1. nanoHUB.org online simulations and more Fouling Mechanisms in Y-shaped Carbon Nanotubes Jason Myers, SeongJun Heo, and Susan B. Sinnott Department of Materials Science and Engineering University of Florida Funded by the Network for Computational Nanotechnology at Purdue University, NSF Grant No. EEC -02288390

2. nanoHUB.org online simulations and more Background Computational Methods System Design Results Conclusions Outline

3. nanoHUB.org online simulations and more The chemical and biomedical fields have a constant demand for solutions of greater purity. Current filtration methods (zeolites) do not offer uniform pore size, and are susceptible to fouling. Carbon nanotubes (CNTs) have the potential to be custom designed for optimal molecular filtration. The Need for Filtration

4. nanoHUB.org online simulations and more Carbon nanotubes (CNTs): Honeycomb graphene lattice rolled into a cylinder Ajayan and Zhou (2001) Sinnott et al. (2002) Nanofluidics: Confinement of fluids to nanopores Discovered by Ijima, et al in 1991 Outstanding mechanical properties Nanometer size enables precise molecular transport Carbon Nanotubes

5. nanoHUB.org online simulations and more Analagous to a rolled graphene sheet. - One-dimensional axial symmetry. - Spiral conformation: Chirality Carbon Nanotubes, cont. Chiral vector C h = (n,m) = na 1 + ma 2 Zigzag (10,0) Armchair (6,6) Chiral (7,4)

6. nanoHUB.org online simulations and more Small arm – large molecule is energetically discouraged from entering Large arm – no similar barrier for large molecules Result? Only the small molecule will pass through the small arm. Y-shaped CNTs

7. nanoHUB.org online simulations and more Background Computational Methods System Design Results Conclusions Outline

8. nanoHUB.org online simulations and more Molecular Dynamics Classical molecular dynamics (MD) simulations (numerically integrating F = ma) Reactive Empirical Bond Order (REBO) Potential Lennard-Jones (LJ) Potential Covalent Interaction van der Waals Interaction For more details on REBO-MD, see Wen-Dung Hsu’s Breeze presentation.

9. nanoHUB.org online simulations and more Background Computational Methods System Design Results Outline

10. nanoHUB.org online simulations and more Y-shaped CNTs (14,0)11.12 (10,0)8.18 (8,0)6.35 “Ytube1”“Ytube2” (8,8)11.00 (6,6)8.23 (5,5)6.92 Branch: Big arm: Small arm: CNTDiameter, ÅCNTDiameter, Å

11. nanoHUB.org online simulations and more Three different molecules: Methane Isobutane N-butane Rigid Argon Box Push-plate 10, 5, 3, and 0 m/s Reservoirs

12. nanoHUB.org online simulations and more Each system consists of a Y-shaped CNT and reservoir. ReservoirBranchArms Direction of Flow Thermostat Rigid Active System Design

13. nanoHUB.org online simulations and more Background Computational Methods System Design Results Methane Isobutane + Methane N-butane + Methane Conclusions Outline

14. nanoHUB.org online simulations and more Methane Ytube1Ytube2 6.35 Å 6.92 Å

15. nanoHUB.org online simulations and more Isobutane + Methane Ytube1Ytube2 Filtered methane Blocking Isobutane

16. nanoHUB.org online simulations and more N-butane + Methane Ytube2, 10 m/s at… 0.48ns 0.27ns Stationary n-butane Aligned n-butane

17. nanoHUB.org online simulations and more Summary Ytube1 shows no tendency for filtration. There is evidence of size-based diffusion in the methane systems. Ytube2 shows no similar behavior. The isobutane + methane systems exhibit fouling. This is attributed to the steric interactions of the isobutane molecule with the junction area, and is not due to a potential energy well. Prior to the formation of the block, filtration occurred in ytube2. There is neither fouling nor filtration in the n-butane + methane systems. Once the driving force is sufficient, the n-butane aligns itself to pass easily down both arms.

18. nanoHUB.org online simulations and more Background Computational Methods System Design Results Conclusions Outline

19. nanoHUB.org online simulations and more Conclusions Y-shaped carbon nanotubes exhibit promising signs of filtration. However, they tend to clog due to molecular steric interactions. Linear molecules (n-butane) avoid fouling, but prevent filtration. System redesign with these factors in mind is needed.