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 T31: Drops: General III |
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Chair: Pengtao Yue, Virginia Tech Room: 239 |
Monday, November 21, 2022 4:10PM - 4:23PM |
T31.00001: Spontaneous motion of a volatile droplet on a warm substrate Pallav Kant, Mathieu Souzy, Nayoung Kim, Devaraj van der Meer, Detlef Lohse When a volatile droplet is deposited on a uniformly heated wettable substrate, one expects to see it spread into a thin film and evaporate at an enhanced rate. However, we demonstrate that at higher substrate temperatures, still below the boiling temperature of the liquid, the deposited droplet starts to contract. Even to that, above a threshold temperature, the droplet surprisingly starts to spontaneously move. We describe and quantify the contraction and the self-propulsion, both arising due to thermocapillary convection induced by a gradient in temperature along the droplet interface. We show that the relative thermal conductivity of the substrate and liquid plays a crucial role in determining the directionality of the themo-capillary flow. Finally, we provide a scaling law that captures the dependence of the moving speed of the droplet on the dynamic contact angle that changes with substrate temperature. |
Monday, November 21, 2022 4:23PM - 4:36PM |
T31.00002: Dynamics and Inner Mixing of a Droplet in a Stokes Trap Gesse A Roure, Alexander Z Zinchenko, Robert H Davis Recently, a flow-based, particle-trapping mechanism called the Stokes trap was developed to allow for trapping and control of small particles in microchannels in the intersection of multiple branches in a microfluidic channel [1]. The motion of such particles can then be controlled by changing the flow rates in the branches. For droplets, in contrast to rigid particles, the Stokes trap allows for extra features such as enhancement of inner mixing and shape control, which can be used in microreactors and fabrication processes. In this work, we analyze the motion and stability of a deformable droplet in a Stokes trap. To this end, the droplet dynamics are analyzed using boundary-integral simulations, which we use to explore the influence of physical parameters such as capillary number and viscosity ratio on drop dynamics and internal mixing. For low values of viscosity ratio and capillary number, there is a steady-state drop configuration, which can be unstable depending on the choice of parameters. Changes in the inlet and outlet flow rates result in changes in equilibrium configurations, allowing for shape control and manipulation of flow pattern inside the droplet, enhancing internal mixing. |
Monday, November 21, 2022 4:36PM - 4:49PM |
T31.00003: Determining micro droplet profiles using reflection interference fringe (RIF) technique Iltai I Kim, Yang Lie, Jaesung Park, Hyun-Joong Kim, Hong-Chul Kim We show that the reflection interference fringe (RIF) technique can successfully determine the micro/macro droplet profiles such as the contact angle, the height, and the dual-profiles by simply measuring the reflected interference fringe formed on the screen away from the sessile droplet. The geometric modeling and the ray-tracing are conducted to show that the interference fringes are formed on the screen The measured droplets have low contact angles from 1 to 15 degrees with the height less than 20 microns and the droplet radius less than 200 microns. The ray-tracing simulation shows that the formed interference is caused by the interaction of the reflected laser beams from the convex spherical profile in the center and the concave hyperbolic profile in the contact line. RIF technique can effectively verify the existence of a dual profile of the microdroplets in a simple setup, not requiring a microscope. It shows a good agreement between the experiments and the ray-tracing simulation in the interference fringe profiles, such as the fringes numbers, the fringe location, the fringe radius (FR). The simulation shows that most droplets have the inflection points close to the edge within 5 % of the droplet radius and the precursor film thickness distribution from a few to ~100 nm. The optical path difference (OPD) is determined analytically as a function of the fringe numbers and the incidence angle, like the OPD in Fizeau interference. The droplet height is determined by counting the fringe numbers in the OPD relation, showing a linear dependency on the fringe numbers and an excellent agreement with the experimental measurements. An analytic expression is obtained between the droplet apparent contact angle and the interference fringe radius to show a good agreement. This research is expected to provide a new insight to understand the microdroplet dynamics and evaporation-induced liquid film formation phenomenon. |
Monday, November 21, 2022 4:49PM - 5:02PM |
