Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session J30: Drops: General I |
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Chair: Satish Kumar, University of Minnesota Room: 238 |
Sunday, November 20, 2022 4:35PM - 4:48PM |
J30.00001: Droplet evaporation on soft solid substrates Satish Kumar, Vasileios Charitatos Droplet evaporation on soft solid substrates is relevant to applications such as fabrication of microlenses and controlled particle deposition. Here, we develop a lubrication-theory-based model to advance fundamental understanding of the important limiting case of a planar droplet evaporating on a linear viscoelastic solid. A set of partial differential equations describing the time evolution of the liquid–air and liquid–solid interfaces is derived and solved with a finite-difference method. A disjoining-pressure/precursor-film approach is used to describe contact-line motion, and the one-sided model is used to describe solvent evaporation. Parametric studies are conducted to investigate the effect of solid properties (thickness, viscosity, shear modulus, wettability) and evaporation rate on droplet dynamics. Our results indicate that softer substrates speed up droplet evaporation due to prolonged pinning of the contact line. Results from our model are able to qualitatively reproduce some key trends observed in experiments. Due to its systematic formulation, our model can readily be extended to more complex situations of interest such as evaporation of particle-laden droplets on soft solid substrates. (V. Charitatos and S. Kumar, Soft Matter 17 (2021) 9339.) |
Sunday, November 20, 2022 4:48PM - 5:01PM |
J30.00002: Elasto-capillary fluid-solid interaction with compound droplets Sthavishtha Bhopalam Rajakumar, Jesus Bueno, Hector Gomez Compound droplets are of pivotal importance in pharmaceutical, biological and food industrial applications. The interaction of deformable solids with compound droplets in some of these applications remains unexplored. This fluid-solid interaction problem at micro- and nano-scales is governed by elasto-capillarity, a phenomenon where a solid is deformed by the capillary forces at fluid-fluid interfaces. We develop a high-fidelity computational model for simulating elasto-capillary problems involving compound droplets. We study the static wetting of a compound droplet on a soft solid and capillary origamis of compound droplets. Our numerical results show that the dynamics of the droplet-solid interaction can be controlled by varying the surface wettability of the solid, fluid-fluid surface tensions and droplet volumes. Our results suggest better ways to self-assemble micro- and nano-structures in additive manufacturing. |
Sunday, November 20, 2022 5:01PM - 5:14PM |
J30.00003: Twisted capillary flow: Sliding drops on twisted fibers Vanessa R Kern, Andreas Carlson Droplet capture by complex fiber networks such as fog nets can greatly increase water security in communities with little to no rainfall. Here we investigate the sliding motion of a single droplet dispensed onto two adjacent vertical fibers that are either parallel or twisted together to an increasing degree. Post deposition the droplet assumes either an axisymmetric (barrel) or asymmetric (clamshell) shape around the fiber bundle. At long times the droplet may lose mass as it coats the fiber. Interestingly, we find that irrespective of viscosity, the groove created by the two adjacent fibers can stabilize the asymmetric shape, allowing for greater downward transport speeds than the axisymmetric configuration. The twisted fibers induce a wide range of sliding dynamics influencing the drop's shape and downward velocity, which are controlled by the drop's surface tension, viscosity, volume, and the fiber's radius and amount of twisting. |
Sunday, November 20, 2022 5:14PM - 5:27PM |
J30.00004: The Fluid Mechanics of splat painting Diego Avila Garcia, Javier Rodriguez-Rodriguez, Roberto Zenit, Lorene Champougny In art, splat painting consists in subjecting a paint-loaded brush to an acceleration (for example by "flicking" the brush or tapping its handle) to detach paint filaments or droplets. These are projected onto the canvas, leaving aesthetic filamentary or spotty patterns. In this technique, perhaps most emblematically used by American painters Jackson Pollock and Sam Francis, the brush never touches the canvas, making the control of the final pattern challenging for the artist. In this study, we develop an experimental setup to produce repeatable tapping on a paint-loaded brush, mimicking the acceleration imposed by the artist's hand. Using a Newtonian model liquid, we characterize how the amount of paint detached and its final spatial distribution on the substrate depend on control parameters such as paint viscosity and brush acceleration. This allows us to identify different splatting regimes and understand how artists adjust these parameters to tune the final splatter pattern. |
