Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session T26: Surface Tension Effects: Textured Substrate and Particle-Particle Interactions |
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Chair: Alban Sauret, UC Santa Barbara Room: North 226 ABC |
Tuesday, November 23, 2021 12:40PM - 12:53PM |
T26.00001: Hairy fluid mechanics: interfacial flows in networks of flexible fibres Christopher M Ushay, Etienne Jambon-Puillet, Pierre-Thomas Brun In natural systems, surfaces are often textured with arrays of slender, filamentous structures that can be significantly deformed not only by viscous drag but also by surface tension. Interfacial flows through these systems are highly coupled: when capillary forces of the moving interface are strong enough to deflect fibres, they in turn modify local flow geometry. As a result, the ability to drain a pre-existing layer of fluid over a deformable surface can significantly differ from the undeformed reference case. By fabricating channels textured with "hairy" elastoporous media, our work here is threefold: we first characterize the mechanics of capillary flow past a confined array of rigid posts and subsequently adapt Kirchhoff beam theory to describe the deflection of beams due to Laplace pressure. Lastly, we couple the aforementioned frameworks to show the impact of deformability on drainage. |
Tuesday, November 23, 2021 12:53PM - 1:06PM |
T26.00002: Collapse of a Granular Raft Ben Druecke, Xiang Cheng, Sungyon Lee Granular rafts consisting of monodispersed glass particles on a fluid-fluid interface collapse when biaxially compressed. Contrary to intuition, small particles are preferentially expelled individually, whereas large particles form a collective crease. We experimentally show that the collective crease is enhanced when the particle weight per unit interface area is large, and is suppressed by a difference in density between the two fluids. We develop a one-dimensional continuum model for the shape of the interface and the concentration of particles along the interface that shows that the particle weight per unit area normalized by the density difference between the two fluids is the key parameter. When this value is large, the raft creases; when small, the raft cannot crease, so steric interactions force particles to be expelled individually. |
Tuesday, November 23, 2021 1:06PM - 1:19PM |
T26.00003: Competition of velocities in the failure of particle rafts under tension Kha-I To, Sidney R Nagel Particle rafts are two-dimensional single-layers of aggregated sub-millimeter polydisperse particles floating at an air-liquid interface. The failure of such rafts under tension shows distinct morphologies that depend on the pulling velocity, Vpull. At high Vpull, numerous small cracks are distributed diffusely throughout the entire system; as Vpull decreases, the distance between adjacent cracks increases; at low Vpull, the raft disconnects into two pieces in a necking event that resembles ductile failure. We model this behavior as a particle chain with inter-particle adhesion provided by lateral capillary forces. As the chain is pulled apart, this model predicts a wave-vector-dependent healing velocity, Vheal(k), for the particles to rearrange and rebond with one another. Vheal(k) competes with the applied Vpull to produce features that are consistent with our experimental measurements. |
Tuesday, November 23, 2021 1:19PM - 1:32PM |
T26.00004: Electro-morphing of particle rafts Kyungmin Son, Jeong-Yun Sun, Ho-Young Kim Materials that turn their shapes in response to external stimuli are actively sought for in soft matter physics, soft robotics, and architecture. Conventional approaches adopting patterns of stretchable soft solids, folds, or cuts have succeeded in inducing dramatically large deformations, but only in a preprogrammed manner at specified locations. Therefore, expanding the realm of shape-morphing technologies strongly requires material systems that can undergo significant shape-shifting without prescribed patterns. Here we present a liquid interface covered by dielectric particles, or a particle raft, which can rapidly and reversibly morph under the external electric field. The initially flat rafts transform to a mound and a tower within seconds and then move horizontally, driven by electrostatic attraction between electrodes and particles, while stabilized by electric discharge. Our study presents a highly deformable and electrically controllable soft material system with potential applications including the human-machine interface. |
Tuesday, November 23, 2021 1:32PM - 1:45PM |