T31.00004: Hybrid measurement of droplet contact angle and refractive index using transmission fringe (TF) technique: Theoretical study Iltai I Kim, Hongkyu Yoon, Jeffery A Greathouse This work presents the development of a hybrid measurement of droplet contact angle and refractive index (RI) using the transmission fringe (TF) technique. Firstly, the transmission fringe (TF) technique is developed, which can determine the droplet contact angle on a surface by measuring the transmitted beam fringe radius on the projected screen. An analytic expression for the contact angle as a function of the measured fringe radius (FR) from geometric modeling has been formulated and simulated as critical angle versus fringe radius at various incidence angles. The analytical formulation shows that the contact angle is a function of the interference FR, the incidence angle, the working distance from the droplet to the interference fringe, and the refractive indices (RI) of the liquid and the substrate. The contact angles in the range of 0 to 90 degrees are simulated for the droplets with a few millimeters in the radius and tens to thousands of micrometers in height. Secondly, different RI liquids are simulated (RI 1.0~2.0) to demonstrate the RI determination of the unknown liquids. The FR from the transmission fringe (TF) and the contact angles from the side view imaging can determine the RI of liquids in a unique way. This new optical detection technique can be used to determine the RI of unknown liquids with a simple optical setup, not requiring a sophisticated microscope. This technique can determine the contact angle of the liquids with known RI by measuring the fringe radius in a simple optical set-up of a TF formation. Furthermore, with a separately measured contact angle from sideview imaging, the RI of unknown liquids can be determined by the measured FR. |
Monday, November 21, 2022 5:02PM - 5:15PM |
T31.00005: An Initial Shape Change during the Freezing of Supercooled Water Microdrops Armin Kalita, Claudiu A Stan Frozen water drops can have complicated shapes due to deformation during freezing. We investigated the deformation induced by freezing in supercooled water microdrops. We imaged hundreds of 40-µm diameter droplets that were supercooled through evaporation in vacuum, and froze after ice nucleation near −39oC. During the first stages of freezing, the images of the drops, which are projections of the three-dimensional shapes of the drops, had a range of shapes from circles to approximate ellipses with ellipticities up to 1.06. The variation of ellipticities in the images could be due to the individual drops having diverse three-dimensional shapes or due to different orientations of drops with the same shape. We investigated if a common three-dimensional shape can explain the distribution of ellipticities in the images, by assuming basic drop shapes and simulating their image projections numerically. By fitting the simulated drop projections against the experimental images, we discovered that the distribution of ellipticities in the images is inconsistent with axially symmetric ellipsoids, but is consistent with a triaxial ellipsoid. We will discuss how the presence or absence of the three-dimensional axial symmetry could be related to the dendritic growth of ice crystals during freezing. |
Monday, November 21, 2022 5:15PM - 5:28PM |
T31.00006: The spitting drop: How a double-emulsion droplet bursts during its cryopreservation Jochem Meijer, Pallav Kant, Detlef Lohse Solidification of a complex liquid is pertinent to numerous natural and industrial processes. Here we examine the solidification of a water-in-oil double-emulsion droplet in water when a freezing front passes over. We show that the solidification of such a complex liquid can trigger the topological transition from the W/O/W state to an O/W single emulsion configuration. Strikingly, during this transition the encapsulated water droplet bursts first, and is then expelled from the oil droplet. This bursting of the encapsulated droplet occurs at extremely short-time-scales ($t \sim 10\mu$s), reflecting the vastly different time-scales involved in this solidification process, spanning over 8 orders of magnitude. We argue that the transition is triggered by the freezing of the encapsulated water droplet from outside in, and discuss the dependence of this phenomenon on the control parameters, i.e., the solidification rate and the water droplet size. Furthermore, we show that upon expulsion of the encapsulated droplet it may migrate within the solidified bulk via a mechanism equivalent to 'brine-diffusion'. Our findings are relevant for exerting better control over cyropreservation procedures of food emulsions and bio-specimen. |
Monday, November 21, 2022 5:28PM - 5:41PM |
T31.00007: Thermodynamically consistent phase-field modeling of three-phase solidification with density variation Pengtao Yue, Jiaqi Zhang, Yichen Li Ice formation plays an important role in many industrial applications as well as natural phenomena. In this talk, we will present a phase-field model for the non-isothermal three-phase system that involves water, ice, and air. The water-ice phase transition is modeled by the Allen-Cahn equation and the water-air interface is tracked by the Cahn-Hilliard equation. The constitutive relations are derived based on non-negative entropy production, such that the whole set of governing equations, including the Navier-Stokes and heat equations, are consistent with the second law of thermodynamics. The three-phase mixture is treated as quasi-incompressible, i.e., the pure phases are incompressible, whereas the mixture density may evolve due to variations in composition. Our model automatically captures the curvature and pressure effects on the melting temperature, as represented by the Gibbs-Thomson and the Clausius-Clapeyron equations, respectively. Meanwhile, the volume change during phase transition is faithfully captured by the quasi-incompressibility of the mixture. In the end, we will present some numerical results on the freezing process of a water droplet deposited on a cold substrate, where a pointy tip forms due to volume expansion. |
