Phase transfer and freezing processes investigated on acoustically levitated aqueous droplets

Area of Science:

• Atmospheric Chemistry
• Physical Chemistry
• Cloud Physics

Background:

• Tropospheric physicochemical processes below 0°C are crucial for cloud formation and atmospheric chemistry.
• Investigating these processes requires precise control over microscale environments and droplet manipulation.
• Existing methods may lack the resolution or control needed for accurate microscale simulations.

Purpose of the Study:

• To design and construct an acoustic trap for microscale investigation of tropospheric physicochemical processes.
• To simulate and study cloud processes, particularly droplet freezing and gas uptake, at sub-zero temperatures.
• To demonstrate the utility of the acoustic trap for accurate, reproducible microscale experiments.

Main Methods:

• Utilized an acoustic trap to levitate and position droplets (0.5 nL to 4 µL) in a cooled reaction chamber.
• Employed a piezo-driven micro pump for reproducible droplet introduction (better than 5% reproducibility).
• Measured hydrogen peroxide (H2O2) gas-phase uptake via in-situ chemiluminescence and droplet freezing using microscopy and video recording.

Main Results:

• Successfully levitated and manipulated droplets in a controlled, sub-zero environment.
• Quantified H2O2 uptake by levitated droplets with calibrated chemiluminescence measurements.
• Observed and documented droplet freezing, validating the technique for cloud process simulation.

Conclusions:

• The acoustic trap is a versatile and effective tool for microscale investigation of tropospheric physicochemical processes.
• The technique allows for precise simulation and study of cloud formation and chemical reactions at low temperatures.
• Microanalysis of levitated droplets using fiber optic luminometry is suitable for detailed physicochemical studies.