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Name: Shannon
Status: educator
Grade: 6-8
Location: IL
Country: USA
Date: Summer 2012


Question:
I am running a summer enrichment science camp. Every day we clean up and place the labware into the dish washer. Sometimes we have extra, and we need to run a second load the next morning. The loads are primarily plastics, glass, and metals. The students have noticed that at the end of the cycle (we do not use the dry cycle), the glassware (ceramics) are nearly dry, while the metals might have a small amount of water on them and the plastics are downright wet. When we put the utensils on a drying rack, the plastics are still wet many hours later, while the other materials are dry. My students have asked many questions that I have been able to direct them to answers or experiments, which we do. This is fun. The stumper question that I do not know enough to direct them to an experiment follows: Does the water evaporation rate differ due to different surface tensions of the different materials?



Replies:
I suspect that the contact angle between the water and the substrate will be important. The greater the angle, the thicker the bead of water, which will make it take longer to evaporate. I think that the more hydrophobic substrates will give greater contact angles.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


Shannon,

There are two possibilities here: that more liquid adheres to particular materials, or - as you suggested - evaporation is quicker from particular surfaces. In both cases it is a question of water adhesion to a surface ... but I suspect that liquid adhesion is more important than evaporation.

Adhesion would mean that the surface activity - a combination of physical and chemical factors - of the different substances allow different amounts of water to stick. Physical factors involve the relative smoothness of a surface (rougher surfaces, particularly at the molecular level, would allow water molecules to be trapped - these water molecules would then hold on to other water molecules by hydrogen bonding). Chemical factors involve the substance having similar intermolecular forces as water so that the substance and water attract each other strongly.

Since evaporation occurs at the surface of the liquid (boundary to air) rather than at the liquid to solid material boundary, the material may have very little to do with the evaporation rate until the liquid bubble gets very small and very close to the material.

I like your approach of having the students design experiments to make determinations. So maybe, this is a good opportunity to have them decide what are the important factors and design experiments accordingly. Hopefully, they will decide that to test for adhesion differences, they could dunk samples - of equal surface area - in water and then measure the mass differences. Hopefully, to test for rate of evaporation, they will add droplets of water on different surfaces and weigh the samples periodically and see if the mass loss is different over time.

Have fun. Greg (Roberto Gregorius) Canisius College


Hi Shannon,

The main reason why some articles come out of the dishwasher dry and others are still wet, is that some materials have a higher Specific Heat than others. The Specific Heat of a material is the amount of heat per unit mass (for example, for 1 gram of a material) needed to raise its temperature 1 degree Celsius.

Conversely, this means that an object made of a high Specific Heat material, when heated, will need to dissipate more of its heat in order to cool down, and thus stay hotter for longer.

Plastic has a very low specific heat, so will cool fast and thus will stay hot for a relatively short time. Since it will stay hot for only a short time, water on its surface will not have time to evaporate before it cools off.

Ceramic and glass materials have a higher Specific Heat, so they will remain hot and cool off more slowly. Since they have more heat to get rid of, they will have more time to evaporate the water on their surface before they finally cool.

Metals generally have a Specific Heat value somewhere between plastics and ceramics.

It is important in this context not to confuse "heat" with "temperature"! In your case, you are heating ceramic objects and plastic objects to the same temperature. But once heated to the same temperature, the ceramic objects contain more heat and therefore cool down more slowly. But nothing is for free! These same ceramic objects, because ceramic has a high Specific Heat, took longer to reach the temperature in the dishwasher, and absorbed more heat to reach this same temperature. So, in absorbing more heat, they take longer to get hot, and therefore longer to liberate this heat when they cool off.

Regards, Bob Wilson


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