Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session M22: Microscale Flows: Interfaces and Wetting |
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Chair: Ying Sun, Drexel University Room: E141/142 |
Tuesday, November 22, 2016 8:00AM - 8:13AM |
M22.00001: Effect of Microstructure and Wettability on Liquid Delivery in Structured Surfaces Diyuan Zong, Arif Rokoni, Yuanyuan Duan, Ying Sun Liquid delivery rate in structured surfaces contributes greatly to the enhancement in critical heat flux during boiling and thin film evaporation. In the present study, the effect of microstructure and wettability on the liquid delivery rate has been investigated using molecular dynamics (MD) simulations for droplet spreading on nanostructured surfaces and liquid infiltration in nanopores. The wicked volume flux, a measure of the liquid delivery rate during droplet spreading, is found to increase with Wenzel roughness ratio regardless of the microstructure and this finding is supported by existing experiments. In addition, the effects of long-range intermolecular interactions and external electric field on liquid delivery rate are investigated. Long-range intermolecular interaction has been found to have negligible effect on liquid delivery rate. Both the wicking length in droplet spreading and the infiltration length of liquid infiltration in nanopores scale with t\textasciicircum (1/2), consistent with the classic theory. The effects of the nanostructure depth, width-to-spacing ratio, wettability, and external electric field on the wicking coefficient have been studied and the results can be used to guide structured surface designs for enhanced liquid delivery. [Preview Abstract] |
Tuesday, November 22, 2016 8:13AM - 8:26AM |
M22.00002: The failure of a superhydrophobic surface under external flow Ying Liu, Matthew Fu, Marcus Hultmark, Alexander Smits, Howard Stone The advantages of superhydrophobic surfaces (SHS), such as ultra water-repellency, drag reduction and enhanced heat transfer, rely on the existence of the air trapped inside the surface geometries. Thus, it is important to study the failure of SHS, i.e., how the air-filled cavities are filled with water. Most of the previous work on this topic focuses on static pressure-driven failure. Here, we study experimentally the dynamic failure of SHS under an external flow. Conditions leading to failure are identified. The effects of both the pressure and the shear from the external flow on the failure of SHS are discussed. [Preview Abstract] |
Tuesday, November 22, 2016 8:26AM - 8:39AM |
M22.00003: Absence of molecular slip on ultraclean and SAM-coated surfaces Justin Pye, Clay Wood, Justin Burton The liquid/solid boundary condition is a complex problem that is becoming increasingly important for the development of nanoscale fluidic devices. Many groups have now measured slip near an interface at nanoscale dimensions using a variety of experimental techniques. In simple systems, large slip lengths are generally measured for non-wetting liquid/solid combinations, but many conflicting measurements and interpretations remain. We have developed a novel pseudo-differential technique using a quartz crystal microbalance (QCM) to measure slip lengths on various surfaces. A drop of one liquid is grown on the QCM in the presence of a second, ambient liquid. We have isolated any anomalous boundary effects such as interfacial slip by choosing two liquids which have identical bulk effects on the QCM frequency and dissipation in the presence of no-slip. Slip lengths are --less than 2 nm-- for water (relative to undecane) on all surfaces measured, including plasma cleaned gold, SiO2, and two different self assembled monolayers (SAMs), regardless of contact angle. We also find that surface cleanliness is crucial to accurately measure slip lengths. Additionally, clean glass substrates appear to have a significant adsorbed water layer and SAM surfaces show excess dissipation, possibly associated with contact line motion. In addition to investigating other liquid pairs, future work will include extending this technique to surfaces with independently controllable chemistry and roughness, both of which are known to strongly affect interfacial hydrodynamics. [Preview Abstract] |
Tuesday, November 22, 2016 8:39AM - 8:52AM |
M22.00004: Apparent and Actual Dynamic Contact Angles in Confined Two-Phase Flows Takeshi Omori, Takeo Kajishima To accurately predict the fluid flow with moving contact lines, it has a crucial importance to use a model for the dynamic contact angle which gives contact angles on the length scale corresponding to the spacial resolution of the fluid solver. The angle which a moving fluid interface forms to a solid surface deviates from an actual (microscopic) dynamic contact angle depending on the distance from the contact line and should be called an apparent (macroscopic) dynamic contact angle. They were, however, often undistinguished especially in the experimental works, on which a number of empirical correlations between a contact angle and a contact line velocity have been proposed. The present study is the first attempt to measure both apparent and actual contact angles from the identical data sets to discuss the difference and the relationship between these two contact angles of difference length scales. The study is conducted by means of numerical simulation, solving the Navier-Stokes equation and the Cahn-Hilliard equation under the generalized Navier boundary condition for the immiscible two-phase flow in channels. The present study also illustrates how the system size and the physical properties of the adjoining fluid affect the apparent and the actual dynamic contact angles. [Preview Abstract] |
Tuesday, November 22, 2016 8:52AM - 9:05AM |
