Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D19: Surface Tension I |
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Chair: Satish Kumar, University of Minnesota Room: 28E |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D19.00001: Liquid Contact Line on Visco-Elastic Gels: Stick-Slip vs Continuous Motions Tadashi Kajiya, Adrian Daerr, Tetsuharu Narita, Laurent Royon, Francois Lequeux, Laurent Limat We studied the dynamics of water sessile drops being inflated on hydrophobic and visco-elastic Poly(styrene-butadiene-styrene)-paraffin gel substrates. While advancing, the droplet contact line exhibits three different motions. When the contact line advances at a high velocity, the contact line moves continuously with a constant contact angle. As the contact line slows down, it starts stick-slip: the contact line stays at a certain position then suddenly slips forward. With further decrease of the velocity, the contact line stops stick-slip and continuously advances again. The type of motions (continuous - stick-slip - continuous) depends on drop radius and mean velocity only through their ratio, $f = v/R$, which is a typical frequency of the contact line motion. The observed transitions of the contact line motions indicate that the rheology of gel drastically contributes to the wetting dynamics on its surface. On visco-elastic gels, the moving contact line exhibits both aspects of wetting on elastic solids and wetting on viscous liquids depending on $f$. At an intermediate regime, the stick-slip motion appears. In this conference, we will present our latest experimental results and suggest possible mechanisms explaining the present phenomena. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D19.00002: Self-cleaning of superhydrophobic surfaces by spontaneously jumping condensate drops Katrina Wisdom, Jolanta Watson, Gregory Watson, Chuan-Hua Chen The self-cleaning function of superhydrophobic surfaces is conventionally attributed to the removal of contaminating particles by impacting or rolling water droplets, which implies the action of external forces such as gravity. Here, we demonstrate a new self-cleaning mechanism, whereby condensate drops spontaneously jump upon coalescence on a superhydrophobic surface, and the merged drop self-propels away from the surface along with the contaminants. The jumping-condensate mechanism is shown to autonomously clean superhydrophobic cicada wings, where the contaminating particles cannot be removed by external wind flow. Our findings offer new insights for the development of self-cleaning materials. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D19.00003: Interaction between counterpropagating Rossby and capillarity waves in planar jets and wakes Luca Biancofiore, Francois Gallaire, Patrice Laure By means of a global linear analysis, Tammisola et al. (2011) have observed a counterintuitive destabilizing effect of the surface tension in planar wakes. They have justified this destabilization by the presence of two different temporal unstable modes found when analyzing the local stability of an extracted velocity profile from the base flow. In the present study, we approximate the velocity profile of a jet/wake flow through a piecewise broken-line. We then explain the presence of these two temporal unstable modes for such flows using the counterpropagating Rossby wave (CRW) perspective (see Heifetz et al., 1999), which associates to each vorticity discontinuity an individual Rossby wave. The introduction of a finite amount of surface tension at the interface creates two capillarity waves (CW) which move with the same velocity but in opposite directions. The interaction of this four waves originates the two temporal unstable modes for both sinuous and varicose symmetries. Analyses of the influence of the shear layer thickness $\delta_w$ and the confinement $h$ on the behaviour of both CRWs and CWs and on their interaction are provided. Finally, comparisons to direct numerical simulations of jets/wakes including surface tension will complete the study. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D19.00004: The dynamics of three-dimensional liquid bridges with pinned and moving contact lines Satish Kumar, Shawn Dodds, Marcio Carvalho Liquid bridges with moving contact lines are relevant in a variety of natural and industrial settings, ranging from printing processes to the feeding of birds. While it is often assumed that the liquid bridge is two-dimensional, three-dimensional effects are prominent in many applications. To investigate this we solve Stokes equations using the finite element method for the stretching of a three-dimensional liquid bridge between two flat surfaces, one stationary and one moving. We find that whereas a shearing motion does not alter the distribution of liquid between the two plates, rotation leads to an increase in the amount of liquid resting on the stationary plate as breakup is approached. This suggests that a relative rotation of one surface can be used to improve liquid transfer to the other surface. We then consider the extension of non-cylindrical bridges with moving contact lines. We find that dynamic wetting, characterized through a contact line friction parameter, plays a key role in preventing the contact line from deviating significantly from its original shape as breakup is approached. By adjusting the friction on both plates it is possible to drastically improve the amount of liquid transferred to one surface while maintaining fidelity of the liquid pattern. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D19.00005: Onset of Dynamic Wetting Failure: The Influence of the Displaced Fluid Eric Vandre, Marcio Carvalho, Satish Kumar Dynamic wetting involves the displacement of fluid on a solid surface by an advancing liquid, and is essential to the successful operation of coating processes. In this work, we consider a model problem in order to examine the influence of the displaced fluid on the failure of dynamic wetting. Full two-dimensional (2D) calculations over a broad range of parameters are performed using the finite element method (FEM), and the results are compared to prior experiments and asymptotic analysis. This comparison motivates the development of a novel and efficient hybrid computational method that combines 2D FEM for the liquid and lubrication theory for the displaced fluid. We will discuss the limits of applicability of the hybrid approach, and its ability to describe realistic coating flows. Overall, our results highlight the significant influence of the displaced fluid on the conditions at which dynamic wetting failure occurs, along with the underlying physical mechanisms. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D19.00006: Pinching of a liquid ligament under surface tension Jerome Hoepffner We study the retraction of a cylinder of fluid caused by surface tension. At the tip of the ligament, a blob if formed which collect progressively the liquid as it retracts. Between the blob and the cylinder, there is the creation of a neck, whose radius decreases progressively following a mechanism close to that of the Rayleigh-Plateau instability. Inside this neck, we observe a jet of the fluid from the cylinder into the blob, a ``capillary Venturi.'' As the radius of the neck decreases the intensity of this jet increases, and we observe by means of numerical simulations and experiments that the detachment of this jet and creation of a vortex ring is able to alter significantly the evolution of the pinching: the pinching is avoided. This phenomenon is significant in the context of atomization because it changes significantly the statistics of the drops which are created from the retraction of the liquid ligament. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D19.00007: Physics, mathematics and numerics of particle adsorption on fluid interfaces Markus Schmuck, Grigorios A. Pavliotis, Serafim Kalliadasis We study two arbitrary immiscible fuids where one phase contains small particles of the size of the interface and smaller. We primarily focus on charge-free particles with wetting characteristics described by the contact angle formed at the interface between the two phases and the particles. Based on the experimental observation that particles are adsorbed on the interface to reduce the interfacial energy and hence the surface tension as well, we formulate a free-energy functional that accounts for these physical effects. Using elements from calculus of variations and formal gradient flow theory, we derive partial differential equations describing the location of the interface and the density of the particles in the fluid phases. Via numerical experiments we analyse the time evolution of the surface tension, the particle concentration, and the free energy over time and reflect basic experimentally observed phenomena. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D19.00008: Double-diffusive Marangoni convection around exothermic chemical fronts L. Rongy, P. Assemat, A. De Wit We study double diffusive Marangoni flows triggered by concentration and temperature gradients across an exothermic chemical front propagating in horizontal uncovered solution layers. We numerically integrate the incompressible Navier-Stokes equations coupled through the tangential stress balance to evolution equations for the concentration of the autocatalytic product and the temperature. Solutal and thermal Marangoni numbers quantify the effect of the concentration and temperature gradients on the surface tension respectively, while the Lewis number measures the ratio of thermal diffusivity over molecular diffusivity. The asymptotic isothermal dynamics is characterized by a steady fluid vortex traveling at a constant speed with the reactive front, deforming and accelerating it. We analyze here the influence of thermal effects on the dynamics of the system in both cases of cooperative and competitive solutal and thermal effects. In particular, because heat and mass diffuse at different rates, new unsteady double-diffusive dynamics such as oscillations of the concentration field can be observed when the solutal and thermal effects act antagonistically on the surface tension. The influence of the various parameters on the flow field is investigated. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D19.00009: Optically controlled Marangoni Tweezers Steffen Hardt, Subramanyan Namboodiri, Sajan George, Tobias Baier, Martina Ewald, Markus Biesalski A novel method for trapping and manipulating small particles is reported. The method relies on photoresponsive surfactants adsorbed to a gas-liquid interface that can be reversibly switched between two isomeric states using a focused laser beam. The principle is based on local changes of the surface tension, giving rise to Marangoni stresses. Depending on the type of surfactant isomer in the region around the laser spot, a flow either radially inward or outward is created. It is studied how the flow field generated depends on the light intensity and on the surfactant concentration. It is shown how the optically-induced inward flow can be utilized to trap and manipulate microspheres adsorbed to the gas-liquid interface. This principle of optically-controlled Marangoni tweezers opens new application perspectives, for example for the manipulation of nanoparticles. This is due to the fact that hydrodynamic stresses exhibit a more favorable scaling with the particle size than the Maxwell stresses utilized in conventional optical tweezers. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D19.00010: Harnessing Compositional Marangoni Flows in Depositing Nanoparticle Films Mainak Majumder, Matteo Pasquali Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the ``coffee-stain'' effect, arising from the convective macroscopic flow into the solid-liquid-vapor contact line of a droplet. We have recently demonstrated that uniform deposition of nanoparticles from aqueous suspensions can be obtained by drying the droplet in an ethanol vapor atmosphere .(.).............(Majumder et al., 2012). This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon{\textregistered} due to absorption of ethanol from the vapor. Visualization of droplet shape and internal flow shows initial droplet spreading and strong re-circulating flow during spreading and shrinkage. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line; thereby generating a surface tension gradient (or Marangoni forces) that drive a strong recirculating flow. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles. .MAJUMDER, M., RENDALL, C. S., PASQUALI, M. et al. 2012. Overcoming the ``Coffee-Stain'' Effect by Compositional Marangoni-Flow-Assisted Drop-Drying. \textit{J.Phys.Chem.B,} 116\textbf{,} 6536-6542. [Preview Abstract] |
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