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Review Article

Nanoscale Fluid Mechanics and Energy Conversion

[+] Author and Article Information
Xi Chen

Department of Earth and Environmental
Engineering, Columbia Nanomechanics
Research Center,
Columbia University,
New York, NY 10027;
International Center for Applied Mechanics,
SV Lab, School of Aerospace,
Xi'an Jiaotong University,
Xi'an 710049, China
e-mail: xichen@columbia.edu

Baoxing Xu

Department of Earth
and Environmental Engineering,
Columbia Nanomechanics Research Center,
Columbia University,
New York, NY 10027

Ling Liu

Department of Mechanical
and Aerospace Engineering,
Utah State University,
Logan, UT 84322

Manuscript received August 22, 2013; final manuscript received February 4, 2014; published online May 29, 2014. Assoc. Editor: Bettina Frohnapfel.

Appl. Mech. Rev 66(5), 050803 (May 29, 2014) (17 pages) Paper No: AMR-13-1063; doi: 10.1115/1.4026913 History: Received August 22, 2013; Revised February 04, 2014

Under nanoconfinement, fluid molecules and ions exhibit radically different configurations, properties, and energetics from those of their bulk counterparts. These unique characteristics of nanoconfined fluids, along with the unconventional interactions with solids at the nanoscale, have provided many opportunities for engineering innovation. With properly designed nanoconfinement, several nanofluidic systems have been devised in our group in the past several years to achieve energy conversion functions with high efficiencies. This review is dedicated to elucidating the unique characteristics of nanofluidics, introducing several novel nanofluidic systems combining nanoporous materials with functional fluids, and to unveiling their working mechanisms. In all these systems, the ultra-large surface area available in nanoporous materials provides an ideal platform for seamlessly interfacing with nanoconfined fluids, and efficiently converting energy between the mechanical, thermal, and electrical forms. These systems have been demonstrated to have great potentials for applications including energy dissipation/absorption, energy trapping, actuation, and energy harvesting. Their efficiencies can be further enhanced by designing efforts based upon improved understanding of nanofluidics, which represents an important addition to classical fluid mechanics. Through the few systems exemplified in this review, the emerging research field of nanoscale fluid mechanics may promote more exciting nanofluidic phenomena and mechanisms, with increasing applications by encompassing aspects of mechanics, materials, physics, chemistry, biology, etc.

Copyright © 2014 by ASME
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References

Figures

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Fig. 1

Representative forms of nanoconfinement: (a) zeolites (Reprinted with permission from Ref. [15]. Copyright 2008 The American Chemical Society.); (b) carbon nanotubes (Reprinted with permission from Ref. [16]. Copyright 2001 Nature Publishing Group.); (c) TEM micrograph of a water strand confined in a carbon nanotube with the diameter of 2 nm (Reprinted with permission from Ref. [17]. Copyright 2008 Springer Science and Business Media.); (d) pores in biological protein (e.g., GlpF, Reprinted with permission from USA Ref. [18]. Copyright 2008 The National Academy of Sciences.); and (e) graphene-based material systems. (Reprinted with permission from Ref. [19]. Copyright 2012 The Royal Society of Chemistry.).

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Fig. 2

(a) Density and (b) velocity profiles of water molecules in CNTs (Reprinted with permission from Ref. [28]. Copyright 2008 Taylor & Francis Publishing Group.). In the density profile, only the region close to CNT wall is shown; different curves correspond to CNTs of different sizes. The velocity profile is shown by discrete data points. (c) MD simulation snapshots of pressurized infiltration of three different electrolyte solutions into a molecular sized silicon dioxide nanotube. (Reprinted with permission from Ref. [29], copyright 2009 The American Physical Society, and reprinted with permission from Ref. [30], copyright 2010 IOP Publishing, respectively) Ions inside the nanotube are found to form couples. Both the separation between two neighboring ion couples and the distance between two paired ions are found to be dependent on ion size.

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Fig. 3

Overview of energy conversion underpinned by the science of nanofluid mechanics. The ultra-large surface of nanopores and unique interactions between functional liquid molecules and solids enable high performance energy conversion between the mechanical, thermal, and electrical forms. Reprinted with permission from New York, NY. from Ref. [64]. Copyright 2012 Columbia University.

