Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session Z01: Drops: General (12:15pm - 1:00pm CST)Interactive On Demand
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Z01.00001: Emergence of Superwalkers Rahil Valani, Tapio Simula, Anja Slim Superwalking droplets emerge when a bath of silicone oil is vibrated simultaneously at a given frequency and its subharmonic tone with a relative phase difference between them. To understand the emergence of superwalking droplets, we have explored their dynamics numerically by extending previously established theoretical models for walkers driven by a single frequency to superwalkers driven by two frequencies. We show that driving the bath at two frequencies with an appropriate phase difference raises every second peak and lowers the intermediate peaks in the vertical periodic motion of the fluid surface. This allows large droplets that could otherwise not walk to leap over the intermediate peaks, resulting in superwalkers. We find that the droplet's vertical and horizontal dynamics are strongly influenced by the relative height difference between successive peaks of the bath motion, a parameter that is controlled by the phase difference. Comparison of our simulated superwalkers with experiments shows good agreement for small- to moderate-sized superwalkers. A novel behavior of superwalking droplets, the stop-and-go motion, is also captured in our simulations. [Preview Abstract] |
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Z01.00002: Magnetic control of breakup of ferrofluid droplets in simple shear flows Md Rifat Hassan, Cheng Wang Dispersion of droplets is prevalent in a number of industrial applications that utilize highly concentrated emulsions where the morphology of droplets plays a vital role in determining the physical and rheological properties of emulsions. Here, we use numerical simulations to analyze the breakup phenomenon of a ferrofluid droplet under uniform magnetic fields in the Stokes flow limit (Re $\le $ 0.03). Our numerical results demonstrate that application of a uniform magnetic field along $\alpha \quad =$ 45\textdegree and 90\textdegree promotes breakup in a ferrofluid droplet at low capillary numbers where the droplet does not typically experience breakup in a shear flow alone. In contrast, implementation of magnetic field along $\alpha $ $=$ 0\textdegree and 135\textdegree suppresses breakup. A critical magnetic Bond number ${Bo}_{cr}$ prevails below which no breakup phenomenon is observed, which also depends on the magnetic field directions and decreases for more viscous droplets. Moreover, large number of satellite droplets are observed at higher magnitudes of magnetic field strengths and viscosity ratios. [Preview Abstract] |
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Z01.00003: Data-Driven Analysis of Contact Line Sahar Andalib, Kunihiko Taira, H. Pirouz Kavehpour Dynamics of droplet evaporation plays an important role in many industrial applications such as ink-jet printing, microfabrication, agricultural irrigation, and bio-diagnostics. Experiments of dropwise evaporation are generally time-consuming and expensive. In the present work, machine learning methods are utilized to analyze methanol droplet in controlled environmental conditions. Relative humidity is predicted by both data-driven classification and regression methods. With the classification approach, Bagged Decision Tree technique is found to outperform Naïve Bayes and is independent of the input data distribution. Furthermore, regression technique enabled the prediction the relative humidity. Additionally, diameter and contact angle are estimated with the regression method. Predictions are more accurate for diameter due to its smooth evolution during droplet lifetime. The current work suggests the potential in using dropwise evaporation as a predictive tool for numerous applications where time-consuming experiments or simulations are not practical options. It also provides valuable insights on the physics of the evaporation phenomena. [Preview Abstract] |
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Z01.00004: Airborne germs transmission through droplet motility Sunny Kumar Generally, droplets are very commonly produced by fragrances spraying, coughing, sneezing and water spilling. The droplet dynamics is dependent upon the several parameters such as surrounding environment, size, shape, composition and external force. The external forces for breaking droplets are gravity, electric field, magnetic field, light, mechanical (pressure) and acoustic force. In case of sneezing or coughing, pressure force breaks the droplets into few microns size droplets with pathogens loaded. These tiny droplets can stay in the air for a few seconds which can transmit airborne infections. Sneeze pathogen loaded droplet motion and staying in air depends upon the speed of sneeze, droplet sizes, height from ground and ambient environment. The droplet contains 97 percent water and other salts, protein, fatty acid and virus. This dynamic of droplets can be analyzed by nondimensional numbers such as Reynold and Weber numbers. This study will help to understand the spreading of the virus through airborne transmission. The various precautions such as face mask, hand sanitization and social distancing can mitigate or reduce the transmission of the infection via respiratory droplets. [Preview Abstract] |
