Why does water tend to form a circular region void of itself around a small piece of soap, present on bathroom tiles, having a very thin layer of water?

Image source: minimalisti.com

This is because of decrease of surface tension of water due to the soap contamination. Due to surface tension the free surface of the water film on a tile is in a stretched state. It is like a rubber sheet pulled all around. If a particular region gets weak it yields. In the case of a water film this tension decreases due to addition of soap and water in this region gets pulled to the regions around where surface tension is higher. Notice that the pull is concentrated near the free surface of the film but the whole layer gets pulled.A similar experiment can be demonstrated in your kitchen with a thin water layer on a thin metal plate and gently warming the plate locally from the other side, say using your thumb or on a candle. Surface tension decreases if temperature increases and again we see water being expelled away from the heated area.Due to the surface tension thin water layers on kitchen utensils or laboratory glassware tend to form lumps or drops and drain out from the surface. But pushing this phenomenon to its logical end one may ask why we see liquid films, say on bathroom tiles, at all. Each film should be inherently unstable due to slight variation of impurities or temperature. Tile surface roughness and also the liquid free surface near the edges of the voids not being exactly flat when it thins comes to the rescue of the film. Hence even though a void appears to be formed an extremely thin water layer remains.

Source: thehindu.com