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 Q34: Flow Instability: Interfacial and Thin Film Elasticity and Substrates |
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Chair: Josh Bostwick, Clemson University Room: 616 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q34.00001: Asymmetric instabilities in the flow of thin films on fibers Chase Gabbard, Joshua Bostwick Understanding the dynamics of thin-film flow down a vertical fiber is valuable for many coating applications. It is well known that such flows give rise to a number of instabilities that define the bead-on-fiber morphology. These include Plateau-Rayleigh breakup, isolated bead formation, and convective instabilities, all of which leave the interface shape axisymmetric. We conduct experiments that reveal an asymmetric instability in the flow of liquid on a fiber, which depends critically upon the liquid properties, flow rate and fiber radius, and we document this dependence on the observed morphology. Multiple liquids were used to change the viscosity and surfactant is used to change the surface tension in order to capture the transition between symmetric and asymmetric. Interestingly, we observe that under certain conditions in the transition region the instability may change from asymmetric to symmetric at some distance down the fiber. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q34.00002: Simulating instabilities of liquid metal alloys on nanoscale using molecular dynamics simulations. Ryan Allaire, Miguel Fuentes-Cabrera, Philip D. Rack, Linda Cummings, Lou Kondic Classical molecular dynamics simulations are used to investigate the influence of phase separation of liquid metal alloys on Rayleigh-Plateau (RP) type instabilities of free standing alloys, as well as alloys deposited as films on substrates. The alloy geometries are created in thin strips with widths modulated by sinusoidal waves of varying amplitudes and wavelengths, corresponding to the fastest growing mode obtained from continuum RP theory. We explore the influence of temperature on the breakup process and on phase separation. Both the ratio of phase separation length scales to wavelengths of the sinusoidal perturbations, and the initial location of phase separation are found to influence the RP instability development, either by modifying the instability growth rate or by changing the spatial positioning of the breakup point. As an outcome, nanoparticles are formed, characterized by morphologies of either core-shell or layered type. Growth rates of instabilities are compared to the predictions of RP theory and resultant nanoparticle compositions are analyzed. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q34.00003: Crystal-Growing with Rayleigh-Plateau Instability Lingzhi Cai, Joel Marthelot, PT Brun We use the Rayleigh-Plateau instability to fabricate structures in a 3D printing context. We deposit threads of glycerol in an immiscible polymeric bath. Owing to capillary effects these threads break into a collection of drops. As the polymer cures, these drops are permanently captured into the matrix, thereby forming a composite material. We propose a methodology to vary the breakup wavelength of liquid threads and reduce the polydispersity of droplets using a solid boundary template with a characteristic wavelength. In addition, by tuning the spacing between successive threads, we are able to fabricate a crystal-like structure in these composite materials. The pattern formation process is robust: the crystal structure exhibits self-healing of initial or accidental defects. Existing theories (for example, spatio-temporal stability analysis) are adapted to our problem so as to rationalize our experimental results. In turn, we aim to take advantage of our model for the directed control of the instability toward the design of material with prescribed properties and function. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q34.00004: Electrohydrodynamic Patterning of non-Newtonian Thin Films Adham Riad, Hadi Nazaripoor, Ali Mohammadtabar, Mohtada Sadrzadeh Electrically driven instabilities of thin liquid films or electrohydrodynamic (EHD) induced patterning has gained popularity for its ability to create nano-sized features for various applications. An initially quiescent thin film of uniform thickness is sandwiched between two electrodes. A voltage is then applied across the electrodes, creating a destabilizing electrostatic pressure on the interface, causing surface deformation and patterns to evolve. A mathematical model is proposed to study the effect of power-law rheology on the EHD patterning process. Simplifying the governing equations using the long-wave approximation leads to a novel thin-film equation that describes the interface's dynamics for both pseudo-plastic and dilatant fluids. The spatiotemporal evolution of the patterning process is then numerically simulated by solving the nonlinear thin-film equation using a finite-difference based discretization scheme and an adaptive time step solver. The results show that the patterning time is strongly influenced by the power-law index and is significantly shorter for shear thickening than shear thinning fluids. Moreover, morphological changes between patterns of shear thinning and shear thickening fluids are correlated to local viscosity variations. