Bubbles are areas of liquid surrounded by gas. They can be formed when a gas is dissolved in a liquid, when a liquid evaporates, or when a solid melts and turns into a vapour. Bubbles are also found in foams and emulsions.
When bubbles form in liquids, they are usually spherical because this shape has the lowest surface tension. This means that the forces between the molecules of liquid at the surface of the bubble are equal in all directions and so the bubble is stable. However, if the surface tension is not uniform (for example, if there is something stuck to the side of the container), then bubbles can be non-spherical shapes such as elliptical or tear-shaped.
The size of a bubble depends on how it was formed. For example, soap bubbles are usually only a few millimetres across because they are made from very thin layers of soap solution with lots of air trapped inside them. In contrast, water droplets forming on cold surfaces can be much larger because they condense directly from water vapour without first passing through an intermediate liquid stage.
The lifetime of a bubble also depends on its size. Soap bubbles last for only a few seconds before popping because their thin walls allow air to escape easily. Larger bubbles made from stiffer materials can last for hours or even days before bursting.
Bubbles have long been associated with fun and games but they also have some important scientific applications:
•They provide insight into how liquids behave – for example, studies of soap bubbles led to our understanding of surface tension which is important in many industrial processes such as coating objects with paint or making sure that drinks don’t spill out of containers;
•they can be used to study fluid dynamics – how fluids flow and interact with each other;
•they can be used to separate different substances – for example, bubbling air through oil creates an emulsion which is useful for making salad dressings;
•they can be used to clean things – for instance ultrasonic cleaning uses cavitation (the formation and collapse of tiny bubbles) to remove dirt from delicate items such as jewellery;
•and finally they just look pretty!