Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session L36: Drops: Particle laden And Drops: Wall-bound w/ Transport |
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Chair: Michael Miksis, Northwestern University Room: Portland Ballroom 251 |
Monday, November 21, 2016 4:30PM - 4:43PM |
L36.00001: Simulations of electrically induced particle structuring on spherical drop surface Yi Hu, Petia Vlahovska, Michael Miksis Recent experiments (Ouriemi and Vlahovska, 2014) show intriguing surface patterns when a uniform electric field is applied to a droplet covered with colloidal particles. Depending on the particle properties and the electrical field intensity, particles organize into an equatorial belt, pole-to-pole chains, or dynamic vortices. Here we present a model to simulate the collective particle dynamics, which accounts for the electrohydrodynamic flow and particle dielectrophoresis due to the non-uniformity of local electrical field. In stronger electric fields, particles are expected to undergo Quincke rotation, inducing rotating clusters through inter-particle hydrodynamical interaction. We discuss how the field intensity influences the width, orientation and periodicity of the particle clusters. Our results provide insight into the various particle assembles discovered in the experiments. [Preview Abstract] |
Monday, November 21, 2016 4:43PM - 4:56PM |
L36.00002: Snap-in of particles at curved liquid interfaces Chao Li, Momene Moradiafrapoli, Jeremy Marston The contact of particles with liquid interfaces constitutes the first stage in the formation of a particle-laden interface, the so-called "snap-in effect". Here, we report on an experimental study using high-speed video to directly visualize the snap-in process and the approach to the equilibrium state of a particle at a curved liquid interface (i.e. droplet surface). We image the evolution of the contact line, which is found to follow a power-law scaling in time, and the dynamic contact angle during the snap-in. Both hydrophilic and hydrophobic particles are explored and we match the lift-off stage of the particles with a simple force balance. We also explore some multi-particle experiments, eluding to the dynamics of particle-laden interface formation. [Preview Abstract] |
Monday, November 21, 2016 4:56PM - 5:09PM |
L36.00003: Irreversible deformation of particle-coated droplets Momene Moradiafrapoli, Jeremy Marston In this presentation, we will explore one particular property of particle-coated droplets, known as liquid marbles - namely - irreversible deformation. Whilst some studies have eluded to certain mechanical properties of particle-laden interfaces such as interfacial rheology and elasticity, the extent of deformation they can sustain has not been well-characterized. We explore this rich area using liquid marbles as the model system for the study. We find that they can withstand a compression of up to approximately 40\% without rupturing, but even even before this threshold, the deformation can be irreversible. We quantify and attempt to explain this feature for a range of particle sizes. Our observations thus add another layer of hidden complexity and non-linearity to seemingly elemental interfacial structures. [Preview Abstract] |
Monday, November 21, 2016 5:09PM - 5:22PM |
L36.00004: Dynamics of Droplet Detachment from a Granular Raft suzie protiere, Matthieu Roche When we sprinkle dense particles at an oil/water interface these particles self-assemble due to long-range capillary interactions into a monolayer that we call a granular raft. Particles can progressively be added to the raft until it destabilizes due to the balance between the local buoyancy forces and the capillary forces at the border of the raft. When the raft destabilizes it sinks and forms oil-in-water armored droplets. We study the formation of such armored droplets and compare its detachment to the behavior observed in suspensions or in pure viscous fluids. Indeed for pure fluids the radius of the neck of the forming droplet decays linearly. Here, we find that depending on the size and on the density of the particles two types of behaviors are observed during droplet formation. Either the raft sinks and no particles are found along the neck during the armored droplet formation, or an ``interfacial granular jet'' forms which breaks, due to a Rayleigh-Plateau-like instability, into a multitude of small millimeter-sized armored droplets. We show that since the particles are adsorbed at the interface, those two types of behaviors depend on a dimensionless parameter that takes into account the particle size and density. Moreover we find that the position of the particles during the formation of the drop dramatically modifies the dynamics, proving that the initial conditions are important during droplet breakup. [Preview Abstract] |
Monday, November 21, 2016 5:22PM - 5:35PM |
L36.00005: Lattice Boltzmann Simulations of Evaporating Droplets with Nanoparticles Mingfei Zhao, Xin Yong Elucidating the nanoparticle dynamics in drying droplets provides fundamental hydrodynamic insight into the evaporation-induced self-assembly, which is of great importance to materials printing and thin film processing. We develop a free-energy-based multiphase lattice Boltzmann model coupled with Lagrangian particle tracking to simulate evaporating particle-laden droplets on a solid substrate with specified wetting behavior. This work focuses on the interplay between the evaporation-driven advection and the self-organization of nanoparticles inside the droplet and at the droplet surface. For static droplets, the different parameters, fluid-particle interaction strength and particle number, governing the nanoparticle-droplet dynamics are systematically investigated, such as particle radial and circumferential distribution. We clarify the effect of nanoparticle presence on the droplet surface tension and wetting behavior. For evaporating droplets, we observe how droplet evaporation modulates the self-assembly of nanoparticles when the droplet has different static contact angles and hysteresis windows. We also confirm that the number of nanoparticles at the liquid-vapor interface influences the evaporation flux at the liquid-vapor interface. [Preview Abstract] |
