Most liquids including water and water-based liquids contain at least some dissolved gas. An example of this is the effect seen when you loosen the top on a bottle containing soda water or any carbonated liquid (yes, beer and champagne too!). Prior to loosening the top and releasing the pressure contained within the bottle, the gas, in this case carbon dioxide, remains dissolved in the liquid because of the pressure in the bottle. Once the top is opened and the contained pressure is released thereby reducing the pressure to that of the atmosphere, the gas is free to come out of solution. Similarly, water under pressure in a water main contains gas which is kept dissolved by the pressure within the water main. When you open the tap and draw out the water, the dissolved gas starts to evolve as atmospheric pressure is not sufficient to keep it dissolved. This is why a glass of water poured and left on the kitchen counter develops little bubbles which attach themselves to the sides of the glass.Why? - As a cavitation bubble starts to form, gasses in the surrounding liquid begin to diffuse into the resulting void or lower pressure area. As the bubble grows, the pressure within the bubble continues to be more and more negative which speeds the process of gas being released into the bubble. Also as the bubble grows, its surface area (the interface between the void and the surrounding liquid) increases . The combination of an increased pressure differential and increasing surface area contribute to the diffusion of gasses previously dissolved in the surrounding liquid into the cavitation bubble. More gas dissolved in the liquid will result in more gas diffusing into the cavitation bubble during its growth. When the time comes for the bubble to implode, any gas that has diffused into the bubble during its growth tries to return back into the surrounding liquid. This process is very similar to the process that caused the diffusion of gas into the bubble in the first place although going in the other direction. There is, however, one significant difference. In the case of bubble growth, the increase of surface area, which increases the rate of diffusion of gas, precedes the decompression of the bubble as the decompression is the result of the bubble growing. As the bubble is collapsing, the decrease in its surface area, which slows the rate of diffusion of gas from the bubble, precedes the increase of pressure within the bubble. It is the decrease in bubble's size that causes the increase in pressure. The result is that any gas that has diffused into the cavitation bubble can not totally escape back into the liquid prior to the ultimate implosion of the bubble. Any gas remaining in the bubble acts to cushion the intensity of the implosion. It's a little like taking a balloon filled with air and trying to collapse it with your hands. Unless the balloon breaks, you can't do it!
Upcoming blogs will explore what determines how much gas can be dissolved in a liquid and what means can be used to quickly and efficiently remove the gas from a liquid prior to use in an ultrasonic process.
- FJF -