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 A7: Microfluids: Interfaces and Wetting I |
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Chair: Kenny Breuer, Brown University Room: 329 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A7.00001: New thermal-sensitive superhydrophobic material Raphaele Thevenin, Zi Liang Wu, Patrick Keller, Robert E. Cohen, Christophe Clanet, David Quere Roughness of superhydrophobic surfaces plays a crucial role in the wetting properties of these surfaces. We propose to modify the roughness of a surface by applying external stimuli to change its wetting properties. In this spirit, we study superhydrophobic surfaces consisting of arrays of micro-pillars made with a liquid crystal elastomer. These liquid crystals change their orientation when heated, so that the height of the pillars decreases when the surface temperature increases; and this is perfectly reversible. We study the impact of such a thermal-actuation on the static and dynamic wetting properties of such surfaces and show superhydrophobicity of this new material can indeed be tuned using temperature stimuli. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A7.00002: A generalised view of high frequency substrate vibration induced wetting (Acoustowetting) Ofer Manor, Amgad Rezk, James Friend, Leslie Yeo High frequency surface vibrations, at frequencies comparable to the HF and VHF radio frequencies O(1--100 MHz), may be used for generating flow at the micron and submicron scale. Such high frequency vibrations are generated by piezoelectric actuators that transfer electric signals to kinetic energy, exciting different types of flow regimes when in contact with viscous fluids that are known in general as acoustic flow. Here we unravel a recently found wetting mechanism, observed in laboratory under the excitation of high frequency vibrations in the form of piston-like substrate motion and surface acoustic waves (SAWs). Wetting is excited by introducing acoustic flow layer of usually submicron thickness near the three phase contact line of liquid/solid systems. This wetting effect further gives rise to various peculiarities including film spreading at different directions according with periodic stability of the film thickness, formation of wave pulse trains, SAW diffraction induced film fingering, etc. We show high frequency vibration induced wetting is governed by a generalised theory that predicts the various physical peculiarities observed. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A7.00003: Dynamics of Wetting of Ultra Hydrophobic Surfaces Alireza Mohammad Karim, Jeong-Hyun Kim, Jonathan Rothstein, Pirouz Kavehpour Controlling the surface wettability of hydrophobic and super hydrophobic surfaces has extensive industrial applications ranging from coating, painting and printing technology and waterproof clothing to efficiency increase in power and water plants. This requires enhancing the knowledge about the dynamics of wetting on these hydrophobic surfaces. We have done experimental investigation on the dynamics of wetting on hydrophobic surfaces by looking deeply in to the dependency of the dynamic contact angles both advancing and receding on the velocity of the three-phase boundary (Solid/Liquid/Gas interface) using the Wilhelmy plate method with different ultra-hydrophobic surfaces. Several fluids with different surface tension and viscosity are used to study the effect of physical properties of liquids on the governing laws. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A7.00004: Dynamic wetting at the nanoscale Gustav Amberg, Yoshinori Nakamura, Andreas Carlson, Junichiro Shiomi Although the capillary spreading of a drop on a dry substrate is well studied, the physical mechanisms that govern the dynamics remain challenging. Here we study the dynamics of spreading of partially wetting nano-droplets, by combining molecular dynamics and continuum simulations. The latter accounts for all the relevant hydrodynamics, i.e. capillarity, inertia and viscous stresses. By coordinated continuum and molecular dynamics simulations, the macroscopic model parameters are extracted. For a Lennard-Jones fluid spreading on a planar surface, the liquid slip on the substrate is found to be crucial for the motion of the contact line. Evaluation of the different contributions to the energy transfer shows that the liquid slip generates dissipation of the same order as the bulk viscous dissipation or the energy transfer to kinetic energy. We also study the dynamics of spreading on a substrate with a periodic nanostructure. Here it is found that a nanostructure with a length scale commensurate with molecular size completely inhibits the liquid slip. This reduces the spreading speed by about 30{\%}. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A7.00005: Hard-sphere and inertial effects in colloidal liquid-gas systems Andreas Nold, Ben Goddard, Serafim Kalliadasis Hard-sphere effects at a liquid-gas contact line on chemically heterogeneous substrates are studied employing density functional theory. For dynamic colloidal liquid-gas systems, a novel extension of dynamic density functional theory (DDFT) to include inertial effects [1,2] is introduced and numerical results for the motion of colloidal droplets are presented. In particular, we present results for the motion of the contact line between a substrate, a colloidal liquid and a colloidal gas phase. The link between the modelling approaches for dynamic colloidal systems using DDFT and the Navier-Stokes equations for molecular systems is discussed. \\[4pt] [1] Goddard, Nold, Savva, Yatsyshin and Kalliadasis, 2013, Unification of dynamic density functional theory for colloidal fluids to include inertia and hydrodynamic interactions: derivation and numerical experiments, J. Phys.: Condens. Matter, 25, 035101; \\[0pt] [2] Goddard, Nold, Savva, Pavliotis, Kalliadasis, 2012, General dynamical density functional theory for classical fluids, PRL, 109, 120603. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A7.00006: Electrostatic line tension resulting from fluid-fluid interfacial deformation Aaron Doerr, Steffen Hardt We investigate the deformation of a fluid-fluid interface due to osmotic and electrostatic forces in the three-phase contact region between an electrolyte, a non-polar fluid, and a dielectric solid substrate with both semi-analytical and numerical methods. It is shown that the interfacial deformation decays exponentially as a function of distance from the three-phase contact line, consistent with a well-defined macroscopic contact angle. As a consequence and on a sufficiently large scale of observation, the physical situation may be modeled by extrapolating the macroscopic interfacial shape to the solid substrate, while the energetic contributions associated with the microscopic configuration near the three-phase contact line may be accounted for by an excess energy per unit length of contact line, an electrostatic line tension. A comparison of the semi-analytical model to numerical calculations is used to examine its limits of quantitative applicability. At the same time, it is demonstrated that beyond those limits the model still qualitatively agrees with the numerical results, corroborating its usefulness for understanding the physics close to the three-phase contact line on large as well as on small observation length scales. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A7.00007: ABSTRACT WITHDRAWN |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A7.00008: Velocity Slip on Curved Surfaces Weikang Chen, Rui Zhang, Joel Koplik The Navier boundary condition for velocity slip on flat surfaces, when expressed in tensor form, is readily extended to surfaces of any shape. We test this assertion using molecular dynamics simulations of flow in linear channels with flat and curved walls and for rotating cylinders and spheres, all for a wide range of solid-liquid interaction strengths. We find that the slip length as conventionally measured at a flat wall in Couette flow is the same as that for all other cases with curved and rotating boundaries, provided the atomic interactions are the same. These results support the idea that the slip length is a material property, transferable between different flow configurations. [Preview Abstract] |
Sunday, November 24, 2013 9:44AM - 9:57AM |
A7.00009: Measurements of a high-speed receding contact line on a hydrophobic surface Joonsik Park, Kenneth S. Breuer We report on the behavior of a non-equilibrium receding contact line generated by the rapid pinchoff of a liquid bridge extending between a capillary tube and a smooth hydrophobic substrate. The motion of the contact line is measured from below using a high-speed camera (10kfps). Three stages are identified according to the power-law scaling between the droplet radius and the receding time. In the initial non-equilibrium phase, the power-law exponent scales with the retraction speed of the tube. As the contact angle approaches the minimum receding angle, the contact line retreats according to a universal power-law exponent. In the final stage, close to the pinch-off, the receding contact angle increases to the minimum equilibrium contact angle and the power-law exponent decreases. The variation of the behavior is measured as a function of the fluid properties (viscosity, elasticity and surface tension), the tube retraction speed, and the substrate properties (hydrophobicity and surface roughness). [Preview Abstract] |
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