Why do air bubbles cling to the walls of a vessel when water is heated?

Why do air bubbles cling to the walls of a vessel when water is heated?

When water is heated, steam bubbles out. Apart from the energy required to heat up to the boiling point (about 100 °C) and the latent heat, additional energy is required (due to surface tension) to form the bubbles since they have a free surface.Pressure inside a bubble is higher than that in the surroundings by an amount 2 T/R, where T is the surface tension coefficient and R is the radius of the bubble. If R is very small this additional pressure can be very large. Hence the bubbles are usually formed where the hot vessel surface is rough and has crevices. 

One can observe boiling water on a stove where the steam bubbles rise from a rough horizontal surface of the vessel. When the bubbles rise due to buoyancy they grow in size and when they are located over the water surface they are formed by a film and have two free surfaces an inside one and an outside one. Additional pressure inside the bubble is then given by (4 T/R) due to two surfaces. Interesting things happen here. This liquid film is fragile and can easily break unless it is made up of milk or oil or soap water. When a bubble comes close to another bubble or an object it gets attracted and clings to it because of surface tension. We can see bubbles clustering on the free surface of coffee. The bubbles in the interior of the water body cling to the wall and have the shape of a truncated sphere. To detach them, there is a need to increase their surface area, which needs additional work. Thus they stick to the wall because of surface tension, resisting buoyancy force or any other disturbance. Since they do not have a film they do not break.Such bubbles can be a great nuisance in a water flow experiment when observations are to be made through a glass window, for example, by shining a laser beam. Bubbles arising from the dissolved air and sticking to the glass window cannot easily be removed even by increasing the water flow rate. They need internal mechanical wiping.

Source: thehindu.com

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?

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