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 M8: Microscale Flows: Interfaces and Wetting |
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Chair: Hooman Tafreshi, Virginia Commonwealth University Room: 108 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M8.00001: Transition States for Submerged Superhydrophobic Surfaces: Partially-Pinned Air-Water Interface Hooman Tafreshi, Ahmed Hemeda The pressure at which a superhydrophobic surface transitions from the Cassie state to the Wenzel state is often referred to as the critical pressure. Our mathematical simulations have shown that the Cassie-to-Wenzel transition is a gradual process that takes place over a range of pressures as oppose to an event that happens at a certain pressure. During the transition period, the air-water interface may go through a series pinned, partially-pinned, and de-pinned states that depend on the geometry of the surface asperities. This in turn indicates that the drag-reduction effect produced by a submerged superhydrophobic surface can vary with the hydrostatic pressure, and is highly dependent on sharpness of the surface asperities. The study reported here reviews our recent discoveries in simulating the wetted area and drag reduction effect of superhydrophobic surfaces with different microstructures. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M8.00002: Instantaneous Slip Length in Superhydrophobic Microchannels Ahmed Hemeda, Hooman Tafreshi Superhydrophobic (SHP) surfaces can be used to reduce the skin-friction drag in a microchannel. This favorable effect, however, can deteriorate over time if the surface geometry is not designed properly. This study presents a mathematical means for studying the time-dependent drag-reduction in a microchannel enhanced with SHP grooves of varying geometries. The performance of an SHP groove is found to be dependent on the interplay between the effects of the apparent contact angle of the air-water interface and the initial volume of the groove. The instantaneous slip length is calculated by solving the Navier--Stokes equations for flow in a microchannel with such SHP grooves, and the results are compared with the studies in the literature. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M8.00003: Predicting apparent slip at liquid-liquid interfaces without an interface slip condition Pietro Poesio, Angelo Damone, Omar Matar We show that if we include a density-dependent viscosity into the Navier-Stokes equations then we can describe, naturally, the velocity profile in the interfacial region, as we transition from one fluid to another. This requires knowledge of the density distribution (for instance, via Molecular Dynamics [MD] simulations, a diffuse-interface approach, or Density Functional Theory) everywhere in the fluids, even at liquid-liquid interfaces where regions of rapid density variations are possible due to molecular interactions. We therefore do not need an artificial interface condition that describes the apparent velocity slip. If the results are compared with the computations obtained from MD simulations, we find an almost perfect agreement. The main contribution of this work is to provide a simple way to account for the apparent slip at liquid-liquid interfaces without relying upon an additional boundary condition, which needs to be calculated separately using MD simulations. Examples are provided involving two immiscible fluids of varying average density ratios, undergoing simple Couette and Poisseuille flows. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M8.00004: Slip length measurement using BBM Adib Ahmadzadegan, Craig Snoeyink We will be presenting experimental characterizations of slip lengths of fluids in nano/micro channels. These channels are becoming increasingly important in sensor and separations applications. However, crucial questions still remain on the mechanisms that govern slip-length behavior. We used Bessel Beam microscopy (BBM), a novel super-resolution imaging system, in conjunction with TIRF system. These two, together led us to be able to do Particle Tracking Velocimetry with significantly higher accuracy than previously possible. We will be presenting results demonstrating the feasibility of this approach and advantages that make this method unique. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M8.00005: Transient coating of substrates with variable topography by viscous films Nikos Lampropoulos, Yiannis Dimakopoulos, John Tsamopoulos We study the transient coating of substrates exhibiting orthogonal trenches. We use the VoF method via OpenFOAM to solve the transient NS eqs on an unstructured grid, which dynamically undergoes local refinement around the interfaces. An Euler implicit method is used with adjustable time-step. The computational cost is reduced by parallel execution via MPI. Completely different wetting patterns result depending on the 3 dimensions of the topography, the capillary and Reynolds numbers and the dynamic contact angle. On one hand, continuous coating can be achieved in which the thin film of fluid wets the entire trench, while a steady flow is established upstream and downstream the topography. This is the desirable pattern in coating microelectronic devices for their protection and planarization. The other extreme possibility is that the film completely bypasses the trench, entrapping air inside it. This pattern reduces the drag coefficient on the film and, therefore, it is desirable in super-hydrophobic surfaces for microfluidic applications. Between these two extremes, a large variety of patterns exists in which the film partially wets the trench forming an air inclusion all along its bottom surface or its upstream or downstream inner corners or the film may breakup periodically. We produce comprehensive maps of film configurations covering a wide range of parameter values. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M8.00006: Hydrodynamic fundamentals of slippage over a superhydrophobic surface Clarissa Sch\"onecker, David Sch\"affel, Kaloian Koynov, Doris Vollmer, Hans-J\"urgen Butt Water easily slips over superhydrophobic surfaces, making such surfaces attractive for the development of functional coatings. While the global behavior of flow past superhydrophobic surfaces has been widely investigated, the local physical fundamentals leading to slippage still remain unclear. Using fluorescence correlation spectroscopy, we performed detailed measurements of the local slip length for water in the Cassie state on a structured superhydrophobic surface. In combination with numerical calculations of the flow, we revealed that the local slip length of a superhydrophobic surface is finite, non-constant and anisotropic. Furthermore, it can be strongly influenced by the presence of surface active substances. All these properties can be explained by the local hydrodynamics within the air layer and at the air-water interface, such as the local flow field depending on the surface geometry or Marangoni forces. More general, these findings are also of relevance for the development of theoretical models of slippery surfaces that rely on a fluid being in the Cassie state. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M8.00007: Robust liquid-infused surfaces through patterned wettability Jason Wexler, Abigail Grosskopf, Melissa Chow, Yuyang Fan, Ian Jacobi, Howard Stone Liquid-infused surfaces display advantageous properties that are normally associated with conventional gas-cushioned superhydrophobic surfaces. However, the surfaces can lose their novel properties if the infused liquid drains from the surface. We explore how drainage due to gravity or due to an external flow can be prevented through the use of chemical patterning. A small area of the overall surface is chemically treated to be preferentially wetted by the external fluid rather than the infused liquid. These sacrificial regions disrupt the continuity of the infused liquid, thereby preventing the liquid from draining from the texture. If the regions are patterned with the correct periodicity, drainage can be prevented entirely. The chemical patterns are created using spray-coating or deep-UV exposure, two economical techniques that are scalable to generate large-scale failure-resistant surfaces. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M8.00008: The shear-driven failure of liquid-infused surfaces and superhydrophobic surfaces. Ying Liu, Jason Wexler, Howard Stone We study experimentally the failure of liquid-infused surfaces under shear. Most of the previous work on this topic focuses on situations where the infused fluid is much more viscous than the external fluid. Here, we study the opposite limit: the viscosity of the infused fluid is much lower than that of the external fluid. Also, we study how the air-filled cavities of superhydrophobic surfaces fill with water under shear, which is another topic that is little studied as compared with pressure-driven failure. In each case we systematically vary the flow rate and characterize both transient and steady-state responses. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M8.00009: A computational DFT study of structural transitions in textured solid-fluid interfaces Petr Yatsyshin, Andrew O. Parry, Serafim Kalliadasis Fluids adsorbed at walls, in capillary pores and slits, and in more exotic, sculpted geometries such as grooves and wedges can exhibit many new phase transitions, including wetting, pre-wetting, capillary-condensation and filling, compared to their bulk counterparts. As well as being of fundamental interest to the modern statistical mechanical theory of inhomogeneous fluids, these are also relevant to nanofluidics, chemical- and bioengineering. In this talk we will show using a microscopic Density Functional Theory (DFT) for fluids how novel, continuous, interfacial transitions associated with the first-order prewetting line, can occur on steps, in grooves and in wedges, that are sensitive to both the range of the intermolecular forces and interfacial fluctuation effects. These transitions compete with wetting, filling and condensation producing very rich phase diagrams even for relatively simple geometries. We will also discuss practical aspects of DFT calculations, and demonstrate how this statistical-mechanical framework is capable of yielding complex fluid structure, interfacial tensions, and regions of thermodynamic stability of various fluid configurations. As a side note, this demonstrates that DFT is an excellent tool for the investigations of complex multiphase systems. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M8.00010: Analysis for $Ca\to 0$ of smearing during gravure printing Umut Ceyhan, S. J. S. Morris During gravure printing, excess liquid must be wiped from the substrate, but wiping itself also smears liquid from the engraved wells onto the substrate. Assuming the wiping blade and substrate to be orthonormal, Ceyhan \& Morris (BAPS.2014.DFD.G10.1) treat the case in which the blade--liquid contact angle $\theta\to\pi/2$. Streamlines are then everywhere quasiparallel, and the evolution equation for thin films describes the entire process. Comparing the solution of that equation for an emerging single well in plane flow with that for 3--dimensional flow, we found that smearing can be treated as a problem in plane flow. We now extend the analysis to cover the range $0\le\theta<\pi/2$. Streamlines are no longer everywhere quasiparallel, and inner--and--outer analysis of an unsteady plane flow is required. Though the problem might appear intractable, it has two redeeming features: (a) the interface, in effect, pins first at the distal edge of a well and, then later, at its proximal edge; (b) except near the pinning points, the pressure is hydrostatic. Using these features, we show that smearing, including the formation of a local maximum in film thickness, can be understood by combining Euclid (geometry of the circle) with Reynolds (squeeze--film flow). [Preview Abstract] |
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