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q34.00005: Electrohydrodynamic instability of viscous falling films: numerical verification of weakly nonlinear models and higher-order effects Tao Wei, Mengqi Zhang A study on the nonlinear stability of an electrified film flowing under gravity down an inclined plate is presented with an electric field acting normal to the plate. An asymptotic expansion is applied to derive a nonlinear evolution equation for the interfacial position in the long-wave limit, which retains terms up to 2nd-order in the small parameter. Numerical solutions to two weakly nonlinear models, considering distinct effects of hydrostatic pressure, inertia and viscous dispersion, are compared to those of the fully nonlinear equation with an electric Weber number well above the critical value from linear analysis. Single- and multiple-hump solitary waves are searched for a wide range of electric field and dispersion, described by a nonlinear eigenvalue problem. Two coupled Ginzburg-Landau equations are derived from a multiple-scale analysis, which suggests that the electric destabilization occurs in the form of travelling waves of finite amplitude. The leading-order solutions show that one order-of-magnitude increase in electric supercriticality will cause one order increase in the growth rate. Reasonable agreement is found between the present results and available experiments. In particular, for realistic parameter values an electric field can diminish the film pressure. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q34.00006: Thin water film rise on a rough surface of a vertical plate in water entry Nayoung Kim, Hyungmin Park A three-phase contact line is formed as a liquid film moves over the solid surface, and the maximum velocity at which the film does not separate from the surface is affected by several liquid properties, such as viscosity, density and surface tension. In addition, it has been shown that surface roughness can also encourage the early separation of a liquid film, but the detailed mechanism still remains unclear. In this study, we investigate the effect of surface roughness on the contact line rise on a falling the acrylic plate (70$\times $150 mm$^{\mathrm{2}})$ into the water tank vertically, using high-speed imaging. The thin liquid film is emitted while the plate passes through the free surface, and initial speed of the film can reach up to \textbf{\textit{O}}(10) m/s. We roughened the plate surface with an array of spanwise grooves (width and depth of 400 $\mu $m, spacing of 800 $\mu $m) and also considered smooth surface as a reference. The motion of liquid film on the rough surface is slower than that of the smooth surface, and the difference increases with increasing impact velocity of the plate. We also vary the wettability (hydrophilic, intermediate and hydrophobic) of the surface and discuss the dynamics of liquid film moving over rough surfaces. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q34.00007: Simultaneous Liquid Flow and Drying on Rotating Cylinders Chance Parrish, Satish Kumar The coating and drying of non-flat discrete objects is an important manufacturing step for a broad variety of products. Flow of a thin, non-volatile liquid film on the outside of a rotating cylinder is commonly used as a model problem for these processes. Here, we use lubrication theory to study the behavior of a volatile, particle-laden coating. Two coupled evolution equations describing variations in coating thickness and composition as a function of time and the angular coordinate are solved numerically. In the limit of a rapidly-rotating cylinder, results from linear stability analysis and nonlinear simulations demonstrate that non-uniform drying at larger drying rates may cause thickness and composition disturbances to regrow after initially decaying. When gravity is reincorporated, poor leveling of the coating thickness at lower rotation rates and larger drying rates leads to less uniform coatings. Colloidal particles hinder leveling at high concentrations through increases in the viscosity, but help prevent coating rupture at more moderate concentrations, leading to a composition "sweet spot". A parametric study is then used to show that thickness and composition variations are minimized at large rotation rate, low drying rate, and moderate particle concentration. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q34.00008: Three-dimensional numerical simulations of a thin film falling vertically down the inner surface of a rotating cylinder Usmaan Farooq, Jason Stafford, Camille Petit, Omar Matar A flow in which a thin film flows due to gravity on the surface of a rotating cylinder is investigated. This was performed using high resolution three-dimensional direct numerical simulations and a volume-of-fluid approach to treat the interface. The variation of the Ekman number (Ek), defined to be proportional to the rotation of the cylinder, and has a significant effect on various parameters of the flow. The centrifugal force increases with rotational speed, producing a stabilising effect (Iwasaki and Hasegawa, 1981), supressing wave formation. Key features, such as the transition from a 2D to a more complex 3D wave regime, and the local thickness of the film, are heavily influenced by this stabilisation and are investigated. Furthermore the imposed rotation results in distinct characteristics in the flow such as the development of angled waves due to the resolved velocity in the axial and azimuthal directions. Fast Fourier Transforms of the interface are performed which show how the wavelength and angle of the waves varies with Ek. Simulations have been conducted for a range of Ek to provide detailed insight on how this parameter affects the flow. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q34.00009: Thermally driven coalescence in thin liquid film flowing down a fiber Claudia Falcon, Hangjie Ji, Abolfazl Sadeghpour, Erfan Sedighi, Y. Sungtaek Ju, Andrea Bertozzi We aim at understanding the dynamics of thin fluid film flowing down a vertical fiber under streamwise thermal effects, both experimentally and theoretically. Recent studies have shown the importance of determining the regime transition from absolute to convective instability. Unlike previous work, our experiments demonstrate that the onset of such irregular wavy regime can also be induced by thermal gradient away from the nozzle. The new model includes spatial-dependent viscosity and surface tension due to inhomogeneous temperature field along the fiber. The predicted coalescence positions based on this theory are useful in the design of heat and mass exchangers for applications that include cooling systems and desalination.~ [Preview Abstract] |
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