Ultrasonics – Degassing – What Gas? and Why?

Although the importance of “degassing” liquids prior to their use in an ultrasonic cleaning or rinsing process is pretty much common knowledge, the source of the gas and details of how to remove it are less well known.  This blog is the beginning of a series which will describe in some detail where that gas comes from, why it prevents effective ultrasonic cavitation and implosion, and, most important, how to eliminate it.

In the blog about ultrasonic cavitation and implosion, it was explained that the implosion of cavitation bubbles produced by ultrasonic waves was at the heart of the ultrasonic cleaning process.  Cavitation bubbles that form but do not violently implode contribute relatively little to the overall cleaning process.  To implode in an ideal way, cavitation bubbles formed during the rarefaction (negative pressure phase) of a sound wave should be voids.  Although it’s not perfectly true, we think of the void within the cavitation bubble as a vacuum supported by the surrounding negative pressure.  As the cavitation bubble comes under the influence of the compression (positive pressure phase) of a sound wave, the surrounding pressure, the surface tension of the liquid surrounding the bubble and the negative pressure within the cavitation bubble all contribute to its violent collapse.  If any of these factors are missing, the implosion of the cavitation bubble will either not occur or will occur but with significantly reduced effect.

What Gas? –

Illustration of glass with bubbles.

Bubbles of gas will form in a glass of water freshly drawn from the tap and left sitting on the kitchen counter. The reduction of pressure from that in the water main to that of atmosphere allows the bubbles to form as dissolved gas comes out of solution.

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!

Illustration of balloon with internal pressure counteracting external pressure.

The gas (air) sealed within a balloon resists external pressure. Unless the balloon breaks, you can not significantly compress the balloon using bare hands. Air confined within a collapsing cavitation bubble similarly prevents the catastrophic collapse of the bubble required for implosion.

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  –

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