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 A32: Drops: Surface Tension Effects |
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Chair: Francois Blanchette, UC Merced Room: 313 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A32.00001: Surfactant-laden drops rising in a stratified ambient Francois Blanchette, David Martin We present results of a numerical study of the dynamics of rising drops in the presence of both surfactants and stratification. Our simulations model oil drops rising in the oceans, where naturally occurring or man-made surfactants are present. We study surfactant covered drops in uniform and density-stratified ambients, as well as clean drops entering a dissolved surfactant layer. We quantify the effects of entrainment for various Reynolds and Marangoni numbers. We also report a brief acceleration followed by a significant deceleration as a clean drop enters a surfactant layer, and describe how the adsorption rate affects those dynamics. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A32.00002: Measurement of strong Marangoni flow near a contact line of a water droplet on hydrophobic surfaces Joonsik Park, Kenneth S. Breuer Strong Marangoni flow from a water droplet on unheated substrate has been theoretically predicted but not been quantitatively measured. Using two different experimental techniques, multi-layer flood illumination and Total Internal Reflection Fluorescence Microscopy (TIRFM), we report Marangoni flows with large ($O$(100 $\mu $m/s)) velocity near a contact line of a water droplet on hydrophobic substrates. The flow is measured by tracking the motion of nanoparticles with respect to the contact line, using statistical particle tracking velocimetry combined with sub-pixel edge detection algorithm. Under multi-layer flood illumination, the recirculating convective flow is identified within 5 $\mu $m vertically from the substrate. From the TIRFM measurement, the changes in the bulk-averaged velocity ($O$(100 $\mu $m/s)) and the shear rate ($O$(100 s$^{\mathrm{-1}}))$ as the distance from the contact line are identified within 550 nm vertically from the substrate, and compared to the characteristic shear rate and speed from Marangoni effect, respectively. Surprisingly, both Flood and TIRFM measurements indicate high slip velocities extending as far as 33 $\mu $m from the contact line. One possible explanation is that the high slip velocity is due to the accumulation of nanobubbles near the contact line which were formed at the deposition of a droplet. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A32.00003: The stability of a rising droplet: an inertialess nonmodal growth mechanism Giacomo Gallino, Lailai Zhu, Francois Gallaire Past modal stability analysis (Kojima et al. 1984) predicted that a rising or sedimenting droplet in a viscous fluid is stable in the presence of surface tension no matter how small, in contrast with experimental and numerical results. By performing a non-modal stability analysis, we demonstrate the potential for transient growth of the interfacial energy of a rising droplet in the limit of inertialess Stokes equations. The predicted critical capillary numbers agree well with that from direct numerical simulations reported in the literature (Koh \& Leal 1989). Boundary integral simulations are used to delineate the critical amplitude of the most destabilizing perturbations. The critical amplitude is negatively correlated with the linear optimal energy growth, implying that the transient growth is responsible for reducing the necessary perturbation amplitude required to escape the basin of attraction of the spherical solution. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A32.00004: Three-dimensional simulations of a rising bubble in a self-rewetting fluid Amarnath Premlata, Manoj Tripathi, Kirti Sahu, George Karapetsas, Khellil Sefiane, Omar Matar The motion of a gas bubble in a square channel with linearly increasing temperature in the vertical direction is investigated via 3D numerical simulations. The channel contains a so-called “self-rewetting” fluid whose surface tension exhibits a parabolic dependence on temperature with a well-defined minimum. An open-source finite-volume fluid flow solver, {\it Gerris}, is used with a dynamic adaptive grid refinement technique, based on the vorticity magnitude and position of the interface. We find that in self-rewetting fluids, the buoyancy-induced upward motion of the bubble is retarded by a thermocapillary-driven flow, which occurs as the bubble crosses the location at which the surface tension is minimum. The bubble then migrates downwards when thermocapillarity exceeds buoyancy. In its downward path, the bubble encounters regions of horizontal temperature gradients, which lead to bubble motion towards one of the channel walls. These phenomena are observed at sufficiently small Bond numbers and have no analogue for fluids whose surface tension decreases linearly with temperature. The mechanisms underlying these phenomena are elucidated by considering how the surface tension dependence on temperature affects the thermocapillary stresses in the flow. