Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q03: Surface Tension III |
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Chair: Emilie Dressaire, University of California, Santa Barbara Room: 201 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q03.00001: Effect of surfactants on jet break-up and drop formation in inkjet printing Evangelia Antonopoulou, Oliver G. Harlen, Mark A. Walkley, Nikil Kapur A key challenge in developing new applications of inkjet technology is to produce inks that can be jetted to form individual droplets and to transport functional components needed for the application. The development of mathematical models that allow fluid jetting behaviour to be determined as a function of fluid properties would allow optimisation to be carried out in-silico before creating the inks and verifying the performance. Surfactants are often added to aqueous inks in order to modify the surface tension. However, the rapid expansion of the free surface during the fast jetting process means local areas of the surface will be depleted of surfactants leading to surface tension gradients. Using high speed video we will compare the jetting behaviour of fluids with and without surfactants in an industrial inkjet print-head. We also present numerical simulation of inkjet break-up and drop formation in the presence of surfactants investigating more closely both the surfactant transport on the interface and the influence of Marangoni forces on break-up dynamics. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q03.00002: The role of surfactants on the interfacial stability of multilayer shear flows Anna Kalogirou This talk will present a theoretical study that utilises mathematical modelling and numerical computations to scrutinise the effect of surfactants on the stability of multilayer shear flows in channels. Understanding stability is essential for efficient flow control in applications where (stable) uniform films or (unstable) interfacial waves are desired. In particular, a shear flow of two immiscible fluid layers in a horizontal channel is considered. One of the fluids is contaminated with surfactants, which can get adsorbed at the interface or form micelles when their concentration is beyond a critical value. An asymptotic model is derived in the long-wave approximation, consisting of a system of highly nonlinear PDEs describing the evolution of the interface as well as interfacial, bulk and micelle surfactant concentrations. The effect of surfactants and their sorption kinetics on the flow stability is investigated via a linear stability analysis, aiming to establish regions in the parameter space where instability and non-trivial dynamics are expected. The identified instabilities are followed into the nonlinear regime via numerical computations of the model system. The underlying physical mechanism responsible for the formation of interfacial waves will also be discussed. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q03.00003: Underwater acrobatics of partially-coated spheres Daren Watson, Joshua Bom, Chris Souchik, Andrew Dickerson Water entry studies traditionally investigate splash physics with homogeneous projectiles by tuning impactor shape, entry speed and surface roughness. Surface heterogeneity is yet another means to tune splash dynamics. In this combined experimental and theoretical study, we systematically investigate splash and cavity dynamics arising from the water entry of smooth, free-falling, partially-coated spheres across various drop heights. Hydrophilic spheres are partially-coated hydrophobic, and splash features for different impact orientations compared with the water entry of homogeneous spheres. Generally, flow separation is tripped when hydrophobic surfaces make contact with the fluid, leading to air-entrainment across the range of entry speeds and impact orientations tested. Spheres having hydrophilic and hydrophobic surfaces entering the fluid simultaneously experience lift forces, resulting in the deviation of trajectories from the axes of water entry. Here, we rationalize the migration of the fluid-sphere contact line and subsequent half-cavity expansion at high impact velocities. Such observations augur well for water entry applications where propulsion and electronic control are not possible. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q03.00004: Dynamic Wetting Failure in Curtain Coating: Comparison of Model Predictions and Experimental Observations Satish Kumar, Chen-Yu Liu, Marcio Carvalho Dynamic wetting failure of Newtonian liquids in a curtain-coating geometry is studied using a hydrodynamic model to predict the onset of wetting failure with curtain heights consistent with prior experimental setups. In the model, a Navier-slip boundary condition and constant contact angle are used to describe the dynamic contact line (DCL). The governing equations are solved with the Galerkin finite-element method and the critical substrate speed is identified at which wetting failure occurs. A boundary of a coating window is constructed which outlines the critical substrate speed for different flow rates of the liquid curtain. The model predictions are compared with prior experimental observations reported by others, and it is found that the model reproduces the non-monotonic behavior of the critical speed as the liquid flow rate increases. When surfactants are absent, our results suggest that the experimental observations can largely be explained with a model that uses the simplest boundary conditions at the DCL and accounts for the air stresses there to accurately calculate interface shapes. When surfactants are present, our results suggest that Marangoni stresses may play an important role (Liu et al., Chem. Eng. Sci. 195 (2019) 74). [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q03.00005: an Armored Droplet Approaching a Fluid-Fluid Interface Alireza Hooshanginejad, Sungyon Lee Droplets coated with a protective armor of particles are relevant in the stabilization of emulsions and drug delivery applications. Here, we consider a stratified system