Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Best angle to blow on coffee?

  1. Oct 16, 2011 #1
    My friends and I were curious as to what the best angle was to blow on coffee to cool it down (actually only the coffee near the surface cools, so that's where we measured)

    We did three trials each for 0°, 30°, 45°, 60°, 75°, and 90°.. all started around 88°C to 91°C measured every 30 seconds for 3 minutes while blowing on it with an electric pump (opening modified to simulate lips)

    We got this result, graph of average degrees per second as a function of blowing angle

    http://img11.imageshack.us/img11/9660/screenshot20111017at102.png [Broken]

    Uploaded with ImageShack.us

    Aside from the air blowing away the vapor-saturated-air to keep evaporation going, we think we got this result because blowing down 60° caused an increase in surface area (by pushing down the water slightly on one side) and we think that the graph went down towards the end because surrounding pressure became greater than vapor pressure so evaporative cooling became less.

    What do you guys think?

    Btw, the red dot was the control, no blowing involved.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 16, 2011 #2
    Wait...did you actually do this experiment? If so, you're awesome. Was it for a school project or something?

    Anyway, there's lots to take into account. I'm guessing the power of the air stream would definitely change the optimal angle. You'd also have to specify exactly what the liquid does when the air hits it. Also, the shape of the container it is in probably matters a great deal.
  4. Oct 16, 2011 #3
    My bet would be the shape of your coffee cup... blowing at the 60 degree angle could be the best by causing the coffee to flow and help convection currents bring the hotter coffee to the surface, more so than the other angles anyways.

    interesting experiment lol
  5. Oct 16, 2011 #4
    Did you have the coffee filled to the brim of the mug, so that the air would always hit the surface of the coffee and not hit the inside of the mug at low angles? Or did you have it filled to a level more characteristic of normal drinking practices?

    My coffee maker doesn't make the coffee scaldingly hot, I never need to worry about this.
  6. Oct 18, 2011 #5


    User Avatar
    Gold Member

    Great experiment.

    Did you stir the coffee to obtain an even temperature of the liquid throughout or just the top surface?

    I don't think I could blow on coffee for 3 minutes.
    Most people will blow on hot coffee a bit and then sip.
  7. Oct 18, 2011 #6
    60 degrees from the normal?
  8. Oct 18, 2011 #7


    User Avatar
    Science Advisor

    If you didn't stir it, blowing at an angle induces convection which hastens cooling.
    Beyond some angle, the air comes in glancing, which reduces its effect.
  9. Oct 18, 2011 #8
    so probably 30 degrees from normal haha.
  10. Oct 18, 2011 #9
    Interesting idea, but graphs of measured data like this are actually nearly useless unless you put error bars on the points, and that is often the hardest part of experimental physics. :smile: The reader has no idea if the results are significant, or if the uncertainty in the measured points is as large as the whole vertical scale, even if you do include 8 significant figures in the rate table. To include error bars, you'll need to consider how accurately the angles were measured, and more importantly how accurately the temperatures and rates of change of temperatures were measured. There are also possible systematic errors, due for example to the height of the level of the coffee relative to the rim of the cup, the distance to the electric fan, etc.
  11. Oct 19, 2011 #10
  12. Oct 19, 2011 #11
  13. Oct 19, 2011 #12
  14. Oct 19, 2011 #13
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook