Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session R09: Drops: General I |
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Chair: Vatsal Sanjay, University of Twente Room: Ballroom I |
Monday, November 25, 2024 1:50PM - 2:03PM |
R09.00001: Marangoni contracted droplets: the role of the solid substrate Huaiyu Zuo, Shenghao Tan, Nate J Cira Marangoni contracted droplets are stabilized by Marangoni flow rather than an energy equilibrium. Recent work has revealed how droplets in the Marangoni contracted state have negligible contact angle hysteresis, allowing them to sense and respond to subtle forces with high sensitivity. This unique ability enables the creation of droplet-based fluidic machines with advanced capabilities. It is known that Marangoni contracted droplets can form on high energy surfaces where liquids are ordinarily expected to spread into a thin film. Here we systematically investigate the role of the solid substrate, showing how texture and surface energy impact whether or not a droplet is in the Marangoni contracted state. We then show how roughness and surface energy can be patterned for tunable control of droplet behavior. We also show how stable surfaces can be created that support the formation of Marangoni contracted droplets for unprecedented amounts of time. Finally, we discuss how these concepts may have application to the cleaning of high energy surfaces with limited equipment and minimal liquid volumes. |
Monday, November 25, 2024 2:03PM - 2:16PM |
R09.00002: Droplet lift force enhancement due to internal 3D flow bifurcation Dominique Legendre, Pengyu Shi, Eric Climent We study numerically the lift force experienced by a spherical droplet with droplet Reynolds numbers of order 100 experiencing a linear shear flow over a wide range of drop-to-fluid viscosity ratio. We reveal that the lift force exhibits, from zero (bubble) to infinite (solid sphere) viscosity ratio, a non-monotonous transition peaking at a viscosity ratio of order unity. This behavior is related to an internal 3D flow bifurcation also observed under uniform-flow conditions. In the presence of an ambiant shear, this internal 3D bifurcation results in the amplification of the inviscid vortex tilting-stretching mechanism responsible for the lift force, making the trailing vortices generated in the droplet wake stronger, and as a consequence an enhanced lift effect. |
Monday, November 25, 2024 2:16PM - 2:29PM |
R09.00003: Characteristics of Wind-Driven Runback Droplet/Rivulet Flows: Part 1 - Experiment Investigation with Simultaneous DIP and PIV Measurements Yuan Zhao, Jincheng Wang, Anthony Michael Socia Settlemier, Saikishan Suryanarayanan, Hui Hu A better understanding of the characteristics of wind-driven water droplet/rivulet flows and their resultant effects on the development of boundary layer airflow is crucial for the safety of aircraft, UAVs, and rotorcraft operation under rainfall and icing conditions. Previous numerical studies suggest that idealized droplet or particle-like disturbances can disrupt the boundary layer airflow by creating turbulent spots, wedges, or unstable low-speed streaks. Given this background, an integrated experimental (part #1) and numerical (part #2) study is conducted to investigate the transient behavior of wind-driven droplet/rivulet flows and their effects on the development of boundary layer airflow over a test surface. In part #1, a systematic experimental campaign was conducted to quantify the dynamic runback behavior of wind-driven droplet/rivulet flows over a flat plate with different incoming wind speeds and water droplet sizes. While a novel Digital Image Projection (DIP) technique was used to quantify dynamic shape changes of the wind-driven droplets/rivulets over a flat plate, the corresponding characteristics of the boundary layer airflow over the test plate are measured simultaneously by using a high-resolution Particle Image Velocimetry (PIV) system. The temporally and spatially resolved DIP measurements of the dynamic shape changes of wind-driven droplet/rivulet flows are correlated to the simultaneous PIV results of the boundary layer airflow over the test surface to elucidate the underlying physics. |
Monday, November 25, 2024 2:29PM - 2:42PM |
R09.00004: Raindrop-Driven Energy Harvesting Inspired by Leaf Fluttering Jisoo Yuk, Katelyn Thomas, Sunghwan Jung To address the climate crisis, the demand for renewable energy sources as alternatives to fossil fuels is steadily increasing. One of the less explored methods is utilizing raindrop energy. When raindrops impact soft surfaces like leaves at high speeds, they generate torque that causes the leaves to bend. Inspired by this phenomenon, we propose an energy harvesting device using a piezoelectric cantilever beam to generate electricity from the impact of raindrops. To optimize the conversion of kinetic energy into electrical energy, we evaluated the influence of beam length scales on energy conversion efficiency. We investigated the vibration and voltage response of a piezoelectric beam to a single water droplet, and further conducted experiments with a series of droplets to simulate more realistic rain conditions. Experimental results demonstrate that the aspect ratio of the beam significantly affects energy conversion. We then measured the power generated by the device under actual rainy conditions to assess its practical applicability. The field test results reveal a power relationship between rainfall intensity and power output. This study highlights the potential of harnessing rainfall for electricity generation, which can be used alongside other renewable energy sources. |
Monday, November 25, 2024 2:42PM - 2:55PM |