comprising three layers of fluids with two immiscible interfaces: a water-Iso Propanol Alcohol mixture, silicone oil, and water. When negatively buoyant particles are added to the system, they self-assemble into a granular raft on the water-IPA and oil interface. As the size of the raft increases, the raft becomes unstable, leading to the encapsulation of water-IPA and the formation of armored droplets in the oil layer. These armored droplets sink down in the oil until they approach the oil-water interface. In this study, we focus on the hydrodynamic interactions between the water-IPA armored drop and the oil-water interface. Two distinct behaviors are exhibited by armored drops: rupture or pinch-off. We demonstrate that the size and the weight of the armored droplet determine the transition between the two regimes. We present our experimental observations and discuss the physical mechanism that underlies them. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q03.00006: A theory for the drag reduction of superhydrophobic surfaces with surfactant in laminar three-dimensional flows Fernando Temprano Coleto, Scott Smith, François Peaudecerf, Julien Landel, Frédéric Gibou, Paolo Luzzatto-Fegiz In recent years, surfactants have been shown to have a crucial impact on the drag reduction of superhydrophobic surfaces (SHS) in laminar flows (Peaudecerf et al. PNAS, 2017; Song et al. PRF, 2018). Even trace amounts of these substances induce adverse Marangoni stresses that can negate the drag reduction of SHS. Including these effects in theoretical models is therefore essential to accurately predict the drag in realistic conditions, where surfactants are unavoidable. Our existing theory for SHS inclusive of surfactant (Landel et al. arXiv:1904.01194, 2019) considers a two-dimensional flow, which is sufficient to capture the streamwise accumulation of surfactant at the air-water interface. Here we build upon this model, expanding it to account for the drag of a fully three-dimensional laminar flow over an array of superhydrophobic rectangular gratings. This extended theory predicts the slip length and drag as a function of ten dimensionless numbers, including two novel ones that account for the spanwise geometry of the gratings. Finally, the performance of the model is tested against numerical simulations of the full problem. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q03.00007: Three-dimensional simulations of surfactant-contaminated flows in superhydrophobic microchannels Scott Smith, Fernando Temprano-Coleto, Francois Peaudecerf, Julien Landel, Frederic Gibou, Paolo Luzzatto-Fegiz Trace amounts of surfactants are now known to severely limit the drag reduction along superhydrophobic surfaces, due to Marangoni stresses, as demonstrated in Peaudecerf et al. (PNAS, 2017), and Song et al. (PRF, 2018). When driving flow in superhydrophobic microchannels, surfactants adsorb onto the air-water interface (known as the plastron) and cause adverse Marangoni stresses in the opposite direction to the driving flow. There is currently an effective model that considers two-dimensional surfactant-laden flow in superhydrophobic channels (Landel et al. arXiv:1904.01194, 2019). Here we perform three-dimensional simulations of the full problem using COMSOL, in order to test our extension of this model to account for the drag of a fully three-dimensional flow. We are able to compute drag reduction as a function of the microchannel geometry, surfactant concentration, and the characteristic dimensionless numbers of the momentum and surfactant transport equations. These simulation results are then used to explore physical trends and validate our extended model. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q03.00008: Suspension coating on a fiber Emilie Dressaire, Brian Dincau, Quentin Magdelaine, Ethan Mai, Martin Bazant, Alban Sauret The thickness of the coating layer entrained by a solid withdrawn from a bath depends on the physical properties of the fluid, the withdrawal speed, but also on the substrate geometry. In particular, many common substrates that are subjected to liquid immersion and withdrawal have the general shape of a thin cylinder such as needles, wires, and fibers. We investigate glass fibers as a model substrate and demonstrate that their diameter plays a dominant role in the particle entrainment and coating by suspensions. We identify experimentally and rationalize different coating regimes of the fiber: at small capillary number, only a liquid film coats the fiber. At intermediate capillary numbers, a heterogeneous coating made of clusters of particles is observed. Finally, at large capillary number, the thickness of the entrained film is captured using the effective viscosity of the suspension. Our results demonstrate that varying the size of the fiber leads to a new degree of control in the entrainment of particles via capillary filtering. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q03.00009: Weakly-nonlinear evolution of surface-tension driven waves in the presence of viscosity Quinton Farr, Rouslan Krechetnikov Weakly nonlinear models describing the evolution of water waves such as the Korteweg-de Vries and nonlinear Schr\"odinger equations have been widely studied because of their ability to capture the behavior of interesting physical phenomena through the reduction of a full problem to a much simpler system retaining only first order nonlinear effects. While a large class of these reductions for gravity-driven waves are well-understood, situations where surface tension is the only driving force require further attention. We consider wave motion on a one-dimensional thin liquid film of infinite extent and on the perimeter of a two-dimensional liquid drop, forced by surface tension in lieu of gravity. Viscosity is also included to understand the effect of dissipation. Our investigation leads to new equations governing surface tension-driven waves. We discuss their properties, special solutions, and physical implications. [Preview Abstract] |
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