1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do coffee grounds fly out of the grinder?

  1. Sep 12, 2009 #1
    Every morning I put whole coffee beans in a small electric coffee mill, fit the cover into the mill and pulverize the beans. Then – no matter how carefully I try to avoid it – when I take the cover off and decant the ground-up coffee into the coffee pot, a thin layer of coffee grinds ALWAYS ends up on the countertop. Friends (who are not into physics) have reported the same experience – with different makes of grinders and coffeepots of glass and/or stainless steel – and have demanded to know why it happens. So far, all I've been able to surmise is that the little coffee bits are (somehow) each ending up with the same electrostatic charge and thus repel each other. But HOW does it happen?
  2. jcsd
  3. Sep 12, 2009 #2


    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    One guess is, a change in pressure as you begin removing the cap followed by rebound of pressure as the cap is separated from the motot-base assembly; the movement of air easily forces the fairly light-mass coffee grinds to move and travel and as they loose their energy, they will fall. I see very little of these coffee grinds flying and falling on the table. How much of this I find depends on how fast I do the process of uncapping.
  4. Sep 12, 2009 #3
    The ginder is one material. The bean flecks are another. One could expect the beam flecks to be like-charged under similar conditions and repell one another, to one extent or another. Where is the mystery?
  5. Sep 12, 2009 #4
    The amount (the total mass, for example) of the coffee particles on the countertop varies somewhat from day to day. These variations may have to do with with how coarse or fine the grind is, and/or with the ambient humidity, the air temperature, etc. I would probably have ignored the phenomenon – writing it off to my clumsiness or the urgency of my early-AM need for caffeine. But it happens to my much more fastidious friends. Whatever its total mass, a fine spray of coffee grinds always ends up on their countertops, too. Plus, having been prevailed upon to "Explain THIS, Mr. Science Guy", I have experimented to see if the phenomenon can be prevented/modified by deliberately slowing down the decanting process, and the net, as far as I can tell, is the same: i.e. even decanting in mloasses-slow motion results in... grinds on the countertop.

    The change in air pressure idea may have something to it. My coffee grinder is an inexpensive Bosch with a fairly tight-fitting lid, and I suppose that very fine particles of coffee produced by the spinning blade covered by the cap could in principle be acting as a gasket to seal the contents of the cap off from the ambient air pressure. Then when the cap is pulled off, the volume of the contents would increase, thus decreasing the pressure inside the cap. There could thus be a momentary pressure differential, with the result that, at the moment when the "gasket" is broken, fewer air molecules move out than in. It's possible. Along similar lines, I have thought that the temperature of everything inside the cap must be increased by the multiple impacts of the beans and the blade. That would INCREASE the pressure inside the cap, and would also result in a pressure differential between inside and outside the cap. To see if this increased temperature could reasonably account for the phenomenon, I let the ground-up coffee sit inside the grinder, with the cap on, for 30 minutes – thinking that would be enough time for the heat energy to dissipate. Same result: a fine spray of coffee grounds on the countertop (abbreviated, hereafter, to CGOC). Another bit of observational evidence that argues against the pressure differential explanation is that the phenomenon happens with a different kind of coffee grinder which does not have a tight-fitting lid. In this type the ground-up beans fall through the grinding wheels into a loose-fitting bin (1/8 inch gap at top) that is then slid out and away from the machine in order to decant the contents. No pressure differential possible, but the same result: CGOC..
  6. Sep 12, 2009 #5
    I'll second the "electrostatic charge" explanation.

    I've seen coffee grounds move in ways that can't be explained by small differences in air pressure, and it's consistent with what I've seen in other cases of fine-grained particles interacting with static charges.
  7. Sep 12, 2009 #6
    wel in simple physics thte coffee grains gained kinetic energy
  8. Sep 14, 2009 #7


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    But they won't take mor than a second to come to a halt.

    Have you experimented to find if waiting improves things?
    I have Bosch grinder too and have observed fine dust sticking to the inside of the lid, suggesting electrostatics at work.
    How about damping the beans a bit first?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook