experimental and theoretical studies of the structure of binary nanodroplets
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Experimental and theoretical studies of the structure of binary nanodroplets. Gerald Wilemski Physics Dept. Missouri S&T. Physics 1 Missouri S&T 25 October 2011. Acknowledgments. - PowerPoint PPT PresentationTRANSCRIPT
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Experimental and theoretical Experimental and theoretical studies of the structure of binary studies of the structure of binary
nanodroplets nanodroplets
Gerald Wilemski Physics Dept. Missouri S&T
Physics 1
Missouri S&T
25 October 2011
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Acknowledgments
• Part I – Supersonic nozzle and small angle neutron scattering (SANS) studies of nucleation and nanodroplet structure
• Barbara Wyslouzil (OSU) • Reinhard Strey (Köln U), • Christopher Heath and Uta Dieregsweiler (WPI)
• Part II – Structure in binary nanodroplets from density functional theory (DFT), lattice Monte Carlo (LMC), and molecular dynamics (MD) simulations
• Fawaz Hrahsheh, Jin-Song Li, and Hongxia Ning (Missouri S&T)
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OUTLINEOUTLINE
Importance of structure for nanodropletsExperimental overview Experimental and theoretical results for
binary nanodropletsSANSDensity Functional TheoryLattice Monte CarloMolecular Dynamics
Conclusions
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simulation
reality
Nucleation occurs all around us…Nucleation occurs all around us…
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Organic matter is a common component of atmospheric particles
Aqueous core + organic layer with polar heads (●)
Inverted micelle model for aqueous organic aerosols was recently revived. (Ellison, Tuck, Vaida, JGR 1999)
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Why is this important ?
Aerosols affect the Earth’s climateAerosols change the properties of clouds Sites for chemical reactions:
heterogeneous chemistry, ozone destruction
Fine particles (<100 nm) affect human health
Particle structure influences particle activity – nucleation and growth rates
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Clouds effect the global energy balance. They modify earth’s albedo and LW radiation.
Radiative forcing by aerosols:
Direct (scattering and absorption) Indirect (affecting cloud formation and cloud properties)
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How are small clusters involved?
… …
Critical cluster properties
growth
Nucleation rates
VV LL
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Supersonic nozzleSupersonic nozzle
neutron or X-rayBeam (λ = 0.1 – 2 nm)
N2(g)
H2O(g)
120
100
80
60
40
20
0
120100806040200
-3.0
-2.5
-2.0
-1.5
N2(g)
H2O(l)
Dp = 2-20 nm 10-6
10-5
10-4
10-3
10-2
10-1
I (c
m-1
)
8 90.01
2 3 4 5 6 7 8 90.1
2 3
q (Å-1
)
3.75 m SDD 2.00 m SDD
Log Normal Distributionrg = 10.25 ± 0.05 nm
ln = 0.184 ± 0.004
N = ( 4.91 ± 0.05 ) × 1011
cm-3
Nozzle APo = 59.7 kPaTo = 308.1 KPD2O,o = 1.37 kPa
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Experimental Setup at NIST
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Is there evidence for structure Is there evidence for structure in larger nanodroplets?in larger nanodroplets?
Well-mixed Core-shell Partly nested or Russian doll
Use small angle neutron scattering (SANS) to find out.
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CoreCore vs.vs. Shell Shell scattering scattering using contrast variationusing contrast variation
In high q region
sphere
I q–4
shell structure
I q–2
[q = (4π/λ)sin(θ/2)]
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Evidence for shell scatteringEvidence for shell scattering
H2O – d-butanol/D2O – (h)butanolWyslouzil, Wilemski, Strey, Heath, Dieregsweiler, PCCP Wyslouzil, Wilemski, Strey, Heath, Dieregsweiler, PCCP 8, 8, 54 (2006)54 (2006)
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Summary
• SANS: first direct experimental evidence for Core-Shell structure in aqueous-organic nanodroplets
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Density Functional Theory applied to nanodroplets
Treat nanodroplets as large critical nuclei in supersaturated binary vapors. The species densities ρi (r) vary with position r.As a typical aqueous-organic system use nonideal water-pentanol mixtures modeled as hard sphere - Yukawa fluids (van der Waals mixtures). Use classical statistical mechanics to find the unstable equilibrium density profiles: Solve Euler-Lagrange Eqs.
D. E. Sullivan, J. Chem. Phys. 77, 2632 (1982).X. C. Zeng and D. W. Oxtoby, J. Chem. Phys. 95, 5940 (1991).
J.-S. Li and G. Wilemski, PCCP 8, 1266 (2006)
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A droplet is a region with higher density than the surrounding fluid
The red line shows how the density (ρ) varies with radial position (r) within the droplet.
This example is fora pure droplet.