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A32.00005: Nature of hydrodynamic causation of Marangoni instabilities for the case of drop rising in a channel: visualisation and statistics Makrand Khanwale, Hrushikesh Khadamkar, Channamallikarjun Mathpati The physics of drop rise with continuous transfer of interfacial tension depressant (acetone), is mainly influenced by the coupling of mass transfer of interfacial depressent fluid, relative motion of two phases, and interface deformation. We present a investigation which focuses on the nature of hydrodynamic causation of aforementioned mass transfer process, which arise due to non-uniform shear at the interface, also known as the Marangoni instabilities. The effects of relative motion of two phases, and interface deformation are eliminated by operating in the spherical shape range (E\"{o}tv\"{o}s number, $Eo = 1.95$, and Morton number, $M= 78.20$) with creeping flow particle Reynolds number($Re_p = 0.053$). A improved technique for measurement and processing of data acquired from simultaneous planar PIV-PLIF is used to obtain velocity and concentration fields around the drop. A progressive non-Gaussian behaviour from large scales to small scales is seen, in scale wise wavelet energy decomposition of vorticity and concentration fields. This suggests similarity with high Schmidt and Reynolds number intermittent turbulence, even in the creeping flow region. Fourier spectra of concentration and velocity shows the plethora of length scales generated by the Marangoni instabilities. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A32.00006: Experimental study of the Marangoni flow in evaporating water droplet placed on vertical vibration and heated hydrophobic surface Chang Seok Park, Hee Chang Lim In general, the heated surface generates a Marangoni flow inside a droplet yielding a coffee stain effect in the end. This study aims to visualize and control the Marangoni flow by using periodic vertical vibration. While the droplet is evaporating, the variation of contact angle and internal volume of droplet was observed by using the combination of a continuous light and a DSLR still camera. Regarding the internal velocity, the PIV(Particle Image Velocimetry) system was applied to visualize the internal Marangoni flow. In order to estimate the temperature gradient inside and surface tension on the droplet, a commercial software Comsol Multiphysics was used. In the result, the internal velocity increases with the increase of the plate temperature and both flow directions of Marangoni and gravitational flow are opposite so that there seems to be a possibility to control the coffee stain effect. In addition, the Marangoni flow was controlled at relatively lower range of frequency 30 $\sim$ 50Hz. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A32.00007: Effect of Marangoni Flows on the Shape of Thin Sessile Droplets Evaporating into Air Yannis Tsoumpas, Sam Dehaeck, Alexey Rednikov, Pierre Colinet With the help of Mach-Zehnder interferometry, we study the (largely) axisymmetric shapes of freely receding evaporating sessile droplets of various HFE liquids. The droplets evaporate into ambient air and, although the liquids are perfectly wetting, possess small finite contact angles reckoned to be evaporation-induced. The experimentally determined droplet profiles are shown here to deviate, under some conditions, from the classical macroscopic static profile of a sessile droplet, as this is determined by gravity and capillarity. These deviations are attributed to a Marangoni flow, due to evaporation-induced thermal gradients along the liquid-air interface, and are mostly observed in conditions of high evaporation. Unlike the classical static shapes, the distorted experimental profiles exhibit an inflection point at the contact line area. When a poorly volatile liquid is considered, however, the temperature differences and the Marangoni stresses are weak, and the measurements are found to be in a good agreement with the classical static shape. Overall, the experimental findings are quantitatively confirmed by the predictions of a lubrication model accounting for the impact of the Marangoni effect on the droplet shape. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A32.00008: Bursting of dilute emulsion-based liquid sheets driven by a Marangoni effect Christian Ligoure, Laurence Ramos, Clara Vernay We study the destabilization mechanism of thin liquid sheets expanding in air and show that dilute oil-in-water emulsion-based sheets disintegrate through the nucleation and growth of holes that perforate the sheet. The velocity and thickness fields of the sheet are not perturbed by holes and hole opening follows a Taylor-Culick law. We find that a pre-hole, which widens and thins out the sheet with time, systematically precedes the hole nucleation. The growth dynamics of the pre-hole follows the law theoretically predicted for a liquid spreading on another liquid of higher surface tension due to Marangoni stresses. Classical Marangoni spreading experiments quantitatively corroborate those findings. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A32.00009: Dancing droplets: Contact angle, drag, and confinement Adrien Benusiglio, Nate Cira, Manu Prakash When deposited on a clean glass slide, a mixture of water and propylene glycol forms a droplet of given contact angle, when both pure liquids spread. (Cira, Benusiglio, Prakash: Nature, 2015). The droplet is stabilized by a gradient of surface tension due to evaporation that induces a Marangoni flow from the border to the apex of the droplets. The apparent contact angle of the droplets depends on both their composition and the external humidity as captured by simple models. These droplets present remarkable properties such as lack of a large pinning force. We discuss the drag on these droplets as a function of various parameters. We show theoretical and experimental results of how various confinement geometries change the vapor gradient and the dynamics of droplet attraction. [Preview Abstract] |
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