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Molecular dynamics study of the lifetime of nanobubbles on the substrate Division of Physics and Astronomy, Graduate School of Science, Kyoto University.

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Presentation on theme: "Molecular dynamics study of the lifetime of nanobubbles on the substrate Division of Physics and Astronomy, Graduate School of Science, Kyoto University."— Presentation transcript:

1 Molecular dynamics study of the lifetime of nanobubbles on the substrate Division of Physics and Astronomy, Graduate School of Science, Kyoto University KOHNO Shunsuke GCOE Symposium 2012 - Links of Hierarchies

2 Outline ･ Introduction – nanobubbles - stability of nanobubbles - preceding studies - purpose of this study GCOE Symposium 201214 Feb. 2012 ･ Molecular dynamics simulations ･ Numerical results – in the bulk liquid vs on the substrate - Knudsen gas ? ･ Summary and remarks

3 Nanobubbles ･ One cause of microscale behavior of fluids GCOE Symposium 201214 Feb. 2012 Schematic illustration of typical nanobubble on a substrate ･ radius ~< 1μm Borkent et al. Langmuir, 26, 260 (2010) ･ Observed in experiments using AFM or IR spectroscopy

4 Nanobubbles are stable ? ･ Theoretical prediction - Nanobubbles are unstable Young-Laplace equation - High pressure in nanobubbles --> Gas in nanobubbles dissolve immediately GCOE Symposium 2012 cf. Ljunggren and Eriksson, Colloids Surf. A, 130, 151 (1997) 14 Feb. 2012 ･ In experiment - Nanobubbles are stable lifetime > 4days lifetime < 100μs (spherical bubble, diameter = 100nm)

5 Suggestions for stability GCOE Symposium 201214 Feb. 2012 1.Young-Laplace equation is NOT applicable to nanobubbles. 2.Young-Laplace equation IS applicable to nanobubbles. But surface tension is reduced and bubbles shrink slowly. 3. Gas atoms in the bubble do dissolve, but outflux is balanced by the influx in nonequilibrium state.

6 Classical theory is applicable ? GCOE Symposium 201214 Feb. 2012 1. Classical diffusion theory is not applicable ? - MD simulations of nucleation of stable nanobubbles ･ single component real nanobubbles are filled with gas Nagayama et al. J. Heat. Mass. Trans., 49, 4437 (2006) Young-Laplace equation is applicable to nanobubbles. ( confirmed in many MD simulations of nucleation evaluating surface tension )

7 Why nanobubbles are stable ? GCOE Symposium 201214 Feb. 2012 2. Reduction of surface tension - Theoretical model and experiments 3. Nonequilibrium stabilizing mechanism : Outflux is balanced by an equivalent influx more than x10 9 lifetime ? -> not enough Das et al. Phys. Rev. E, 82, 056310 (2010) -> Nanobubbles shrink slowly ･ Impurities on gas-liquid interface - Reduce surface tension - Prevent dissolution of gas

8 Dynamic Stabilizing Mechanism GCOE Symposium 201214 Feb. 2012 Gas enrichment near the hydrophobic wall - in MD simulations Dammer and Lohse, Phys. Rev. Lett., 96, 206101 (2006) influx near the contact line balanced outflux - theoretical model Brenner and Lohse, Phys. Rev. lett., 101, 214505 (2008) the origin of the nonequilibrium : wall = heat bath -> heat flux Knudsen gas circulating flow - theoretical model and experiments Seddon et al. Phys. Rev. Lett., 107, 116101 (2011) need to be nonequilibrium ･ These models have not confirmed How Nonequilibrium state sustained for a long time ?

