Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F11: Drops IRecordings Available
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Sponsoring Units: DFD Chair: Siddhartha Das, University of Maryland Room: McCormick Place W-181B |
Tuesday, March 15, 2022 8:00AM - 8:12AM |
F11.00001: Inertial coalescence of a liquid drop surrounded by viscous liquid Yukina M Koga, Ko Okumura Coalescence of liquid drops is a phenomenon familiar in everyday life. In recent years, the dynamics of coalescence of liquid drops has been actively explored. Many groups have focused on the bridge formed between drops and revealed its growing dynamics such as viscous and inertial regimes. The coalescence where drops are surrounded by another liquid was also studied [1], and it was concluded that regardless of the viscosity ratio of the surrounding liquid to the droplet liquid, early neck dynamics is always governed by the viscosity of the drop [2]. |
Tuesday, March 15, 2022 8:12AM - 8:24AM |
F11.00002: Statics and dynamics of droplets on lubricant infused surfaces Zhaohe Dai, Dominic J Vella Slippery liquid infused porous surfaces are formed by coating surface with a thin layer of oil lubricant. This thin layer prevents other droplets from reaching the solid surface and allows such deposited droplets to move with ultra-low friction, leading to a range of applications. In this talk we will discuss the static and dynamic behaviour of droplets placed on lubricant infused surfaces. We will show that the layer thickness and the size of the substrate are key parameters in determining the final equilibrium. However, the evolution towards the equilibrium is extremely slow (on the order of days for typical experimental parameter values). As a result, we suggest that most previous experiments with oil films lubricating smooth substrates are likely to have been in an evolving, albeit slowly evolving, transient state. |
Tuesday, March 15, 2022 8:24AM - 8:36AM |
F11.00003: Controlling emulsion elasticity by bridging telechelic triblock copolymers Ryan Poling-Skutvik, Daniel Keane Suspension elasticity controls the performance of complex fluids in applications ranging from 3D printing to therapeutics to consumer products. Here, we add telechelic triblock copolymers consisting of polystyrene (PS) endblocks and a poly(ethylene oxide) (PEO) midblock to a suspension of cyclohexane droplets stabilized by non-ionic surfactants. The PS-PEO-PS chains bridge between droplets to form an elastic network. We measure the elasticity of these suspensions using standard linear oscillatory rheology and find that the telechelic polymers drastically increase the suspension elasticity relative to neutral non-associating polymers. We tune the emulsion elasticity by increasing the Mw of the endblocks to partition them more strongly within the droplets, the Mw of the midblock to increase the bridging-to-looping ratio, and by increasing the polymer concentration. Furthermore, for high Mw endblocks, we observe strong thermal dependence of the emulsion elasticity across the upper-critical solution temperature between polystyrene and cyclohexane. Whereas the emulsions undergo terminal relaxations at low temperatures, they exhibit sustained plateau moduli at elevated temperatures, indicating a fully elastic response. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F11.00004: Universal behaviour in coalescence of polymeric droplets Sarath Vegesina, Aniruddha Saha, Siddhartha Mukherjee, Aditya Bandopadhyay, Aloke Kumar, Suman Chakraborty One of the most common and frequent rheological phenomena observed in nature is the coalescence of droplets. It encompasses the importance of phenomena like raindrops formation in clouds, printing techniques, and even multiphase flows. The process of merging of two droplets towards forming a single daughter drop is known to exhibit formation of liquid bridge. The dynamics of bridge growth for fluids exhibiting characteristic relaxation time scales like polymer solutions is yet to be probed. In polymeric solutions the interplay between surface-forces along with the inertial, viscous, and elastic forces propel this phenomenon. By the balance of the pertinent forces in the phenomenon, we unveil a universal temporal evolution of the neck radius, R, as R/(νoλ)0.5 ~ (t/λ)0.36 Oh-0.36(c/c*)-0.432. Our findings are substantiated by a theoretical model using linear Phan-Thein-Tanner constitutive equation. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F11.00005: Symmetric and Asymmetric Coalescence of Polymeric Droplets: Effect of Weber number and Viscosity Siddhartha Das, Vishal Sankar Sivasankar, Sai A Etha, Daniel R Hines Understanding droplet dynamics (e.g., impact, spreading, coalescence, etc.) is key to understand the deposition process of any droplet based direct-write printing processes such as Aerosol Jet printing (AJP) and inkjet printing (IJP). In this study, we employ direct numerical simulations for probing the coalescence dynamics of two equal-sized (case of symmetric coalescence) as well as two unequal-sized (case of asymmetric coalescence) polymeric liquid droplets impacting on a solid substrate. We investigate the effect of Weber number and the liquid viscosity on the coalescence dynamics, which in turn is quantified by time evolution of the height (h0) and width (W) of the bridge formed by the coalescing droplets. The numerical results dictating this growth of the height and the width of the bridge are established to obey distinct physics-based scaling laws. