Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session ZC18: Complex Fluids |
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Chair: Henry Chu, University of Florida Room: 146A |
Tuesday, November 21, 2023 12:50PM - 1:03PM |
ZC18.00001: Central Collapse of non-Newtonian Droplets Ziwen He, Huy Tran, Min Y Pack This study focuses on the impact dynamics of non-Newtonian droplets compared to Newtonian droplets, specifically investigating the central collapse phenomenon induced by capillary waves when non-Newtonian droplets impact a lubricated surface with low Weber numbers (We=ρU2R/γ < 10). The experimental techniques employed include laser-sheet visualization, reflection interference microscopy (RIM), and total internal reflection microscopy (TIRM), which enable the visualization and analysis of the re-entrant jet and the evolution of the air layer beneath the droplets. By studying Xanthan gum (XG) droplets, known for their shear-thinning behavior leading to an effective Ohnesorge number (Oheff) below the critical Ohnesorge number (Ohcr), the excitation of capillary waves along the liquid-air interface is observed. These capillary waves generate a downward jet that contributes to the inversion of the air dimple, ultimately causing the central collapse of the droplets. In contrast, the capillary wave is dampened in viscous Newtonian droplets with a zero-shear viscosity above the critical Ohnesorge number (Oh>Ohcr), and thus, central collapse is not observed. The experimental findings provide valuable insights into the impact dynamics of droplets with non-Newtonian properties, and a physical model is proposed to explain the formation of a squeezing film between the jet and the air dimple. |
Tuesday, November 21, 2023 1:03PM - 1:16PM |
ZC18.00002: Arrested by heating: controlling the motility of active droplets by temperature Corinna C Maass, Prashanth Ramesh, Yibo Chen, Svenja Morsbach, Maziyar Jalaal One of the challenges in tailoring the dynamics of active, self-propelling agents lies in arresting and releasing these agents at will. Here, we present an experimental system of active droplets with thermally controllable and reversible states of motion, from unsteady over meandering to persistent to arrested motion. These states depend on the Péclet number of the chemical reaction driving the motion, which we can tune by using a temperature sensitive mixture of surfactants as a fuel medium. We quantify the droplet dynamics by analysing flow and chemical fields for the individual states, comparing them to canonical models for autophoretic particles. In the context of these models, we are able to observe in situ the fundamental first transition between the isotropic, immotile base state and self-propelled motility. |
Tuesday, November 21, 2023 1:16PM - 1:29PM |
ZC18.00003: Shape morphing with expanding foam drops Tom Marzin, Julien Le Dreff, Abigail Plummer, Pierre-Thomas Brun Polyurethane foams find extensive application in a wide range of industries due to their remarkable thermal properties and structural resilience. These foams can expand many times their original volume during curing, making them ideal for lightweight casting and backfilling. We investigate how expanding foams with free surfaces can be used to create 3D forms from quasi-2D molds. In our experiments, we drill an array of shallow wells of different sizes and fill them by pouring uncured reactive liquid foam. The expanding foam overflows out of the wells and forms a domed drop shape. We model the final foam shape using a quasi-static drop approximation based on the balance between surface tension and gravity. We explore various configurations, including colliding foam droplets and make progress toward solving the inverse design problem. By understanding the connections between foam expansion and well morphologies, we work toward developing a novel strategy for shape morphing that offers an alternative to classical 3D printing. |
Tuesday, November 21, 2023 1:29PM - 1:42PM |
ZC18.00004: Elastocapillary Worthington jets in viscoelastic liquids Omar K Matar, Konstantinos Zinelis, Thomas Abadie, Detlef Lohse, Maziyar Jalaal, Uddalok Sen The impact of droplets of viscoelastic liquids on non-wetting substrates is relevant for various deposition processes such as spray cooling, spraying pesticides, and inkjet printing. The retraction of an impacting droplet on a non-wettable substrate is often associated with the formation of a Worthington jet, which is fed by the retracting liquid. Here, we investigate the critical role of viscoelasticity in a characteristic transition from an inertiocapillary to an elastocapillary regime in the stretching Worthington jet. To achieve this, we perform drop impact experiments on non-wetting solid surfaces for identifying the impacting conditions with respect to the inertia and elasticity effects, revealing that viscoelasticity results in a pinned contact line on the non-wettable substrate. In addition, we confirm via numerical simulations using the open-source Volume-Of-Fluid code Basilisk (with adaptively-refined grid to capture the interface) that pinning conditions on the substrate are key for the development of beads-on-the-string structures, characteristic of the elastocapillary thinning of slender viscoelastic liquid filaments. |
Tuesday, November 21, 2023 1:42PM - 1:55PM |
ZC18.00005: The Influence of Breaking Waves and Photo-oxidation on the Morphology, Viscosity, and chemical composition of Water-in-Crude Oil Emulsions Diego F Muriel, Carlos Fuentes-Cabrejo, Nicolas Escobar-Castaneda, Robert J Faragher, Keval Shah, Zeyu Yang, Chun Yang, Bruce Hollebone, Benjamin Fieldhouse, Joseph Katz This study examines the time evolution and effect of photo-oxidation on the microscopic morphology, viscosity, viscoelasticity, and chemical composition of water-in-crude oil emulsions formed by the impact of breaking waves on 2 mm thick oil slicks. Emulsions involving Alaskan North Slope (ANS) and Hibernia, both light crude oils, and Cold Lake oil, a diluted bitumen, form by exposure to periodic plunging waves for 12 days. Samples are analyzed using Rheometry, microscopy, and chemical fingerprinting. ANS emulsifies water faster than Hibernia, resulting in higher concentration of water droplets and a more interconnected structure. Hence, the non-Newtonian viscosity of ANS emulsion is at least 3 times higher than that of Hibernia. Cold Lake emulsified by waves has double the water content compared to emulsions created by mixing in a flask at similar energy level, resulting in higher viscosity. Photo-oxidation of Hibernia accelerates the emulsification and formation of droplet clusters, and increases the water volume, hence the viscosity. It also reduces the droplet sizes and increases the biomarker concentration. For the first 4 days, photo-oxidation increases the concentration of alkylated PAHs and aromatic compounds, which increase the toxicity. They decrease in later days. |
