Saturday, August 29, 2015


We were silly enough to get a low-noise dishwasher, and found the hard way that grinding up stuff makes noise: no noise means little grinding means the thing clogged more often. When it broke we went back to manual operations. Since we had to pre-clean the dishes for the machine anyway, it isn't a hardship to do without--though certain family members are more thorough than others.

My habit is to wash plates and silverware and some of the glasses as the first load. Sometimes someone comes to dry the dishes so I can continue. It seems a waste of time and effort: just wait 5 minutes and the dishes will be dry by themselves. (Also it makes my "first load" a much larger affair. But I digress.) Later loads require the towel; they don't dry quickly.

My impressions are that ceramics tend to dry very quickly, plastics very slowly, and metal somewhere in between. I wonder how much this has to do with wetting and how much with thermal conductivity and how much with thermal mass. And how much with the effects of thin films of oil, that is so hard to get off plastic, spreading to cover water droplets and block evaporation.

Wetting and evaporation may not be as trivially linked as you might guess; apparently there's a body of literature on the subject. One little detail, obvious in retrospect, is that a drop of water forms a shape governed by its surface tension and how strongly it wets the surface. OK. Now some evaporation happens, and the shape of the droplet is different--if water escapes uniformly according to air speed (higher farther away from the surface) the drop flattens out, and no longer has the ideal shape for balancing the forces involved. What happens next?

Thermal conductivity matters too. If the surface is still hot from the hot water, evaporation will cool it down. But if more heat is quickly conducted from the inside, the surface stays relatively hot and the water keeps evaporating. So you'd think iron would dry faster than ceramic. But water's heat capacity is much larger than iron's, and a little evaporating water can cool quite a bit of thin iron. (Some plastics have heat capacities larger than ceramics.)


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