Fog Droplet Size and Neutral Buoyancy
Applied Physics public AP Series material commonly references approximately 2–3 µm fog droplets. That is a useful manufacturer planning claim, but real behavior depends on temperature, humidity, distribution width, evaporation, coalescence, outlet momentum and the measurement method.
Droplet size is a distribution
A fog cloud is not made of one exact diameter. Any credible characterization should define the measurement location, instrument, environmental conditions and whether the number represents a mean, median, mode or range. A nominal 2–3 µm statement should not be converted into an unqualified guarantee at the end of a long hose or inside a warm, dry room.
Neutral buoyancy is conditional
The phrase describes fog that follows the surrounding airflow with limited independent rise or fall over the observation period. Gravitational settling, evaporation, thermal gradients and gas momentum still exist. The introduction system should minimize its own momentum so the tracer reveals the airflow rather than creating a new one.
What to document
- Published or measured droplet-size basis
- Room temperature and relative humidity
- Air velocity at the observation zone
- Hose length and outlet configuration
- Distance between outlet and process
- Camera frame rate, exposure and contrast
- Any visible condensation, wetting or dropout
Build a defensible fogger requirement
Send Applied Physics the room, process, utilities, documentation and safety constraints—not just a model number.