Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L34: Drops XII: Elastic Surfaces and Fibers |
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Chair: Siddhartha Das, University of Alberta Room: 405 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L34.00001: Dew-driven folding of insect wings Andrew Dickerson, Sam Beadles, Courtney Clement, David Hu Small insect wings fold into tacos when exposed to dewfall or fog for extended times. Such shapes are tightly held together and require great force or long evaporation times for the wings to unfold. In this experimental investigation, we use time-lapse and high-speed videography on a mosquito wing exposed to fog to characterize the folding process from a flat wing to a taco. We observe a taco is formed through a series of processes involving wing bending, unbending, and subsequent tight folding of the wing following the sliding of the drop off the wing. We use a simplified 2D model to determine the forces coalescing drops exert on the wing, and present folding-resistant design suggestions for micro-aerial vehicle wings. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L34.00002: Aperture-Embedded Polymer Microlenses for Ultra-Low-Cost Microscopy Platforms (Foldscope) Laurel Kroo, George K. Herring, Manu Prakash Our lab has recently introduced an ultra-low cost microscopy platform: Foldscope, an origami based print-and-fold paper microscope for applications in disease diagnostics and science education. This current study introduces the concept of aperture-embedded microlenses made of ultra-violet curable polymers as a solution for high-throughput roll-to-roll manufacturing of micro-optical components utilized in Foldscope. The approach employs fluid droplets trapped in an aperture via capillary forces to induce various characteristic lens surfaces. By implementing static and dynamic pressure as a method to manipulate the droplet, a large and versatile range of optical surface shapes become viable. When the polymer droplet acquires the desired shape on either side of the aperture, the lens is frozen \textit{in situ} within milliseconds with a high-power UV source. We explore the dynamics of ultra-fast curing of polymeric droplets through both experimental and analytical means. The presented capillary induced printed lens manufacturing enables ultra-low cost optical instruments. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L34.00003: Interaction of Drops on a Soft Substrate Luuk A. Lubbers, Joost H. Weijs, Siddhartha Das, Lorenzo Botto, Bruno Andreotti, Jacco H. Snoeijer A sessile drop can elastically deform a substrate by the action of capillary forces. The typical size of the deformation is given by the ratio of surface tension and the elastic modulus, $\gamma/E$, which can reach up to 10-100 microns for soft elastomers. In this talk we theoretically show that the contact angles of drops on such a surface exhibit two transitions when increasing $\gamma/E$: (i) the microsocopic geometry of the contact line first develops a Neumann-like cusp when $\gamma/E$ is of the order of few nanometers, (ii) the macroscopic angle of the drop is altered only when $\gamma/E$ reaches the size of the drop. Using the same framework we then show that two neighboring drops exhibit an effective interaction, mediated by the deformation of the elastic medium. This is in analogy to the well-known Cheerios effect, where small particles at a liquid interface attract eachother due to the meniscus deformations. Here we reveal the nature of drop-drop interactions on a soft substrate by combining numerical and analytical calculations. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L34.00004: Elastically dominated viscous spreading Jerome Neufeld, Marcin Malinowski The spreading of viscous liquid drops on a floating elastic sheet is a rich problem exhibiting striking new phenomena across a wide array of scales, from the buckling of nanoscale elastic sheets to the deformation of the Indian subcontinent by the Tibetan plateau. Here we show that when density of the fluid exceeds that of the ``ocean'' supporting the elastic sheet ($\rho_f > \rho_o$) a steady state radius is reached. In contrast, for relatively light liquid droplets ($\rho_f < \rho_o$) two modes of propagation are found. At early times bending provides a small correction to the classic viscous gravity current, while at late times a new bending-dominated mode of propagation emerges. The results are applicable to the spreading of droplets on elastic membranes and, at very much larger scales, to the dynamics and topology of the Tibetan plateau. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L34.00005: Droplet impact on an elastic beam; a leaf-raindrop system Sean Gart, Daniel Chique, Sunghwan Jung We investigate a leaf-drop system exhibiting a unique system of coupled elasticity and drop dynamics by studying water droplet impact on an elastic cantilever beam with a wettable and non-wettable surface. We found that wettable beams experience much higher torque and bending energy than non-wettable beams. This is because a drop sticks to a wettable beam and rolls off of a non-wettable beam. Simple analytical models can explain the difference in bending energy and torque of wettable and non-wettable beams, which is verified with experimental observations. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L34.00006: Compound droplets on fibers Floriane Weyer, Laurent Dreesen, Marjorie Lismont, Nicolas Vandewalle The development of a fiber-based digital microfluidics device mimicking biological membranes requires the formation and the manipulation of compound droplets in order to prevent contamination and evaporation. In the present work, we propose a study of compound water-oil droplets on fibers. Although the case of pure droplets is well established, we show herein that the situation becomes more complex for compound fluidic systems. In particular conditions, contact lines are merging and quadruple contact lines are formed. Depending on the formation of this quadruple line, the behavior of the compound droplet is different from pure systems. Two different aspects are then addressed: the equilibrium position and the maximum size of the droplets on the fiber, being obtained by the balance of capillary and gravity forces. Finally, we show that the characteristic length is the fiber diameter or the size of the core droplet depending on whether a quadruple line is formed. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L34.00007: Wetting and drying of liquid on crossed fibers Alban Sauret, Alison D. Bick, Howard A. Stone Fibrous media are common in various engineered systems such as filters, paper or the textile industry. Many of these materials can be described as a network of fibers in which a wetting liquid tends to accumulate at its nodes and changes the bulk properties. Here we study a drop of silicone oil sitting on the simplest element of the array: two rigid crossed fibers. In particular, we investigate experimentally how the structure of the material affects the wetting and drying dynamics of that liquid drop. We first show that the liquid can adopt different shapes from a long liquid column to a drop. The transition between these morphologies depends on the volume of liquid, the tilting angle between the fibers, as well as the fiber radius. The wetting length in the column state can be predicted analytically. Because of these different shapes, the liquid exhibits different drying kinetics, which effects the overall drying time. Our study suggests that shearing a wetted array of fibers, by tuning the liquid morphology, may enhance the drying rate. [Preview Abstract] |
Monday, November 25, 2013 5:06PM - 5:19PM |
L34.00008: Drops moving along and across a filament Rakesh P. Sahu, Suman Sinha-Ray, Alexander Yarin, Behnam Pourdeyhimi The present work is devoted to the experimental study of oil drop motion both along and across a filament due to the air jet blowing. In case of drop moving along the filament, phenomena such as drop stick--slip motion, shape oscillations, shedding of a tail along the filament, the tail capillary instability and drop recoil motion were observed which were rationalized in the framework of simplified models. Experiments with cross-flow of the surrounding gas relative to the filament with an oil drop on it were conducted, with air velocity in the range of 7.23 to 22.7 m s$^{-1}$. The Weber number varied from 2 to 40 and the Ohnesorge number varied from 0.07 to 0.8. The lower and upper critical Weber numbers were introduced to distinguish between the beginning of the drop blowing off the filament and the onset of the bag-stamen breakup. The range of the Weber number between these two critical values is filled with three types of vibrational breakup: V1 (a balloon-like drop being blown off), V2 (a drop on a single stamen being blown off), and V3 (a drop on a double stamen being blown off). The Weber number/Ohnesorge number plane was delineated into domains of different breakup regimes. [Preview Abstract] |
Monday, November 25, 2013 5:19PM - 5:32PM |
L34.00009: Drops in wedges Etienne Reyssat Constrictions or widening of porosity result in non-balanced capillary forces acting at the interface between two fluid phases in porous media. Gradients of confinement can thus be used to produce, break up or manipulate drops and bubbles. We investigate experimentally the motion of oil drops and air bubbles confined between two quasi-horizontal plates forming a sharp wedge. The confinement gradient drives the migration of drops of wetting fluid toward the apex of the wedge. The capillary driving force is balanced by viscous dissipation occurring both in the bulk of the drop and along the contact lines. We provide a minimal model that quantitatively explains the migrations dynamics. In particular, we observe and explain two asymptotic regimes associated to both dissipation modes. We also present various possibilities to trap, expel or transport fluid using confinement gradients. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L34.00010: Snail droplets: How fast is a flattened droplet transported by a more viscous wetting carrier fluid in a thin microchannel? Francois Gallaire, Mathias Nagel It has been known for more than hundred years that a spherical droplet of fluid \#1 immersed in an unconfined environment of a more viscous carrier fluid \#2 travels at a relative velocity outreaching the carrier fluid. This result does not hold when the droplet is squeezed in-between walls, an ubiquitous situation in microfluidics. Indeed, the presence of confining walls results in thin lubricating films of fluid \#2 lying in-between the walls and the droplet interface, which introduce an additional source of drag that increases as the film thickness decreases. Following Park and Homsy (1984), the lubrication film thickness may be shown to vary as $Ca^{2/3}$ where $Ca$ is the capillary number that compares the viscous damping and surface tension effects. These films also affect the pressure jump across the interface, which, combined with the Brinkman equations for the flow in Hele-Shaw cells, allows to determine the flow field and the resulting deformations of the droplet interface. The obtained results appear to be in good agreement with experimental measurements. A multipole expansion of the flow field created by the droplet is then coupled to the nonlinear boundary condition at the interface yielding a simple analytical expression for the relative droplet velocity. [Preview Abstract] |
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