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Fig. 4

(a) Schematic setup of the pressure-volume controller fabricated in stainless steel that can be used as the apparatus for testing the system combining nonwetting liquids with nanoporous materials (Reprinted with permission from New York, NY. Ref. [65]. Copyright 2010 Columbia University.). (b) Schematic illustration of typical volume change with applied pressure at quasi-static loading for non reusable systems. The area enclosed by the hysteresis roughly equals to the total energy absorbed. (c) Loading-unloading sorption isotherm for a reusable system (Reprinted with permission from New York, NY. Ref. [65]. Copyright 2010 The American Chemical Society, and reprinted with permission from Ref. [66]. Copyright 2009 Columbia University, respectively). (d) Temperature variations (raw data measured from thermocouple and the calibrated pressure-dependency-corrected net increment) during the first three loading–unloading cycles for the reusable system.

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Fig. 5

(a) The two effects governing nanofluidic infiltration, defiltration, and transport: capillary effect and viscosity effect. During infiltration, both effects impede liquid invasion. In defiltration, viscosity effect continues to resist the liquid flow while the capillary effect offers assistance. The molecular mechanisms of both effects are illustrated in (b) and (c), respectively.

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Fig. 6

(a) Schematic of the loading–unloading hysteresis of a nanoporous energy absorption system with two possible defiltration results: full-defiltration (dashed line) and partial/non-defiltration (solid line). (b) Infiltration pressure and transport slope may be fine-tuned by a number of system and material variables, including the pore diameter (D), half apex angle of conical pores (α), flow velocity (v), and temperature (T).

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Fig. 7

Schematic of the energy trapping mechanism. Upon an impact or blast stress wave, the liquid molecules become highly compacted as they intercalate into the nanopores. Within a short period of time, a large amount of impact/blast energy can be converted into the potential energy of liquid confined in nanopores, leading to substantial mitigation of the impact/blast stress wave. (Reprinted with permission from Ref. [80]. Copyright 2014 Elsevier.).

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Fig. 8

(a) The MD computational cell consisting of an impactor, piston, water reservoir, boundary plane of reservoir, nanopore CNT, and receiver (from left to right). The left open-end of the nanopore is inserted into the reservoir and is tied with the right rigid boundary plane of the reservoir, and the right end of the nanopore is closed; the receiver is fixed to deduce the transmitted force through the nanopore/water system. (b) Snapshots of water molecules invading and receding from the (16, 16) CNT under impacting velocity (v) of 50 m/s, where the impactor, piston, and bound of reservoir are not shown. The right blue line represents the receiver position. (c) The history of transmitted force F. (d) Number of infiltrated water molecules in the CNT during impacting process. The diameter (D) and length (L) of the CNT are 2.17 nm and 9.84 nm, respectively. (Replotted with permission from Ref. [80]. Copyright 2014 Elsevier.).

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Fig. 9

(a) The reduction of the peak transmitted force ΔF and the total energy mitigation ΔW upon impact loading. (b) The ratio between the trapped potential energy of the intercalated water molecules and the total impact energy ETrap/EInput. The CNT size: D = 2.17 nm, L = 9.84 nm. (Reprinted with permission from Ref. [80]. Copyright 2014 Elsevier.).

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Fig. 10

(a) Normalized density profile, ρ(L)/ρ0, of water molecules along the axial direction of the CNT/water system (i.e., the impacting direction) at different moments under the impacting velocity of v = 50 m/s. The CNT size: D = 2.17 nm, L = 9.84 nm and ρ0 = 998.0 kg/m3 is the density of bulk water at 300 K and 1 atm (Reprinted with permission from Ref. [80]. Copyright 2014 Elsevier.); and (b) illustration of “freight train” energy trapping and releasing mechanism by water molecules into and out of CNTs, where each train reflects highly compacted water cluster with high trapped energy, and the arrow is the impacting direction.

