Using Evaporation to Check Spray Dryness | Generated by AI
Question: Is it correct that when applying Yunnan Baiyao spray on the feet, we can tell if it is dry by swaying the feet — if we feel cold, it means the spray/liquid is still evaporating into air, and if we feel nothing, then it is already dry?
Answer:
Yes, this method is basically correct and makes good physical sense.
When you spray Yunnan Baiyao (or almost any alcohol-based topical spray/liquid), the main cooling sensation you feel while waving the foot comes from evaporative cooling.
- The spray usually contains a high percentage of ethanol/alcohol (common in many Chinese herbal aerosol sprays including variants of Yunnan Baiyao).
- Alcohol evaporates much faster than water.
- During evaporation, it absorbs heat from the skin → the skin temperature drops → you perceive coldness.
- When you gently wave/sway the foot, you increase airflow → evaporation happens faster → the cooling effect feels stronger and more obvious.
Practical signs in real use:
- Strong/prolonged cold feeling when waving = still quite a bit of liquid/alcohol on the surface → not fully dry yet.
- No cold feeling anymore (or only very faint sensation) even when you wave the foot = most of the volatile solvents have evaporated → the remaining material is mostly the non-volatile herbal components and film formers → basically dry.
A few small caveats / more precise understanding:
- Very thin layers dry extremely fast (often 10–60 seconds depending on how much was sprayed, room temperature, humidity, and airflow).
- Even after the strong cold sensation disappears, there can still be an extremely thin film that is tacky/sticky for another 30–120 seconds.
- If you sprayed a very thick/wet layer, you might feel only mild coolness even when it’s still quite wet (because thick liquid evaporates slower from the deeper parts).
But in normal usage (1–3 seconds of spraying per area), your described method — waving the foot and checking for obvious cold sensation — is one of the most reliable and commonly used practical ways people judge whether a medicated alcohol spray has dried sufficiently.
So your understanding is correct in everyday practical terms.
References:
- None (explanation based on basic principles of evaporative cooling and common properties of Yunnan Baiyao aerosol formulations)