Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P15: Drop II |
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Sponsoring Units: DFD DSOFT Chair: Daphne Klotsa, University of North Carolina at Chapel Hill Room: 210/212 |
Wednesday, March 4, 2020 2:30PM - 3:06PM |
P15.00001: Microfluidics platform for generation of microparticles and capsules through Electrohydrodynamics Danish Eqbal, Venkat Gundabala In this work we present a microfluidics based implementation of electrohydrodynamics for generation microparticles and capsules. Electric fields are applied through a novel interface-crossing of the precursor single and double emulsion droplet. The particles are alginate based and the capsules have oil core with alginate shell. The particle and capsule sizes were investigated as functions of the applied electric field strength, when the droplet generation happened in the electrodripping mode. It was observed that the obtained particle/capsule sizes were much lower than non-EHD methods and the monodispersity is much better than external EHD approaches. Thus we show an efficient approach that couples EHD with microfluidics to generate uniformly sized microparticles and microcapsules. |
Wednesday, March 4, 2020 3:06PM - 3:18PM |
P15.00002: Numerical Modeling of In-Situ Curing of a Photopolymerizing and Spreading Drop with Applications in 3D-Printing Siddhartha Das, Vishal Sivasankar, Harnoor Sachar, Shayandev Sinha, Daniel Hines The key to understand droplet-based direct writing (DW) printing processes like Aerosol Jet Printing (AJP), Ink-jet printing, syringe printing, etc. is to quantify the behavior of ink droplets during deposition. For instance, the resolution of the print depends on minimum drop radius and also to the extent of the droplet spread (spreading radius). In this study, we investigate the spreading behavior of a polymer drop in conditions similar to that of the Aerosol Jet Printing (AJP) process. Here, the drop is undergoing simultaneous spreading and photopolymerization with timescales of τs and τp respectively. We conduct a comparative study of the droplet behavior for two cases, τs << τp , where the spreading is much faster than polymerization and τs ~ τp, where the spreading and polymerization have similar timescales. Droplet spreading is unaffected by the polymerization process in the former case. However, in the latter case the spreading of the droplet is hindered by the polymerization process due to a tremendous increase in viscosity. We further study this complex thermo-fluid-solutal dynamics by comparing the time dependent curing profiles for the two cases. |
Wednesday, March 4, 2020 3:18PM - 3:30PM |
P15.00003: A Comparative Study of the Pattern Formation in the Drying Protein Droplets suspended in the De-ionized water Anusuya Pal, Germano S Iannacchione Pattern formation in drying bio-relevant systems continues to attract major attention of the researchers because of its medical applications. Here, an extensive description of drying evolution and the final patterns are presented, highlighting the concentration dependence (from 1 to 13 wt%) on two globular proteins, lysozyme (Lys) and bovine serum albumin (BSA) prepared in de-ionized water. The drying process starts with a constant contact radius mode and shifts to a mixed-mode where both contact radius and contact angle changes. The contact angle decreases monotonically, whereas, the contact radius exhibits two regimes: an initial linear, and a later non-linear regime. Unlike linear regime, the non-linear regime is faster for Lys droplets. This results in the formation of a `mound'-like structure and a new feature, a `dimple' is observed in this structure and found to be influenced by the initial concentration. The cracks which are only seen near the edge of BSA droplets, are spread throughout Lys droplets. In particular, Lys shows a hierarchy from delaminated to spiral cracks, and the spirals obey the well-known logarithmic equations. The mean crack spacing is found to be smaller than BSA droplets, and finally, these patterns are interpreted using a mechanical shear mechanism. |
Wednesday, March 4, 2020 3:30PM - 3:42PM |
P15.00004: Flow of Emulsions with Insoluble Surfactant through Porous Media Jacob Gissinger, Alexander Z Zinchenko, Robert H Davis While our understanding of surfactants’ effect on single-drop dynamics has recently been improving, the general rheology of confined and contaminated emulsions remains less characterized. In particular, complex multiscale behavior results when the drop size is larger than granular interstices. In such cases, the emulsion cannot be treated as a single phase and a droplet-resolved model is required. A pressure-driven, concentrated emulsion flowing through a packed bed of spheres is modeled using a three-dimensional boundary-integral algorithm. The emulsion is initially monodisperse and uniformly coated with surfactant, and subsequently advected using a recently-developed robust algorithm for insoluble surfactant transport, with added options for complex equations of state as well as drop breakup. The ensemble-averaged permeabilities of the drop and continuous phases are studied versus degree of contamination and capillary number, with an emphasis on the effect of surfactant on the droplet-breakup cascade. The possibility of daughter drops having a significantly different surfactant concentration, and therefore capillary number, compared to the original drops results in particularly rich local and global dynamics. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P15.00005: Surface roughness control of water-air dynamic contact angle hysteresis on metal substrates Marina Machado de Oliveira, Joseph R Murphy, John Ackerman, WIlliam Rice, Vladimir Alvarado Ice adhesion measurements for impact ice are critical for solving important aerospace challenges. Previous measurements have shown that the ice adhesion on surfaces of apparently identical materials differ significantly under similar measuring conditions. This striking observation indicates that surface properties, e.g. roughness, may significantly affect ice formationand its adhesion response. Roughness properties can regulate the water-air-substrate contact-line dynamics. In this work, we focus on water-air contact angle hysteresis on metal substrates of interest, e.g. aluminum, using a pendant-drop system over a frequency range 0.1-10 Hz. Surface roughness of metal substrates is measured using confocal microscope profilometry. To establish a quantitative correlation between roughness and contact angle hysteresis, we analyzed the spatial roughness correlation function, including its angular dependency. Contact angle hysteresis results reveal a weak dependency on frequency, but a strong function on anisotropy, spatial correlation and surface roughness. The effect depends highly on drop size, which might explain differences in ice accretion under atmospheric conditions. |