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Z01.00005: Droplet Migration on Contrasting Micro-Striated Surfaces: Vertical Oscillations and Coupled Horizontal Motion Hongyu Zhao, Daniel Orejon, Khellil Sefiane, Martin Shanahan Hydrophobic, micro-structured solid surfaces comprising structural solid fraction gradients are proposed for liquid droplet spontaneous motion. In this study, we examine, experimentally, the migration of a droplet deposited at the boundary of two micro-striated surfaces comprising different structural solid fractions. Upon being deposited onto the boundary, the vertical oscillatory behaviour of the droplet is observed in addition to the alternate, leading and trailing motion of the contact line, resulting in the horizontal migration of the droplet. To model the vertical oscillatory behaviour of the droplet shape, truncated spheroids featured for different droplet shapes with a flatted region in contract with the solid surface are proposed. The vertical oscillatory behaviour is believed to help the droplet overcome wetting hysteresis. In the modelling approach the vertical droplets oscillating truncated spheroids are then coupled to the alternate contact line motion and to the horizontal migration. A good qualitative and quantitative agreement is achieved when comparing the experimental results with the proposed model. [Preview Abstract] |
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Z01.00006: Spitting: droplets, transmission, and infection probability during COVID-19 pandemic Khushboo Pandey, Saptarshi Basu, Roven Pinto, Shashi Prabha Pandey The physical phenomenon of spitting is characterized by the act of forcibly ejecting saliva or any other unwanted substances from the buccal cavity (mouth). Spitting in open is universally accepted as unhygienic as germs present in the saliva may cause serious health issues for those who come in the contact with the expelled saliva and droplets. Spitting is very common in India due to cultural usage of tobacco but during this pandemic of COVID-19, spitting in public and open places cast a serious issue towards public health as spread of COVID 19 is predominantly attributed to respiratory droplets. Since, India is a highly populated country there is a greater possibility of people getting infected through inhalation of droplets carrying the virus. Also, coming in contact with settled droplets on surrounding objects and then touching eyes, nose or mouth, can infect other individuals. Here, we present several pertinent aspects of spitting; generation of droplets, consequent settling on nearby surfaces or suspension in air, and their propensity towards fomite formation. We present droplet size distribution curves and their respective probable distance of travel. We also present an infection probability for spitting droplets settled on various common substrates such as glass, paper, and wood by using the unique method of Voronoi tessellation. [Preview Abstract] |
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Z01.00007: Physics behind the droplet detachment from a surface Neda Ojaghlou, Ali Moghadam, Dusan Bratko, Hooman V. Tafreshi, Alenka Luzar The study of liquid droplet adhering to flat solid surfaces has received considerable attention due to its importance in many different engineering applications, such as filtration, spray coating, and oil recovery, liquid water removal in PEM (proton exchange membrane or polymer electrolyte membrane) fuel cells, and resuspension of an aerosol from solid surfaces. In this project, we focus on understanding how the adhesion of the droplet on a hydrophilic surface affects the detachment behavior and how the applied force can change the residue of droplet on the flat surface. We analyze the process using atomistic molecular dynamic (MD) simulations. We address the fundamental questions about the droplet size dependence of the minimal force capable of detaching a droplet from the surface, and the effects of droplet size and applied force on the amount of the liquid residue left on the surface after the detachment. We perform multiple MD simulations for droplets on a smooth hydrophilic surface at varied system sizes and applied forces. Our modelling studies of the droplet breakup show the amount of residual water to be maximal near the minimum detachment force strengths whereas a complete or near-complete detachment of the droplet can be achieved with very strong force. Because of its fundamental appeal and importance for applications, we hope the work will inspire experimental investigations and theoretical analyses of liquid retention and its control through varied stimuli for droplet detachment from the surface. [Preview Abstract] |
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Z01.00008: Enhanced Anti-Frosting Strategies for Macrotextured Surfaces Christian Machado, Kyoo-Chul Park Condensation frosting is a nearly ubiquitous two stage process that affects thermal systems: first, vapor-liquid phase change occurs on a surface cooled to the saturation limit of the fluid; then, the condensed fluid undergoes a liquid-solid phase change when the surface temperature is below the liquid's freezing point. Oftentimes, frost propagation on various types of surfaces is undesired, resulting in a reduction of heat transfer for thermal systems, mechanical degradation caused by freezing and thawing, and reduction of functional surface area. Most prior research has focused on reducing the adhesion between the surface-ice interface, so that removal can be facilitated easily. The problem still exists, though, of creating a surface design that intrinsically resists the formation of frost. Building upon our previous work identifying the formation of a frost-free zone on macrotextured surfaces, we introduce new experimental results that substantially increase the surface area of the frost-free zone, and establish its stability over long term frosting conditions. By focusing on the impact of diffusion flux coupled with heat transfer at millimetric length scales, these surface designs create thermodynamically stable frost-free regions. [Preview Abstract] |
