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Decompression Theory

The concepts behind bubbles mechanics and some ideas

This section is base on Dan Reinders presentation on The Varying Permeability Model which you can find free on our library or at Decompression.org

  • Bubble is a body of gas within a liquid, does the pressure of a gas in a bubble is equal to the surrounding hydrostatic  pressure plus a contribution from surface tension.
  • Surface tension is the attractive force felt by surface molecules of a liquid from the molecules beneath that tends to draw the surface molecules into the mass of the liquid and makes the liquid take the shape having the least surface area.

How Gas diffuses from bubbles:

  • If the pressure inside a bubble is greater than the pressure of the dissolved gas in the surrounding tissue, the bubble will shrink. Otherwise will grow, with one exception, after decompression, because dissolved gas pressure is lower than the ambient pressure due to oxygen metabolism. Despite the theory, the reality is, bubbles don’t always dissolve. To explain this phenomenon (why bubbles don’t always dissolve), a lot of ideas has been suggested. One of the best explanation until now is that the tiny bubbles become stabilized by active molecules on the surface. Surface active molecules are molecules that embed themselves in the gas-water interface.
  • Just as a water molecules pulls towards each other in surface tension, active molecules push against the others. This countereffects the process of gas diffusion, and no diffusion means no bubble dissolution. we can assume that Surfactants are tiny springs pushing against each-other at the interface between water and bubble.

Take a look at the process:

  • When you descend presents bubbles are compressed, which means that the available Surfactant area diminish; basically each spring push more against its neighbor. But at some point the spring runs out of space for travel, them the springs will start popping off the bubble surface. This mean that it’s more advantage for the spring to leave the surface than to compress further countering the effect of the surface tension stabilizing the bubble and making it smaller (the bubble).
  • Surfactant definition: compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid.
Surfactant can be thought as tiny springs
Surfactant

Understanding how bubbles grow

  • Bubbles grow when dissolved gas pressure is greater than the bubble interior pressure, this means that supersaturation is require in order to stimulated growth, therefore crushed nuclei are better for divers than stabilized bubbles.
  • In reality, at first bubbles expands thus making the spring loosing contact with each other. Then they surface tension does not get a countereffect.

What other effects have the Surfactant?

  • They form a barrier to diffusion
  • The more squeezed they are, the stronger the barrier to diffusion.

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