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 H32: Drops: Elastic Surfaces and Fibers |
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Chair: Jesse Belden, Naval Undersea Warfare Center Room: 313 |
Monday, November 23, 2015 10:35AM - 10:48AM |
H32.00001: Please comply: the water entry of soft spheres Jesse Belden, Randy Hurd, Tate Fanning, Michael Jandron, John Rekos, Allan Bower, Tadd Truscott The typical phenomena associated with sphere water impact are significantly altered when the sphere material is highly compliant rather than rigid. We describe the water impact physics of homogenous and hollow elastic spheres. The homogeneous spheres undergo large oscillatory deformations throughout entry that carve nested disturbances into the normally smooth air cavity, altering cavity shape and pinch off. Using an analytical model, we relate the maximum sphere deformation to the material properties and impact velocity. This characteristic deformation is used to reconcile the differences between cavities formed by compliant and rigid spheres. In addition to the nested disturbances seen with the homogeneous spheres, we observe azimuthal irregularities on the cavity during water entry of hollow elastic spheres. Based on experiments and finite-element modeling, we suggest that these disturbances are initiated by vibration mode shapes excited in the hollow spheres upon impact. For all sphere types, we compare the forces throughout water entry to the rigid sphere case. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H32.00002: Capillarity-driven folding of a thin floating annular film Joseph D. Paulsen, Vincent D\'emery, K. Bugra Toga, Zhanlong Qiu, Benny Davidovitch, Thomas P. Russell, Narayanan Menon A thin elastic sheet that is compressed on a substrate will form wrinkles to gather excess material while conforming to its foundation. When compressed further, the sheet may form folds, which can be understood as minimizing the sum of the bending energy and the energy to deform the substrate$^1$. Here we demonstrate in a simple planar geometry a folding transition that is independent of the mechanical properties of the sheet. We study the deformations of a thin polymer film that is cut into an annular shape and floated onto a flat air-water interface. By increasing the concentration of surfactant outside the film, we reduce the surface tension that pulls on the outer boundary of the annulus. The larger, inward tension causes the film to wrinkle and fold. Folding occurs at a threshold ratio of inner to outer tension that depends on the geometry of the sheet, but is independent of its bending rigidity. Our results are consistent with the simple geometric principle that the sheet assumes the unstretched shape that minimizes the interfacial energy of the exposed liquid surface. A similar principle was found to control how a thin elastic sheet wraps a liquid drop$^2$.\\\\ 1. Pocivavsek et al., Science 320, 912 (2008).\\ 2. Paulsen et al., BAPS.2015.MAR.G34.4 [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H32.00003: Drops spreading on flexible fibers Katarzyna Somszor, Fran\c{c}ois Boulogne, Alban Sauret, Emilie Dressaire, Howard Stone Fibrous media are encountered in many engineered systems such as textile, paper and insulating materials. In most of these materials, fibers are randomly oriented and form a complex network in which drops of wetting liquid tend to accumulate at the nodes of the network. Here we investigate the role of the fiber flexibility on the spreading of a small volume of liquid on a pair of crossed flexible fibers. A drop of silicone oil is dispensed at the point of contact of the fibers and we characterize the liquid morphologies as we vary the volume of liquid, the angle between the fibers, and the length and bending modulus of the fibers. Drop morphologies previously reported for rigid fibers, i.e. a drop, a column and a mixed morphology, are also observed on flexible fibers with modified domains of existence. Moreover, at small inclination angles of the fibers, a new behavior is observed: the fibers bend and collapse. Depending on the volume, the liquid can adopt a column or a mixed morphology on the collapsed fibers. We rationalize our observations with a model based on energetic considerations. Our study suggests that the fiber flexibility adds a rich variety of behaviors that can be crucial for industrial applications. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H32.00004: Self-Propelled Droplet Removal from Hydrophobic Fiber-Based Coalescers Kungang Zhang, Fangjie Liu, Adam Williams, Xiaopeng Qu, James Feng, Chuan-Hua Chen Fiber-based coalescers are widely used to accumulate droplets from aerosols and emulsions, where the accumulated droplets are typically removed by gravity or shear. We report self-propelled removal of drops from a hydrophobic fiber, where the surface energy released upon drop coalescence overcomes the drop-fiber adhesion, producing spontaneous departure that would not occur on a flat substrate of the same contact angle. The self-removal takes place above a threshold drop-to-fiber radius ratio, and the departure speed is close to the capillary-inertial velocity at large radius ratios. [K. Zhang et al., Phys. Rev. Lett., in press.] [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H32.00005: Liquid Droplets on a Highly Deformable Membrane Rafael Schulman, Kari Dalnoki-Veress We present measurements of the deformation produced by micro-droplets atop thin elastomeric and glassy free-standing films. Due to the Laplace pressure, the droplets deform the elastic membrane thereby forming a bulge. Thus, there are two angles that define the droplet/membrane geometry: the angle the liquid surface makes with the film and the angle the deformed bulge makes with the film. The contact line geometry is well captured by a Neumann construction which includes contributions from interfacial and mechanical tensions. Finally, we show that a droplet atop a film with biaxial tension assumes an equilibrium shape which is elongated along the axis of high tension. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H32.00006: Sophisticated compound droplets on fiber networks Floriane Weyer, Marjorie Lismont, Laurent Dreesen, Nicolas Vandewalle Droplets on fibers are part of our everyday lives. Indeed, many phenomena involve drops and fibers such as the formation of dew droplets on a spiderweb, the trapping of water droplets on cactus spines or the dyeing of cotton or wool fibers. Therefore, this topic has been widely studied in the recent years and it appears that droplets on fibers can be the starting point for an open digital microfluidics. We study the behavior of soapy water droplets on a fiber array. When a droplet slides along a vertical fiber and encounters a horizontal fiber, it can either stick there or continue its way. In the latter case, the droplet releases a tiny residue. We study the volume of these residues depending on the geometry of the node. By using this technique, a large number of small droplets can be trapped at the nodes of a fiber array. These residues can be encapsulated and collected by an oil droplet in order to create a multicompound droplet. Moreover, by using optical fibers, we can provoke and detect the fluorescence of the inner droplets. Fibers provide therefore an original way to study compound droplets and multiple reactions. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H32.00007: Abstract Withdrawn
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(Author Not Attending)
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H32.00008: Drop impact and capture on a thin flexible fiber Jean Comtet, Bavand Keshavarz, John W.M. Bush When a drop impacts a thin fiber, a critical impact speed can be defined, below which the drop is entirely captured by the fiber, and above which the drop pinches-off and fractures. We discuss here the capture dynamics of both inviscid and viscous drops on flexible fibers free to deform following impact. We characterize the impact-induced elongation of the drop thread for both high and low viscosity drops, and show that the capture dynamics depends on the relative magnitudes of the bending time of the fiber and deformation time of the drop. In particular, when these two timescales are comparable, drop capture is less prevalent, since the fiber rebounds when the drop deformation is maximal. Conversely, larger elasticity and slower bending time favor drop capture, as fiber rebound happens only after the drop has started to recoil. Finally, in the limit of highly flexible fibers, drop capture depends solely on the relative speed between the drop and the fiber directly after impact, as is prescribed by the momentum transferred. Because the fiber speed directly after impact decreases with increasing fiber length and fiber mass, our study identifies an optimal fiber length for maximizing the efficiency of droplet capture. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H32.00009: Droplet impact on a fiber Sung-gil Kim, Taehong Kim, Wonjung Kim We present the results of a combined experimental and theoretical investigation of drop impact on a fiber. We use high-speed videography to characterize the dynamics of drops impacting fibers. Our systematical experiments reveal that the outcome of droplet collision critically depends on the relative magnitude of inertial to capillary forces and the ratio of the thickness of fiber to the diameter of the drop. We identify three outcomes of the collision using a non-dimensional regime map. The selection among the modes of single capturing, single drop falling, and divided drop falling is explained through a scale analysis of forces. We also examine the droplet retention on the fiber after impact. For each mode, we suggest the mathematical models to predict the amount of residual water on the fiber. Our study can be extended to predicting the remaining droplet, the critical problem in air filtration, water collection, and fiber coating. [Preview Abstract] |
Monday, November 23, 2015 12:32PM - 12:45PM |
H32.00010: Elastocapillary mist collector Camille Duprat, Romain Labbé, Ana Rewakowicz Fibrous media are commonly used to collect droplets from an aerosol. In particular, woven textiles are used to harvest fresh water from fog, and coalescing filters made of non-woven entangled fibers are used to extract oil drops from gas streams. We propose a novel mist collector made of a forest of vertical flexible threads. As the droplets accumulate on the fibers, capillary bridges are formed, leading to the collapse of adjacent fibers thus forming liquid columns. This improve the liquid collection by preventing clogging, enabling high capture and precluding re-entrainment of drops in the gas stream due to the immediate coalescence of incoming droplets, and promoting fast drainage. We find that the collection flow rate is constant and can be adjusted by varying the fibers arrangement and flexibility. We show that there is an optimal situation for which this collection rate, i.e. the global efficiency, is maximal due to an elastocapillary coupling that we further characterize with a model experiment. Specifically, we study the drainage between two flexible fibers. Depending on the geometry and the fiber deformations, several flow regimes are observed. We characterize these regimes, and discuss the consequences on the drainage velocity, and thus the collection efficiency. [Preview Abstract] |
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