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F11.00006: Theories of elastic shell buckling in drying colloidal droplet Angkur J Shaikeea, Saptarshi Basu Evaporation of drops is ubiquitous in any real environment. We have all experienced drying of coffee on surfaces and rolling of water drops on lotus leaf to the extent that these natural phenomena are translated into engineering products _ from inkjet printing to water repellent surface sprays. They are a delight for fluid mechanics thinkers. Not very often researchers from diverse fields, like solid and fluid divisions discuss science until a drop of coffee spills on the tablecloth and they begin to surprise with their scientific insights. A drying droplet is once such phenomena. Sessile drops dispersed with solid constituents evaporating in a natural environment intersects science across disciplines. In this study, we discuss theories of elastic shell buckling to explain the eventual collapse of a drying colloidal droplet that had self-assembled to form a shell. Why? and how? does this fluid drop fracture when evaporation is about to drive away the fluid. This can bring interesting insights into the Physics of drying droplets. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F11.00007: How a leak can stop itself Caroline D Tally, Katharine E Jensen, Rose B Tchuenkam, Heather E Kurtz Small fluid leaks are common—and often troublesome—occurrences in everyday life. We often consider what action is required to stop a leak, or to prevent one from starting in the first place, but here we consider a somewhat different question: how can a leak stop itself? We perform simple experiments characterizing the leakage of fluid from a small circular hole in a vertical tube. We observe flow transitions from jetting to rivulet flow to dripping to spontaneous arrest with decreasing driving pressure. Using high-speed imaging, we capture the mechanism of flow-stop, and observe that the leaking fluid undergoes a series of breakup instabilities leading to the creation of a final, static "capping droplet" whose surface curvature prevents further leakage. Measurements of attempted capping events reveal that the equilibrating droplets behave as lightly damped harmonic oscillators. Combining our experiments with an energetic analysis, we find that a leak can stop itself by generating a spherical cap of fluid whose total energy is less than the potential barrier at that driving pressure and fluid contact geometry. |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F11.00008: Prediction and Measurement of Leaky Dielectric Drop Interactions Jeremy I Kach, Aditya S Khair Application of an electric field across the curved interface of two fluids of low but non-zero conductivities, or ‘leaky dielectrics,’ can give rise to electric stresses that drive sustained fluid flow. In a uniform DC electric field of sufficiently weak magnitude, the electric and velocity fields around an isolated, neutrally buoyant leaky dielectric drop at zero Reynolds number are fore-aft and azimuthally symmetric about the applied field axis. Consequently, the drop remains stationary. The presence of a second drop breaks these symmetries, resulting in relative motion of the drop pair. Recently, Sorgentone et al. (Sorgentone et al. JFM, 2021) derived an analytical expression for the relative velocity of a pair of widely separated drops of identical constitution, asymptotic in the inverse separation distance between the drop centroids. In the present work, we generalize the theory of Sorgentone et al. to interactions of dissimilar drops (of different size or constitution), and the pairwise additive interactions of three or more drops. We perform experiments on silicone oil drops suspended in castor oil and compare to asymptotic predictions of the drop pair trajectories. Experimental trajectories of drops with their line of centers initially at an arbitrary angle to the field direction are shown to be qualitatively predicted by our theory. We show results of experiments of dissimilar drops and of three and four drops, again observing qualitative agreement with our theoretical predictions. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F11.00009: The Formation of Instabilities In Oxidizing Eutectic Gallium Indium on an Inclined Plane Keith D Hillaire, Hangjie Ji, Karen E Daniels Surfactant-covered droplets and streams can go unstable via a variety of different mechanisms, driven by gravity, surface tension gradients, the Rayleigh-Plateau instability, and altering droplet wettability and elasticity. Oxidizing liquid metals, such as eutectic gallium-indium (EGaIn), provide a system where surface tension effects can be reversibly tuned electrochemically using anodization. We pin a droplet of EGaIn to a small copper patch at the the top of an inclined substrate, and observe the spreading and pinch-off behavior of EGaIn as a function of the applied voltage during anodization. This delicately controls the surface tension of the interface via the resulting concentration of oxide molecules acting as surfactants. The interplay of the instabilities that form as the droplet spreads leads to droplets with regions of positive, negative, and neutral curvature, with the droplets taking on a variety