Tuesday, November 21, 2023 1:55PM - 2:08PM |
ZC18.00006: High-fidelity and reduced-order modelling of viscoelastic filaments Konstantinos Zinelis, Sarah Beetham, Thomas Abadie, Gareth H McKinley, Omar K Matar, Jesse Capecelatro Accurate and stable simulations of the hyperbolic constitutive equations describing viscoelastic flows remain challenging, especially for flows containing complex topological transitions such as spray formation and fragmentation. The development of singularities in the solution to the constitutive equations at high strain rates or large polymer relaxation times is known to lead to numerical instabilities that challenge convergence. Here, we consider axisymmetric simulations of an impulsively-started low-speed viscoelastic jet exiting a nozzle using the open-source Eulerian code Basilisk. We identify the distinct flow regimes characterizing the thinning and the “pinch-off singularities” in the viscoelastic filament, revealing the effects of the jet speed and the polymeric extensibility. We also perform three-dimensional simulations at higher injection flowrates, exploiting the capabilities of adaptive grid refinement to capture the interface accurately within a volume-of-fluid framework. In addition, a less expensive frame-invariant data-driven rheological model is employed and compared to DNS. We demonstrate that key dynamical features of the stretching viscoelastic filaments can be captured by transporting the trace of the conformation tensor and reconstructing the full constitutive model algebraically using symbolic regression. |
Tuesday, November 21, 2023 2:08PM - 2:21PM |
ZC18.00007: Rheology of diluted ferrofluid emulsions in planar extensional flows Taygoara F Oliveira, Arthur L Guilherme, Roney L Thompson, Lucas H P Cunha In this talk, we present a 3D computational study on how external magnetic fields impact the rheology of dilute ferrofluid emulsions in planar extensional flows. The intensity and direction of uniform magnetic fields influence the planar extensional rheology by altering the shape, orientation, and magnetization of the ferrofluid droplets in suspension. The conventional extensional viscosities associated with the normal stresses of the bulk emulsion either remain constant or increase with the field intensity, except when the field is perpendicular to the extension plane. In that case, the planar extensional viscosity stays constant, and the second extensional viscosity slightly decreases. The droplets tilt in the flow when the external field is not aligned with one of the main flow directions, resulting in a change in the recirculation pattern and flow topology inside the droplet. At the microscopic level, the droplets experience a magnetic torque due to a misalignment between their magnetization and the external field direction. At the macroscopic level, the bulk emulsion experiences a field-induced internal torque leading to a nonsymmetric stress tensor with unexpected shear components in extension. To describe this unconventional stress-strain response, we introduce new extensional material functions, to provide a complete rheological description of ferrofluid emulsions in planar extensional flows. This study provides fresh perspectives on the practical applications involving the manipulation of ferrofluid emulsions through the assistance of magnetic fields. |
Tuesday, November 21, 2023 2:21PM - 2:34PM |
ZC18.00008: Contact line mobility of nanoparticle suspension Hyun-Woo Kim, Sohyun Jung, Ho-Young Kim Microdroplets of complex fluids or molten substances have played a pivotal tool as building blocks in bioprinting and bottom-up fabrication. Control of their behavior, specifically their spreading on solid surfaces, is paramount to the precision and accuracy of these technologies. While the spreading dynamics of pure liquids or microparticle suspensions have been extensively investigated for years, the spreading behavior of nanoparticle suspension microdrop has been rarely studied to date. Here, utilizing an in-house inkjet system that can generate single microdroplets in the diameter range from 50 μm to 80 μm on demand, we observed the spreading behavior of aqueous microdrops containing particles of various sizes on hydrophilic surfaces. Notably, the expanding radius of nanoparticle suspension surpassed that of pure water microdrops at the same Weber number. To understand this behavior, we visualized the flow field near the moving contact line of the nanoparticle suspension drops. We suggest possible mechanisms behind enhanced contact line mobility of nanoparticle suspension based on our experimental results. Our findings shed new light on the wetting behavior of nanoparticle suspensions, and they hold potential for further refining the precision and accuracy of microdroplet-based applications. |
Tuesday, November 21, 2023 2:34PM - 2:47PM |
ZC18.00009: Viscoelastic drop spreading: Cox-Voinov theory with normal stress effects Minkush Kansal, Vincent Bertin, Charu Datt, Jens Eggers, Jacco H Snoeijer The dynamics of slowly spreading drops is dictated by the contact line motion. In Newtonian fluids, the classical Cox-Voinov theory links the macroscopic contact angle to the microscopic contact angle and the contact line velocity. Here, we investigate the effects of viscoelastic normal stresses on wetting dynamics. We first analytically derive an asymptotic expression for the radius of a spreading drop, and find the existence of two qualitatively different regimes. For weak viscoelasticity, the contact line dynamics follows a modified Cox-Voinov theory, where the microscopic contact angle is now replaced by an apparent microscopic angle dependent on the magnitude of viscoelasticity. By contrast, at larger values of viscoelasticity, the wetting dynamics, although affected by viscoelasticity, is independent of the microscopic properties, as had been previously anticipated in the case of complete wetting. We then discuss the intricate differences between spreading and retraction dynamics in the presence of viscoelasticity. |
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