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 G22: Microscale Flows: General |
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Chair: Andres Tejada-Martinez, University of South Florida Room: E141/142 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G22.00001: Fabrication of Converging and Diverging Polymeric Microlens Arrays By A Thermocapillary Replication Technique Soon Wei Daniel Lim, Kevin R. Fiedler, Sandra M. Troian Thermocapillary forces offer a powerful method for sculpting air/liquid interfaces at microscale dimensions. Here we demonstrate how square arrays of slender chilled pins in close proximity to a molten nanofilm enforce periodic distributions of thermocapillary stresses suitable for fabricating microlens arrays with ultrasmooth surfaces and excellent focusing capability. We applied this technique to shape and then solidify polystyrene films on quartz to form converging and diverging microlens arrays. By adjusting the growth time, width of the chilled pins, and pin pitch, we created simple convex, simple concave, caldera-like and even hierarchical microarray components. The latter two tend to form when the pitch and pin width are comparable in size. The diverging arrays were incorporated into a Shack-Hartmann wavefront sensor for imaging spatial fluctuations in refractive index caused by bursts of cooled spray. The caldera-like arrays were used to collimate an incident beam into annuli. These demonstrations illustrate how spatiotemporal control over thermocapillary distributions can be used to fabricate a multiplicity of micro-optical components in a single, non-contact step. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G22.00002: Control of smearing during wiping stage of gravure printing of electronics umut ceyhan, s. j. s. morris During gravure printing, a blade wipes the excess liquid from the engraved gravure roll, the objective is leaving liquid filled cells defining the image to be printed. Capillarity, however, draws some liquid from cell into a meniscus connecting to the blade; and the continuing motion of gravure roll smears that meniscus over its surface. Smearing delivers features lacking in sharpness at the micron scale. Ceyhan and Morris (BAPS.2015.DFD.M8.10) analyze smear formation for the blade--liquid contact angles covering the range $0\leq\theta<\pi/2$ and show that the problem can be treated in plane. Using numerical solutions of the corresponding free boundary problem for the Stokes equations of motion, we show\footnote{Ceyhan and Morris, {\it J. Fluid Mech.} submitted 2016} that hydrostatic theory provides an upper bound for the smear volume for finite $Ca$. The approach of trailing edge of the cell to the meniscus isolates a certain volume of liquid from the cell, the isolated liquid is deformed with the continuing motion of gravure roll. The problem of controlling smear formation now reduces to the simpler problem of reducing the quantity of liquid drawn into the meniscus. The theory explains why polishing to reduce the tip radius of the blade is an effective way to control smearing. [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G22.00003: The unified slip boundary condition: addressing the breakdown of the no-slip boundary condition Joseph Thalakkottor, Kamran Mohseni The no-slip boundary condition has been contested for over a century. Although it has been successful in reproducing most continuum and macroscopic results, the condition breaks down in situations such as contact line motion, corner flow and in many micro- and/or nano-scale applications. The widely used Maxwell and Navier slip boundary conditions make an implicit assumption that velocity varies only in the wall normal direction. This assumption is not applicable in the vicinity of a contact and a corner point, where velocity varies in wall-normal and wall-tangential directions. Here, we present a generalized velocity boundary condition that shows that slip velocity is a function of not only the shear rate but also the linear strain rate. In addition, we present a universal relation for slip length which shows that, for a general flow, slip length is a function of the principal strain rate. The universal relation for slip length along with the generalized velocity boundary condition provides a unified slip boundary condition to model a wide range of steady Newtonian fluid flows. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G22.00004: Enhanced oil recovery with polymer flooding. Shima Parsa, David Weitz Polymer flooding is a method for enhanced oil recovery, however the mechanism responsible for the effectiveness of polymer flooding is not well understood. We use confocal microscopy and bulk transport measurements to probe the effectiveness of different molecular weight and concentrations of Polyacrylamide solution in imbibition of crude oil in 3D micromodel. We show that large molecular weight and moderate to high concentration of polymer is required for enhanced recovery. By directly measuring the pore level velocities in the medium, we show that polymer retention in the medium results in diversion of flow in some pores. The inhomogeneous changes in the flow velocities result in redistribution of viscous forces and enhanced recovery of oil. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G22.00005: Nanoscale measurement of apparent slip velocity near a moving contact line Joonsik Park, Kenneth Breuer We report the nanoscale flow measurements within tens of microns from a moving contact line on hydrophobic substrates. A moving contact line was generated using a liquid bridge instability induced by retreating syringe. Contact line speeds ranging from 0.15 to 3 mm/s were recorded. The motions of tracer nanoparticles were measured using two independent experimental techniques: multi-layer flood illumination and Total Internal Reflection Fluorescence Microscopy. The flow field was derived using a novel probabilistic particle tracking velocimetry, which allows the accurate estimation of the rapidly changing flow field near a contact line without bias due to binning or fitting. The results confirm that for distances larger than a few microns from the contact line, the velocity field scales with the instantaneous contact line speed and agrees well with the corner flow solution predicted by the biharmonic equation. A significant slip velocity is shown to exist close to the contact line, decaying rapidly within a few microns. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G22.00006: Fluid mechanical proximity effects in high-resolution gravure printing for printed electronics Gerd Grau, William J. Scheideler, Vivek Subramanian Gravure printing is a very promising method for printed electronics because it combines high throughput with high resolution. Recently, printed lines with 2 micrometer resolution have been demonstrated at printing speeds on the order of 1m/s. In order to build realistic circuits, the fluid dynamics of complex pattern formation needs to be studied. Recently, we showed that highly-scaled lines printed in close succession exhibit proximity effects that can either improve or deteriorate print quality depending on a number of parameters. It was found that this effect occurs if cells are connected by a thin fluid film. Here, we present further experimental and modeling results explaining the mechanism by which this thin fluid film affects pattern formation. During the transfer of ink from the roll to the substrate, ink can flow in between connected cells. Asymmetry in the fluid distribution created by the preceding doctor blade wiping process results in net fluid flow from cells that transfer first to cells that transfer subsequently. The proximity of these cells thus affects the final ink distribution on the substrate, which is critically important to understand and design optimally when printing highly-scaled patterns of electronic materials. [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G22.00007: Interfacial melting of ice under a high-speed slider: real-time visualization and friction modeling Hyung-Seok Kim, Chang-Ho Yun, Dong-Jo Kim, Ho-Young Kim When a solid plate slides on ice, frictional heat melts asperities on the ice surface causing the real contact area to increase. Previous studies indicate the significance of contact area growth for ice friction, yet its quantitative understanding is far from clear mainly because the direct observation of the melting process at the interface has been extremely difficult. Here we describe a novel experimental setup that visualizes the interface of a rapidly rotating ice disc (up to the linear velocity of 10 m/s) and a transparent quartz surface in real time using the total internal reflection. We find that the melted area of the ice surface is a sensitive function of both sliding speed and temperature. We rationalize such quantitative measurements numerically and analytically, which allows us to predict the friction coefficient of ice as a function of relative velocity and temperature. This work can be used to develop friction-controlling mechanisms on ice surface, which are important in traffic safety as well as winter sports. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G22.00008: A unifying framework for mass transfer dynamics in the Taylor flow of a dissolving train of bubbles Ghata Nirmal, Arun Ramchandran Operation in the Taylor flow regime in microfluidics for estimation of mass transfer coefficients in multiphase flows has gained popularity due to the presence of high interfacial areas and well-characterized flow profiles. Although there are multiple models available for data interpretation, these are accompanied by two major limitations. First, mass transfer from the lubricating liquid film to the bulk liquid segment between bubbles has been incorrectly estimated. Second, the liquid segment is assumed to be well mixed. Both assumptions fail in the normal operating limits for Taylor flow experiments of dissolving bubbles. In this work, we rectify the two limitations described above and present a unifying framework to comprehend experimental results in a dissolving train of bubbles in microchannels. Based on a scaling analysis, the experiments can be operated in four regimes controlled by L$_{\mathrm{B}}$ / R, L$_{\mathrm{L}}$/ R, Peclet number and capillary number where L$_{\mathrm{B}}$, L$_{\mathrm{L}}$ and R are the bubble length, the liquid segment length and the tube radius, respectively. Finally, we present the differences in the results due to a rectangular cross-sectional shape instead of a circular one, and in particular, on the additional leakage flux through the lubricating film around the corners of the cross-section. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G22.00009: Conformation and stretching of end-tethered polymers in pressure-driven flow under confinement. Tamal Roy, Steffen Hardt Understanding of the conformation and dynamics of polymers under confinement is important for both fundamental studies and applications. We experimentally study the conformation and stretching of surface-tethered polymer chains confined between parallel surfaces and exposed to a pressure-driven flow. $\lambda $-DNA molecules are tethered to the wall of a microchannel of height smaller than the contour lengths of the molecules. The DNA molecules, stained with a fluorescent dye, are visualized by epifluorescence and laser-scanning confocal microscopy (LSCM). The effects of the channel height, flow rate and contour length on the extension of the molecules are determined from epifluorescence images. From LSCM images the complete conformation and orientation of the DNA molecules is inferred. We find that the fractional extension of the molecules is uniquely determined by the fluid shear stress at the tethering surface and the chain contour length. There is no explicit influence of the channel height in the range of contour lengths we consider. We also derive analytical scaling relationships (in the weak and strong extension limits) that explain the experimentally observed stretching characteristics. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G22.00010: Comparison of Simulated and Measured Fluid-Surface Oscillation Frequencies in a Channel Matthew Trapuzzano, Kiesha Pierre, Emre Tufekcioglu, Rasim Guldiken, Andres Tejada-Martinez, Nathan Crane Many important processes from agriculture to manufacturing depend on the wetting of fluids on rough or textured surfaces. This has traditionally been studied from a macro-perspective. The effects of these surface features can be dramatically altered by vibrations that overcome energy barriers to contact line motion caused by surface roughness. In order to study these effects in confined geometries and at different length scales, a validated model is required. This presentation will compare the measured and simulated frequencies of capillary vibrations in a cylindrical glass tube. Fluid surface vibrations are excited externally through deformation of the interface. The resulting surface oscillations are observed with a high speed video camera and the dominant oscillation frequencies are calculated. The measured oscillation frequencies are compared to predictions from transient CFD simulations across a range of interface diameters from 400 um to 1.5 mm. These results may be used to inform studies of wetting under vibration. [Preview Abstract] |
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