Homogeneous Nucleation Experiments
Download
Report
Transcript Homogeneous Nucleation Experiments
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
• 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)
OUTLINE
Importance of structure for nanodroplets
Experimental overview
Experimental and theoretical results for
binary nanodroplets
SANS
Density Functional Theory
Lattice Monte Carlo
Molecular Dynamics
Conclusions
Nucleation occurs all around us…
simulation
reality
Organic matter is a common component
of atmospheric particles
Inverted micelle model for aqueous organic aerosols was
recently revived. (Ellison, Tuck, Vaida, JGR 1999)
Aqueous core + organic layer with polar heads (●)
Why is this important ?
Aerosols affect the Earth’s climate
Aerosols 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
Radiative forcing by
aerosols:
Direct (scattering and
absorption)
Indirect (affecting cloud
formation and cloud
properties)
Clouds effect the global
energy balance. They
modify earth’s albedo
and LW radiation.
How are small clusters involved?
V
L
growth
…
…
Nucleation rates
Critical cluster properties
Supersonic nozzle
10
-1
Log Normal Distribution
rg = 10.25 ± 0.05 nm
ln = 0.184 ± 0.004
10
N = ( 4.91 ± 0.05 ) × 10
-2
11
-3
cm
-3
-1
I (cm )
10
10
10
Dp = 2-20 nm
-4
Nozzle A
Po = 59.7 kPa
To = 308.1 K
PD2O,o = 1.37 kPa
-5
3.75 m SDD
2.00 m SDD
10
-6
8 9
2
3
120
5
6 7 8 9
0.1
-1
q (Å )
-1.5
100
80
-2.0
60
-2.5
40
20
N2(g)
4
0.01
-3.0
0
0
20
40
60
80
100
120
H2O(g)
N2(g)
H2O(l)
neutron or X-ray
Beam (λ = 0.1 – 2 nm)
2
3
Experimental Setup at NIST
Is there evidence for structure
in larger nanodroplets?
Use small angle neutron scattering (SANS) to find out.
Well-mixed
Core-shell
Partly nested
or Russian doll
Core vs. Shell scattering
using contrast variation
In high q region
[q = (4π/λ)sin(θ/2)]
sphere
I q–4
shell structure
I q–2
Evidence for shell scattering
Wyslouzil, Wilemski, Strey, Heath, Dieregsweiler, PCCP 8, 54 (2006)
H2O – d-butanol/D2O – (h)butanol
Summary
• SANS: first direct experimental evidence for
Core-Shell structure in aqueous-organic
nanodroplets
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 waterpentanol mixtures modeled as hard sphere - Yukawa
fluids (van der Waals mixtures).
Use classical statistical mechanics to find the
unstable equilibrium density profiles: Solve EulerLagrange 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)
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 for
a pure droplet.
Two types of droplet structures
well-mixed
core-shell
1.0
1.0
Water
Pentanol
BDS
0.8
3
aP=1.001602
aW=1.178168
xP=2.64%
0.6
WW
WW
3
0.8
0.4
Water
Pentanol
BDS
0.6
0.4
0.2
0.2
0.0
0.0
0
1
2
3
4
Distance (nm)
5
6
aP=1.001602
aW=1.178168
xP=2.64%
0
1
2
3
4
Distance (nm)
5
6
Structural Phase Diagram from DFT
at 250 K
DFT predicts nonspherical
oil( )/water( ) droplets
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
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
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
Ideal binary droplet at kT=2.5
1400 ● + 3264 ● (Erb=0)
Nonideal binary droplet at kT=2.5
1400 ● + 3264 ● (Erb=0.5)
Density profile indicates surface enrichment of red beads.
Core-Shell droplet at kT=2.5
1400 ● + 3400 ● (Erb=0.8)
Interior depletion and surface enrichment of red beads.
Russian doll droplet at kT=2
1400 ● + 3400 ● (Erb=0.8)
Russian doll axial density profile at kT=2
1400 ● + 3400 ● (Erb=0.8)
Dimensionless Number Density
1.2
1.0
kT=2.0
N1=1400
0< r<1
0<r<1
component 1
component 2
N2=3400
0.8 E3=0.8
0.6
0.4
0.2
0.0
-20
-10
0
Axial (z) position
10
20
Core-Shell droplet at kT=2.5
formed by heating Russian Doll
1400 ● + 3400 ● (Erb=0.8)
Antonow’s Rule: Interfacial Tensions
and Wetting Transitions
γ(bv) < γ(rv) + γ(rb)
Partial wetting
γ(bv) = γ(rv) + γ(rb)
Perfect wetting
By Analogy with Antonow’s Rule
and Wetting Transitions
Partial wetting
Perfect wetting
heat
cool
Russian doll
γ(bv) < γ(rv) + γ(rb)
Core-shell
γ(bv) = γ(rv) + γ(rb)
Cool the Core-Shell droplet to
observe the dewetting transition
1400 ● + 3400 ● (Erb=0.8)
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)
kT=2.3
kT=2.2
As the temperature is reduced further, the droplet elongates.
Cool the Core-Shell droplet to
observe the dewetting transition
1400 ● + 3400 ● (Erb=0.8)
kT=2.1
T=2.0
kT=2.0
At the lowest temperatures dewetting and elongation are
pronounced.
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
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
MD of nonane/water droplet
initial
Nonane molecules (blue-green)
surround a droplet of water (red-white).
final
The water droplet partly emerges
from the oil droplet.
Double click on the slide to see the simulation.
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