Ultrasonics – Number and Size of Cavitation Bubbles

The size of cavitation bubbles produced in an ultrasonic cleaning bath depends primarily on the ultrasonic frequency.  The number of cavitation bubbles depends on both the ultrasonic frequency and the ultrasonic power being introduced into the cleaning tank.

Frequency -

Higher ultrasonic frequencies produce smaller cavitation bubbles than lower frequencies.  This is at least partially due to the fact that cavitation bubbles have more time to grow due to the increased wavelength at lower frequency and, therefore, become larger before they implode.  It may be useful for the reader to review the blogs entitled Ultrasonics – Sound – Frequency and Ultrasonics – Consequences of Increased Frequency to understand why the cavitation bubbles have a longer time to grow.  The following chart shows the relationship between frequency and bubble size.

Increasing the ultrasonic frequency while maintaining the same ultrasonic power will result in a larger number of smaller cavitation bubbles. 

The energy required to produce a cavitation bubble and, in turn, the energy released as it implodes are directly related to its size.  Larger cavitation bubbles require more energy to produce and, in turn, release a larger amount of energy when they implode.

Chart showing the relationship between frequency and bubble size

As the ultrasonic frequency is increased, the size of the cavitation bubbles decreases. This illustration was revised on 9Jan12 by the author.

 

Since the overall energy in the liquid must remain the same and, since the energy released by the implosion of a cavitation bubble depends on its size, it is logical that more smaller bubbles result in a conservation of energy.

Power -

Increasing the ultrasonic power in a cleaning bath results in a larger number of cavitation bubbles being produced.  Since the size of the cavitation bubbles is dictated primarily by the ultrasonic frequency, an increase in the number of bubbles is the only way that the energy balance can be resolved.  As we have discussed in earlier blogs, cavitation bubbles are created as defects in the structure of the liquid act as “seeds” for the growth of cavitation bubbles.  At lower energy, only the most most favorable “seed” sites grow into cavitation bubbles.  As power is increased, there is sufficient to cause less favorable seed sites to grow into cavitation bubbles and the overall number of bubbles increases.

Illustration showing that there are more cavitation bubbles at higher power.

As ultrasonic power is increased, the size of the cavitation bubbles remains the same (since bubble size is frequency dependant) and the energy in each bubble remains the same. The number of cavitation bubbles increase in proportion to the power level to preserve the conservation of energy.

Again, conservation of energy dictates that as power is increased, there need to be more bubbles if the size of the bubbles (hence the energy) of each of the bubbles remains the same.

Depending on the application, ultrasonic cleaning may benefit from the fewer but more intense cavitation implosions created by lower frequency or the larger number of smaller energy implosions created by higher frequency.  Power and frequency are important variables in the ultrasonic cleaning process for this reason.

Upcoming blogs will discuss how frequency and power can be varied to produce the number and size of cavitation bubble implosions required for an effective ultrasonic cleaning process.

-  FJF  -

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