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 A22: Nano Flows: General |
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Chair: Jonathan Freund, University of Illinois at Urbana-Champaign Room: E141/142 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A22.00001: Atomic-scale thermocapillary flow in focused ion beam milling Kallol Das, Harley Johnson, Jonathan Freund Focused ion beams (FIB) offer an attractive tool for nanometer-scale manufacturing and material processing, particularly because they can be focused to a few nanometer diameter spot. This motivates their use for many applications, such as sample preparation for transmission electron microscopy (TEM), forming nanometer scale pores in thin films for DNA sequencing. Despite its widespread use, the specific mechanisms of FIB milling, especially at high ion fluxes for which significant phase change might occur, remains incompletely understood. Here we investigate the process of nanopore fabrication in thin Si films using molecular dynamics simulation where Ga$^{\mathrm{+}}$ ions are used as the focused ions. For a range of ion intensities in a realistic configuration, a recirculating melt region develops, which is seen to flow with a symmetrical pattern, counter to how it would flow were it is driven by the ion momentum flux. Such flow is potentially important for the shape and composition of the formed structures. Relevant stress scales and estimated physical properties of silicon under these extreme conditions support the importance thermocapillary effects. A continuum flow model with Marangoni forcing reproduces the flow. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A22.00002: Origin of dynamic contact angle at the nanoscale. Alex Lukyanov, Alexei Likhtman Generation of a dynamic contact angle in the course of wetting is a fundamental phenomenon of nature. Dynamic wetting processes have a direct impact on flows at the nanoscale, and therefore, understanding them is exceptionally important to emerging technologies. Here, we reveal the microscopic mechanism of dynamic contact angle generation, which is demonstrated using large-scale molecular dynamics simulations of bead−spring model fluids. It has been shown that the main cause of local contact angle variations is the distribution of microscopic force acting at the contact line region. We were able to retrieve this force with high accuracy to understand its nature and its characteristic physical parameters. It has been directly established that the force distribution can be solely predicted on the basis of a general friction law for liquid flow at solid surfaces first formulated by Thompson \& Troian on the basis of molecular dynamics simulations of Lennard-Jones liquids. The relationship with the friction law provides both an explanation of the phenomenon of dynamic contact angle and a methodology for future predictions. The mechanism is intrinsically microscopic, universal, and irreducible and is applicable to a wide range of problems associated with wetting phenomena. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A22.00003: Flow rate and slip length measurements of water in single nanopipes David Mallin, Peter Taborek, Angel Velasco Measurements of pressure driven water flows in hydrophobic and hydrophilic fused quartz capillaries of 200 nm diameter are compared. Typical flow rates on the order of 100 femtoliters and pressure drops up to 50 Atm were used. Water exited the capillaries into an oil reservoir where the volume of the pendant drop was monitored using time lapse photography. The typical growth rate for the drop diameter was \textasciitilde 50 um per day. Flow through a single nanotube can be continued for several weeks. For the untreated quartz capillary, the results are consistent with a no-slip boundary condition. The hydrophilic capillaries are chemically treated with polydimethylsiloxane (PDMS) to form hydrophobic surfaces. Successful surface preparation is confirmed with pressure threshold behavior of the water flow. Our technique can detect slip lengths above 3 nm. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A22.00004: Massive radius-dependent flow slippage in carbon nanotubes alessandro siria, Eleonora Secchi, Sophie Marbach, Antoine Niguès, Derek Stein, Lydéric Bocquet Nanofluidics is the frontier where the continuum picture of fluid mechanics confronts the atomic nature of matter. Recent reports indicate that carbon nanotubes exhibit exceptional water transport properties due to nearly frictionless interfaces and this has stimulated interest in nanotube-based membranes for desalination, nano-filtration, and energy harvesting. However, the fundamental mechanisms of water transport inside nanotubes and at water-carbon interfaces remain controversial, as existing theories fail to provide a satisfying explanation for the limited experimental results. We report a study of water jets emerging from single nanotubes made of carbon and boron-nitride materials. Our experiments reveal extensive and radius-dependent surface slippage in carbon nanotubes (CNT). In stark contrast, boron-nitride nanotubes (BNNT), which are crystallographically similar to CNTs but electronically different, exhibit no slippage. This shows that slippage originates in subtle atomic-scale details of the solid-liquid interface. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A22.00005: Macroscopic nanoporous graphene membranes for molecular-sieving-based gas separation Michael Boutilier, Rohit Karnik, Nicolas Hadjiconstantinou Nanoporous graphene membranes have the potential to exceed permeance and selectivity limits of existing gas separation membranes. This is made possible by the atomic thickness of the material, which can support sub-nanometer pores that enable molecular sieving while presenting low resistance to permeate flow. The feasibility of gas separation by graphene nanopores has been demonstrated experimentally on micron-scale areas of graphene. However, scaling up to macroscopic membrane areas presents significant challenges, including graphene imperfections and control of the selective nanopore size distribution across large areas. Towards this goal, gas permeance experiments are conducted on single and few layer graphene membranes to understand leakage pathways and a model is developed to predict conditions under which molecular sieving can occur in macroscopic membranes. Approaches to seal or mitigate the effects of micron and nanometer scale defects in graphene are investigated and methods of creating a high density of selectively permeable nanopores are explored. Experimental results demonstrating separation ratios exceeding the Knudsen effusion limit, indicating molecular sieving in agreement with the model predictions, are presented and discussed. