Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V48: Drops IIIFocus
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Sponsoring Units: DFD GSOFT GSNP Chair: A Hirsa, Rensselaer Polytech Inst Room: BCEC 251 |
Thursday, March 7, 2019 2:30PM - 2:42PM |
V48.00001: The Effect of Detaching Force on the Droplet Residue on a Fiber Neda Ojaghlou, Hooman Tafreshi, Dusan Bratko, Alenka Luzar The adherence to, and removal of droplets from cylindrical fibers underlie applications from fog harvesting, oil–water, and water transport in fuel cells. When the droplets are forcibly removed from hydrophilic fibers, the ease of detachment strongly depends on droplet volume and the rate of the process controlled by the applied force. In the present work, we study the mechanism of water droplet detachment and retention of residual water on smooth hydrophilic fibers using nonequilibrium molecular dynamics simulations. We extract scaling relations that allow extrapolation of our findings to larger length scales that are not directly accessible by molecular models. Our studies of the droplet breakup uncover a strongly nonmonotonic influence of external force, with the amount of residual water maximized under the intermediate force strengths whereas a complete or near-complete detachment of the droplet can be achieved in both extremes, with the applied force only slightly, or considerably exceeding the minimal force of detachment. The strength of this force decreases with the size of the drop, while the maximal residue increases with the droplet volume, V, sub-linearly, in proportion to the V2/3. |
Thursday, March 7, 2019 2:42PM - 2:54PM |
V48.00002: Effects of surfactant transport on the electro-deformation of viscous prolate drops Herve Nganguia, On Shun Pak, Yuan-nan Young We report quantification of effects of surfactant transport on a viscous prolate drop under a DC electric field, focusing on characterization through the dimensionless Peclet number. Our findings reveal distinct equilibrium deformations that depend on the type of drops (leaky dielectric versus conducting) and the transport regime (convection or diffusion). |
Thursday, March 7, 2019 2:54PM - 3:06PM |
V48.00003: Migration of droplets on a conical fiber Carmen Lee, Kari Dalnoki-Veress In arid climates, nature has developed an efficient method of harvesting water from the air. Organisms, like cacti, are covered in a multitude of needle-like conical spines. Water droplets that condense on the tip of the fiber are spontaneously driven toward the base by Laplace pressure. The changing curvature of the conical shape is the mechanism responsible for the motion. We examine the effect of geometry on the droplet movement and compare the motion of multiple droplets on a conical glass fiber to that of a single droplet. |
Thursday, March 7, 2019 3:06PM - 3:18PM |
V48.00004: Preliminary observations of bursting of molten steel thick films in a steel plate irradiated by a high energy laser Mary Lanzerotti, Kenneth Brakke, Kenneth Allen, John Hartke This talk presents preliminary observations of the bursting of films of molten steel following illumination of a thin vertical steel plate by a 1075-nm continuous-wave 1000W Ytterbium fiber laser. Molten steel formed in the illuminated region persists as a molten disk for several seconds before a hole forms. Gravity is responsible for the formation of a dimple in the upper part of the molten disk and a bulge in the lower part. After several seconds, a hole appears at the dimple. Hole enlargement is quite sudden, like a soap film popping. Following this, a molten drop forms and falls under the influence of gravity below the laser beam, leaving behind a hole in the plate. Images of the initial hole captured by a high speed digital camera show that the hole forms first in the top portion of the molten disk, not in the center. |
Thursday, March 7, 2019 3:18PM - 3:30PM |
V48.00005: Mechanism of Contact between a Droplet and an Atomically Smooth Substrate Hau Yung Lo, Yuan Liu, Lei Xu When a droplet gently lands on an atomically smooth substrate, it will most likely contact the underlying surface in about 0.1 s. However, theoretical estimation from fluid mechanics predicts a contact time of 10–100 s. What causes this large discrepancy, and how does nature speed up contact by 2 orders of magnitude? To probe this fundamental question, we prepare atomically smooth substrates by either coating a liquid film on glass or using a freshly cleaved mica surface, and visualize the droplet contact dynamics with 30-nm resolution. Interestingly, we discover two distinct speed-up approaches: (1) droplet skidding due to even minute perturbations breaks rotational symmetry and produces early contact at the thinnest gap location, and (2) for the unperturbed situation with rotational symmetry, a previously unnoticed boundary flow around only 0.1 mm=s expedites air drainage by over 1 order of magnitude. Together, these two mechanisms universally explain general contact phenomena on smooth substrates. The fundamental discoveries shed new light on contact and drainage research. |
