Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X12: General Fluid Mechanics: Surface and Thermal Effects |
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Sponsoring Units: DFD Chair: Amber Krummel, Harvard University Room: B110-B111 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X12.00001: Watching heat flow near a nucleating bubble Scott Parker, David Cahill, Steve Granick When a liquid boils on surface, heat must flow out of the solid substrate and into both the nucleating bubble and the fluid surrounding the bubble. We have developed a high speed thermal imaging technique to observe the spatial distribution of the temperatures on functionalized surfaces in contact with water. This system is used to observe temperatures while growing individual vapor bubbles from a local hot spot on the surface. By varying the average surface temperature and fluid pressure, we tune the growth of the bubble. We report on how the static contact angle and local curvature of a bubble near the surface affect localized heat transfer and the corresponding bubble dynamics. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X12.00002: Fluctuations of interfacial forces near a moving contact line Yongjian Wang, Shuo Guo, Ping Sheng, Penger Tong Atomic force microscope (AFM) is used as a force sensor to measure the capillary forces on a long vertical glass fiber with one end glued onto a rectangular shaped cantilever beam and the other end immersed through a liquid-air interface. Using a cleaned glass fiber of $\sim 2 \mu$m in diameter, we were able to determine the surface tension of a class of liquids with good accuracy. For this class of liquids, no detectable hysteresis is found when the contact line between the liquid and glass fiber moves at a constant speed. The AFM force measurements, however, revealed considerable fluctuations in the force amplitude. The probability density function of the force fluctuations all shows a Gaussian form. Investigations are carried out to find the relationship between the mean squared value of force fluctuations and the chemical properties of the liquid. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X12.00003: Slip boundary conditions for the moving contact line in molecular dynamics and continuum simulations Anoosheh Niavarani, Nikolai Priezjev The problem of the moving contact line between two immiscible fluids on a smooth surface is revisited using molecular dynamics (MD) and continuum simulations. In MD simulations a finite slip is allowed by choosing incommensurate wall-fluid densities and weak wall-fluid interaction energies. The shear stresses and velocity fields are extracted carefully in the bulk fluid region as well as near the moving contact line. In agreement with previous studies, we found slowly decaying partial slip region away from the contact line. In steady-state shear flows we extract the friction coefficient along the liquid-solid interface, the local slip length, and the dynamic contact angle. The MD results show that both dynamic contact angle and slip velocity near the contact line increase with increasing the capillary number (Ca). Also, at high Ca the break up of fluid-fluid interface is observed. The slip boundary conditions near the moving contact line extracted from MD simulations were then used in the continuum solution of the Navier-Stokes equation in the same geometry to reproduce velocity profiles and the shape of the fluid-fluid interface. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X12.00004: Sub-Micron Velocity Measurements near a Moving Contact Line Jeremiah Zimmerman, Mark M. Weislogel, Derek C. Tretheway The displacement of one fluid by an immiscible second fluid (i.e. dynamic wetting), governs many natural and technological processes. Despite extensive studies, understanding and modeling the displacement process remains one of the outstanding problems in fluid mechanics. In this work, we explore the physics of the moving contact line (the idealized line of intersection between two fluids and a solid) with micron resolution particle image velocimetry ($\mu $PIV), which enables sub-micron two-dimensional velocity measurements. The measured flow is generated by dynamic wetting in a glass microchannel. The microchannel is mounted on an automated microscope stage with precise velocity control allowing for the static placement of the contact line within the field of view. Full-field velocity measurements within 1 $\mu $m of the contact line were made in water/glycerol and fructose/glucose/water solutions. Preliminary results appear to show remarkable similarity to controversial theoretical predictions. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X12.00005: Drop Impact on Superhydrophobic Electrospun Nanomats Alexander Yarin, Andreas Lembach, Iliya Roisman, Tatiana Gambaryan-Roisman, Cameron Tropea, Peter Stephan Experiments were conducted to study peculiarities of drop impact on electrospun polymer nanofiber mats. The nanofiber cross-section diameters were of the order of several hundred nanometers, the pore sizes in the mats of about several microns. Polymers which are partially wettable by water, and non-wettable by water were used to electrospin nanofiber mats. The experiments revealed that drop impacts on nanotextured surfaces of nanofiber mats produce spreading similar to the one on impermeable surfaces. However, at the end of the spreading stage the contact line is pinned and drop receding and bouncing is completely prevented. At higher impact velocities, prompt splashing events with formation of tiny drops were observed. It was shown that the well-known splash parameter $K_d $ can be used as an acceptable scaling for splashes, however the threshold value of number$K_{d,s} $for the nanomats is higher than that for dry flat substrates. The enhanced efficiency of drop cooling in the presence of nanofiber mats was also observed experimentally. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X12.00006: Controlling ice formation on nanostructured superhydrophobic surfaces Tom Krupenkin, Lidiya Mishchenko, Benjamin Hatton, J. Ashley Taylor, Vaibhav Bahadur, Joanna Aizenberg In this work we describe anti-icing properties of nanostructured superhydrophobic surfaces with well-defined regular arrays of micron and submicron surface features. Both open-cell and closed-cell structures are investigated. Dependence of ice formation dynamics on the temperature, details of the surface topography, substrate material, and other factors are investigated. We find that ice formation on these surfaces can be substantially retarded, with some of the surfaces showing no ice accumulation at temperatures as low as -20 C. The experimental results are in good quantitative agreement with the simple theoretical model based on the classical heterogeneous nucleation theory and wetting dynamics. The results of the work can provide new insight into design and optimization of anti-icing structures and coatings. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X12.00007: Morphology of air nanobubbles trapped at hydrophobic nanopattened surfaces Antonio Checco, Tommy Hofmann, Elaine DiMasi, Charles Black, Benjamin Ocko By using wettability and X-ray scattering measurements we study the trapping of air nanobubbles at the interface between water and a hydrophobic silicon surface patterned with 20 nm-wide cavities. Hydrophobic cavities of various depths were fabricated over a large area of the substrate using diblock-copolymer lithography followed by silane functionalization. We have found that the contact angle of millimiter-sized water drops wetting the nanostructured surfaces increases with the cavity's depth eventually reaching a plateau. This behavior results form the stable trapping of air in the cavities consistent with Small-angle X-rays scattering (SAXS) measurements. The latter also show that water always penetrates slightly in the cavities independent on their depth which can be rationalized considering the geometry of the cavities. The ability to form high-density arrays of nanobubbles of well-defined morphology at the water/solid interface is relevant to the fabrication of surfaces with reduced liquid slippage for integration in micro- and nano-fluidic devices. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X12.00008: Correlation of Bulk Viscosity and Liquid Slip on Smooth Hydrophobic Surfaces Sean McBride, Bruce Law In this study we examine the slip behavior of eighteen Newtonian liquids from the two homologous series, the n-alcohols and n-alkanes, with viscosities covering the range $\sim $ 0.4 -- 11.0 mPa s. Colloidal probe atomic force microscopy (AFM) is used to extract the slip length from experiments, which were conducted against molecularly smooth n-hexadecyltrichlorosilane (HTS) coated surfaces. The primary feature of this work is that the slip length b is found to be a function of the bulk viscosity $\eta$ with b $\sim \eta ^x$ where x $\sim $ 0.33. The slip length is also shown to be independent of the shear rate, therefore, validating the use of Vinogradova's slip theory. An important aspect of this study is that the same surfaces are used for each liquid, allowing any relative trends in slip behavior to be attributed to the properties of the liquid. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X12.00009: On Features of Capillary Flows as Predicted from Direct Comparison between Network Models and Experiments B. Markicevic, K. Hoff, H. Li, A. Zand, H. K. Navaz Having imbibed a particular volume of a wetting liquid by porous medium, a spontaneous capillary flow of liquid within porous medium itself takes place. The flow has been investigated experimentally for the unidirectional flow conditions, where the spatial and temporal changes of the liquid saturation are measured. The axial saturation profiles and their changes in time are also predicted numerically using the capillary network models. For each specific porous medium$\vert $liquid pair, the experimental and numerical saturation profiles are matched, and from numerical calculations, a unique capillary pressure and relative permeability as functions of saturation are found. The fine and medium grain sands as porous media are used, which are not significantly different in their structure. Starting with this assumption, we are able to reduce both relative permeability and capillary pressure into single dependencies. It is found that the relative permeability falls onto the same curve without any further reducing factor, whereas for the capillary pressure, the Leverett J-functions scales are sufficient. For two distinct liquids, the liquid surface tension and contact angle are used. For two sands, the scale is obtained from the sand permeability and porosity. Two network parameters: pore size distribution and liquid residual saturation are used in predicting the experimental data. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X12.00010: Direct Imaging of Two-phase Flow in Porous Media at the Pore Level Amber Krummel, Stefan Munster, Stefan Lindstroem, David Weitz The dependence of residual oil saturation on capillary number is investigated during a series of two-phase flow experiments. We exploit the spatial and time resolution of confocal microscopy to collect three-dimensional images during the course of two-phase flow experiments. The engineering of an optically transparent, three-dimensional micromodel affords direct imaging of the fluid configurations while the physical characteristics of the flow are measured. An optimal capillary number for oil production is observed. Beyond this point the entrapped residual oil actually increases while the relative permeability of the medium increases. The origin of this counterintuitive behavior lies in the size distribution of the residual oil ganglion. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X12.00011: Surface Tension Driven Instability in the Regime of Stokes Flow Zhenwei Yao, Mark Bowick, Xiangjun Xing A cylinder of liquid inside another liquid is unstable towards droplet formation. This instability is driven by minimization of surface tension energy and was analyzed first by [1,2] and then by [3]. We revisit this problem in the limit of small Laplace number, where the inertial of liquids can be completely ignored. The stream function is found to obey biharmonic equation, and its analytic solutions are found. We rederive Tomotika's main results, and also obtain many new analytic results about the velocity fields. We also apply our formalism to study the recent experiment on toroidal liquid droplet[4]. Our framework shall have many applications in micro-fluidics. [1] L.Rayleigh, On The Instability of A Cylinder of Viscous Liquid Under Capillary Force, Scientific Papers, Cambridge, Vol.III, 1902. [2] L.Rayleigh, On The Instability of Cylindrical Fluid Surfaces, Scientific Papers, Cambridge, Vol.III, 1902. [3] S.Tomotika, On the Instability of a Cylindrical Thread of a Viscous Liquid surround by Another Viscous Fluid, Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Volume 150, Issue 870, pp. 322-337. [4] E.Pairam and A.Fern\'{a}ndez-Nieves, Generation and Stability of Toroidal Droplets in a Viscous Liquid, Physical Review Letters 102, 234501 (2009). [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X12.00012: Thermal Performance of Surface Wick Structures. Yongkang Chen, Noel Tavan, John Baker, Lawrence Melvin, Mark Weislogel Microscale surface wick structures that exploit capillary driven flow in interior corners have been designed. In this study we examine the interplay between capillary flow and evaporative heat transfer that effectively reduces the surface temperature. The tests are performed by raising the surface temperature to various levels before the flow is introduced to the surfaces. Certainly heat transfer weakens the capillary driven flow. It is observed, however, the surface temperature can be reduced significantly. The effects of geometric parameters and interconnectivity are to be characterized to identify optimal configurations. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X12.00013: Cold Water Jets on a Hot Si surface Ji Yong Park, Chang-Ki Min, David Cahill, Steve Granick We are using a femtosecond pump-probe apparatus to study heat transfer when a pulsed jet of liquid water impinges on a hot Pt-coated Si surface (Leidenfrost Effect). The light source in the experiment is a 100 mW Er:fiber laser operating at a wavelength of $\lambda $=1550 nm; the total volume of the pulsed water jet is $\sim $0.9 mm$^{3}$. The temperature change within the Si substrate at a distance of 50 microns from the interface is measured by a novel time-resolved thermometry based on two-photon absorption. We measure the thermal conductance of the water layer within 50 nm of the interface by time-domain thermo-reflectance; changes in the thermal conductance provide a direct measurement of the contact time of the liquid. We convert the integral of the temperature excursion to the energy transferred using a Green's function solution of heat conduction in the Si substrate. Both the energy transferred and contact time show a smooth evolution from high values at 110C to low values at 210C without any clear indication of a Leidenfrost point. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X12.00014: Optimal convective mixing by forced two-dimensional Stokes flows David Saintillan, Qizheng Yan Numerous mixing strategies in the Stokes flow regime rely on time-dependent body forces. The question of determining the required forcing function to achieve optimal mixing at a given power input remains however open. Using optimal control theory, we numerically determine general optimal mixing flows in a two-dimensional periodic geometry as truncated sums of time-modulated Fourier modes. The time-averaged power spectra of these flows are calculated to investigate the effect of scale, and demonstrate that best mixing is achieved when a wide range of scales are present in the flow. We also determine the frequency spectra of the time-modulating functions and characterize the importance of non-harmonic forcing. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X12.00015: Self-sustaining oscillations of the falling sphere in some viscoelastic fluids Young Ju Lee, Chensong Zhang We investigate the mathematical models for the unusual phenomenon observed in motion of the sphere falling through the wormlike micellar fluids;a sphere falling in a wormlike micellar fluids undergoes nontransient and continual oscillations. Using a novel numerical techniques, we identified right models in our simulations by exploring the parameter regimes of models that have been unexplored previously for the flow past a sphere and reproduce the self-sustaining, continual, (ir)regular and periodic oscillations. Our results show that the flow instability can be correlated with the critical value of the velocity gradient as observed in experiments. [Preview Abstract] |
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