Liquid bubble

A bubble is a globule of one substance in another, usually air in a liquid.

Due to surface tension, bubbles may remain intact when they reach the surface of the immersive substance.

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Common examples

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 given off in chemical reactions, e.g. baking soda + vinegar
• As a gas trapped in glass during its manufacture

Physics and chemistry of bubbles

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.

The appearance of bubbles

Humans can see bubbles because they have a different refractive index (IR) than the surrounding substance. For example, the IR of air is approximately 1.0003 and the IR of water is approximately 1.333. Snell's Law describes how electromagnetic waves change direction at the interface between two mediums with different IR; thus bubbles can be identified from the accompanying refraction and internal reflection even though both the immersed and immersing mediums are transparent.

One should note that the above explanation only holds for bubbles of one medium submerged in another medium (e.g. bubbles of air 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.

Applications

Nucleation can be intentionally induced, for example to create bubblegram art.

Pulsation

When bubbles are disturbed, they pulsate (that is, they oscillate in size) at their natural frequency. Large bubbles (negligible surface tension and thermal conductivity) undergo adiabatic pulsations, which means that no heat is transferred either from the liquid to the gas or vice versa. The natural frequency of such bubbles is determined by the equation:^[1][2]

where:

• γ is the specific heat ratio of the gas
• R₀ is the steady state radius
• p₀ is the steady state pressure
• ρ is the mass density of the surrounding liquid

Smaller bubbles undergo isothermal pulsations. The corresponding equation for small bubbles of surface tension σ (and negligible liquid viscosity) is^[2]

Excited bubbles trapped underwater are the major source of liquid sounds, such as when a rain droplet impacts a surface of water.^[3][4]

References

1. ^ Minnaert, Marcel, On musical air-bubbles and the sounds of running water, Phil. Mag. 16, 235-248 (1933).
2. ^ ^{a b} Leighton, Timothy G., The Acoustic Bubble (Academic, London, 1994).
3. ^ Prosperetti, Andrea; Oguz, Hasan N. (1993). "The impact of drops on liquid surfaces and the underwater noise of rain" (PDF). Annual Review of Fluid Mechanics 25: 577-602. doi:10.1146/annurev.fl.25.010193.003045. Retrieved on 2006-12-09.
4. ^ Rankin, Ryan C. (June 2005). Bubble Resonance. The Physics of Bubbles, Antibubbles, and all That. Retrieved on 2006-12-09.

See also

• Sonoluminescence
• Bubble fusion
• Underwater acoustics
• bubble physics -- touches upon vapor pressure, bubble formation, bubble dynamics, cavitation, acoustic oscillations, sound of raindrops underwater, Rayleigh-Plesset equation, snapping shrimp, lithotripsy, ultrasonic cleaning, sonochemistry, sonoluminescence, medical reperfusion imaging, and micro-bubble therapy