Monday, November 21, 2022 5:41PM - 5:54PM |
T31.00008: Withdrawn Abstract
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Monday, November 21, 2022 5:54PM - 6:07PM |
T31.00009: Velocity and size quantification of drops in single and collective bursting bubbles experiments Baptiste Neel, Luc Deike Mechanisms of droplet production from bursting bubbles have been extensively studied for single bubbles, but remain sparsely investigated in more complex collective settings. We discuss jet and film drop velocity-size relationships from physics-based mechanisms, as a potential means to further differentiate between various mechanisms and correctly determine the drops origin. In a collective bubbling experiment, quasi-monodisperse subsurface bubbles rise up to the surface where, depending on the surfactant concentration, they can either merge or assemble in rafts of monodisperse bubbles. Drop trajectories are recorded, analyzed and shown to exhibit uniquely distinctive features for the different production mechanisms: centrifuge film drops are ejected sideways, jet drops are ejected vertically. Different single-burst scalings are finally compared to the experimental size-velocity relationships, and reveal that drops coming from collective bubble bursting appear slower and more scattered than when coming from single bursting bubbles. |
Monday, November 21, 2022 6:07PM - 6:20PM |
T31.00010: Droplet pinchoff inside a tube: applications for droplet-on-demand devices Jacob A Hale, Benjamin Wilkerson, Nanami Mezaki Recent Droplet-on-demand (DOD) devices use a voltage pulse on a piezoelectric disk to eject droplets from a nozzle. By replacing the standard nozzle with a short tube, doplet pinchoff can occur within the tube when activating the piezoelectric disk for certain pulse amplitudes and pulse widths. We will present the dynamics of droplet pinchoff inside the tube and its effect on droplet size in a modified DOD device. |
Monday, November 21, 2022 6:20PM - 6:33PM |
T31.00011: gallium-based liquid metal patterning, deformation, and manipulation for soft electronic applications Woochan Kim, Jinwon Jeong, Jeong-Bong Lee, Sang kug Chung, Daeyoung Kim in this paper, we report a gallium-based liquid metal patterning, and magnetic field-driven deformation and manipulation for soft electronics. in this report, we propose a simple liquid metal patterning technology onto a paper based on the imprinting lithography. we investigated the super-lyophobic paper among various papers by measuring contact angle, sliding angle, and wetting area. we demonstrated the Korean Peninsula shape liquid metal patterning on the paper. we also studied magnetic field-based liquid metal deformation and manipulation. in order to manipulate the liquid metal to be non-wettable and controlled by magnetic field, we obtained magnetic liquid metal marble by coating the surface of liquid metal with iron particles using diluted hydrochloric acid solution. then, we investigated magnetowetting behavior of the magnetic liquid metal marble according to the amount of iron particles. we demonstrated dynamic behavior of the magnetic liquid metal marble with various applied magnetic fields. we expect that the proposed pattering, deformation, and manipulation method of the liquid metal may be useful for various soft electronic applications. |
Monday, November 21, 2022 6:33PM - 6:46PM |
T31.00012: Droplet propagation from wind instruments for different playing conditions Pablo L. L Rendón, Federico Hernández, Aarón Lozano Droplets and aerosols are efficient agents for the airborne transmission of viruses, such as COVID-19, and musical groupings which include wind instruments must take this into account in order to minimise the risk of transmission through adequate placement of the wind players. For this reason, the dynamics of large droplets produced by playing a variety of wind instruments are examined. Playing instruments such as the flute naturally results in a steady stream of droplets propelled away from the player, whereas for other instruments the emission of droplets is heavily dependent on the register of the instrument, the articulation employed and the dynamic level at which the instruments are played. Players are filmed while playing single notes and short musical phrases corresponding to these different conditions using a high-speed camera. The trajectories of individual droplets are then mapped digitally, and the range which they cover can be estimated for each instrument. The effect of varying dynamic levels, articulation and general register of the different instruments is discussed, mainly in terms of safe placement of the instruments within an ensemble. Shock formation is also known to occur for certain brass instruments when playing loudly, and this phenomenon is expected to affect the trajectory of droplets noticeably. |
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