Skin of earthworms stays clean while moving through moist soil because of electroosmotic flow near the body.
"[S]oil does not seem to adhere to earthworm surface…An important phenomenon of earthworms moving in moist soil is that the electric potential exists on an earthworm tissue…The electroosmotic flow near an earthworm body surface is a basic electrokinetic phenomenon that takes place when the earthworm moves in moist soil. The flow in a micro thin layer of water is formed in the vicinity of the earthworm body surface as a result of the electric double layer (EDL) interaction. Such a micro scale electroosmotic flow plays the role of lubrication between the earthworm body surface and the surrounding medium of moist soil and reduces surface adhesion." (Zu and Yan 2006:179)
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