Thursday, March 7, 2019 3:30PM - 3:42PM |
V48.00006: Manipulating drop shapes in a microchannel Rocio Navarro, Alexander Z Zinchenko, Robert Davis We use a boundary-integral algorithm that simulates the motion of a freely suspended, three-dimensional deformable drop in a plane-parallel microchannel at small Reynolds number, large Peclet number, and moderate capillary number. The drop size is comparable to the channel height, which is much smaller than the channel depth. In this problem, the final shape of the drop is extremely dependent on the channel shape, flow ratios and time, so we have studied a variety of conditions. By changing the number of inlet and outlet channels, as well as the physical geometry of the channel and the drop properties, we are able to create a diverse set of droplet shapes, including shapes with interesting geometric properties such as deltoids, dumbbells and oblate spheroids. In principle, desired shapes could subsequently be “frozen” by a temperature-induced or flow-induced phase change, to yield particles with desired geometries and properties for drug delivery, tissue scaffolds, etc. This presentation will describe the simulation method and present example results of how different shapes may be achieved. |
Thursday, March 7, 2019 3:42PM - 3:54PM |
V48.00007: Flow in a ring-sheared drop with finite surface shear viscosity. Frank Riley, Shreyash Gulati, Juan Manuel Lopez, A Hirsa In the ring-sheared drop, a liquid drop is constrained by two contact rings, where the rings differentially rotate. This differential rotation generates a flow in the bulk primarily through surface shear viscosity. Numerical predictions of bulk flow and mixing within ring-sheared drops were recently reported. The previous work assumed that the surface shear viscosity, non-dimensionalized by the size of the drop, is large. However, in many cases, especially for large drops, which can be studied in microgravity, the dimensionless surface shear viscosity can be arbitrarily small but still able to affect the bulk flow. Computations reveal that an increasing Reynolds number correlates to an increase in sensitivity to the effects of changing surface shear viscosity. Furthermore, upon decreasing surface viscosity, the coupling between the surface and the bulk flows strengthens. The system is then driven towards solid-body rotation. |
Thursday, March 7, 2019 3:54PM - 4:06PM |
V48.00008: Droplet Motion on Superhydrophobic Surfaces Alexander Smith, Rebecca Sutton, Keoni Mahelona, Shaun C Hendy In this talk, molecular dynamics simulations are used to investigate the roles of droplet size and surface geometry on the equilibrium velocity of droplets moving down superhydrophobic surfaces. An extension of prior theoretical descriptions, accounting for interfacial slip, is used to interpret these results. This approach yields three limiting cases for the drop’s steady-state velocity, where energy losses are dominated by viscous dissipation, surface friction or contact line friction respectively. |
Thursday, March 7, 2019 4:06PM - 4:18PM |
V48.00009: Numerical Investigation of Hypoxic Effects on the Biodegradation of Oil Microdroplets George Kapellos, Nicolas Kalogerakis, Patrick Doyle We present an extended compound particle model for the biodegradation of solitary oil microdroplets moving through a seawater column. The compound particle is of the core-shell type and consists of an oily core that is successively surrounded by an ultrathin bioreactive skin of oleophilic microbes and another bioreactive biofilm shell. The extended model accounts for the counter transport of dissolved oxygen and multiple oil components of varying bioavailability and toxicity within the biofilm that covers and degrades the oily droplet. The governing advection-diffusion-bioreaction PDEs are solved numerically to calculate the droplet shrinking rate as a function of the drifting speed, the non-linear microbial kinetics, the biofilm thickness, and other key parameters. A system of coupled ODEs is also formulated for the evolution of the compound particle dimensions. The impact of hypoxic and inhibitory conditions on the droplet biodegradation rate is quantified and critically discussed in connection with the potential for the - very slow, but feasible- anaerobic hydrocarbon degradation in oxygen-depleted marine waters and sediments. |
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