of shapes: split fingers, pearling, snowmen, and wavy pendant drops. We capitalize on these instabilities to improve understanding of the fundamental fluid mechanics of oxidation liquid metals. |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F11.00010: Scale-free measurements of oil droplet rising speeds during bacterial bio-degradation Vincent Hickl, Gabriel Juarez In marine environments, spilled oil droplets can be transported over kilometers in the water column. As this oil is biodegraded, growing bacteria on the surface of these microscopic droplets give rise to complex emergent phenomena, including interfacial deformations which can generate complex shapes and significantly enlarge droplets. A complete understanding of the fate and transport of spilled oil requires bridging the gap between these length scales and determining how microscale processes affect large-scale transport of oil. Here, we describe experimental observations of rising oil droplets using a scale-free vertical microscope. Droplets of radii 10-100 µm are imaged in a rotating fluid chamber as bacteria attach to and grow on the interface. The rotation rate is matched to the droplet rising speed to allow imaging over 24 hours, corresponding to vertical distances of 3-300 m. The formation of a dense bio-aggregate due to bacterial colonization of the interface significantly alters the rising speed. This effect depends on the droplet radius, and can cause oil to remain in the water column for months longer than previously predicted. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F11.00011: Dynamics of an unconstrained droplet in the transitional boiling regime Pranjal Agrawal, Susmita Dash The impact of droplets on heated surfaces and subsequent droplet dynamics plays a crucial role in several applications, including spray cooling, solidification, and pharmaceuticals. At wall temperatures above the boiling point of the liquid, boiling initiates, and the regime is dependent on the wall superheat. The transition from vigorous boiling to conduction-limited film boiling regime is marked by intermittent solid-liquid contact. This transition boiling regime is characterized by an increase in the total evaporation time of the droplet with an increase in substrate temperature, unlike the evaporation and nucleate boiling regimes where the total evaporation time decreases with an increase in substrate temperature. We show that an unconstrained droplet in the transitional boiling regime undergoes trampolining behavior and high rebounds. The droplet dynamics result in a non-monotonic variation in the total evaporation time with temperature– an observation that is absent for a constrained droplet. The high rebound droplet height in the transition boiling regime is attributed to the nucleation and escape of the bubble at the liquid-substrate interface that results in a gain in momentum of the droplet before take-off. |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F11.00012: Proposal for studying bulk supercooled water using a magnetically levitated drop Giovanna Truong, Yiqi Wang, Ananya Rai, Chitres Guria, Yogesh S. S Patil, Jack G. E Harris Levitation has become a useful technique in science and engineering to isolate, measure and manipulate macroscopic objects with high precision. We will describe a tabletop magneto-gravitational trap (fabricated from permanent magnets) that stably levitates diamagnetic materials, including silica and water. We have characterized the trapping potential by levitating silica microspheres with diameters ~50 μm and measuring their oscillations. The resonant frequencies measured along the trap's three principal axes agree with those predicted by numerical simulation. We also measure the sphere's first- and second- harmonic responses to an applied drive, which stem from the sphere's net charge and its dielectric response, respectively. A longer-term goal of this work is to address outstanding puzzles concerning supercooled water [1-3]. Since a levitated drop is removed from solid walls, vibrations, and impurities (all of which tend to reduce the stability of the supercooled phase), this approach represents a promising way to study the supercooled phase of water. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F11.00013: The Dynamics of Fluid Droplets in Acoustic Traps Jacqueline Sustiel Acoustically levitated particles constitute wave-matter composite systems in which waves exert forces on objects, the objects scatter the waves, and the scattered waves mediate interactions that organize the objects. This interplay of influences causes solid spheres to organize themselves into floating monolayer crystals within the nodes of acoustic standing waves. The behavior of wave-matter composites becomes substantially more complex when the levitated objects are deformable, as in the case of acoustically levitated emulsion droplets. Compliant droplets couple more strongly than solid spheres to solenoidal components in a sound wave's velocity field and therefore can spin rapidly. The resulting hydrodynamic coupling contributes to new dynamical states. This talk briefly reviews the principles of acoustic radiation forces and combines acoustokinetic analysis with computer simulation to explain recent observations of anomalous behavior in acoustically levitated droplets. |
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