Wednesday, March 4, 2020 3:54PM - 4:06PM |
P15.00006: Modeling of hydrophobic particle capture by expanding droplets Gesse Roure, Robert H Davis The method of froth flotation by small air bubbles has been traditionally used in industry to capture fine minerals and other hydrophobic particles. This method, however, has shown to not be efficient for capturing smaller particles. The present work is motivated by a new agglomeration method proposed to overcome this lack of efficiency. It consists of mixing a particle suspension and saltwater-filled droplets covered with semi-permeable oil layers. The present work investigates the two-particle dynamics of a solid particle and an expanding semi-permeable spherical drop in an external pure extensional flow field. The computational results from the numerical integration determine a collision efficiency, which describes the influence of hydrodynamic interactions and osmotic flow on particle capture. An engulfment parameter measures the relative effects of droplet growth and convective flow. |
Wednesday, March 4, 2020 4:06PM - 4:18PM |
P15.00007: Experiments in liquid-spray physics Gary Lapham, John McHugh Waves on the ocean surface interacting with solid objects create complex and visually spectacular patterns of spray. The solid object can be a breakwater, drilling rig, or a ship. Another spray-related case is the presence of large industrial tanks of liquid, and often dangerous liquids, that exist throughout the world. These tanks are becoming obsolete in great numbers. When such tanks burst it is often catastrophic. Recent experience has shown, that when such tanks burst, the resulting spray may shoot several hundreds of meters from the tank—distances that are not readily explained. These tanks often have a wall or dam (containment barrier) surrounding them in an attempt to contain a violent breach or leakage. When the tank bursts it is akin to the dam-break problem. A wall of water rushes forth and impinges on the barrier creating spray. Previous experiments (McHugh and Watt, 1998) considered the related configuration of a solitary wave impinging on a vertical wall. Present experiments discussed will include tank experiments that more closely model the bursting tank case and smaller-scale experiments that attempt to identify some of the fundamental mechanisms of spray formation. |
Wednesday, March 4, 2020 4:18PM - 4:30PM |
P15.00008: The Cheerios Effect under a thin elastic film Carmen Lee, Abigail Buller, Kari Dalnoki-Veress An air bubble placed in a liquid bath will rise through the bath until it reaches the liquid-air interface. The surface tension of the interface will keep the bubble from escaping into the air, but the bubble will in turn deform the flat interface. An equilibrium is reached between the buoyant force of the bubble and the surface tension of the liquid. Furthermore, an air bubble will move along the interface to reach a minimum in potential energy, in this case, the maximum height. Two bubbles in proximity to each other will attract each other due to the surface deformation from the other bubble. This mechanism is analogous to the famous Cheerios effect. Here, we examine the effect of replacing the liquid-air interface with a thin, elastic film. By floating a thin film onto the surface of a clean water bath, we can precisely place two monodisperse air bubbles below and observe the attraction between the bubbles, while altering the interface. We examine the effect of the thin film on the equilibrium position of the bubble as well as the attractive force between the two bubbles and present a theoretical model. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P15.00009: Stability limit of electrified droplets and bubbles Justin Beroz, John Hart, John W M Bush In many physical processes, including cloud electrification, electrospray and electrocoalescence, droplets and bubbles are exposed to electric fields and may either remain whole or burst in response to electrical stresses. Determining the stability limit of a droplet exposed to an external electric field has been a century-long mathematical challenge, and the only analytical treatment to date is an approximate calculation for a free-floating droplet. In this talk, we demonstrate, experimentally and theoretically, that the stability limit of a conducting droplet exposed to an external electric field is described by a power law with broad generality that applies to the cases in which the droplet is pinned or sliding on a conducting surface or free floating. The power law is simply derivable by the variational principle, provided the physical quantities defining the instability are selected to be independent of an arbitrary variation of the system. Practically, our finding provides a simple formula that captures the parameter range of bubbles and droplets that can be supported on electrified surfaces. |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P15.00010: Superpropulsion of liquid droplets in sharpshooter insects Elio Challita, Raghav Acharya, Rodrigo Krugner, Saad Bhamla Sharpshooters are plant-sucking agricultural pests that constantly discharge large quantities of discrete water-based droplet excreta while feeding on plant’s xylem. Owing to a catapult-like mechanism powered by a biological spring located at their anal stylus, sharpshooters are able to blast away discrete droplet excrements at high accelerations reaching 200 m/s2. Here, we unravel the propulsion mechanism exploited by sharpshooters that enables them to optimize the droplet ejection kinematics. Unlike the take-off speed of a rigid projectile that is set by maximum speed of the launching spring, we find that the elastic liquid droplets in sharpshooters can achieve take-off velocities that are up 3X faster than the underlying spring. This ‘superpropulsion’ behavior reveals that the frequency of the insect stylus is tightly tuned to the Rayleigh frequency of droplets (f0~R-3/2), providing insight into the physical limits on the maximum size and speed droplets in these extraordinary insects. |
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