Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L07: Nanoflows: Basics and Modeling |
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Chair: Narayana Aluru, University of Illinois Urbana-Champaign Room: Georgia World Congress Center B212 |
Monday, November 19, 2018 4:05PM - 4:18PM |
L07.00001: A Molecular-Kinetic Scaling Relation for Fluid Slip at Solid Boundaries Gerald J Wang, Nicolas G Hadjiconstantinou Fluid slip at solid boundaries is perhaps the most well known nanoscale-related phenomenon associated with fluid flow. Despite the attention it has received, predictive models of slip based on ab-initio (molecular) considerations have yet to be fully developed. Using the observation that slip in simple fluids at low to moderate shear rates is a thermally activated process driven by the shear stress in the fluid close to the solid boundary, we use kinetic arguments to develop a model for fluid slip at solid boundaries in the form of a universal scaling relation that connects fluid slip and shear rate. The proposed model reduces to the well known Navier-slip condition under low shear conditions, providing a direct connection between molecular parameters and the slip length. Molecular-dynamics simulations strongly support the predicted dependence of slip on system properties, including the temperature and fluid-solid interaction strength. Finally, we explore the connections between our model and previous theories, simulation results, and experiments studying fluid-solid slip. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L07.00002: Response of Molecularly Thin Films to an Applied Shear Stress: Theory vs Experiment Kishan Makwana, Sandra Troian Derjaguin and co-workers in 1946 introduced the "blow-off technique" for inferring the boundary viscosity of ultrathin liquid films supported on a solid substrate. Air blown through a slender horizontal slit is used to create a constant wall shear stress that ideally deforms an initially flat free surface film into a streamwise wedge shape whose slope decreases in time. The boundary viscosity can then be inferred from the slope value. High resolution measurements of the film shape are normally obtained by interferometry for microscale films or ellipsometry for molecular scale films. Over the years, it has become evident that many molecular scale films exhibit shear response that deviates significantly from ideal linear behavior. Various physical mechanisms have been proposed to help resolve discrepancies between theory and experiment. Here we present results of computational studies of the 1D thin film equation which describes the liquid deformation process in time. We evaluate the influence of different proposed mechanisms based on quantitative comparison to experimental data and discuss which candidate mechanisms best fit the trends observed. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L07.00003: Effect of charge inversion on Poiseuille flow of multivalent electrolyte solutions in nanochannels: an atomistic study Andrés Rojano, Andrés Córdoba, Jens H Walther, Harvey A Zambrano Miniaturized devices integrated by nanoconduits have a great potential for clinical and biotechnological analysis due to amplified sensibility, faster response and increased portability. In nanoconduits, wherein the electrical double layer can occupy a considerable part of the cross section, Electro-Kinetic Phenomena (EKP) play a key role in determining transport properties of electrolytes. Hence, a comprehensive understanding of EKP and related phenomenology such as charge inversion (CI), is essential to develop more efficient nanodevices. Here, atomistic simulations of Poiseuille flow of aqueous multivalent electrolyte solutions in silica nanochannels are conducted to study the influence of CI on fluid properties. The solutions consist of water as solvent, chloride as co-ion and different amounts of counter-ions i.e. sodium, magnesium, aluminum and calcium. From atomistic trajectories, the relation between the concentration of different cations and, local and effective viscosities is analyzed considering the particular hydration shell around each ionic species. Moreover, the effect of CI on flow velocity, stick boundary condition, shear stress and friction coefficient is examined.
