Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G32: Drops: Particle-Droplet Interactions |
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Chair: Pengtao Yue, Virginia Tech Room: 313 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G32.00001: ALE-Phase-field simulations of floating particles Pengtao Yue In this talk, we will present a hybrid Arbitrary-Lagrangian-Eulerian(ALE)-Phase-Field method for the direct numerical simulation of multiphase flows where fluid interfaces, moving rigid particles, and moving contact lines coexist. Practical applications include Pickering emulsions, froth flotation, and biolocomotion at fluid interface. An ALE algorithm based on the finite element method and an adaptive moving mesh is used to track the moving boundaries of rigid particles. A phase-field method based on the same moving mesh is used to capture the fluid interfaces; meanwhile, the Cahn-Hilliard diffusion automatically takes care of the stress singularity at the moving contact line when a fluid interface intersects a solid surface. To fully resolve the diffuse interface, mesh is locally refined at the fluid interface. All the governing equations, i.e., equations for fluids, interfaces, and particles, are solved implicitly in a unified variational framework. In the end we will present some recent results on the water entry problem and the capillary interaction between floating particles (a.k.a. the Cheerios effect), with a focus on the effect of contact-line dynamics. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G32.00002: Capillary Thinning of Particle-laden Drops Brayden Wagoner, Sumeet Thete, Matt Jahns, Pankaj Doshi, Osman Basaran Drop formation is central in many applications such as ink-jet printing, microfluidic devices, and atomization. During drop formation, a thinning filament is created between the about-to-form drop and the fluid hanging from the nozzle. Therefore, the physics of capillary thinning of filaments is key to understanding drop formation and has been thoroughly studied for pure Newtonian fluids. The thinning dynamics is, however, altered completely when the fluid contains particles, the physics of which is not well understood. In this work, we explore the impact of solid particles on filament thinning and drop formation by using a combination of experiments and numerical simulations. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G32.00003: Drop floating on a granular raft Etienne Jambon-Puillet, Christophe Josserand, Suzie Protiere When a droplet comes in contact with a bath of the same liquid, it coalesces to minimize the surface energy. This phenomenon reduces emulsion stability and is usually fought with surfactant molecules. Another way to slow down coalescence is to use colloidal solid particles. In this case the particles spontaneously migrate to the interface to form ``Pickering'' emulsions and act as a barrier between droplets. Here we use dense, large particles ($\sim 500\:\mu m$) which form a monolayer at an oil/water interface that we call a granular raft. When a droplet is placed on top of such a raft, for a given set of particle properties (contact angle/size), the raft prevents coalescence indefinitely. However, in contrast to what happens when a droplet is placed on a hydrophobic surface and never wets the surface, here the droplet is strongly anchored to the raft and deforms it. We will use this specific configuration to probe the mechanical response of the granular raft: by controlling the droplet volume we can impose tensile or compressive stresses. Finally we will show that the drop, spherical at first, slowly takes a more complex shape as it's volume increases. This shape is not reversible as the drop volume is decreased. The drop can become oblate or prolate with wrinkling of the raft. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G32.00004: Ring formation on an inclined surface Robert Deegan, Xiyu Du A drop dried on a solid surface will typically leave a narrow band of solute deposited along the contact line. We examined variations of this deposit due to the inclination of the substrate using numerical simulations of a two-dimensional drop, equivalent to a strip-like drop. An asymptotic analysis of the contact line region predicts that the upslope deposit will grow faster at early times, but the growth of this deposit ends sooner because the upper contact line depins first. From our simulations we find that the deposit can be larger at either the upper or lower contact line depending on the initial drop volume and substrate inclination. For larger drops and steeper inclinations, the early lead in deposited mass at the upper contact line is wiped out by the earlier depinning of the upper contact line and subsequent continued growth at the lower contact line. Conversely, for smaller drops and shallower inclinations, the early lead of the upper contact line is insurmountable despite its earlier termination in growth. Our results show that it is difficult to reconstruct \textit{a postiori }the inclination of the substrate based solely on the shape of the deposit. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G32.00005: Capillary and elastic failure of particle-stabilized droplets Nivi Samudrala, Raphael Sarfati, Jin Nam, Eric Dufresne Colloidal surfactants robustly stabilize fluid interfaces against spontaneous phase separation. Like molecular surfactants, they improve the thermodynamic and kinetic stability of the interface. Here, we investigate the mechanical stability of particle-stabilized droplets using micro-pipette aspiration. We observe two distinct modes of failure. In capillary failure, fluid is pulled through the gaps between the particles. In elastic failure, the particle-laden interface buckles like an elastic shell. We explore the impact of the fluid surface tension and particle interactions on these two modes of failure. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G32.00006: Electro-induced manipulations of liquid marbles for chemical reactions Zhou Liu, Xiangyu Fu, Bernard P. Binks, Ho Cheung Shum Liquid marbles, liquid droplets coated by non-wetting particles, have been well demonstrated as a promising template for various droplet-based applications, in particular for chemical reactions. In these applications, controlled manipulations on liquid marbles, including coalescence and mixing, are highly demanded but yet rarely investigated. In this work, we study the coalescence and mixing of liquid marbles controlled by an electric field. We found that a sufficiently large applied voltage can cause the coalescence of two or multiple marbles arranged in a chain. This critical voltage, leading to the consequent coalescence, increases with the number of the liquid marbles. In addition, the imposed electric stress can induce convective liquid flow within the different liquid marbles, resulting in rapid and efficient mixing. The mixing efficiency can be conveniently tuned through varying the applied voltage. Our approach based on electro-assisted manipulations of liquids marbles represents a robust and feasible template for chemical or biomedical reactions involving multiple reagents and steps. We have demonstrated its potential by performing a chemiluminescence to detect the hydrogen peroxide encapsulated in liquid marbles. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G32.00007: Lattice Boltzmann Method for Liquid-Gas-Particle Systems with Compact Discretization Taehun Lee, Samaneh Farokhirad We have developed a liquid-gas-particle (LGP) lattice Boltzmann method (LBM) that utilizes only the nearest neighbor lattice sites for the computation of intermolecular forcing terms. Previous LGP-LBM requires larger number of lattice sites to model the interaction of fluid interfaces with immersed solid particles. This makes the treatment of contact line on a particle cumbersome when the partially wetting particle interacts with liquid-gas interface. The new model is capable of suppressing spurious currents at equilibrium. Many existing multi-component solvers suffer from spurious currents and the inability to employ components with sufficiently large density differences due to stability issues. Due to their finite size and wetting properties, particles deform an interface locally, which can lead to capillary interactions that dramatically alter the behavior of the system, relative to the particle-free case. We will present the liquid-gas-particle algorithm and its validations, which include two-particles on a flat liquid-gas interface approaching each other due to capillary effects, and a particle-laden drop impact with various impaction velocities. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G32.00008: Colloidal Drop Deposition on Porous Substrates Ying Sun, Min Pack, Han Hu, Dong-Ook Kim, Xin Yang Printable electronics and in particular paper and textile-based electronics have fueled research in inkjet printing on porous substrates. On nonporous substrates, the particle motion of the particles and evaporation of the solvent are the two main mechanisms that drive the final deposition morphology. For porous substrates another factor, mainly infiltration, adds a layer of complexity to the deposition patterns that has not yet been elucidated in literature. In this study, a high-speed camera was used to capture the imbibition of picoliter-sized polystyrene nanoparticles in water droplets into nano-porous anodic aluminum oxide substrates of various porosities and wettabilities. For water, the infiltration rate is much faster than both evaporation and particle motion and thus when the substrate fully imbibes the droplet, the well-known ``coffee ring'' is suppressed. However, when a residual droplet forms upon the termination of the infiltration regime, the competing particle motion and evaporation regimes, $t_{\mathrm{P}} $and$ t_{\mathrm{EI\thinspace }}$respectively, define the critical time scales for which the coffee ring will be formed ($t_{\mathrm{P/}}t_{\mathrm{EI\thinspace }}$\textless 1) or suppressed ($t_{\mathrm{P/}}t_{\mathrm{EI\thinspace }}$\textgreater 1). [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G32.00009: Does a particle encapsulated in a droplet always migrate towards its center? Lailai Zhu, Francois Gallaire The behavior of anuclear cells like red blood cells in flow have been extensively investigated. However, the dynamics of nuclear cells are much less explored. The objective of this work is to investigate the interplay between the stiff organelles and the surrounding deformable cell membrane and we consider a finite-size spherical particle inside a droplet subjected to an unbounded shear flow. A three-dimensional boundary integral implementation was developed to fully resolve the interface-structure interaction characterized by capillary number $Ca$ and particle-droplet size ratio (between $0.2$ to $0.6$). For low $Ca$, the particle approaches the center of droplet. For $Ca$ above a critical value, the time invariance is broken and the particle migrates to a closed orbit, reaching a limit cyle. We identify a supercritical Hopf bifurcation as a result of the balance between interfacial energy and viscous dissipation. [Preview Abstract] |
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