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Two types of droplet structureswell-mixed core-shell
1.0
0.8
0.6
0.4
0.2
0.0
W W
3
6543210
Distance (nm)
aP=1.001602
aW=1.178168
xP=2.64%
Water Pentanol BDS
1.0
0.8
0.6
0.4
0.2
0.0
W W
3
6543210
Distance (nm)
aP=1.001602
aW=1.178168
xP=2.64%
Water Pentanol BDS
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Structural Phase Diagram from DFT at 250 K
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DFT predicts nonspherical oil( )/water( ) droplets
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Why interested in oil/water droplets?
• Offshore natural gas wells produce high pressure mixtures of methane, water, and higher hydrocarbons (i.e., oils)• Gas must be cleaned before pumping to shore and clean-up may involve droplet formation
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DFT Summary
• DFT: provides a vapor activity “phase diagram” for the nanodroplet structures– bistructural region implies hysteresis for transitions
between well-mixed and core-shell structures
• Also predicts nonspherical shapes for droplets with immiscible liquids
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Lattice Monte Carlo Simulations of Large Binary Nanodroplets
• Generalize the lattice MC approach of Cordeiro and Pakula, J. Phys. Chem. B (2005) for pure droplets
• Each site of an fcc lattice is occupied by a different particle type (red or blue beads) or by a vacancy.
• Beads and vacancies interact repulsively– Ebv = 1, Erv = 2/3, Erb = 0, 0.5, 0.8– Red beads ↔ lower surface tension, higher volatility (~alcohol)
Blue beads ↔ higher surface tension, lower volatility (~water)
• T range: 2.8 ≥ kT ≥ 2.0; Blue triple point is at kT= 2.8
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Ideal binary droplet at kT=2.5
1400 ● + 3264 ● (Erb=0)
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Density profile indicates surface enrichment of red beads.
1400 ● + 3264 ● (Erb=0.5)
Nonideal binary droplet at kT=2.5
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Core-Shell droplet at kT=2.5
Interior depletion and surface enrichment of red beads.
1400 ● + 3400 ● (Erb=0.8)
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Russian doll droplet at kT=2
1400 ● + 3400 ● (Erb=0.8)
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Russian doll axial density profile at kT=2
1400 ● + 3400 ● (Erb=0.8) 0<r<1
-20 -10 0 10 200.0
0.2
0.4
0.6
0.8
1.0
1.2kT=2.0N1=1400
N2=3400
E3=0.8
0< r<1 component 1 component 2
D
imen
sio
nle
ss N
um
ber
Den
sity
Axial (z) position
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Core-Shell droplet at kT=2.5formed by heating Russian Doll
1400 ● + 3400 ● (Erb=0.8)
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Antonow’s Rule: Interfacial Tensions and Wetting Transitions
γ(bv) < γ(rv) + γ(rb) γ(bv) = γ(rv) + γ(rb)
Partial wetting Perfect wetting
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By Analogy with Antonow’s Rule and Wetting Transitions
Russian doll Core-shell
Partial wetting Perfect wetting
heat
cool
γ(bv) < γ(rv) + γ(rb) γ(bv) = γ(rv) + γ(rb)
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kT=2.5
The backside is more evenly covered.
kT=2.4
There is a large dewetted patch; the backside is evenly covered.
Cool the Core-Shell droplet to observe the dewetting transition
1400 ● + 3400 ● (Erb=0.8)
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As the temperature is reduced further, the droplet elongates.
kT=2.2kT=2.3
Cool the Core-Shell droplet to observe the dewetting transition
1400 ● + 3400 ● (Erb=0.8)
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Cool the Core-Shell droplet to observe the dewetting transition
At the lowest temperatures dewetting and elongation are pronounced.
T=2.0kT=2.1 kT=2.0
1400 ● + 3400 ● (Erb=0.8)
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LMC Summary
• LMC: the core-shell - Russian doll structural change is a reversible wetting-dewetting transition that modulates the shape of the nanodroplet – May ultimately be a cause of droplet fission ?
• The RD droplet resembles the nonspherical structure found with DFT for oil/water droplets
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Molecular Dynamics (MD)
• Solve Newton’s equations of motion for large numbers of interacting molecules
• Time step = 1 or 2 fs (10-6 ns)• Average over 2 ns long trajectories to
calculate properties of interest
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MD of nonane/water droplet
initial final
Nonane molecules (blue-green) surround a droplet of water (red-white).
The water droplet partly emergesfrom the oil droplet.
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Double click on the slide to see the simulation.
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Grand Summary• SANS: experimental evidence for Core-Shell structure of
aqueous-organic nanodroplets• DFT: vapor activity “phase diagram” for CS and well-
mixed nanodroplet structures• DFT: nonspherical droplet shapes• LMC: core-shell - Russian doll structural transition
changes the shape of the nanodroplet• MD: realistic simulations of droplets with large numbers
of molecules