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 L6: Microfluidics: Capillary I |
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Chair: Pirouz Kavehpour, University of California, Los Angeles Room: 24B |
Monday, November 19, 2012 3:35PM - 3:48PM |
L6.00001: Breakup of liquid filaments on a partially wetting solid substrate Giovanni Ghigliotti, Chunfeng Zhou, James J. Feng We report direct numerical simulations of liquid filaments breaking up into droplets on partially wetting substrates. It is motivated by recent experiments, linear stability analyses, and lubrication-based calculations. The fluid flow is governed by the Stokes equations and the contact line motion is handled by a phase-field model, which also serves to capture the interfacial motion. The coupled Stokes and Cahn-Hilliard equations are solved using a finite-element algorithm in three dimensions. This avoids additional approximations of the fluid flow or contact line motion, and allows us to consider arbitrary contact angles $0\leq\theta\leq\pi$ on the substrate. We simulate both the breakup of infinite liquid filaments via growing capillary waves and that of finite liquid filaments with drops pinching off from the ends. The results show the differences in the two processes and in the final drop size, spacing and polydispersity, as well as the dependence of these two phenomena on the value of the contact angle with the substrate. The development of capillary waves agrees well with prior linear analysis and the end-pinching results offer new insights into this poorly understood phenomenon. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L6.00002: Movement of a droplet on a structured substrate: a dissipative particle dynamics simulation Guohui Hu, Yi Yao The last decade has witnessed the explosive development of microfluidic systems. They allow the integration of various steps of chemical or biological analyses on a single chip, which significantly minimize reagent volume and material cost. Droplet manipulation is one of the crucial technologies in microfluidic devices design and optimization. The wettability gradient employs an imbalance of surface tension on the circumference of a droplet base, thus it can be applied to actuate a droplet on a substrate in the direction of wetting gradient. In the present study, dissipative particle dynamics is applied to investigate movement of a liquid droplet actuated on a structured substrate. The wetting property ranging from hydrophilic to hydrophobic is implemented by adjusting the coefficient of solid-liquid attraction, which results in variation of solid-liquid surface tension. The structure of internal flow is analyzed based on numerical results, as well as the movement of the contact line. The influences of wettability gradient, thermal fluctuation are intensively discussed, as well as the wall structures. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L6.00003: Investigation of drag reduction properties of liquid impregnated micro-textured surfaces Vamsi Spandan, Gareth McKinley, Sarit Das The liquid repelling and drag reducing properties of superhydrophobic surfaces are attributed to microscopic pockets of air trapped in between topographical structures with low surface energy. In this work we numerically investigate the drag reduction properties of textured surfaces which are impregnated with an immiscible liquid lubricant instead of air. Although these surfaces outperform conventional superhydrophobic surfaces in dynamic characteristics such as droplet movement (due to lack of contact line pinning), the question whether such surfaces can exhibit drag reduction when immersed in a fluid remains open. We employ a continuum approach (using Volume of Fluid simulations) to investigate changes in skin friction on such surfaces. This method enables the simulation of two incompressible, immiscible fluids with the inclusion of capillary effects between the phases and specification of a finite contact angle between the phases and the textured wall. We study the effect of viscosity ratio, interfacial tension, and topography of the microtextures on the drag reducing properties of such liquid impregnated surfaces. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L6.00004: Wetting transitions on silicon nanowires with different surface functionalizations XiuMei Xu, Guy Vereecke, Erik van den Hoogen, Jens Smeers, Silvia Armini, Tinne Delande, Herbert Struyf The wetting property of an ideal smooth surface is well described by Young's equation, on the other hand, the wetting behavior on a patterned substrate can be quite different due to the topographic influence. To date the transition from Wenzel or hemi-wicking state to the metastable Cassie-Baxter wetting state is still an active research topic. In literature the patterned substrates have a typical feature size in the micrometer scale, and it has been found that although a complete wetting is achievable on such dimensions, there can be a Cassie state of wetting in mesoscopic scale due to the air trapping by the nanometric roughness on the features. In this work we investigate the wetting properties of nano-patterned substrates. Silicon nanopillars (30-40 nm in diameter and 90 nm in pitch) are fabricated with different heights ranging from 70 to 450 nm, and different treatments are applied to functionalize the pillar surface to have a good coverage of water contact angles. Three wetting regimes are observed: hemi-wicking state for contact angles lower than 50 degrees, Cassie-Baxter state for contact angles larger than 80 degrees, and in between there is a sharp transition with the apparent contact angles increasing from 30 to 150 degrees. Wenzel model does not agree with the measurements for the entire range of the contact angle, indicating the wetting mechanism might be very different in the nanometer scale. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L6.00005: Wetting and spreading behaviors of impinging microdroplets on textured surfaces Dae Hee Kwon, Sang Joon Lee Textured surfaces having an array of microscale pillars have been receiving large attention because of their potential uses for robust superhydrophobic and superoleophobic surfaces. In many practical applications, the textured surfaces usually accompany impinging small-scale droplets. To better understand the impinging phenomena on the textured surfaces, the wetting and spreading behaviors of water microdroplets are investigated experimentally. Microdroplets with diameter less than 50 \textit{$\mu $}m are ejected from a piezoelectric printhead with varying Weber number. The final wetting state of an impinging droplet can be estimated by comparing the wetting pressures of the droplet and the capillary pressure of the textured surface. The wetting behaviors obtained experimentally are well agreed with the estimated results. In addition, the transition from bouncing to non-bouncing behaviors in the partially penetrated wetting state is observed. This transition implies the possibility of withdrawal of the penetrated liquid from the inter-pillar space. The maximum spreading factors (ratio of the maximum spreading diameter to the initial diameter) of the impinging droplets have close correlation with the texture area fraction of the surfaces. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L6.00006: Precipitation of salt in saline water drop on superhydrophobic surface Bongsu Shin, Myoung-Woon Moon, Ho-Young Kim In the membrane distillation process, water vapor of heated, pressurized saline water is transported across the membrane to be collected as pure water. While the water-repellency of the membrane surface has been considered an important parameter affecting the distillation efficiency, the resistance of the membrane to the contamination due to salt has gathered little scientific interest thus far. Here we experimentally investigate the precipitation of salt in sessile saline water drops, to find drastic differences in salt crystallization behavior depending on the water-repellency of solid surface. On a moderately hydrophobic surface with a static contact angle with water being about 150 degrees, salt crystals are aligned and stacked along the initial contact line, forming an interesting structure resembling an igloo. On a superhydrophobic surface with about 164 degrees of static contact angle with water, salt crystallizes only at the center of the drop-solid contact area, forming a pebble-shaped structure. We explain this difference by comparing the evaporation modes (constant contact radius versus constant contact angle) of the sessile drops on those surfaces. We also visualize the liquid flow within drops undergoing evaporation and precipitation at the same time using PIV. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L6.00007: Pressure and Heating Effects on Superhydrophobic Friction Reduction Tae Jin Kim, SungYun Hann, Carlos Hidrovo Slip in internal flows is known to reduce friction and thus decrease the required pumping power. One method to achieve slip is by roughening the surface to induce Cassie state (a phenomenon in which a liquid rests on top of a rough surface with a gas layer formed underneath). While most work in this area has concentrated on optimizing the surface microtexturing geometry to maximize the friction reduction effects, less attention has been paid to the effects of partially wetted conditions. Our research goal is to control/track the air-water interface location within the roughness elements and study the interface effects on the microchannel friction. The frictional behavior of the flow suggests that (1) the air-water interface resembles closer to a no-slip boundary than a shear-free one, (2) the friction is rather insensitive to the degree of microtexturing wetting, and (3) the fully wetted microtexturing provides lower friction than the non-wetted ones. In accordance to the high frictional nature of the air-water interface, the effective slip length was varied by controlling the location of the air-water interface through heating. Results have shown that the Cassie state can be maintained at higher pressures through heating, but the flow may be pinched if excessive heat is applied to the microchannel. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L6.00008: Facile fabrication of super-hydrophobic surfaces with 3D pillar structures Shengjie Zhai, Hui Zhao, Yingtao Jiang Super-hydrophobic surfaces have attracted growing interest due to their unique properties, including drag reduction, facilitation of heat transfer, self-cleaning, anti-corrosion, anti-sticking, and anti-contamination. However, the method of fabricating super-hydrophobic surfaces with regular 3D micro/nano pillars structures is still complicated. Here we present a simple, reliable, and low-cost fabrication method which can create complex 3D structures. Briefly, the commercial nanostamping products like CD, DVD,and bluray disc serve as the PDMS mold The pit size (LxWxH) of CD, DVD, and Blueray is 0.8$\mu$m x 0.15$\mu$m x 0.1$\mu$m, 0.4$\mu$m x 0.15$\mu$m x 0.1$\mu$m, and 0.15$\mu$m x 0.15$\mu$m x 0.1$\mu$m. The PDMS surface with the relevant structures can be directly replicated from the molds by the soft lithography technology. The precise geometric structures including height, width, and density of pillar arrays can be readily controlled by using different optical discs. The contact angle is measured about 136-140 degree. We also study the relationship between the contact angle and different feature size. Finally, we measure the slip length for different structures [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L6.00009: Friction Reduction and Robustness for Laminar Fluid Flow on Spray-Coated Superhydrophobic Mesh Surfaces Siddarth Srinivasan, Wonjae Choi, Kyoo-Chul Park, Shreerang Chhatre, Robert Cohen, Gareth McKinley ~We measure the effective Navier slip length for flow over a liquid-repellent non-wetting surface (fabricated using a spray-deposition technique) which supports a composite solid-liquid-air interface. The morphology of the hydrophobic textured substrate consists of randomly distributed corpuscular microstructures that stabilize a layer of trapped air upon immersion in liquid. The reduction in viscous skin-friction due to this ``plastron layer'' is evaluated using torque measurements in a parallel plate rheometer, and results in measured slip lengths of b$_{slip}\approx $40 $\mu $m, that are comparable to the mean periodicity of the microstructure. The use of dual-textured spray-coated woven meshes increases the magnitude of the effective slip length to between b$_{slip} \approx $ 90 $\mu $m to 200 $\mu $m depending on the mesh dimensions. We compute the wetted-solid fraction $\varphi _{s}$ from surface evolver simulations, and we demonstrate that the experimentally obtained slip-lengths are consistent with the logarithmic prediction of Davis {\&} Lauga. Finally we define a robustness parameter (A$^{\ast })$ to quantify the stability of the plastron. And illustrate the inverse correlation between A$^{*}$~and b$_{slip}$ by means of a design chart. [Preview Abstract] |
Monday, November 19, 2012 5:32PM - 5:45PM |
L6.00010: Spreading of Emulsions on Glass Substrates Alireza Mohammad Karim, Pirouz Kavehpour The wettability of emulsions is an important factor with explicit influence in an extensive variety of industrial applications ranging from the petroleum to food industries. Surprisingly, there is no comprehensive study of emulsion spreading to date; this is due to the complexity of the structure of the emulsions and non-homogeneity of the dispersed phase bubbles in size as well as distribution through the emulsion. The spreading of water/silicone oil emulsions on glass substrates was investigated. The emulsions were prepared with varying volume fractions of water dispersed in silicone oil, with addition of small amounts of surfactant to stabilize the emulsion structure. The time dependent variation of dynamic contact angle, base diameter, and the spreading rate of the droplets of an emulsion are different from a pure substance. The effect of water/silicone oil weight percentage as well as the droplet size and dispersed phase bubble size were also investigated. The weight percentage of water/silicone oil emulsion and droplet size did not have significant influence on the spreading dynamics; however the dispersed phase drop size affected the spreading dynamics substantially. [Preview Abstract] |
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