When I open my dishwasher, which has a dry heat/dry finish, the china, glassware and cutlery are dry, but plastic containers are still covered in drops of water. This is regardless of position or proximity in the wash. Why is the plastic wet, when the rest is dry?
鈥� as to why china, glass and cutlery dry more completely than plastic items, missed out an important additional factor, which is the effect of surface energy on wettability. Plastics generally have a lower surface energy than glass and metals, which leads to the formation of the drops observed on the plastic containers. Glass and metal surfaces are more easily wetted and the water spreads out as a thin film that evaporates more easily than a water droplet with its much-reduced contact area.
Keith Davis, Stourbridge, West Midlands, UK
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鈥� The predominant effect is that of surface wetting. Where this is minimal, as on the plastic, the water drops cannot pick up so much heat from the surface below and the drop shape means there is less surface area between water and air. Both clearly limit evaporation.
In contrast, with maximum wetting, as on clean glassware, a continuous, thin water film is optimal for evaporation. This argument is supported by the observation that the very on a saucepan also results in droplets and poor drying.
鈥淲ith maximum wetting, as on clean glassware, a thin water film is optimal for evaporation鈥�
Martin Wilson, Kendal, Cumbria, UK
鈥� The rinse cycle on dishwashers injects a 鈥渞inse aid鈥�, a surfactant which is designed to produce a very thin film of liquid over the washed items, rather than leave droplets. When this dries, the low volume of fluid that has dried on the surface leads to a lack of visible smears. This surfactant also creates a ratio of low liquid volume to high surface area that leads to fast drying.
However, these surfactants are relatively ineffective on plastics, particularly polyethylene, so this leads to droplet formation. These drops have a large volume-to-surface-area ratio and so they dry more slowly than the thin film on the metallic or china contents of the dishwasher.
That is why these droplets can still be present when one empties the dishwasher.
Mark Radnor, Wangaratta, Victoria, Australia