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Z01.00009: A molecular dynamics approach to modeling effects of detaching force on the compound droplet residue on spherical surfaces Ali Moghadam, Neda Ojaghlou, Dusan Bratko, Hooman V. Tafreshi Droplet detachment from a surface is involved in many industrial applications. While single-phase droplet detachments have been broadly explored, the detachments of multi-phase (compound) droplets attracted less attention. Compound droplets, consisting of magnetic and non-magnetic components, have recently shown promise to detach the non-magnetic component by applying a force on the magnetic one. We study the compound droplet detachment from a sphere via Nonequilibrium Molecular Dynamics (NEMD) simulations. The compound droplet is simulated via coarse-grain models for water (polar) and hexane (non-polar), where water is fully cloaked with hexane owing to their interfacial tensions and the positive spread parameter. The compound droplet is subjected to a vertical force, and we gradually increase its magnitude until detachment. In our study, the force acts on hexane molecules and is varied as a function of the hexane bead's position to emulate applications of magnetic forces for the droplet detachment. The outcome strongly depends on the dynamics of the detaching process. At relatively large forces and faster detachments, the hexane (oil) is capable of detaching water. However, weak forces and slow detachments result in oil separation from the aqueous phase, with most of water remaining as a residue on the surface. [Preview Abstract] |
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Z01.00010: Transparent photothermal metasurfaces amplifying superhydrophobicity by absorbing sunlight Efstratios Mitridis, Henry Lambley, Sven Tröber, Thomas Schutzius, Dimos Poulikakos Designing robust superhydrophobic surfaces has received significant attention in the last decade, empowered by advancements in surface micro/nanoengineering. Researchers have investigated the effect of temperature on droplet-superhydrophobic surface interactions, which poses additional challenges when liquid nucleation manifests itself, due to ensuing surface condensation compromising its anti-wetting behavior. The few existing solutions fail to prevent condensation nucleation, which limits their working envelope, and are not engineered for applications requiring visible transparency. Here we employ and explore the working limits of plasmonic photothermal metasurface composites harvesting sunlight that can sustain water repellency and transparency under challenging environmental conditions where condensation would otherwise be strongly promoted. We demonstrate that a dramatic increase in microtexture filling time with light-induced heating prevents impalement of impacting droplets even for droplet-surface temperature differences of 50 degC. We also evidence how our metasurface works in symbiosis with nanotexture to offset inherent undesirable nucleation, which causes failure under moderate supersaturation conditions, for enhanced impalement resistance. [Preview Abstract] |
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Z01.00011: Superhydrophobic Surfaces for Extreme Environmental Conditions Henry Lambley, Thomas Schutzius, Dimos Poulikakos Superhydrophobicity against impinging water droplets predominantly relies on designing surfaces with capillary pressures larger than that of the impinging droplets. More recent research has explored the compression and drainage of the air layer beneath an impacting droplet and how this can enhance impalement thorough local droplet deformation and increases in the local curvature. However, comparatively little consideration has been given to how the both the thermodynamic state and composition of the intervening gas layer, as well as compressibility effects, affect the outcome of impacts when departing from ambient conditions. Here, by varying the ambient pressure and relative humidity, we probe the limits of the working envelope for robust superhydrophobic surfaces and explore the different failure mechanisms exhibited beyond this. Using engineered materials synthesised with a variety of micro and nanoscale features, we are able to propose additional design constraints to mitigate against this and provide superhydrophobic and icephobic solutions for applications spanning a broad range of environmental conditions. [Preview Abstract] |
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Z01.00012: Environmental friendly and scalable fabrication of antibacterial ZnO-nanostructured surfaces Abinash Tripathy, Athanasios Milionis, Matteo Donati, Chander Shekhar Sharma, Fei Pan, Maniura, Katharina Weber, Qun Ren, Dimos Poulikakos In this work we show that nanostructured ZnO surfaces, fabricated by a fully water-based protocol, exhibit enhanced bactericidal action against \textit{E. coli}, both on the surface itself as well as remotely, and in the absence of sunlight (bacterial killing efficiency of 9250 cells cm$^{\mathrm{-2}}$hr$^{\mathrm{-1}})$. They are also able to disinfect contaminated water (\textgreater 99.98{\%} elimination of bacteria) while satisfying the legal limits on the released Zinc ion concentration (0.73 \textpm 0.15 ppm). Using the same ZnO nanostructures and adding hydrophobization, we obtain self-cleaning surfaces with ultra-low bacterial adhesion. This is achieved either by deposition of a water-soluble fluoroalkylsilane or ethanol-soluble stearic acid. The resulting impressive and tunable antibacterial behavior, combined with the `green' and scalable fabrication, render these materials excellent candidates in sustainable antibacterial and water purification applications. [Preview Abstract] |
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