Monday, November 21, 2016 5:35PM - 5:48PM |
L36.00006: Colloidal Deposition of Ellipsoidal Particles: Competition between Capillary and Hydrodynamic Forces Dong-ook Kim, Min Pack, Hyoungsoo Kim, Ying Sun Ellipsoidal particles have previously been shown to suppress the coffee-ring effect in millimeter-size colloidal droplets. Compared to their spherical counterparts, ellipsoidal particles experience stronger adsorption energy to the drop surface where the anisotropy-induced liquid-air interface deformation leads to much greater capillary attractions between particles. Using inkjet-printed colloidal drops of varying drop size, particle concentration, and particle aspect ratio, the present work demonstrates how the suppression of the coffee-ring is not only a function of the particle anisotropy, but rather a competition between the propensity for particles to assemble at the drop surface via capillary interactions and the evaporation-driven particle motion to the contact line. For ellipsoidal particles on the drop surface, the capillary force increases with particle concentration and aspect ratio, while the hydrodynamic force increases with aspect ratio but decreases with drop size. When the capillary force dominates, the surface ellipsoids form a coherent network inhibiting advection and the coffee-ring effect is suppressed, whereas when the hydrodynamic force dominates, the ellipsoids move to the contact line resulting in coffee-ring deposition. [Preview Abstract] |
Monday, November 21, 2016 5:48PM - 6:01PM |
L36.00007: Targeting Sessile Droplets with Electrospray to Form Nanoparticle Deposits Paul Chiarot, Matthias Daeumer, Sepehr Maktabi, Xin Yong The ability to print ordered deposits of nanoparticles has significant implications for electronics and photonics manufacturing. In this work, electrospray atomization was used to deliver dry nanoparticles to the surface of sacrificial sessile droplets. The particles were subsequently mapped to a glass substrate upon complete evaporation of the target droplet to create a deposit. The influence of the key electrospray operating parameters on the final deposit structure were explored, including: spray time, nanoparticle concentration, and initial sacrificial droplet volume. Once the nanoparticles were delivered to the interface, evaporatively-driven transport of the particles across the surface of the sessile droplet played a significant role in determining the structure of the deposit. When the contact line of the target sessile droplet was pinned during evaporation, the final deposit had greater particle density at the edge and center. The particles were distributed more uniformly across the deposit when the contact line of the target droplet moved during evaporation. The influence of thermal gradients on the final deposit structure was investigated by heating the substrate to increase the sessile droplet temperature. We also conducted computational simulations of evaporating particle-laden droplets and explored the influences of contact line behavior and nanoparticle surface chemistry on the deposit structure. [Preview Abstract] |
Monday, November 21, 2016 6:01PM - 6:14PM |
L36.00008: Controlling the Growth Modes of Femtoliter Sessile Droplets Nucleating on Chemically Patterned Surfaces Xuehua Zhang, Lei Bao, Zenon Werbiuk, Detlef Lohse Femtoliter droplet arrays on immersed substrates are essential elements in a broad range of advanced droplet-based technologies, such as light manipulation, sensing, and high throughput diagnosis. Solvent exchange is a bottom-up approach for producing those droplets from a pulse of oil oversaturation when a good solvent of the droplet liquid is displaced by a poor solvent. The position and arrangement of the droplets are regulated by chemical micropatterns on the substrate. Here we show experimentally and theoretically that the growth modes of droplets confined in planar micropatterns on the surface can be manipulated through the laminar flow of the solvent exchange. The control parameters are the area size of the micropatterns and the flow rate, and the observables are the contact angle and the final droplet volume. For a given pattern size, the Peclet number of the flow determines whether the growing droplets switch from an initial constant contact angle mode to a subsequent constant contact radius mode. Good agreement is achieved between the experimental results and our theoretical model that describes the dependence of the final droplet size on Pe. [Preview Abstract] |
Monday, November 21, 2016 6:14PM - 6:27PM |
L36.00009: Dissolution of Droplets on a Substrate with Engraved Concentric Rings Jose Manuel Encarnacion Escobar, Erik Dietrich, Pengyu Lv, Harold Zandvliet, Xuehua Zhang, Steve Arscott, Detlef Lohse The nucleation of nano and micro sized drops and bubbles often occurs on catalytic surfaces lowering its efficiency. The contact angle hysteresis, which is a consequence of the pinning on heterogeneities of the surface, can dramatically affect the stability and lifetime of the drop. The stability of a surface bubble can, in fact, be theoretically calculated thanks to the assumption of the pinning of the bubble [Lohse and Zhang, Lohse, D.; Zhang, X., Phys. Rev. E 2015, 91, 031003.]. Our experiments try to shed light on the understanding of the pinning of droplets caused by micro structures during their dissolution. It is possible to predict the depinning angle of a drying drop as a function of the geometry of the defect and the receding contact angle. Additionally, the jump from one defect to another happens fast but is not an immediate change. This dewetting happens showing the so called zipping behavior. We present quantitative data from experiments as well as the experimental techniques used, including confocal microscopy and the first analysis and comparison with the already existent theoretical models. [Preview Abstract] |
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