Sunday, November 20, 2022 5:27PM - 5:40PM |
J30.00005: Mixing in drops on a fiber with a crossflow Jessica L Wilson, Philippe Bourrianne, Howard A Stone Drops on fibers in crossflows have complex dynamics. At an intermediate Reynolds number, the imposed crossflow leads to a wake asymmetry which induces translation along the fiber. The internal flows that are generated have much lower Reynolds number. These viscous flows are three-dimensional. We use PIV to image the flows and report on the trajectories of the fluid particles, which we aim to rationalize with analytical and numerical models for the internal flows and particle trajectories. |
Sunday, November 20, 2022 5:40PM - 5:53PM |
J30.00006: A particle-based exoskeleton robot with extreme transformability Hyobin Jeon, Keunhwan Park, Jeoung-Yun Sun, Ho-Young Kim Soft robots that are flexible due to their transformable skeleton can be widely used in various fields where rigid endoskeleton robots are impossible to perform. However, soft robots still have limitations in that they do not work properly when damaged, torn, or separated. Here we show that a particle-based exoskeleton robot, which is encapsulated with far more amounts of hydrophobic particles compared to conventional liquid marbles, can continue to perform various functions under severe damages or deformations. This is thanks to the external particle-laden layer that sufficiently covers the increased surface area of the robot when severely deformed. We quantify the capability of the robot to endure bursting (fracture of external layer) as a function of the particle density. Then we demonstrate that this unconventional robot can perform a variety of functions including merging, penetrating, traversing, stair-descending, engulfing, and discharging, as remotely driven by an ultrasonic field. |
Sunday, November 20, 2022 5:53PM - 6:06PM |
J30.00007: How does a spray cloud behave? antoine Parrenin, Stefan Kooij, Daniel Bonn Spraying is important for many applications, from medication administration and disease propagation to pesticide use in agriculture, it is always important to know how and to what extent spray clouds form, propagate and disappear. We use nozzles with micrometric holes to produce water sprays, and record the cloud propagation with time for different spray speed. We find that the shape remains the same, but the propagation distance of the cloud varies greatly with the speed. We also change the salt concentration in the water to provoke friction induced charging of the droplets. Doing this, we obtain repulsion between droplets, which drastically limits coalescence and changes the droplets size. We notice an important change of behaviour between a charged cloud and a non charged cloud. Since droplet size seems to have a great effect on the cloud behaviour, we then measure the droplets size within different spray clouds and multiple configurations. We remark some coalescence effects increasing the droplets size, followed at a greater distance by some evaporation decreasing the droplets size. Since we still observe droplets far from the nozzle, we conclude that the cloud disappearance for the naked eye is due to the low droplet density far away from the nozzle and not evaporation. |
Sunday, November 20, 2022 6:06PM - 6:19PM |
J30.00008: Shear-induced pair-trajectories of viscous drops in a viscoelastic medium: transition from passing to tumbling Kausik Sarkar, Anik Tarafder, Abhilash Reddy Malipeddi We simulated pair-interactions between viscous drops suspended in a viscoelastic matrix. Simulations revealed a phase plot showing that at small Capillary number Ca, below a critical Weissenberg number Wi drops slide past each other separating in the stream-wise direction, while above this number they rotate around each other in a tumbling/rotating trajectory. We offer a crude physics-based analytical model for this phenomenon, predicated on the premise that the elasticity results in a large region of spiraling streamlines around a single drop which during a pair interaction traps the second drop. We treat the additional tension along the curved streamlines due to the first normal stress difference as an enhancement to the interfacial tension. An approximate force balance between the imposed planar shear flow and the perturbation flow driven by the interfacial tension and elastic tensions around the drop delineates the region of the curved streamlines. Demanding that it be large enough to trap the second drop gives us the scaling between the critical Ca and Wi values defining the phase boundary seen in the simulation. The model based on single drop dynamics is admittedly crude and entirely neglects the direct strong elastic interactions seen between the separating drops, which could play a critical role in determining the trajectories. [NSF Award 2019507, ACI-1548562 (CTS180042)] |
Sunday, November 20, 2022 6:19PM - 6:32PM |
