Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session ES: Drops V |
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Chair: Alberto Aliseda, University of Washington Room: Long Beach Convention Center Grand Ballroom A |
Sunday, November 21, 2010 4:10PM - 4:23PM |
ES.00001: Dynamics of water droplet on a superhydrophobic carbon nanotube array Adrianus Aria, Christina Shu, Anirban Ghosh, Morteza Gharib Among diverse types of superhydrophobic materials, arrays of vertically aligned carbon nanotube have attracted significant attention, mainly because of their exceptional properties. In this study, we look at the dynamic behavior of water droplet upon impact on the carbon nanotube array and subsequent primary rebound at wide range of Weber number. At small Weber number, the water droplet deforms and bounces off completely of the array, while at large Weber number, the water droplet splashes with fingering patterns. Our study shows that no pinning of the water droplet is observed on the array at both small and large Weber number, confirming that the water droplet does not have the capability to wet the array even at high impact velocity. In addition, the coefficient of restitution and spread factor of the bouncing and splashing water droplet will be discussed, along with the critical Weber numbers which predict when the droplet start to split and splash. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
ES.00002: Spreading and breakup of a compound drop over a partially wetting substrate James J. Feng, Peng Gao The spreading of a encapsulated compound drop over a partially wetting solid substrate is numerically simulated using a diffuse-interface method. Compared with a single-phase drop, the spreading of a compound drop can exhibit much more complex behavior. Depending on the radius ratio and the contact angle, three flow regimes of interfacial morphology evolutions are identified. The core can remain suspended inside the outer drop or attaches onto the substrate if the outer interface does not rupture during the spreading. Otherwise, the compound drop spontaneously breaks up and releases the inner drop into the ambient fluid. A series of breakup scenarios are observed depending on the location of the initial rupture. In some regimes, the spreading and breakup can generate secondary drops, which can either attach to the substrate or stay away. The viscosity ratio mainly affects the spreading rate, and plays a secondary role in the morphology evolution. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
ES.00003: Droplets on shallow grooved hydrophobic surfaces Olesya Bliznyuk, Vasilisa Veligura, Stefan Kooij, Bene Poelsema The equilibrium shape of water droplets on shallow-grooved hydrophobic surfaces is studied experimentally. The dependence of the two final states, notably metastable Cassie-Baxter and Wenzel, on the underlying geometric pattern is analyzed and discussed. The anisotropy of the patterns, including variation of the relative groove and ridge widths, allows studying the influence of different mechanisms of spreading in orthogonal directions and geometrical parameters on the final shape of the droplets. The validity of the Cassie-Baxter and Wenzel models in case of anisotropic surfaces is investigated, comparing the experimental data with theoretical predictions in the two respective regimes. The transition from one regime to another for different ridge width is discussed in relation to existing literature on this subject. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
ES.00004: Effects of the Secondary Length Scale on Hierarchical Superhydrophobic Surfaces Fabricated by Double-Layer Electron Beam Lithography Jiansheng Feng, Jonathan Rothstein Surface topology is a key to superhydrophobicity. Many superhydrophobic surfaces found in nature have more than one characteristic roughness length scales. Very often the primary length scale is on the order of 10$\mu $m and the secondary length scale is on the order of 100nm. The secondary length scale is thought to play a key role in the stability and hysteresis of the hydrophobic surface. In our study, a novel method, double-layer electron beam lithography on SU-8 followed by surface silanization on thermally deposited silicon dioxide coating, is used to fabricate superhydrophobic surfaces with well-ordered and controllable secondary length scale patterns. The feature size and spacing of the secondary patterns are varied to study their effects. The size and spacing of the primary scale features will also be varied independently, as well as the surface chemistry. Results of contact angle measurements and hysteresis will be presented. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
ES.00005: The effect of contact angle hysteresis on droplet motion and collisions on superhydrophobic surfaces Michael Nilsson, Jonathan Rothstein The effect of varying the contact angle hysteresis of a superhydrophobic surface on the characteristics and dynamics of water droplet motion and their subsequent collision are investigated using a high-speed camera. The surfaces are created by imparting random roughness to Teflon through sanding. With this technique, it is possible to create surfaces with similar advancing contact angles near 150 degrees, but with varying contact angle hysteresis. This talk will focus on a number of interesting experimental observations pertaining to drop dynamics along a surface with uniform hysteresis, drop motion along surfaces with transition zones from one hysteresis to another, and the collision of droplets on surfaces of uniform hysteresis. For single drop studies, gravity is used as the driving force, while the collision studies use pressurized air to propel one drop into the other. For the case of droplet collision, the effect of hysteresis, Weber number, and impact number on the maximum deformation of the drops, and the post-collision dynamics will be discussed. For the single droplet measurements, the resistance to motion will be characterized as well as the transition from rolling to sliding as a function of drop size, inclination angle, and hysteresis. Additionally, we will quantify the effect of surface transitions on the resulting motion, mixing, and deflection of the drops. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
