Dot Physics

As part of the Ask a ScienceBlogger series, reader Jim Swanson asks:

When I open the dishwasher after washing and the contents are still hot, why do the glass and ceramic items dry off more quickly than the plastic items?

This is a great question. Great because it is something most everyone can relate to and great because there is some good science. Really, this shows the difference between temperature and thermal energy. I think the common idea is that temperature is a measure of the energy something has – but this isn’t quite true.

Ok, let me first start with a sample case. Suppose you are outside with your grill cooking up some hot dogs or something. The lid is closed and it is quite hot. Now, it begins to rain. As rain hits the top, it forms drops of water that quickly disappear. The hot grill lid has energy. Some of this energy is transferred to the water. When the water molecules have enough energy, they can break out of the drop going from water to gas. (evaporation is another story)

The point is that water on a hot thing will evaporate.

Back to the dishwasher. After the dishes are all clean and still wet, there is a heater in there to help them dry. Let’s assume that this heater brings everything in the washer to the same temperature (maybe it is like 180 degrees F). Also, let me assume there are two cups in there. A ceramic coffee cup and a plastic cup that is the same mass as the coffee cup. After a while, they are at the same temperature (180 F).

Although the two cups are at the same temperature, they do not have the same amount of thermal energy. You can think of thermal energy as a measure of how much the particles in the cups are moving around. It depends on how many particles make up the cup and also the types of particles. So, these two materials are different and thus have different amounts of thermal energy.

The ceramic cup has more thermal energy per mass at that temperature, so it can give more energy to the water sitting on it to make it evaporate. Of course, there is not an infinite amount of thermal energy in the coffee cup, but it is enough to evaporate all of the water sitting on it. You are left with a dry cup. For the plastic, there is not as much thermal energy. Not enough to evaporate all of the water. Oh – also, the surface of the plastic could be different enough that it can also hold more water on the surface.

Comments

  1. #1 Joseph
    June 7, 2010

    Thanks for the interesting post. I’d not really given it much thought before, but the explanation makes a great degree of sense.

  2. #2 lordaxil
    June 7, 2010

    Except that porcelain has specific heat around 1.0 kJ/(kg.K) whereas polystyrene is 1.6-1.8 kJ/(kg.K). Probably more to do with thermal conductivity and surface wetting properties.

  3. #3 Mr. H
    June 7, 2010

    Would the heat/thermal conductivity also make a difference in this case (e.g. conductor such as a metal versus an insulator such as a ceramic tile from the space shuttle)? Is it a negligible effect (if there is one)?

  4. #4 Rhett Allain
    June 7, 2010

    @Mr H and @Lordaxi,

    You are probably right about the conductivity – I didn’t think about that.

  5. #5 Tim Eisele
    June 7, 2010

    There’s also the point that, in my experience, plastic cups are almost always significantly less massive than ceramic cups (the plastic is inherently only about 1/2 to 1/3 the density of ceramics, and the plastic cups have significantly thinner walls). I’d estimate that an average plastic cup only has about a quarter the mass of an average ceramic or glass cup.

    I should weigh some when I get home.

  6. #6 NoAstronomer
    June 7, 2010

    I think Tim has the right answer.

    Also I never use heated dry on my dishwasher. I crack the door open a notch and let it all air dry. The heater is a colossal energy hog.

  7. #7 Anonymous Coward
    June 7, 2010

    I’m with Tim at #5. The effect is accentuated greatly with those super-thin-walled plastic tupperware containers, which always come out of my dishwasher wet, while the surrounding thick-walled glass and ceramics (and usually the thick-walled plastics) are dry.

  8. #8 Eric Lund
    June 7, 2010

    Tim is right, the higher density of ceramics trumps the lower heat capacity. I don’t have an accurate enough scale to measure, but to me it feels as if my coffee/tea mugs are as heavy or maybe even heavier than the plastic containers that hold 3-4 servings worth of leftovers. I also observe that the ceramics and glassware come out dry (unless they are located so that water can get trapped inside), but plastics always come out wet, even if I run the dishwasher after dinner and leave the dishes in there overnight.

  9. #9 Anthony
    June 7, 2010

    I think that surface properties are probably much more important than heat capacity here, because until you actually open the dishwasher, heat capacity isn’t relevant — the only thing heating the dishes is hot air, which means they aren’t any hotter than the air in the dishwasher, which means they don’t cool down regardless of their actual heat capacity. Other than that, no container has enough heat capacity to eliminate significant quantities of water, wine glasses that are maybe 1mm thick will come out dry and at typical dishwasher temperatures (maybe 30C warmer than outside) that’s only enough thermal energy to eliminate a layer 0.03mm thick.

  10. #10 Tim Eisele
    June 7, 2010

    Thinking about it a bit more, there might be something to the surface properties, too. Plastics are hydrophobic, so the water has more tendency to bead up on them (like water drops on waxed paper). Meanwhile, ceramics are hydrophilic, and so the water can more easily spread out as a sheet covering the surface. This corresponds to what I see coming out of the dishwasher: fat drops of water on the plastics, thin drops or films on the ceramics. The beads of water on the plastic have less surface area relative to the volume, which would make them evaporate more slowly than the film of water on the surface of the ceramics.

    Between the mass difference providing more evaporation from the ceramics, and the difference in the way the water beads up changing the surface available for evaporation, there would be a big difference in evaporation rates. As for which effect is more important, the answer is obvious: Experiments must be conducted!

  11. #11 qetzal
    June 7, 2010

    As for which effect is more important, the answer is obvious: Experiments must be conducted!

    That would make for a great science fair project. Much better than the typical “Do Plants Grow Faster Classical or Rap Music.”

  12. #12 KeithB
    June 7, 2010

    Another thing that complicates things is the amount of water vapor already in the air. Most dishwashers have a little fan that removes the moisture saturated air. My Bosch has no heater or fan, but the stainless steel tub is surrounded by cool air and the moisture condenses and runs down into the bottom of the tub.

  13. #13 Rhett Allain
    June 7, 2010

    @qetzal,

    You are right – perfect for science fair. Quick, someone write this down on the science fair list idea (actually that is a good idea to keep such a list).

  14. #14 Bryce Hand
    April 8, 2011

    Drops of equal volume (from a medicine dropper) survive much longer on plastic or wax paper than they do on glass, even at room temperature on the kitchen counter, and in a time frame that guaranteed the whole system had to be isothermal!

    In other words, differential evaporation is real (and it’s substantial) without recourse to temperature differences, thermal conductivities, thermal inertia….