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A22.00006: Quantifying pore size and density for membranes in the Knudsen and transitional-flow regimes Richard Castellano, Matthew Purri, Erick Hernandez, Jerry Shan, Ngoc Bui, Chiati Chen, Eric Meshot, Francesco Fornasiero Membranes with well-controlled nanoscale pores have interest for applications as diverse as chemical separations, water purification, and ``green'' power generation. For instance, membranes incorporating carbon nanotubes (CNTs) as through-pores have been shown to pass fluids orders-of-magnitude faster than predicted by theory.$^{\mathrm{1}}$ However, the efficient characterization of the pore size and density of membranes is an important area of focus, particularly for membranes fabricated from bulk nanotubes.$^{\mathrm{2}}$ Here, we report on a new technique to identify the pore size ($d)$ and number of open pores ($N)$ in membranes. A nanoporous membrane is characterized with a combination of pressure-driven gas flow, and electrical-conductance measurements in aqueous solution. For the conductance measurements, the electrical current passing through the membrane scales as $d^{2}N$. For pressurized gas flow, the scaling with molecular weight ($M)$ and gas viscosity ($\mu$) identifies the flow as either Poiseuille or Knudsen, scaling as either $d^{4}$\textit{N/$\mu$ } or $d^{3}N/M^{1/2}$, respectively. With this combination of measurements, the pore size and number of pores in the membrane can be calculated. We validate this technique using track-etched polycarbonate membranes and CNT membranes with known pores, and show that it can be used to count open pores and identify defects in CNT membranes.\newline 1) N. Biu, et al., Adv. Mat. (2016) 2) R. J. Castellano, et al., J. Applied Physics. (2015) [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A22.00007: Effect of meniscus contact angle during early regimes of spontaneous capillarity in nanochannels N.K. Karna, Elton Oyarzua, J.H. Walther, Harvey Zambrano In capillary imbibition, the classical Lucas-Washburn equation predicts a singularity as the fluid enters the channel consisting in an anomalous infinite velocity of the capillary meniscus. The Bosanquet’s equation overcomes this problem by taking into account fluid inertia predicting an initial imbibition regime with constant velocity. Nevertheless, the initial constant velocity predicted by Bosanquet's equation is much greater than experimentally observed. In the present study, we conduct atomistic simulations to investigate capillary imbibition of water in silica nanochannels with heights between 4 and 18 nm. We find that the meniscus contact angle remains constant during the inertial regime and its value depends upon the height of the channel. We also find that the meniscus velocity computed at the channel entrance is related to the particular value of the meniscus contact angle. Moreover, after the inertial regime, the meniscus contact angle is found to be time dependent for all the channels under study. We propose an expression for the time evolution of the dynamic contact angle in nanochannels which, when incorporated in Bosanquet’s equation, satisfactorily explains the initial capillary rise. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A22.00008: Carbon nanotube-based coatings to induce flow enhancement in hydrophilic nanopores. Enrique Wagemann, J. H. Walther, Harvey A. Zambrano With the emergence of the field of nanofluidics, the transport of water in hydrophilic nanopores has attracted intensive research due to its many promising applications. Experiments and simulations have found that flow resistance in hydrophilic nanochannels is much higher than those in macrochannels. Indeed, this might be attributed to significant fluid adsorption on the channel walls and to the effect of the increased surface to volume ratio inherent to the nanoconfinement. Therefore, it is desirable to explore strategies for drag reduction in nanopores. Recently, studies have found that carbon nanotubes (CNTs) feature ultrafast water flow rates which result in flow enhancements of 1 to 5 orders of magnitude compared to Hagen-Poiseuille predictions. In the present study, CNT-based coatings are considered to induce water flow enhancement in silica nanopores with different radius. We conduct atomistic simulations of pressurized water flow inside tubular silica nanopores with and without inner coaxial carbon nanotubes. In particular, we compute water density and velocity profiles, flow enhancement and slip lengths to understand the drag reduction capabilities of single- and multi-walled carbon nanotubes implemented as coating material in silica nanopores. [Preview Abstract] |
Sunday, November 20, 2016 9:44AM - 9:57AM |
A22.00009: CNT based thermal Brownian motor to pump water in nanodevices Elton Oyarzua, Harvey Zambrano, J. H. Walther Brownian molecular motors are nanoscale machines that exploit thermal fluctuations for directional motion by employing mechanisms such as the Feynman-Smoluchowski ratchet. In this study, using Non Equilibrium Molecular Dynamics, we propose a novel thermal Brownian motor for pumping water through Carbon Nanotubes (CNTs). To achieve this we impose a thermal gradient along the axis of a CNT filled with water and impose, in addition, a spatial asymmetry by fixing specific zones on the CNT in order to modify the vibrational modes of the CNT. We find that the temperature gradient and imposed spatial asymmetry drive the water flow in a preferential direction. We systematically modified the magnitude of the applied thermal gradient and the axial position of the fixed points. The analysis involves measurement of the vibrational modes in the CNTs using a Fast Fourier Transform (FFT) algorithm. We observed water flow in CNTs of 0.94, 1.4 and 2.0 nm in diameter, reaching a maximum velocity of 5 m/s for a thermal gradient of 3.3 K/nm. The proposed thermal motor is capable of delivering a continuous flow throughout a CNT, providing a useful tool for driving liquids in nanofluidic devices by exploiting thermal gradients. [Preview Abstract] |
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