Berker, A. Nihat

Bocquet, L.

Bonthuis, D. J.

Falk, K.

Horinek, D.

Kaplan, C. N.

Netz, R. R.

English

D. J. Bonthuis

K. Falk

C. N. Kaplan

D. Horinek

A. N. Berker

L. Bocquet

R. R. Netz

Crossref

Physical Review Letters

Comment on “Pumping of Confined Water in Carbon Nanotubes by Rotation-Translation Coupling”

Sabanci University Research Database

Comment on ‘‘Pumping of Confined Water in CarbonNanotubes by Rotation-Translation Coupling’’Carbon nanotubes (CNT) allow easy entrance and pas-sage of water [1] and are thus of interest for microfluidics.In electro-osmosis, charged double layers are set intomotion by external electric fields. In recent simulations,electro-osmosis of water inside a CNT in the absence offree charges was observed and rationalized by invoking anelectric-field-driven torque-vorticity conversion at orientedinterfacial water molecules [2]. Though exciting at firstsight, this result seems at odds with both Onsager’s recip-rocal theorem (since an externally applied pressure dropcannot cause a steady electric current due to the absence offree charges) and thermodynamics (since the electric fieldperforms no work in the steady state). In this Comment, wepresent evidence that results presented in [2] stronglydepend on the cutoff scheme implementation in GROMACS.We concentrate on (16, 0) CNTs for which a pronounceddependence of the water flux on the electric field strengthwas demonstrated [2]. In the thinner (10, 0) CNTs, a long-lived nonzero flux is also obtained without electric field,showing that the simulations do not converge well enoughto yield the true steady state [2,3]. We perform moleculardynamics simulations of SPC/E water in a 9.8 nm long(16, 0) CNT connected to a reservoir (4:0 3:6 2:0 nm)in the presence of an axial electric field using GROMACS [4].We use a Nose´-Hoover thermostat, an anisotrotropicParrinello-Rahman barostat, periodic boundary conditions,and particle mesh Ewald summation for the electrostatics.For the Lennard-Jones interaction we compare the simplecutoff scheme, which sets the force to zero beyond rc andthus creates a force discontinuity at r ¼ rc, as employed in[2], with the shifted cutoff scheme, where the force decaysto zero [4]. We simulate for 5 ns and collect the last 3 ns foranalysis, for rc ¼ 1:0 nmwe extend the simulation time by40 ns. In the inset of Fig. 1 we show the cumulative flux ofwater molecules through the cross section of the CNT atone end (open symbols) versus time for both cutoffschemes. We also show the conditional cumulative flux(filled symbols), according to [2] defined as the number ofmolecules exiting the tube on one end that have previouslyentered on the other end. Except the lag time in the condi-tional flux of about 8 ns, caused by the time it takes a watermolecule to traverse the CNT, the results for the two fluxdefinitions are similar and show a striking yet unphysicaldifference between the cutoff schemes: For the simplecutoff we find an average flux (obtained from a linear fitto the cumulative flux) of 22 8 ns1, comparable to34:0 5:2 ns1 from [2], while the shifted cutoff exhibitsvanishing flux. In Fig. 1 we plot the average flux versus rc.In the limit rc ! 1, the spurious difference between thecutoff schemes disappears and the water flux vanishes, inaccord with basic thermodynamics and generalized hydro-dynamic theory [5]. Using the alternative simulation pack-age LAMMPS [6] the flux vanishes, even for small rc,regardless of the cutoff scheme (see Fig. 1). The meanwater orientation does not depend on the cutoff scheme andthus is unrelated to the flux [3]. We conclude that electric-field-induced steady flow of pure water in a CNT is asimulation artifact related to the implementation of thesimple cutoff in GROMACS.D. J. Bonthuis,1 K. Falk,2 C. N. Kaplan,1 D. Horinek,1A.N. Berker,1,3 L. Bocquet,1,2 and R. R. Netz11Physik DepartmentTechnische Universita¨t Mu¨nchen85748 Garching, Germany2Laboratoire PMCNUniversite´ Lyon 1, CNRSUMR 5586 69622 Villeurbanne, France3Sabanc U¨niversitesi,Orhanl-Tuzla, 34956 Istanbul, TurkeyReceived 12 June 2009; published 8 November 2010DOI: 10.1103/PhysRevLett.105.209401PACS numbers: 47.61.k, 68.15.+e, 81.07.De[1] A. Berezhkovskii and G. Hummer, Phys. Rev. Lett. 89,064503 (2002).[2] S. Joseph and N. R. Aluru, Phys. Rev. Lett. 101, 064502(2008).[3] D. J. Bonthuis et al. (unpublished).[4] D. van der Spoel et al., J. Comput. Chem. 26, 1701 (2005).[5] D. J. Bonthuis et al., Phys. Rev. Lett. 103, 144503 (2009).[6] S. Plimpton, J. Comput. Phys. 117, 1 (1995).0.6 0.8 1.0 1.2 1.4 1.6rc [nm]0200400600800average flux [#/ns]time [ns]cumulative flux [#]20 30 401000200400600800 simple shiftedconditional fluxfluxrc = 1.0 nm, GROMACSsimple shiftedGROMACSLAMMPSFIG. 1 (color online). Axial average flux (water molecules perns) through a (16, 0) CNT for electric field Ez ¼ 1:0 Vnm1versus cutoff length rc for simple and shifted cutoff schemes inGROMACS and LAMMPS. The inset shows the conditional(as defined in [2]) and the unconditional cumulative flux forrc ¼ 1:0 nm in GROMACS as a function of time.PRL 105, 209401 (2010) P HY S I CA L R EV I EW LE T T E R Sweek ending12 NOVEMBER 20100031-9007=10=105(20)=209401(1) 209401-1  2010 The American Physical Society

Comment on "pumping of confined water in carbon nanotubes by rotation-translation coupling"

Bonthuis, DJ

Falk, K

Kaplan, CN

Horinek, D

Berker, AN

Bocquet, L

Netz, RR

Oxford University Research Archive

Comment on "pumping of confined water in carbon nanotubes by rotation-translation coupling".

Oxford University Research Archive (ORA)

http://research.sabanciuniv.edu/15107/1/Nanotube.pdf

Comment on "pumping of confined water in carbon nanotubes by rotation-translation coupling"

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