Water nucleation at extreme supersaturation.

Area of Science:

• Physical Chemistry
• Chemical Physics
• Thermodynamics

Background:

• Understanding nucleation processes is crucial for various fields, including atmospheric science and materials engineering.
• Previous studies on nucleation have focused on different substances and conditions, providing a basis for comparison.
• The behavior of water clusters under extreme conditions remains an area of active research.

Purpose of the Study:

• To investigate water cluster formation in a Laval nozzle under low-temperature and high-supersaturation conditions.
• To determine critical cluster sizes and experimental nucleation rates.
• To compare experimental findings with theoretical predictions and established nucleation behavior trends.

Main Methods:

• Experiments were conducted in a Laval nozzle with uniform postnozzle flow at 87.0 K and 47.5 K.
• High supersaturations (ln S ~ 41 and 104) were achieved.
• Cluster size distributions were measured using soft single-photon ionization (13.8 eV) and mass spectrometry.

Main Results:

• Critical cluster sizes of 6-15 and 1 were determined at the respective experimental conditions.
• Experimental nucleation rates were found to be 5 x 10^15 cm^-3 s^-1 and 2 x 10^15 cm^-3 s^-1.
• A systematic trend in nucleation behavior was observed, differing from previous data for propane and toluene.

Conclusions:

• Experimental nucleation rates align well with predictions from a master equation model based on ab initio transition state theory.
• The results suggest barrierless growth at 47.5 K.
• A more complex nucleation behavior is indicated at 87.0 K, warranting further investigation.