Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session Q27: Surface Tension Effects: Interfacial Phenomena II |
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Chair: Douglas Bohl, Clarkson University Room: Georgia World Congress Center B315 |
Tuesday, November 20, 2018 12:50PM - 1:03PM |
Q27.00001: Stick-Slip Dynamics in Non-Isothermal Flow Elaheh Alizadeh-Birjandi, Ryan Patrick McGuan, H. Pirouz Kavehpour We have investigated the effect of a solidifying contact line on the dynamics of flowing fluid down an inclined plate. A complete set of experimental studies were performed for continuous flow at constant flow rate on an aluminum substrate for advancing contact angle values less than 90 degrees. As the fluid propagates downhill, the contact line gets pinned to the surface due to solidification. The liquid builds up behind the pinned contact line till it reaches a critical angle. The fluid then over flows the solid barrier, and the process repeats over the length of the inclined plate at a specific frequency. This work focuses on understanding the physics behind this stick-slip behavior and determining the governing characteristics such as critical pinning contact angle, critical spreading length, and pinning frequency as well as the important physical parameters involved in non-isothermal spreading fluids . The results of experimental and analytical studies are presented. |
Tuesday, November 20, 2018 1:03PM - 1:16PM |
Q27.00002: Thermocapillary Energy Transportation at Evaporating Liquid Surface Fei Duan, Lu Qiu |
Tuesday, November 20, 2018 1:16PM - 1:29PM |
Q27.00003: Abstract Withdrawn We provide a laminar, weakly compressible, numerical investigation of surface effects on the problem of a water droplet, initial radius R=1mm, de-wetting a horizontal plate that is oscillating in the vertical direction with velocity -Vcosωt, V=2800m/s and ω=280s^{-1}. We distinguish surface effects into cohesive and adhesive forces. We model cohesion by imposing the interface tension force γ along the interface. The approach is validated for two static cohesion problems: pressure inside a bubble and contact angle. We model adhesion following peeling theory, where the adhesive force σ decays with distance from the triple line. Our results, in agreement with the available experimental data, show that during upward acceleration the water droplet wets the plate. As the plate decelerates, the rim of the droplet continues an upward motion while the bottom gradually de-wets the plate, generating a conical cavity. We show that our results depend primarily on the integrated adhesive energy, not the exact form of the decay function. Our key new finding for this problem, where the Weber number is We=ρ2RV^{2}/γ ≈O(10^{8}), is that the de-wetting speed depends on adhesion, but not surface tension. Surface tension is important only after the droplet has separated from the plate. |
Tuesday, November 20, 2018 1:29PM - 1:42PM |
Q27.00004: Capillary Adhesion on Rough Surfaces: Is Splitting Droplets Beneficial? Matthew D Butler, Dominic J Vella A wetting droplet confined between two surfaces will pull them together due to capillary forces. This mechanism is believed to (at least partially) explain how many species of insects adhere to surfaces so well — they secrete a thin film of fluid beneath their feet to attach themselves. Close observations of the fluid in certain species have revealed it to be a water-in-oil emulsion. One suggestion for why this is preferred is that the emulsion droplets themselves form liquid bridges, and that splitting into a large number of small bridges strengthens adhesion. However, there has been debate as to whether this is actually the case in reality: does droplet splitting really increase the adhesive force for physiologically relevant wetting properties? We suggest that droplet splitting may be particularly beneficial in ensuring good adhesion to a rough surface, and discuss some fluid mechanical features of drops on rough surfaces that give rise to this improvement. |
Tuesday, November 20, 2018 1:42PM - 1:55PM |
Q27.00005: Universal solution for the retracting edge of a stretched viscous sheet John Lister, James Munro Surface tension acting alone causes the edge of a fluid sheet to retract and thicken. However, whether at the edge of the hole in a bursting bubble or at the edge of a falling ribbon of molten glass, such retracting edges are often simultaneously being stretched parallel to the edge, thus modifying the flow, the rate of retraction and even thinning the edge. Remarkably, a universal similarity solution for Stokes flow in a stretched edge shows that the scaled shape is actually independent of the stretching rate, and decays rapidly to the far-field thickness. This solution justifies the use of an integrated normal-stress boundary condition in long-wavelength models of viscous sheets, and gives the detailed shape of the edge, resolving its position to the order of the sheet thickness. |
Tuesday, November 20, 2018 1:55PM - 2:08PM |
Q27.00006: Discussion of the impact of surfactant on the drag-reduction potential of superhydrophobic surfaces Julien R. Landel, Fernando Temprano-Coleto, François J. Peaudecerf, Frederic Gibou, Paolo Luzzatto Fegiz Recent studies, Peaudecerf et al. (PNAS 2017) and Song et al. (PRF 2018), have investigated the negative effect of surfactant on the drag-reduction performance of superhydrophobic surfaces (SHS). As SHS could have a large impact in reducing energy utilisation for many internal and external flow applications, it is important to understand and predict how surfactant-Marangoni stresses affect the flow over SHS. We review experimental and numerical work which show how surfactant can deteriorate the effective slip of SHS and lead to the undesired no slip boundary condition. By studying the governing equations of surfactant transport over SHS, we show the complexity of modelling this coupled nonlinear problems where up to eleven dimensionless parameters can appear. We reveal different regimes in surfactant dynamics and highlight the important dimensionless parameters. We show the impact on the effective slip and the drag of the SHS. Finally, we discuss how surfactant effect could be mitigated through changes in the geometry and flow conditions. |
Tuesday, November 20, 2018 2:08PM - 2:21PM |