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Fig. 11

A typical impacting experiment on nanoporous silica gel with the average nanopore size of 100 nm under an effective strain rate of around 3 × 102/s

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Fig. 12

Mechanisms of nanofluidic actuation: similar to (a) thermocapillary and electrocapillary effects (Reprinted with permission from Ref. [81]. Copyright 1992 Taylor & Francis Publishing.), (b) a liquid flow can be actuated by temperature or electric field in a nanochannel. Similar mechanisms also apply to (c) nanoporous materials that have numerous nanopores immersed in non wetting liquid

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Fig. 13

Thermal dependence of nanofluid infiltration. (a) MD model of water infiltration into CNTs. (b) Applied pressure versus the number of water molecules entering a (10,10) CNT. Infiltration pressure Pin is defined by the threshold pressure for water to enter the CNT. (c) Temperature and size dependencies of Pin. CNT size is indicated by its chirality (c,c); a larger “c” represents a larger size of CNT. (Replotted with permission from Ref. [64]. Copyright 2012 Columbia University.). (d) Experimental results to demonstrate the temperature dependence of infiltration pressure for nanoporous carbon.

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Fig. 14

(a) For a representative themo-actuation system [(18,18) CNT/water], temperature is varied to show the dynamic infiltration/defiltration processes driven by temperature change. (b) The maximum energy density Wmax per mass. Here the error bar represents the small variation of Wmax with respect to the temperature range considered. (c) Output power density per mass. (d) Efficiency, η, of the thermo-actuation system (Reprinted with permission from Ref. [75]. Copyright 2011 The Royal Society of Chemistry.).

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Fig. 15

Experimental results of infiltration pressure and output power for a thermal actuation system (ZSM-5 zeolite/water). (Reprinted with permission from Ref. [75]. Copyright 2011 The Royal Society of Chemistry.)

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Fig. 16

(a) For a representative electro-actuation system with CNT and water, Pin is shown to have strong dependencies on the size of CNT, and the intensity of the applied electric field, E. (Reprinted with permission from Ref. [74]. Copyright 2011 The American Chemistry Society.) (b) Energy conversion efficiency, η, of the system also varies with E and CNT size. (Reprinted with permission from Ref. [88]. Copyright 2011 The American Institute of Physics.)

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Fig. 17

(a) Radial number density of Cl and H3O+ ions in a (20,20) CNT. Tube size is 2.712 nm, and the concentration of HCl solution is 0.9 mol/l. The left axis is aligned with the tube axis. (b) Accumulated displacement of Cl ions inside FSS at the applied velocity v0=9 m/s and temperature of 300 K. (c) Variation of the drifting velocity of Cl ions in FSS and (d) induced voltage in axial direction, as a function of the flow rate at different temperatures. (Reprinted with permission from Ref. [95]. Copyright 2013 The Royal Society of Chemistry.).

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Fig. 18

(a) Average radial density profiles for different atoms and ions inside a (20,20) CNT at 300 K. (b) Distribution of the electrical potential, φ(r), across the CNT. NaCl solution is 1.5 mol/l and the left axis is aligned with the tube axis. (Reprinted with permission from Ref. [105]. Copyright 2012 Elsevier.).

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Fig. 19

(a) Schematic of energy generation by two liquid–nanopore interfaces operated under different temperatures. (b) Variation of the net output voltage ΔU with respect to heat grade. (c) Illustration of ions in FSS with a NaCl concentration of 0.5 mol/l, where R = D/2 represents the radius of the CNT, and a is the inner radius of FSS; (d) effect of pore size and ion concentration on the thermal-to-electrical energy convention efficiency; the baseline temperature TL = 300 K. (Reprinted with permission from Ref. [105]. Copyright 2012 Elsevier.).

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Fig. 20

(a) Schematic of experimental setup for harvesting thermal energy from heat grade based on a nanoporous carbon/NaCl solution system; (b) the output voltage versus temperature difference between the cells (Reprinted with permission from Ref. [105]. Copyright 2012 Elsevier.)

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Fig. 21

(a) Schematic illustration of a nanofluidic mechanical-to-electric energy conversion system: as the piston is driven to move by external mechanical excitations, an electrolyte solution is forced to flow through a nanoporous electrode (Reprinted with permission from Ref. [51]. Copyright 2012 The Japan Society of Applied Physics.). The confined electrolytic flow varies the cross-interface electric potential and generates a voltage of ΔU between the nanoporous electrode and a reference electrode residing in the bulk electrolyte solution. (b) U versus flow velocity, v, in a CNT with D = 20.34 Å.

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