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Monday, November 19, 2018 4:44PM - 4:57PM |
L07.00004: Tuning electroosmotic flow of aqueous electrolytes in nanochannels with implanted wall electrodes Manash Pratim Borthakur, Gautam Biswas, Dipankar Bandyopadhyay The accurate prediction and precise control over electroosmotic flows at the nanoscale plays a pivotal role for the design and development of integrated micro-nano fluidic systems. In the present work, the electroosmotic flow of aqueous electrolytic solutions confined inside a nanochannel with implanted wall electrodes is thoroughly investigated. In the simulations, each channel wall consists of four layers of atoms in a face-centered cubic (FCC) lattice. The surface atoms of the walls are maintained at a constant charge density and different charging conditions are applied for the implanted electrode atoms. The channels are filled with different monovalent aqueous electrolytes at a desired pressure and an external homogeneous electric field is applied along channel axis. The ion concentration and electrical potential distribution inside the EDL strongly depends on the ion type and the charging condition of the electrodes. The velocity and concentration profiles show striking dissimilarity from previous investigations for homogenously charged surfaces including local flow reversal and modified ion distribution. By tuning the electrode charge, the local electro-kinetic driving force can be altered which can effectively manipulate the local flow direction of the electrolyte. |
Monday, November 19, 2018 4:57PM - 5:10PM |
L07.00005: Atomistic simulations of water flow in graphene channels driven by imposed thermal gradients Harvey A. Zambrano, Elton E. Oyarzua, J. H. Walther Nanofluidics has become interesting as the basis for further device miniaturization. Different from macro and microfluidics, nanoconfined flows are significantly influenced by fluid-wall interaction. In this context, recent studies have reported the potential exploitation of imposed thermal gradients as mechanism to transport water in nanoconduits. Moreover, graphene-based materials have attracted increasing attention in nanofluidic applications due to their unique thermal, structural and hydrodynamic properties. Here, we conduct atomistic simulations to investigate water transport in graphene nanoslit channels driven by thermal gradients. The study is focused in understanding the relation between phonon currents induced in the walls by imposed thermal gradients and the corresponding measured flow rates. Furthermore, a comprehensive analysis of the influence of wettability, multi-layer graphene in the walls and geometrical asymmetries is performed. Our results provide valuable information for the design of thermal graphene-based nanopumps and contribute to the understanding of suitable driving mechanisms for liquids in nanoconduits. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L07.00006: Slip Flow of Water through Carbon Nanotubes: Pronounced Role of Many-Body Polarization Effects on the Friction Coefficient of Nanoconfined Water Rahul Prasanna Misra, Daniel Blankschtein Recent experimental and molecular dynamics (MD) simulation studies have demonstrated that carbon nanotubes (CNTs), which are 1D cylindrical allotropes of carbon, offer very little hydrodynamic resistance to water flow, thereby generating tremendous hope for practical applications of CNTs in membrane-based applications. Although several MD simulation studies have already been carried out to investigate the slip flow of water through CNTs, in all reported studies, the interaction of water molecules with the CNT has been modeled using a pair-wise additive Lennard-Jones potential. In this talk, we discuss our formulation of polarizable force fields which can self-consistently model the polarization of CNTs resulting from the finite electric fields exerted by water molecules under confinement. By carrying out classical MD simulations using polarizable force fields, we investigate the dependence of the friction coefficient of water on the CNT diameter and chirality. We also develop a microscopic theory to elucidate the static and dynamic origins of the water friction coefficient, and show that many-body polarization effects have a pronounced influence on the density and on the hydrogen-bond structure of the interfacial water molecules. |
Monday, November 19, 2018 5:23PM - 5:36PM |
L07.00007: A Coarse-grained Transport Model of Water for Nano-fluidic Systems Nabil Ramlawi, Narayana Aluru Molecular Dynamics(MD) is an important tool to simulate flows at the nanoscale. The limitation of MD in simulating important biological and chemical systems having a large length and time scale increased the interest in efficient coarse-grained (CG) models. Although many existing CG models for various fluids are able to capture the structure and dynamics of the bulk fluid accurately, these models are not suited to describe transport phenomena involving explicit walls in nano-channels. Here we introduce a complete CG transport model for water in nano-channels having explicit walls. The model, which was applied to the water-graphene system, was able to demonstrate a very good match, with the structure (error<5%) and dynamical (error<1%) equilibrium properties of MD simulations. Moreover, the CG model was able to reproduce the MD results for water transport in a Poiseuille flow configuration with an error of < 5%. The accuracy of the model was transferable through different configurations and forcing conditions up to a critical force, where the MD slip velocity starts to deviate from the equilibrium prediction. Finally, the CG model was able to achieve ≈20 x speedup compared to MD simulations, making it more suitable for flows close to experimental conditions, where MD produces a poor signal to noise ratio. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L07.00008: Molecular Dynamics study of the hydrodynamics in a polymeric slit pore with graphitic wall coating Diego Becerra, Andrés Córdoba, Harvey Zambrano Lab-On-a-Chip units are often designed to transport water solutions through nanopores. High resolution separations of solvated species and reduction of flow resistance are required in such systems encouraging the design of more efficient nanomembranes. Recently, scalable techniques to deposit graphene coatings on polymeric substrates have been developed. In this work, we propose to achieve drag reduction in polymeric nanochannels by using 2D graphitic materials as wall coating. Atomistic simulations are conducted to study water flow through polyamide nanoslits coated with layers of graphene and graphene oxide. We perform a detailed characterization of the effects that the polymeric matrix has on water transport through the nanoslits. The force fields describing interactions between the polyamide, graphene layers and water were calibrated by reproducing experimental contact angles. From the simulations, we extract density and velocity profiles, shear stress and orientation of water molecules at interfaces. The relation between fluid structure, flow enhancement and slip lengths is examined. Data are then used to evaluate drag reduction capabilities of graphitic coatings in polymeric nanochannels. |
