Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session M26: Nanofluids III |
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Chair: Petros Koumoutsakos, ETH Zurich Room: 329 |
Tuesday, November 22, 2011 8:00AM - 8:13AM |
M26.00001: Laminar Channel Flow Over Superhydrophobic Porous Surfaces Ozgur Ozsun, Victor Yakhot, Kamil L. Ekinci We have fabricated mesh-like porous superhydrophobic surfaces with solid area fraction $\phi_{s}$, which can maintain intimate contact with both air and water reservoirs on either side. Typical structure has linear dimensions of $1mm \times 1mm \times 200nm$ and pore area of $10\mu m \times 10\mu m$. Recent experiments on such surfaces have shown anomalous hydrodynamic response in oscillating flows [1]. This effect is attributed to a Knudsen layer of gas at the solid-liquid interface. In this study, we investigate laminar channel flow over porous superhydrophobic surfaces. The surfaces are enclosed with PDMS microchannels, where pressure driven flow of DI water is generated. Pressure drop across the microchannels is measured as a function of flow rate. Slip lengths are inferred from the Poiseuille relation as a function of $\phi_{s}$ and compared to that of similar standard superhydrophobic surfaces, which lack intimate contact with air reservoir.\\[4pt] [1] http://arxiv.org/abs/1107.5873 [Preview Abstract] |
Tuesday, November 22, 2011 8:13AM - 8:26AM |
M26.00002: Micron-scale measurement of dynamic contact angles in the vicinity of moving contact lines Bian Qian, Kenneth Breuer Directly probing the liquid-vapor interface shape near moving contact lines is critical to understanding the microscopic physics of dynamic contact line behavior. Using a novel high resolution optical technique, we report on the meniscus shape in the immediate vicinity of moving contact lines. The liquid is fluorescently-dyed and illuminated with a highly focused laser beam. The strongest illumination intensity occurs at the laser focus and this bright and tiny spot serves as a probe which changes in intensity and shape near the liquid-vapor interface. This experimental setup enables us to resolve the meniscus shape with submicron resolution within 10 microns of the contact line. Using this technique, we report on the dependence of the interface shape on the contact line speed and compare it with theoretical predictions. [Preview Abstract] |
Tuesday, November 22, 2011 8:26AM - 8:39AM |
M26.00003: Uncertainty Quantification in MD Simulations: Forward Propagation and Parameter Inference Francesco Rizzi, Omar Knio, Habib Najm, Bert Debusschere, Khachik Sargsyan, Maher Salloum, Helgi Adalsteinsson This work focuses on quantifying uncertainty in molecular dynamics (MD) simulations accounting for both intrinsic noise and parametric uncertainty. We consider isothermal, isobaric MD simulations of TIP4P water at ambient conditions. Due to the thermal noise in the system, this yields non-deterministic, noisy predictions for the water observables. Selected macroscale observables are expressed in terms of polynomial chaos (PC) expansions using both a non-intrusive spectral projection (NISP) and Bayesian inference approach. We show that the effect of the thermal noise can be controlled, and that the two methods yield similar results. We illustrate the possibility of determining or refining a set of force-field parameters for water using a reformulated Bayesian approach based on PC expansions. We present a synthetic problem where presumed ``true'' values of the TIP4P model parameters are used to generate a collection of noisy data of selected water observables. Exploiting the PC representation, we show how the ``true'' force-field parameters can be accurately and efficiently recovered using low-order surrogate models. [Preview Abstract] |
Tuesday, November 22, 2011 8:39AM - 8:52AM |
M26.00004: Molecular Dynamic Simulation of Flow in a Channel with Oscillating Wall Joseph Thalakkottor, Kamran Mohseni Molecular dynamic simulations are employed in order to investigate the effect of fluid inertia on slip at a solid surface. The accuracy of the numerical technique is verified by reproducing the steady state slip boundary condition for Couette flow at a solid surface reported by Thompson and Troian, Nature 1997. Numerical experiments are also conducted on Couette flow with oscillatory wall velocity in order to investigate the effect of fluid inertial on the slip boundary condition. Preliminary results indicate that the accelerating/decelerating wall velocities could have some effect of the slip value at the wall. [Preview Abstract] |
Tuesday, November 22, 2011 8:52AM - 9:05AM |
M26.00005: Wall Boundary Conditions in 3D Smooth Dissipative Particle Dynamics Simulations Jun Yang, Raffaele Potami, Nikolaos Gatsonis The mesh-free thermodynamically consistent smooth dissipative particle dynamics (SDPD) method is implemented in 3D using a block structure to minimize computational time and the Verlet neighbor-list algorithm for efficient search of particles. A new method for solid wall boundary conditions is presented that enforces non-penetration and minimizes the density fluctuations. The geometry of the wall is described using wall pseudo-particles. This geometry is used to generate a set of ghost fluid particles obtained by mirroring the neighbor particles of a given fluid particle in the proximity of a boundary. The ghost particles are used for minimizing the density fluctuations near walls, for generating the repulsive force that ensures the impenetrability condition, and for generating a dissipative force that ensure the no slip condition. Numerical results for isothermal flows show only minimal density fluctuations in the proximity of the solid wall and an overall good agreement between the velocity profiles from numerical simulations and analytical results. [Preview Abstract] |
Tuesday, November 22, 2011 9:05AM - 9:18AM |
M26.00006: Behavior of the Kapitza Resistance at Liquid/Solid Interfaces: Insights from Molecular Dynamics and Continuum Studies Anoosheh Niavarani, Sandra Troian While Kapitza (1941) first proposed the concept of thermal boundary resistance in superfluid helium, Khalatnikov (1952) soon realized that this phenomenon was caused by acoustic phonon mismatch across any interface separating dissimilar materials. Acoustic phonon mismatch has also been invoked as the source of instability in viscous nanofilms exposed to a large thermal gradient [1]. Liquid films approaching nanoscale dimensions are especially prone to thermal boundary resistance effects which can substantially alter the internal temperature profile. For this purpose, non-equilibrium molecular dynamics (NEMD) simulations provide an especially convenient tool for investigation [2-4]. Using NEMD and continuum studies, we explore the origins of the discontinuity in temperature at liquid/solid interfaces and specify regimes leading to deviations from Fourier's law. These findings suggest that although thermal convection effects are often negligible in nanofluidic systems, the Kaptiza resistance can nonetheless strongly influence hydrodynamic flow at small scales. [1] E. Schaffer et al., Macromolecules 36, 1645 (2003) [2] L. Xue et al., Int. J. Heat Mass Transfer 47, 4277 (2004) [3] B. Kim, A. Beskok and T. Cagin, J. Chem. Phys. 129, 174701 (2008) [4] S. Murad and I. Puri, Appl. Phys. Lett. 92, 133105 (2008) [Preview Abstract] |
Tuesday, November 22, 2011 9:18AM - 9:31AM |
M26.00007: Submicron flows of polymer solutions Amandine Cuenca, Hugues Bodiguel We study flow properties of high molecular weight polymer solutions below the micrometer scale. Fluorescence photobleaching is used as a non-invasive technique to evaluate the velocity of pressure-driven flows in channels from 200 to 4000 nm height. We observe a striking reduction of the effective viscosity. The latter is reduced up to 50 times at 400 nm. This effect increases with molecular weight and concentration. Using a Rabinowitsch-like approach, we correlate the data at different thicknesses to obtain both the slippage at the wall and the rheological flow curve at sub-microscale. Those properties are also evaluated in bulk using respectively PIV (Particle Image Velocimetry) and rheometer. Comparing the measurements in bulk and in confined geometries, we conclude that the viscosity reduction can not solely be explained by slippage. We can think of two origins of the confinement effect, either it modifies the polymer solution on itself, in terms of concentration (size- dependent filtration of coils) or it induces a change in the rheological behaviour. [Preview Abstract] |
Tuesday, November 22, 2011 9:31AM - 9:44AM |
M26.00008: Ultra-fast Thinning of Nanoscale Films by Convection Markus Abel, Michael Winkler, Rumen Krastev We present experimental results on flows on very thin, nanometer--scale membranes. More specific we observe an enormous speed-up of the thinning of a film with surfactants towards its equilibrium of a few nanometers when we drive it thermally by cooling locally on a spot in the upper third of the film. Interesting and beautiful to watch by itself, the thinning of films is one of the most important topics for applications and research: Interfaces in general are important for all two-phase flows, as for colloids, aerosols, droplets, or for the production of ultra-thin materials, etc.. The involved processes are macroscopic in 2 dimensions and at nanometers in the third one. Consequently, the system is just at the edge of the hydrodynamic description; furthermore, the chemical and electrical forces of the surfactant and bulk material become important. We have combined convection with thin films in order to observe the effects of thermal driving on thinning and vice versa. As a result, mixing was observed which does not only accelerate the thinning but changes the thinning law qualitatively from linear to expoential. [Preview Abstract] |
Tuesday, November 22, 2011 9:44AM - 9:57AM |
M26.00009: Near wall velocity measurement of nanofluids using evanescent wave based PIV technique Anoop Kanjirakat, Reza Sadr Recently nanofluids, dilute suspension of nano particles in a based fluid, have emerged as a possible candidate as a coolant in variety of applications based on some reports of enhanced conductivity for these fluids. However, there are controversies in the reported properties of nanofluids and their applicability's, specially, since there is no fundamental understanding explaining these enhancements. A better understanding of these fluids and how they interact with the solid boundary is achieved by detailed near wall fluid flow study at nano scale. In this work nanofluids of different concentrations are prepared by dispersing silicon dioxide particles (10-20nm) in water as the base fluid. Pressure driven nanofluids flow inside a micro channel is studied by adding fluorescent polystyrene particles of 100nm to the flow. Nano Particle Image Velocimetry (nPIV) is used to measure near-wall velocity fields with an out of plane resolution of less than 250nm. The measured near wall velocity field is then compared with that of the basefluid at the same condition. Initial observations have shown that nPIV techniques can be successfully extended for nanofluids for better understanding the flow characteristics in the near wall region. [Preview Abstract] |
Tuesday, November 22, 2011 9:57AM - 10:10AM |
M26.00010: Surface Nanobubble Stability James Seddon, Harold Zandvliet, Detlef Lohse We provide a model for the remarkable stability of surface nanobubbles to bulk dissolution. The key to the solution is that the gas in a nanobubble is of Knudsen type. This leads to the generation of a bulk liquid flow which effectively forces the diffusive gas to remain local. Our model predicts the presence of a vertical water jet immediately above a nanobubble, with an estimated speed of $\sim 3.3m/s$, in good agreement with our experimental atomic force microscopy measurement of $\sim 2.7m/s$. In addition, our model also predicts an upper bound for the size of nanobubbles, which is consistent with the available experimental data. [Preview Abstract] |
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