MD simulation of methane in nanochannels

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1 MD simulation of methane in nanochannels COCIM, Arica, Chile M. Horsch, M. Heitzig, and J. Vrabec University of Stuttgart November 6, 2008

2 Scope and structure Molecular model for graphite and the fluid-wall interaction Scalability of the MD simulation Poiseuille/Couette flow of methane through graphite channels Cluster criteria for systems with vapor-liquid coexistence Vapor-liquid equilibria of methane under confinement Contact angle dependence on the fluid-wall interaction

3 Methane confined in a nanochannel z Poiseuille flow: The fluid is accelerated in z direction Couette flow: x y The walls are accelerated in z direction Contact angle: Meniscus perpendicular to the z axis

4 State of the art: system size Vishnyakov et al L 1,5 nm Langmuir 15: 8736 Werder et al L 7,5 nm Nano Lett. 1: 697 Diameter / number of particles Sokhan et al L 2,8 nm N wall = 2000 J. Chem. Phys. 117: 8531 Cui und Cochran 2004 L 200 nm (ρ Ionen 0,02 mol/l) Mol. Sim. 30: 259 Dimitrov et al L 20 σ LJ 6 nm N fluid = PRL 99: Present work 2008 L up to 75 nm N up to 4,8 million

5 Scaling with isotropic domain decomposition Molecular dynamics code Mardyn (developed by the ls 1 project): n P x runtime / N x time steps [µs] wall fluid+wall fluid number of concurrent processes n P N = N = 3000n P N = 32000n P

6 State of the art: potential models fluid wall fluid-wall Vishnyakov et al CLJQ rigid Steele Langmuir 15: 8736 (CO 2 ) (10-4-3) Werder et al SPC Walther et al. LJ Nano Lett. 1: 697 (water) (carbon) Sokhan et al LJ Tersoff-Brenner LJ J. Chem. Phys. 117: 8531 (carbon) Liu und Wang 2005 SPC rigid LJ Phys. Rev. B 72: (water) Dimitrov et al LJTS LJTS, elastic LJTS PRL 99: (LJTS LJ, truncated and shifted at r ij = 2,5σ) Present work 2008 LJTS Tersoff LJTS (also: springs)

7 Reparametrization of the Tersoff potential U(bond length) Bond length Tersoff potential: Å Actual graphite: Å RDF new old length in units of Å More adequate potential parameters for graphite: Cutoff Attraction radius in units of Å Repulsion R = 2.0 Å (1.8 Å) S = 2.35 Å (2.1 Å) µ = Å -1 ( Å -1 ) λ = Å -1 ( Å -1 )

8 Integration time step U [ev] p [kpa] t [ps] A time step of 1 fs leads to an acceptable accuracy.

9 Uniform acceleration (PI controller) Poiseuille, v target = 10 m/s, τ = 3 ps, L = 15 nm, liquid CH 4 at 166 K flow velocity in units of m/s τ aɺ = v v τvɺ target current a z mean over 50 ps time in units of ps acceleration in units of nm/ns 2

10 Fluid-wall interaction Lennard-Jones energy parameter: fluid density in units of mol/l Lennard-Jones size parameter: ε σ FW FW = ξ ε = η σ ξ = η = 1 ξ = η = 1 ξ = η = ξ = η = Wang et al. Kalra et al y coordinate in units of nm FF FF T = 0.95 ε/k ρ = ρ

11 MD simulation of Couette flow

12 Poiseuille and Couette flow: velocity profile T = 0.95 ε/k, ρ = ρ, v target = 10 m/s, ξ = 0.353, η = (Wang et al.) v z [m/s] ns ns y [nm] v z [m/s] ns ns y [nm] Couette flow Poiseuille flow

13 Comparison of cluster criteria criterion a 2 criterion g 2

14 Dependence of the contact angle on ξ y [σ] T = 0,73 ε/k B 75 o ξ = 0,16 ξ = 0, o y [σ] o 120 o T = 1,00 ε/k B ξ = 0,11 ξ = 0, z [σ] 8 10 z [σ]

15 Conclusion System size: acceptable scaling of Mardyn, L up to 75 nm easily possible the system geometry requires (at least static) load balancing Flow simulations: were carried out in the canonical ensemble unknown interaction parameters ξ and η Vapor-liquid interface: dependence of the contact angle on ξ was studied suitable criterion for the interface: ρ ρ ' ρ '' J. Chem. Phys. 128: Phys. Rev. E 78:

Molecular modeling of hydrogen bonding fluids: vapor-liquid coexistence and interfacial properties

12 th HLRS Results and Review Workshop Molecular modeling of hydrogen bonding fluids: vapor-liquid coexistence and interfacial properties High Performance Computing Center Stuttgart (HLRS), October 8,