Monday, November 19, 2018 5:49PM - 6:02PM |
L07.00009: Molecular dynamics study of water evaporation enhancement through a capillary graphene; bilayer with tunable hydrophilicity Hieu Trung Kieu, Bo Liu, Hui Zhang, Kun Zhou, Adrian Wing-Keung Law The acceleration of the rate of water evaporation is critically important to thermal processes in various industrial and manufacturing applications. Recently, graphene nanostructures have exhibited promising potential in the evaporation enhancement due to their porous structure, which can take advantage of the capillary effect. However, the mechanism of water evaporation through graphene nanochannels and the effect of surface properties remain unexplored. The present study (Kieu et al., 2018, Applied Surface Science, 452, 372-380) investigates the evaporation behavior of nanoscale water through a vertically aligned graphene bilayer using molecular dynamics simulations. The effects of the capillary layer distance and temperature are also examined in detail. The results show that significant enhancement of evaporation occurs when the graphene bilayer is tuned from hydrophobic to hydrophilic, and the evaporation behavior is controlled by two main factors, namely, the morphology of the liquid-gas interface and the interaction energy between the water molecules and the graphene layer. |
(Author Not Attending)
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L07.00010: All-Atoms Molecular Dynamics Study of Multilayer Holey Graphene Frameworks for Membrane Distillation Yung Chak Anson Tsang, Hieu Trung Kieu, Hui Zhang, Kun Zhou, Adrian Wing-Keung Law Membrane distillation (MD) is an emerging water desalination technique which has critical advantages over traditional pressure-driven membrane-based counterparts, including a 100% salt rejection rate. The challenge still remains in designing a membrane for MD that allows for a high level of permeate flux with superior anti-wetting and mechanical properties. Recently, holey graphene frameworks have gained much popularity due to its intrinsic network porous structure. Their low tortuosity, tunable hydrophobicity, and wide range of pore size should also make them good candidates for MD. In this study, an all-atom molecular dynamics model is developed to simulate a flexible holey graphene framework, with varied interlayer spacing and pore size as a membrane for MD. Furthermore, we investigate the permeation performance when the framework surface is doped with fluorine atoms, which has been shown to exhibit super-hydrophobicity and low thermal conductivity. GPU acceleration and OpenMP multithreading are used to increase simulation speed. The overall results shall be presented at the conference. |
Monday, November 19, 2018 6:15PM - 6:28PM |
L07.00011: Quantifying the Influence of Surface Roughness on Conductive Heat Transfer at Liquid/Solid Interfaces Hiroki Kaifu, Sandra Marina Troian Rapid developments in extreme ultraviolet lithography are soon expected to produce integrated circuit components with feature sizes of about 10 nm. Heat transfer at this scale is known to differ substantially from Fourier’s law. Interfacial or so-called Kapitza resistance, which tends to increase with diminishing system size, hinders effective heat evacuation thereby undermining device reliability. Although numerous experiments and simulations over the past few decades have elicited trends in the Kapitza resistance which correlate with system size and surface roughness, good quantitative understanding for atomistically rough interfaces is still lacking. In this talk, we discuss some of the effects on Kapitza resistance induced by surface roughness at liquid/solid interfaces as investigated by non-equilibrium molecular dynamics simulations using realistic solid walls modeled by an interacting 12-6 Lennard-Jones potential. Detailed comparison between atomistically smooth and rough interfaces contrasting such behavior as the vibrational density of states will be examined in an effort to interrelate the phonon spectrum and phonon modes transmitted across the interface with various quantitative measures characterizing interfacial roughness. |
Monday, November 19, 2018 6:28PM - 6:41PM |
L07.00012: Computational study of multivalent binding of deformable nanocarriers to the cell surface inspired by a soft matter approach Ravi Radhakrishnan, Sreeja Kutti Kandi, Portonovo S. Ayyaswamy, David M. Eckmann, Samaneh Farokhirad Functionalized nanocarriers (NCs) adhered to the target cell through multivalent interactions of their surface ligands with their corresponding cell surface receptors promise to revolutionize the biomedical field for drug delivery and remote–sensing diagnostic applications. The binding efficacy of the NC is governed by a complex set of physicochemical and physiological parameters that include its shape, size, and surface chemistry, the receptor expression levels and the mechanical state of the cell membrane and relatively little is known about the role of carrier flexibility on drug delivery. We focus on the adhesion of deformable crosslinked polymeric NCs to receptors on the cell membrane. We have developed a statistical mechanics-based mesoscale model by accounting for the mechanical properties of the NC and the cell membrane, the configurational degrees of freedom of the NC, membrane and receptor-ligand interactions. The emergent properties of the model are obtained using Monte Carlo simulations and free energy calculations. We address how the NC binding is sensitive to NC composition from very soft NC to rigid sphere and how the interplay between energetic and entropic terms of NCs, the cell membrane and the receptors play a role on the enhancement of the NC avidity. |
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