R09.00005: Characteristics of Wind-Driven Runback Droplet/Rivulet Flows: Part 2 - Direct Numerical Simulations of Boundary Layer Transition Anthony Michael Socia Settlemier, Jincheng Wang, Yuan Zhao, Saikishan Suryanarayanan, Hui Hu The prediction and modeling of the effect of rainfall on boundary layer transition is of interest to ensuring the safe and efficient operation of aircraft, UAVs and rotorcraft in inclement weather conditions. This requires understanding the coupling between the evolution of droplet shapes driven by wind and their impact on the boundary layer flow. Our previous simulations suggest that idealized droplet or particle-like disturbances can disrupt the boundary layer by creating turbulent spots, wedges and/or unstable low-speed streaks. With this background, the evolution of droplets into rivulets and their effect on the surrounding airflow on a flat-plate boundary layer are studied using a combination of experiments and simulations. A sequence of wind driven runback droplet geometries, experimentally measured using digital image projection (as discussed in part 1), are introduced in direct numerical simulations using immersed-boundary methods to study their effect on boundary layer transition. The differing timescales of droplet and flow evolution enables a quasi-static analysis using instability and vorticity viewpoints to reveal the structure of the flow as a function of droplet geometry and the associated roughness Reynolds numbers. |
Monday, November 25, 2024 2:55PM - 3:08PM |
R09.00006: Can a combined shear and pressure-driven mechanism control the optimal droplet migration in Microfluidics? Arindam Basak, Rajaram Lakkaraju, G P Raja Sekhar Studies on surfactant-laden droplets in tubular Poiseuille flows have revealed intriguing cross-stream migration behaviours under low surface Péclet number limits. However, droplet dynamics in rectangular channels, driven by combined shear and pressure, present new opportunities in biomedical lab-on-a-chip devices. So, what could be the optimal design for a microfluidic device—shear-driven, pressure-driven, or a combination of both—to achieve controlled droplet migration? To answer this, we investigate surfactant-laden droplet steering in combined flows, emphasising the influence of a thermal dipole generated by living cells within the droplet and its interaction with a non-isothermal environment. Our findings highlight that subject to specific conditions, thermocapillary effects and the strength of the thermal dipole can significantly enhance the Marangoni effect and modify the cross-stream migration velocities. Moreover, we explore how varying the internal dipole strength modulates fluid jet velocities near the droplet centre, which is crucial for microfluidic control. Further, transitioning from Poiseuille to Couette profiles in rectangular channels proves advantageous for precise droplet manipulation, offering insights for optimising microfluidic assays. |
Monday, November 25, 2024 3:08PM - 3:21PM |
R09.00007: Mixing dynamics of droplets impacted on liquid films Blake Andersen, Kusal Uprety, Abhishek Saha Droplet impact is essential for many industrial and natural phenomena, including 2D/3D inkjet printing, spray coating, fuel injection in engines, and rain droplets falling on the ocean surface. In many of these systems, post-coalescence mixing of impacted droplets with the liquid film controls the heat-mass transfer and chemical reactions; hence, it is critical for many of these processes. Recognizing such criticality, we present an experimental investigation of mixing dynamics during droplet impacts on liquid films. Experimental diagnostics used for this study include a combination of shadowgraphy, particle image velocimetry (PIV), and planar laser-induced fluorescence (PLIF). Simultaneous PIV and PLIF characterize the vorticity fields and scalar mixing processes. PIV data shows how toroidal vortices are generated after the droplet coalesces with the impacted film surface and translates through the film. From PLIF images, we will quantify the concentration distribution of the droplet liquid and the mixing dynamics using statistical measures. Additionally, simultaneous shadowgraphy and PLIF images will show how the cavity geometry strongly affects the vortex dynamics. We will primarily discuss the evolution and correlation between the vortical structure and the mixing dynamics. |
Monday, November 25, 2024 3:21PM - 3:34PM |
R09.00008: Collision efficiency of uncharged conducting spheres in a uniaxial compressional flow and an external electric field Pijush Patra, Anubhab Roy, John Scott Wettlaufer We study collisions of a pair of uncharged conducting spheres subject to a uniaxial compressional flow and an external electric field. The near-field asymptotic expression for the electric-field-induced attractive force between two spherical conductors can overcome the lubrication resistance leading to surface-to-surface in finite time. We demonstrate the role of the external electric field in the particle trajectories and estimate how the shear-induced collision efficiency depends on electric-field-induced forces. Because the continuum lubrication interactions are no longer valid when the suspending medium is a non-continuum gas, we include the non-continuum lubrication interactions in our analysis. We show how collision efficiencies vary with the particle size ratio, the Knudsen number, and the dimensionless parameters capturing the relative strengths of electric-field-induced and the van der Waals forces. |
Monday, November 25, 2024 3:34PM - 3:47PM |
R09.00009: Double droplet formation and flow characteristics in microchannel Chen Tang, Loïc Chagot, Panagiota Angeli Double emulsions, such as water-oil-water, are oil droplets that contain an aqueous droplet, which are dispersed within another aqueous phase. These double emulsions can be formed via encapsulation in microchannels, which provide a controllable and efficient environment for applications in pharmaceuticals and food. However, due to the difficulty in stabilizing double emulsions, the inner droplets can easily coalesce. Addition of surfactants can enhance the stability of double emulsions. |
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