Q27.00007: Interface Deformation Neat the Moving Contact Line on a Periodically Roughened Surface Takahiro Ito, Tatsuya Tsuneyoshi, Yoshiyuki Tsuji, Kenji Katoh, Tatsuro Wakimoto The dynamic contact angle on non-ideal solid surface generally deviates from the theoretical predictions (e.g. Voinov(Fluid Dyn. 1976), Cox (J. Fluid. Mech., 1985)). In the present study effect of the repetition of the defect on the dynamic contact angle is investigated. The defects are simulated by periodically distributed two-dimensional-like micrometer-sized grooves prepared on thermally oxidized Si wafer with photo lithography techniques. The interface geometry is measured by high-speed video camera for the Capillary number (=μU/γ, μ:viscosity, U:average contact line speed, γ:surface tension) ranged 0.5 ~ 3.5×10^{-5}. The contact line shows stick-slip motion resulting in the induction of surface wave near the contact line which decays within the length from the contact line of ~0.1L where L=(γb^{2}/2πρU^{2})^{1/3}. The dissipation of this wave should enhance the dynamic contact angle. |
Tuesday, November 20, 2018 2:21PM - 2:34PM |
Q27.00008: Surface tension of nanoparticles in electrolyte solutions Saheed Olawale Olayiwola, Morteza Dejam The equilibrium surface tension of nanoparticles (NPs) in both deionized water and electrolyte solutions is mathematically modeled. The concept of Gibbs dividing surface is revisited in order to include the effect of dipole-dipole interaction and oscillatory energy. NPs in deionized water behave like electrolytes. The Li and Lu method [Chemical Engineering Science, 56(8):2879-2888, 2001] is combined with the Debye-Hückel constants to estimate the mean activity coefficient for calculating the surface tension of NPs in deionized water. NPs in an electrolyte solution behave different than mixed electrolyte solutions. Their behavior is similar to surface active agents. The Borwankar and Wasan technique [Chemical Engineering Science, 43(6):1323-1337, 1988] is extended by considering the effect of dipole-dipole interaction and oscillatory energy. In order to take into account the adsorption of NPs, the resulting equation is combined with the Langmuir approach to calculate the surface tension of NPs in an electrolyte solution. The proposed model is compared and validated with experimental data from previous studies. |
Tuesday, November 20, 2018 2:34PM - 2:47PM |
Q27.00009: Capillary Induced Particle Motion in Thin Liquid Films Mahesh S Tirumkudulu, Abhishek Yadav, E J Hinch We demonstrate a new form of capillary force experienced by neutrally buoyant spherical particles adsorbed simultaneously at both interfaces of a thin liquid film of spatially varying thickness. The force is proportional to the slope of the interface and the difference between the local contact angle and the equilibrium value, and exists even when the two bounding interfaces have zero curvature. We derive the expression for the force, which when balanced against the hydrodynamic drag gives the trajectory of the particle. The measured trajectories for spherical particles of varying diameters in thin films compare well with predictions. |
Tuesday, November 20, 2018 2:47PM - 3:00PM |
Q27.00010: Dynamic analysis of the flotation of small spheres Bingqiang Ji, Qiang Song, Jiaheng Liu, Kai Shi, Qiang Yao Small spheres experience a dynamic process to float or sink after they contact the liquid surface. Thus, their final status should be judged by dynamic analysis instead of static equilibrium. The motion equation of small spheres after contacting the liquid surface with zero velocity is established in consideration of the gravity, surface tension, and buoyancy, combining a quasi-static assumption of liquid surface shape, and the motion process of spheres is numerically simulated. The results show that a finally floating sphere experiences a sinking process of first accelerating and then decelerating, during which the static equilibrium position is crossed due to sphere inertia. There exists a limit position for the sphere, beyond which the surrounding liquid surface will collapse. The sphere will float if its velocity decreases to zero before reaching the limit position, else it will sink ultimately. Based on such dynamic analysis, the critical contact angle and limit density for small spheres to float are determined, and agree well with the published experimental results, while the limit density predicted by static equilibrium has a large deviation. |
Tuesday, November 20, 2018 3:00PM - 3:13PM |
Q27.00011: Dynamic super menisci: spontaneous film climbing of colloidal particle solutions Min Young Pack, Nan Xue, Howard A Stone We report experiments showing that suspensions can form a “super meniscus” above a critical interfacial concentration of particles. As a pre-wetted film is inserted into a liquid interface covered with particles, we observe a climbing liquid front as well as a lagging particle front. Previously, the Marangoni effect has been used to produce the growth of climbing films, where a surface tension gradient drives flow with the aid of temperature, surfactant or solute gradients. Particles at an interface may also change the effective interfacial properties. A high-speed interferometric technique is used to track in-situ the film thickness of a climbing colloidal solution. We find that the particle size and particle concentration control the rising and falling rates of the climbing film as well as the final deposition patterns of the particles. We also present a mathematical model to rationalize our results. |
Tuesday, November 20, 2018 3:13PM - 3:26PM |
Q27.00012: Controlling Flow Patterns inside a Sessile Droplet using External Volatile Liquid Jonghyeok Park, Hyung Jin Sung, Hyoungsoo Kim Controlling flow patterns inside droplets has been the subject of many studies because it can be applied to mixing sample, uniform coating and particle separation. Usually, in droplet-based microfluidics, to control internal flows, an external energy is needed, e.g. acoustic, electric, and magnetic fields. Here, we show that when volatile liquid is placed next to the water droplet, vapor-driven solutal Marangoni effects are induced, which can create vortices inside a sessile droplet and mix samples without any active control device. Furthermore, we present that the total number of vortices (M) can be determined by the number of volatile components (N), i.e. M = 2N. The flow patterns are measured using particle image velocimetry. In this talk, physical arguments to support our experimental observations will be discussed. |
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