ES.00006: Wetting and Dewetting on Superhydrophobic Surfaces with Two-Tier Roughness Jonathan Boreyko, Chuan-Hua Chen Many natural superhydrophobic structures, such as the lotus leaf, demonstrate hierarchal two-tier roughness. The hierarchal roughness is empirically known to promote robust superhydrophobicity, but the mechanism is still under debate. Here, we report the wetting and dewetting properties of two-tier roughness as a function of the wettability of the working fluid, where the surface tension of the water/ethanol mixture is tuned by the mixing ratio. On both natural and synthetic two-tier surfaces, externally deposited drops of increasing ethanol concentration exhibit two distinct wetting transitions, first for the impalement of the microscale texture and then for the nanoscale. The impaled drops are subsequently subjected to vibration-induced dewetting [1]. Drops impaling only the micro-scale roughness exhibited a metastable superhydrophobicity, as sufficient vibrational energy can enable a complete dewetting with no residual drops. In contrasct, drops impaling both the micro and nano-scale roughness can not be completely dewetted. Our work suggests that the nanoscale roughness is essential for preventing catastrophic, irreversible wetting of superhydrophobic surfaces. \\[4pt] [1] J.B. Boreyko and C.H. Chen, Phys. Rev. Lett. \textbf{103,} 174502 (2009). [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
ES.00007: Dynamic wetting and hysteresis on superhydrophobic surfaces: an experimental observation of contact line motion Adam Paxson, Katherine Smyth, Hyuk-min Kwon, Kripa Varanasi Contact angle and width are sampled at a high frequency to quantify advancing and receding behavior. As the contact angle increases, the contact line moves smoothly along the surface. As the contact angle recedes, instead of approaching a steady value, a stick-slip behavior occurs. The contact line sticks on the micro-pillars and forms capillary bridges, and slips when the bridges are stretched and then ruptured. The frequency of contact angle stick-slip behavior increased with contact line velocity. For the range of velocities tested, contact line velocity is not dependent upon pillar density, and does not appear to have an effect on measured contact angle values. This model of the moving contact line is verified by images captured using multiple methods. First, a silica nanoparticle solution is imaged under high magnification to observe contact line behavior during volume addition and subtraction. Additionally, the contact line of a sliding droplet is imaged with environmental scanning electron microscopy. This paper experimentally establishes for the first time advancing and receding behavior on micro-textured surfaces, and investigates the dependence of this behavior on contact line velocity. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
ES.00008: Droplet growth and coalescence on nanostructured surfaces during condensation Ryan Enright, Matthew McCarthy, Benjamin Hatton, Evelyn Wang In this work, we investigated the condensation behavior of water on nanostructured surfaces fabricated using a self-assembled virus template resulting in typical feature dimensions of 40 nm. These surfaces were first functionalized with a hydrophobic silane coating and, subsequently, some of the surfaces were selectively coated with hydrophilic PVA to create a chemically heterogeneous surface. The condensation process of water on these surfaces was characterized by microscopic imaging of the droplet growth behavior. The dynamics of energetic droplet coalescence events were obtained using high-speed imaging. Condensation on both the chemically homogenous and heterogeneous surfaces showed a preference for the unpinned Cassie droplet wetting mode. However, observed differences between the chemically homogenous and heterogeneous surfaces in both droplet growth and coalescence behavior demonstrate the effects of locally lowered nucleation energy barriers and increased droplet adhesion. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
ES.00009: Preferential Condensation of Water Droplets Using Hybrid Hydrophobic-Hydrophilic Surfaces Kripa Varanasi, Tao Deng, Adam Paxson, Rajeev Dhiman Heterogeneous vapor-to-liquid nucleation of water is an everyday phenomenon and plays an important role in the formation of rain drops, dew, heat transfer, water recovery, etc. Classical nucleation theory predicts that an energy barrier that depends strongly on the intrinsic wettability of the surface has to be overcome for the formation of initial liquid nuclei. Since the intrinsic wettability of regular surfaces is spatially uniform, heterogeneous nucleation of water droplets seems to occur in a random fashion without any particular spatial preference. This effect accounts for the recent observations on the loss of superhydrophobic properties of lotus leaves and associated synthetic surfaces under condensation. By taking advantage of the strong dependence of the nucleation energy barrier on wettability, we show for the first time that heterogeneous nucleation can be spatially controlled by the manipulation of the local intrinsic wettability of a surface. Using an environmental scanning electron microscope, we show that water droplets preferentially nucleate on the hydrophilic regions of the hybrid hydrophobic-hydrophilic surfaces we fabricated. Such ability to control water nucleation could address the condensation-related limitations of superhydrophobic surfaces and has implications for efficiency enhancements in energy, water, and electronics cooling systems. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
ES.00010: Deceleration-driven wetting transition of ``gently'' deposited drops on textured hydrophobic surfaces Kripa Varanasi, Hyukmin Kwon, Adam Paxson, Neelesh Patankar Many applications of rough superhydrophobic surfaces rely on the presence of droplets in a Cassie state on the substrates. A well established understanding is that if sessile droplets are smaller than a critical size, then the large Laplace pressure induces wetting transition from a Cassie to a Wenzel state, i.e., the liquid impales the roughness grooves. Thus, larger droplets are expected to remain in the Cassie state. In this work we report a surprising wetting transition where even a ``gentle'' deposition of droplets on rough substrates lead to the transition of larger droplets to the Wenzel state. A hitherto unknown mechanism based on rapid deceleration is identified. It is found that modest amount of energy, during the deposition process, is channeled through rapid deceleration into high water hammer pressure which induces wetting transition. A new ``phase'' diagram is reported which shows that both large and small droplets can transition to Wenzel states due to the deceleration and Laplace mechanisms, respectively. This novel insight reveals for the first time that the attainment of a Cassie state is more restrictive than previous criteria based on the Laplace pressure transition mechanism. [Preview Abstract] |
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