J30.00009: Vapour field between neighbouring sessile drops probed by background-oriented schlieren technique Yutaku Kita, Anushka Kapoor, Khellil Sefiane Despite the extensive progress in the studies of the evaporation of isolated sessile drops for the past decades, little is understood about how they evaporate in a group, which is rather relevant to actual applications such as printing, nanofabrication, cooling and bioassays. While recent theoretical analysis has concerned the evaporation of a pair of thin drops adjacent each other, to the best of our knowledge, no accurate and direct quantitative characterisation of vapour diffusion in such systems has been made, which could assist in a more detailed description of the competitive evaporation. In this contribution, we develop a technique capable of visualising vapour concentration fields around evaporating drops based on the background-oriented schlieren (BOS) principle. Acetone is used in the present study for its sufficient refractive-index contrast of the vapour with air. So far, relative vapour concentration profiles have been obtained and validated. Furthre analysis has been undertaken to investigate the effect of drop size and distance on the evaporative flux distributions. |
Sunday, November 20, 2022 6:32PM - 6:45PM |
J30.00010: Extracting droplet height using the pattern distortion technique: from single droplets to arbitrary arrays Joseph J Kilbride, Fouzia F Ouali, David J Fairhurst Recent theoretical and numerical models describe the reduced evaporation rates of multiple droplets on surfaces [1,2]. However, the experimental work required to validate these models has fallen behind due to difficulties in determining the transient volumes of droplets arranged in arbitrary 2D patterns [3]. We present a simpler method [4] for determining individual droplet volumes for any arrangement of droplets. In this technique each droplet acts like a lens which magnifies a pattern placed beneath it. We use an optical transfer matrix method to trace the paraxial rays from the pattern, through the substrate and droplet, to the camera, providing a relationship between the measured magnification and the height of each droplet. The technique is validated against a traditional side-camera method finding excellent agreement. Finally, we take experimental data [5] from various 2D droplet arrays and compare quantitatively to the different theoretical models [1,2]. |
Sunday, November 20, 2022 6:45PM - 6:58PM |
J30.00011: The evaporation of multiple nearly-circular droplets and applications to the coffee stain effect Alexander W Wray, Matthew R Moore The diffusion-limited evaporation of droplets has received significant interest in recent years due to its fundamental significance in a variety of industrial processes such as printing, patterning, DNA microarray technologies and the manufacture of OLEDs. However, the only available analytical solutions are for circular or elliptical droplets. We improve this state of affairs by finding asymptotic solutions for the evaporative flux for nearly-circular droplets. Via matched asymptotic expansions, we show that this allows for the determination of the resultant profiles, and corresponding coffee-stain residue, in large droplets. This is then extended to polygonal drops, before the framework is extended to allow for the complete calculations to be performed for arbitrary arrays of multiple droplets. |
Sunday, November 20, 2022 6:58PM - 7:11PM |
J30.00012: Droplet impact on highly deformable substrate and three-dimensional stress fields measurement by photoelastic tomography Yuto Yokoyama, Sayaka Ichihara, Benjamin R Mitchell, Ali Nassiri, Brad L Kinsey, Yannis P Korkolis, Yoshiyuki Tagawa Understanding the droplet impact behavior on soft substrates is important in many industrial technologies, such as 3D inkjet bioprinters. When the droplet impacts on the deformable substrate, energy loss due to substrate deformation, which depends on the impact stress, must be considered. Therefore, measurement of the three-dimensional (3D) stress field within the soft substrate is essential to understand the droplet impact. To do so, we use photoelasticity. When circularly polarized light enters a material under a stressed state, it exits as elliptically polarized light with photoelastic parameters, which depend on the stress state. Photoelastic tomography can be used to reconstruct the 3D stress field from the measured photoelastic parameters. We developed a novel photoelastic tomography algorithm, combining vector tomography and conventional photoelastic tomography. Linear interpolation and matrix calculations are used to improve the conventional method. We applied this tomography algorithm for the droplet impact onto a soft substrate which is an unsteady phenomenon. We simultaneously measured droplet impact on a highly deformable substrate (gelatin gel) and the stress field in the substrate using a high-speed polarization camera. |
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