moment of contact, which implies the capillary number is the dominant parameter in the present situation. [Preview Abstract] |
Monday, November 20, 2017 12:32PM - 12:45PM |
G12.00010: Hydrate crystallization at oil-water interface, the effect of nonionic surfactants Liat Rosenfeld, Kevin Dann Gas hydrates pose economic and environmental risks to the oil and gas industry when plug formation occurs in pipelines. A novel approach using interfacial rheology and visualization techniques was applied to understand cyclopentane clathrate hydrate formation in the presence of nonionic surfactant to achieve hydrate inhibition at low percent weight compared to thermodynamic inhibitors. The hydrate-inhibiting performance of various surfactants on a manually nucleated $2~\mu L$ droplet showed a morphological shift in crystallization from planar shell growth to conical growth. Monitoring the internal pressure of the water droplet undergoing hydrate crystallization provides information on the change of interfacial tension during crystallization process. At low surfactant concentrations, planar hydrate crystal was formed and decreasing interfacial tension was observed. At high surfactant concentration, crystal morphology was shifted to conical. Interfacial tension measurements reveal oscillations of interfacial tension during the crystallization process. The oscillatory behavior of the interfacial tension is a result of the growth and release of the hydrate cones from the surface of the droplet. [Preview Abstract] |
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