Bubble (physics)

A bubble is a globule of a gas substance in a liquid. In the opposite case, a globule of a liquid in a gas, it's called a drop. Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance. Bubbles are seen in many places in everyday life, for example: As spontaneous nucleation of supersaturated carbon dioxide in soft drinks As water vapor in boiling water As air mixed into agitated water, such as below a waterfall As sea foam As a soap bubble As given off in chemical reactions, e.g., baking soda + vinegar As a gas trapped in glass during its manufacture As the indicator in a spirit level Bubbles form and coalesce into globular shapes because those shapes are at a lower energy state. For the physics and chemistry behind it, see nucleation. Bubbles are visible because they have a different refractive index (RI) than the surrounding substance. For example, the RI of air is approximately 1.0003 and the RI of water is approximately 1.333. Snell's Law describes how electromagnetic waves change direction at the interface between two mediums with different RI; thus bubbles can be identified from the accompanying refraction and internal reflection even though both the immersed and immersing mediums are transparent. The above explanation only holds for bubbles of one medium submerged in another medium (e.g. bubbles of gas in a soft drink); the volume of a membrane bubble (e.g. soap bubble) will not distort light very much, and one can only see a membrane bubble due to thin-film diffraction and reflection. Nucleation can be intentionally induced, for example to create a bubblegram in a solid. In medical ultrasound imaging, small encapsulated bubbles called contrast agent are used to enhance the contrast. In thermal inkjet printing, vapor bubbles are used as actuators. They are occasionally used in other microfluidics applications as actuators. The violent collapse of bubbles (cavitation) near solid surfaces and the resulting impinging jet constitute the mechanism used in ultrasonic cleaning.