M22.00005: From catastrophic acceleration to deceleration of liquid plugs in prewetted capillary tubes. Juan Magniez, Michael Baudoin, Farzam Zoueshtiagh Liquid/gas flows in capillaries are involved in a multitude of systems including flow in porous media, petroleum extraction, imbibition of paper or flows in pulmonary airways in pathological conditions. Liquid plugs, witch compose the biphasic flows, can have a dramatic impact on patients with pulmonary obstructive diseases, since they considerably alter the circulation of air in the airways and thus can lead to severe breathing difficulties. Here, the dynamics of liquid plugs in prewetted capillary tube is investigated experimentally and theoretically, with a particular emphasis on the role of the prewetting films and of the driving condition (constant flow rate, constant pressure). For both driving conditions, the plugs can either experience a continuous increase or decrease of their size. While this phenomenon is regular in the case of imposed flow rate, a constant pressure head can lead to a catastrophic acceleration of the plug and eventually its rupture or a dramatic increase of the plug size. A theoretical model is proposed to explain the transition between theses two regimes. These results give a new insight on the critical pressure required for airways obstruction and reopening. [Preview Abstract] |
Tuesday, November 22, 2016 9:05AM - 9:18AM |
M22.00006: Liquid/liquid/solid contact angles Marine Borocco, Charlotte Pellet, Jean-René Authelin, Christophe Clanet, David Quéré Many studies have investigated solid/liquid/air interfaces and their corresponding wetting properties. We discuss what happens in less-studied liquid/liquid/solid systems, and focus on questions of dynamical wetting in a tube, having in mind applications in detergency. We use a capillary tube filled with water and containing a slug of silicone oil (or vice-versa), and present a series of experiments to determine static and dynamic wetting properties corresponding to this situation. We also discuss interfacial aging of such systems. [Preview Abstract] |
Tuesday, November 22, 2016 9:18AM - 9:31AM |
M22.00007: Dynamic wetting of a liquid film in a vertical hydrophobic tube Franck Pigeonneau, Pascaline Hayoun, Etienne Barthel, Francois Lequeux, Emilie Verneuil, Alban Letailleur, Jeremie Teisseire The drop of a liquid plug through a tube occurs for instance in vending machine. In such a system, the fouling is linked to the creation of the liquid film at the rear of the liquid plug. Consequently, the conditions leading to the film creation are important to know. We study numerically the dynamic wetting transition of a liquid plug undergoing gravity on hydrophobic surface in a vertical tube. Using a lubrication theory, the liquid film thickness obeys the mass conservation equation with a volume flow rate depending on the relative motion of the tube, capillary and gravity forces. An ad hoc friction at the triple line is used to take into account the wetting dynamics. The lubrication equation is solved using a finite difference technique in space and a time integrator for stiff system with an adaptive time step. The numerical results are compared to experimental data. The complex film morphology due to the transients and the critical slowing down at the dynamic transition are reproduced. However, several experimental features are not predicted numerically especially the width of the transition. Our preliminary calculations suggest that the dispersion relation of the liquid film mode can explain the discrepancy. [Preview Abstract] |
Tuesday, November 22, 2016 9:31AM - 9:44AM |
M22.00008: ABSTRACT WITHDRAWN |
Tuesday, November 22, 2016 9:44AM - 9:57AM |
M22.00009: Comparison of the Cahn-Hilliard-Navier-Stokes and Molecular Dynamics Approaches for the Simulation of Droplet Coalescence and Wetting Phenomena Urbain Vaes, Benjamin Aymard, Srikanth Ravipati, Petr Yatsyshin, Amparo Galindo, Serafim Kalliadasis Diffuse-interface/Cahn-Hilliard equations, coupled to Navier-Stokes (CHNS), have been used extensively over the last few years in fluid dynamics including interfacial phenomena in multiphase systems. Applications range from turbulent two-phase flows to rheological systems and microfluidic devices. But despite the considerable attention CHNS have received, little work has been undertaken to investigate the extent to which they agree with ``first-principles" physical models such as those provided by molecular dynamics (MD). Here we compare MD simulations with solutions of the CHNS system obtained numerically using an efficient and systematic finite-element methodology we have developed recently. For this purpose, we consider two paradigmatic model systems: droplet coalescence and droplet motion on a substrate with varying wettability. [Preview Abstract] |
Tuesday, November 22, 2016 9:57AM - 10:10AM |
M22.00010: Rayleigh-Plateau instability of slipping viscous filaments in v-shaped grooves Martin Brinkmann, Tak Shing Chan, Ralf Seemann Since the seminal works of Rayleigh and Plateau on the break-up of free-standing liquid jets, a large number of studies have addressed capillary instability of cylindrical interfaces in various settings. Here, we report the numerical results of a linear stability analysis of cylindrical liquid filament wetting v-shaped grooves employing a boundary element formalism. It is found that slip affects the wavelength $\lambda^{\rm max}$ of the fastest growing mode whenever the transverse dimension $W$ of the filaments is comparable, or smaller than the Navier slip-length $B$. The corresponding timescale of the decay, $\tau^{max}$, grows logarithmically with increasing $B/W$. In the opposite limit $B/W \ll 1$, however, $\lambda^{max}$ grows unboundedly with increasing $B/W$ while $\tau^{max}$ saturates to a finite lower bound, similar to the situation observed for free-standing viscous liquid cylinders in the absence of inertial effects. Long wavelength approximations of the flows for $B/W \ll W$ and $B/W\gg 1$ are in good agreement with the numerical results only for contact angles $0<\theta-\psi \ll 1$ where the neutrally stable wavelength $\lambda^\ast<\lambda^{max}$ is large compared to the transverse filament dimension $W$. [Preview Abstract] |
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