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Vapor bubble nucleation by rubbing surfaces: Molecular dynamics simulations
3. K. G. Ikels, “Production of gas bubbles in fluids by tribonucleation,” J. Appl. Physiol. 28, 524–527 (1970).
5. P. McDonough and E. Hemmingsen, “Bubble formation in crabs induced by limb motions after decompression,” J. Appl. Physiol. 57, 117–122 (1984).
9. S. Wildeman
, H. Lhuissier
, C. Sun
, A. Prosperetti
, and D. Lohse
, “Writing bubbles
,” Bulletin of the American Physical Society – 65th Annual Meeting of the APS Division of Fluid Dynamics, San Diego, CA, 18–20 November 2012
. Available online at http://meetings.aps.org/link/BAPS.2012.DFD.A11.2
12.The apparent contact area Aap between a sphere and a solid plate can be estimated from Hertz's equation as , where E1 and E2 are the Young's moduli of the sphere and the plate, respectively.25 Substituting E1 = 420 GPa (sapphire) and E2 = 400 GPa (alumina which is expected to cover the surface of the aluminum plate) yields the value for Aap, while Aac follows from Eq. (1) with M ∼ 15 GPa for alumina.
13.The simulation system is large enough to behave as infinite with respect to nucleation. Indeed, if the kinetic energy of the released molecules was dissipated equally throughout the whole of the bulk, the liquid temperature would only increase by about 2 K, which is only 5% of the actual superheat (= 41 K). This means that the transfer of energy across the (non-physical) periodic boundaries has a negligible influence on the nucleation process.
14. J. Weijs, J. H. Snoeijer, and D. Lohse, “Formation of surface nanobubbles and the universality of their contact angles: A molecular dynamics approach,” Phys. Rev. Lett. 108, 104501 (2012).
15. V. G. Baidakov, S. P. Protsenko, and Z. R. Kozlova, “Shear and bulk viscosity in stable and metastable states of a Lennard-Jones liquid,” Chem. Phys. Lett. 517, 166–170 (2011).
LAMMPS is an open source code for a classical molecular dynamics on, e.g., solid-state materials, soft matter and droplets, distributed by Sandia National Laboratories. Available at http://lammps.sandia.gov/
18.The Nosé-Hoover method,26 by which a canonical ensemble is able to be obtained, was used with success for the equilibration of the liquid. However, it did not work for the thermostat layer in the solids. We think that this is because the molecules of which kinetic energy should be controlled were bonded with molecules constrained to be motionless.
19. B. R. Novak, E. J. Maginn, and M. J. McCready, “Comparison of heterogeneous and homogeneous bubble nucleation using molecular simulations,” Phys. Rev. B 75, 085413 (2007).
20.During the depressurization, only the number of molecules and the temperatures of the block and the substrate are actually constrained to be constant. However, as for a real liquid far away from the critical point, the simulated liquid is almost incompressible, and therefore its temperature is almost constant under depressurization (the global variation we observed was below the statistical fluctuations). Eventually, both the liquid and solid temperatures were unchanged by the depressurization process.
21.We consider here a two-dimensional critical bubble, since for a three-dimensional configuration the critical diameter 2Rc = 2γ/(psat − p∞) ≃ 7.6 nm is much larger than the computational domain width Y, which means that the system behaves as a two-dimensional fluid with respect to nucleation.27
24.The values are averaged over 5.6 ps over a rectangular mesh of 0.5 × 4.8 × 0.5 nm3. The temperature is defined as m⟨v − ⟨v⟩⟩2/3k, where v is velocity of each molecule, k is the Boltzmann constant, and ⟨·⟩ denotes the averaging.
25. F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids (Oxford University Press, New York, 1950).
27. V. P. Skripov, Metastable Liquids (Wiley, New York, 1974).
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We propose a new mechanism for bubble nucleation triggered by the rubbing of solid surfaces immersed in a liquid, in which the fluid molecules squeezed between the solids are released with high kinetic energy into the bulk of the liquid, resulting in the nucleation of a vapor bubble. Molecular dynamics simulations with a superheated Lennard-Jones fluid are used to evidence this mechanism. Nucleation is observed at the release of the squeezed molecules, for squeezing pressures above a threshold value and for all the relative velocities between the solids that we investigate. We show that the total kinetic energy of the released molecules for a single release event is proportional to the number of molecules released, which depends on the squeezing pressure, but is independent of the velocity.
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