9 Purpose of this study GCOE Symposium 201214 Feb. 2012 ･ Investigate dynamics of nanobubbles -> Molecular dynamics simulations ･ Only a few MD simulations of nanobubbles in ternary systems ( liquid + gas + wall ) have performed. -> Create stable nanobubbles and survey shrinking nanobubbles ･ Introduce nonequilibrium ( transient heat flux ) -> Thermal wall

10 Molecular Dynamics Simulations GCOE Symposium 201214 Feb. 2012 ･ Interaction – Lennard-Jones 6-12 potential ･ N,V,(T) = const. ･ Liquid and gas – Ar and Ne respectively ･ Fixed wall and thermal wall

11 Two Steps of Simulations GCOE Symposium 2012 Step1 : Creation of nanobubbles T = 85K ･ Nanobubbles are stable ･ Bubble nucleation occurs spontaneously ･ Negative system pressure ( stretched state ) 14 Feb. 2012 Step2 : Simulations of shrinking nanobubbles T = 104K ･ Nanobubbles are unstable ･ Bubble nucleation does not occur ･ Positive system pressure Step2 -> Step1 : Quenching ( bubble nucleation )

12 Lifetime of Nanobubbles GCOE Symposium 201214 Feb. 2012 In a bulk liquid < 50ps On a fixed wall > 8000ps Averaging 2000ps – 3000ps ?

13 Inhomogeneity of the System GCOE Symposium 201214 Feb. 2012 0.06MPa 2.80MPa 7.60MPa 12.68MPa 16.47MPa 4.73MPa Low pressure near the bubble Increases lifetime of the bubble Averaged pressure ･ 2000ps – 3000ps ･ 5 regions parallel to the wall

14 14 Feb. 2012 Knudsen Gas Behavior ? GCOE Symposium 2012 ･ No anisotropic motion of gas atoms observed transient heat flux exists : wall -> liquid & gas(thermal wall)

15 Summary GCOE Symposium 201214 Feb. 2012 ･ Lifetime of nanobubbles on a substrate is more than 100 times longer than in a bulk liquid. ･ Inhomogeneity of the system increases the lifetime of nanobubbles ･ Knudsen gas behavior is not observed. ･ Lifetime of simulated nanobubbles was shorter than 1μs. ( No stabilizing mechanism is implemented in simulations. ) ･ Thermal wall is also employed to realize transient nonequilibrium state.

16 Remarks GCOE Symposium 2012 ･ Knudsen number in simulated nanobubble Kn~2 is slightly higher than proposed condition. -> Create lower density nanobubbles 14 Feb. 2012 ･ More detailed analysis is needed to investigate flux of atoms. ･ Simulated nanobbubbles are smaller than real nanobubbles. -> Larger calculations

17 Microfluidics* Microscale behavior of fluids - Apparent slip on walls - Attractive force between walls Technological application Surface tension Hydrophobicity of the wall GCOE Symposium 201214 Feb. 2012 Lab on a chip for DNA sequencing diameter of the wafer = 100mm Nature 444, 985 (2006) = differ from macroscale - Flow or mixing control in microfluidic devices …and more

18 Molecular Dynamics Simulations Interaction between different species - Berthelot-Lorentz low Motion of the center of mass is removed in order to avoid “flying ice cube” GCOE Symposium 201214 Feb. 2012 ･ Solve equation of motion for each particle ･ Interaction – Lennard-Jones 6-12 potential Cut off length = 2.24nm ( ) ･ NVT = const.

19 Particle Settings GCOE Symposium 201214 Feb. 2012 ･ Liquid and gas – Ar and Ne respectively Baidakov and Protsenko, J. Chem. Phys., 26, 1701 (2008) ･ Fixed wall – fixed to FCC lattice ( Thermostat is applied to liquid and gas ) ･ Thermal wall – confined to FCC lattice by harmonic potential - behaves as a heat bath ( Thermostat is applied only to wall particles )

20 Simulations GCOE Symposium 201214 Feb. 2012 Liq : 50688 Gas : 4608 Wall : 24336 ･ Initial Conditions ･ System configurations (I) In a bulk liquid(II) On a fixed wall (III) On a thermal wall(IV) On a thermal wall (Temperature raised immediately) (Temperature raised slowly)

21 Visualized Shrinking Nanobubbles GCOE Symposium 201214 Feb. 2012 After temperature suddenly raised

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