T26.00005: Dip-coating of bidisperse suspensions Deok-Hoon Jeong, Michael Ka Ho Lee, Virgile Thievenaz, Alban Sauret Dip coating is an efficient method to entrain a thin layer of fluid when withdrawing a solid object from a liquid bath. Whereas the situation is well-understood for homogenous fluids, heterogeneities, such as particles dispersed in the liquid, lead to more complex situations. Indeed, in addition to the thickness of the coating film, the presence of monodisperse spherical particles introduces a new length scale: the particle diameter. Recent studies have shown that the thickness of the coating film for a monodisperse suspension of particles can be captured by an effective capillary number based on the viscosity of the suspension, providing that the film is thicker than the particle diameter. However, many practical applications involve polydisperse suspensions, characterized by a wide range of particle sizes, introducing additional length scales and complexities. In this study, we experimentally describe how the approach developed for monodisperse suspensions can be modified to account for bidisperse suspensions. The effective viscosity of bidisperse suspensions is smaller than that of monodisperse suspensions for the same particle volume fraction, but we show that the effective viscosity approach is still valid, providing that the thickness of the coating film is larger than the diameter of the largest particles. We also observe and rationalize an intermediate coating regime for a certain range of withdrawal velocities where large particles are filtered out of the liquid film owing to a capillary filtration mechanism. |
Tuesday, November 23, 2021 1:45PM - 1:58PM |
T26.00006: Drag force on particles at a fluid interface in creeping flows ZHI ZHOU, Michael J Miksis, Petia M Vlahovska The problem of particles attached to an interface between two immiscible fluids has been extensively studied and has a wide variety of engineering and medical applications. Here we present an asymptotic/numerical investigation of the fluid motion past spherical particles attached to a deformable interface between two immiscible fluids undergoing uniform creeping flows in the limit of small Capillary number. Under the assumption of a constant three-phase contact angle, we analytically obtain the interfacial deformation around a single particle and numerically the two-particle deformation. Applying the Lorentz reciprocal theorem to the zeroth-order approximation for spherical particles at a flat interface and to the first correction in Capillary number allows us to obtain explicit analytical expressions for the hydrodynamic drag in terms of the zeroth-order approximations and the correction deformations. The drag coefficients are computed as a function of the three-phase contact angle, the viscosity ratio of the two fluids, the Bond number, and the separation distance between the particles. |
Tuesday, November 23, 2021 1:58PM - 2:11PM |
T26.00007: Solid Beads on Liquid-Infused Materials Saurabh Nath, Armelle Keiser, David Quéré It is well-known that ants get trapped in carnivorous pitcher plants, owing to the plant's unique ability to achieve ultra-slippery surfaces. Along this vein, we study the friction on a solid sphere moving on a liquid-infused material. We identify three distinct regimes of motion - sticking, rolling, and bouncing – respectively determined by the existence of a symmetric meniscus, of an asymmetric meniscus (or tail), and by the breaking of this tail. Based on experimental observations, we propose that contrary to the general conception, solids on tilted infused materials do not necessarily slip and fall. |
Tuesday, November 23, 2021 2:11PM - 2:24PM |
T26.00008: Microdroplet Mobility Mediated by Dynamic Oil Menisci on Lubricant-Infused Surfaces Jianxing Sun, Patricia B Weisensee During dropwise condensation on lubricant-infused surfaces (LISs), we show that oil menisci that form surrounding microdroplets lead to a dynamically changing formation of oil-rich and oil-poor regions which in turn affect the mobility of the microdroplets. Microdroplets robustly self-propel long distances (3 - 6 times their diameters) towards larger droplets in the oil-rich regions, independent of gravity. The sliding velocity depends on droplet sizes, the distance between droplets, and the oil viscosity. The overlapping oil menisci between droplets create an anisotropic oil profile around the moving droplets, leading to unbalanced lateral components of surface tension force along its (apparent) three-phase contact line. We also reveal strong outward flow at the oil-vapor interface of the menisci, effectively transporting nucleated droplets to oil-poor regions at high subcooling degrees. We attribute the internal dynamics to temperature-driven Marangoni flow. Mathematical models based on force balance and scale analysis show good agreement with the experimental data. This work will help inform the selection of lubricant for LISs to enhance droplet mobility and heat transfer performance, and improve our understanding of microscale interfacial thermal-fluid dynamics. |
Tuesday, November 23, 2021 2:24PM - 2:37PM |
