Transcript Nucleation Rates of Supercooled Water

Freie Universitaet Berlin
Department of physics and phys. chemistry.
The Homogeneous Nucleation Rates of
Supercooled Aqueous Non-Electrolytic
Mixtures
Petr Kabath
SET for Europe
August 2005
Brno
Why nucleation?
It is important in the processes
in the atmosphere like creation of
clouds.
The nucleation is of a great importance for the
species living under extremal conditions like
polar fishes (Arctic Cod is on the figure), insects
or plants. It must be prevented or inhibited.
Outline
• How does it freez? Brief description of nucleation
• Electrodynamical trap and scatterd light
• The rates of homogeneous nucleation (results)
• Summary and outlook
How does it freez?
• We investigate homogeneous nucleation rates.
• The solid phase embryos are created spontaneous in the
liquid phase.
• If the embryo reaches a critical size then it continues to
grow. If the critical size is not reached then it shrinks.
G
is an energy of formation of a
critical germ
Freezing of water
Creation of the critical nucleus
leading to freezing. of water
with 512 molecules.
Computation by Matsumoto,
Saito and Ohmine, Nature,
Vol. 416, 2002
Nucleation statistics
• Freezing process in a bulk volume is a stochastic process
and it is described by the Poisson distribution
dN
  JV f N
dt
J is the rate constant of the freezing process
What do we need for evaluating of J?
• The nucleation rate J could be estimated from the equation
N 
ln  u    JVt
 N0 
where V is the freezing volume and t is the nucleation
time, N0 is the number of all droplets and Nu is the number
of unfrozen droplets.
• Thus we need to estimate V and t from the experiment.
Electrodynamical trap and scattered light
(Experimental part)
• Electrodynamical trap
• Optical setup and Mie-Scattering
• Measurement
Naked Trap
The „hearth“ of an experimental setup is the inner chamber with
ring electrodes.
Potential in the trap
Voltage in the trap has a form of
U  U 0 cos  t   U DC
This gives us a time dependent
field with a zero potential in the
middle of the trap. The droplet
moves on the Lisajous curve but for
our purposes it is stable.
Setup
Creation and trapping of the droplet
• The quality of the droplet is controled by stroboscope effect.
The droplet injection frequency is synchronized with the
frequency of the AC voltage in the trap.
• Usual droplet diameter varies between 120-30 micrometers
Mie Scattering
• if the drop is liquid then the laser
light has the same polarization as
before the interaction with the drop
• if the droplet freezes then the laser
light will become depolarized
• number of Mie stripes is
proportional to the diameter of the
drop
Optical Setup
Phase transition
The rates of homogeneous nucleation
(Data processing)
Nucleation rate J is the slope of the
logarithmic equation for Nu/N0
Factors influencing the measurements
• The diameters of the
generated droplets are not
constant
• Evaporation
The rates of homogeneous nucleation for
water
Homogeneous nucleation rates
A concentration dependency of the
mixture on the supercooling for a
given value of J.
• the distribution of the clusters
of the mixture molecules probably
do not influence the linearity or
they are statisticaly homogeneous
mixed
The rates of homogeneous nucleation a
comparison
The most recent measurements
• Measurements of rates of homogeneous nucleation of
simple alcohol-water mixtures
• Measurements of dioxane-water mixtures
Summary and outlook
• precise method for measurements of homogeneous
nucleation rates
• non-contact method
• we have two traps currently, one fully automatised
• measurements of alcohol-water mixtures, dioxane-water
mixtures, some biological relevant mixtures (amino acids)
Acknowledgements
• To Prof. H. Baumgaertel
• To my collegues I. R. Türkmen, A. Lindinger who are
involved within this project
• Especially to P. Stöckel for his help with the operating of
the trap
• To Mr. E. Biller for the technical support and for all
perfectly running injectors
• To DFG for the financial support
Our publications
1.
2.
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4.
5.
6.
B. Krämer, O. Hübner, H. Vortisch, L. Wöste, T. Leisner, M. Schwell,
E. Rühl, H. Baumgärtel: Homogeneous Nucleation Rates of
Supercooled Water Measured in Single Levitated Microdroplets, J.
Chem. Phys, 111, (1999), 6521
H. Vortisch, B. Krämer, I. Weidinger, L. Wöste, T. Leisner, M. Schwell,
H. Baumgärtel, E. Rühl: Homogeneous freezing nucleation rates and
crystallization dynamics of levitated sulfuric acid droplet
PCCP 7, (2000), 1407
P. Stöckel, H. Vortisch, T. Leisner, H. Baumgärtel: Homogeneous
Nucleation of Supercooled Liquid Water in Levitated Microdroplets, J.
Mol. Liq., 96-97, (2002), 153
I. Weidinger, J. Klein, P. Stöckel, and H. Baumgärtel, T. Leisner:
Nucleation Behavior of n-Alkane Microdroplets in an Electrodynamic
Balance, J. Phys. Chem. B, 107, (2003), 3636
Stöckel P., Inez M. Weidinger, Helmut Baumgärtel, Thomas Leisner:
Rates of homogeneous ice nucleation in levitated H2O and D2O
Droplets, J. Phys. Chem. A, 109 (2005), 2540
P. Stoeckel, P. Kabath, A. Lindinger and H. Baumgaertel: A kinetic
two-step model of homogeneous nucleation of ice in supercooled
supercooled liquid H2O and D2O, paper submitted to J. Mol. Liq.
The device itself