T26.00009: Leidenfrost levitation of a spherical particle above a liquid bath: evolution of the vapor-film morphology with particle size Rodolfo Brandao, Ory Schnitzer We consider a spherical particle levitating above a liquid bath owing to the Leidenfrost effect, where the vapor of either the bath or sphere forms a thin insulating film whose pressure supports the sphere's weight. Starting from a reduced formulation based on a lubrication-type approximation, we use scaling arguments and matched asymptotics to describe the morphology of the vapor film assuming that the sphere is small relative to the capillary length (small Bond number). We find that this regime is comprised of two formally infinite sequences of distinguished limits, the limits being defined by the smallness of an intrinsic evaporation number relative to the Bond number. As the drop size increases, the vapor-film morphology initially forms a neck-bubble structure, similar to that in the classical Leidenfrost scenario of a liquid drop levitating above a flat substrate; in contrast to the classical scenario, however, the morphology then continues to evolve and ultimately consists of a nearly uniform film bounded by localised capillary waves. |
Tuesday, November 23, 2021 2:37PM - 2:50PM |
T26.00010: Levitating granular cluster: statics and oscillations Evgeniy Khain The granular Leidenfrost state consists of a dense granular cluster levitating above a hot granular gas. The density of particles inside the cluster can be very high and even close to the density of crystalline packing. To describe this state theoretically, one needs to know the density dependence of constitutive relations (pressure, heat losses, thermal conductivity) at these very high densities. We measured the constitutive relations at high densities in molecular dynamics simulations in three different settings: a uniform freely cooling dense granulate (to measure heat losses), a uniform ensemble of elastically colliding particles (to measure pressure), and a dense granular medium between two thermal walls under gravity (to measure thermal conductivity). Next, the hydrodynamic equations with the resulting expressions were solved to describe the levitating cluster state in various parameter regimes. Separate molecular dynamics simulations were performed to test the theoretical predictions, and a good agreement with theoretical results was observed. In some cases, however, the agreement with the static levitating cluster theoretical solution was not good, since the cluster developed high amplitude low frequency oscillations. We will discuss these oscillations in some detail. |
Tuesday, November 23, 2021 2:50PM - 3:03PM |
T26.00011: Impact of insoluble surfactants on the flow of a liquid over a surface containing gas-filled grooves perpendicular to the flow direction Tobias Baier, Steffen Hardt A gas-liquid interface can become immobilized when surfactants gather upstream of an obstacle, with Marangoni stress balancing viscous shear, effectively rendering the surface rigid. This effect, well-known for influencing the rate of ascend of small air bubbles in a liquid, has also been proposed to crucially influence the flows near partially wetted superhydrophobic surfaces in Cassie state. To investigate this, we analytically and numerically study laminar shear flow of an incompressible liquid over a surface with periodic gas inclusions and insoluble surfactants attached to the gas-liquid interface. Assuming advection-dominated transport of the surfactants along the interface, a semi-analytical expression for the fraction of the gas-liquid interface immobilized by surfactants is found. The resulting effective slip on the surface is compared with the ideal situation of flow across a surface not contaminated by surfactants. |
Tuesday, November 23, 2021 3:03PM - 3:16PM |
T26.00012: Surface tension and energy conservation in a moving fluid Tomas Bohr, Bernhard Scheichl In what sense do the surface tension forces in a fluid surface contribute to the energy budget, and what are the correct boundary conditions at a free surface of a moving viscous liquid? In two recent papers (Bhagat et al. J. Fluid Mech. (2018), 851, R5 and Bhagat & Linden, J. Fluid Mech. (2020), 896, A25) it has been claimed that surface tension forces, F, give a power term F·v, where v is the fluid velocity at the surface, and that this energy, for a stationary flow is balanced by viscous dissipation at the surface, thereby changing the standard dynamic free surface boundary condition into a balance between surface tension and viscosity. In this work we show that this is not correct: the velocity v determining the power is not the velocity of the liquid, but the velocity of the control surface. For a static control volume, all surface energy contributions from surface tension thus disappear, except the terms coming from the Laplace pressure. We show this by deriving a simple conservation equation for the surface area of a part of a moving liquid, which clearly reveals the contribution of the Laplace pressure at the free surface and the tangential surface tension forces at its boundary. |
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