Ultrasonics – Pulse – More About Pulsed Ultrasonics

“Pulse” in ultrasonic terms simply means turning the ultrasonic energy on and off repeatedly in either a fixed or random pattern.  At first it seems this would would be counter-productive to the goal of maximizing the ultrasonic cleaning effect.  In fact, the blogs Ultrasonics – Ultrasonic Generators – What Is “Pulse” and Ultrasonics – Ultrasonic Generators – Power Control both indicated that pulsing the ultrasonic generator reduces overall ultrasonic power.  It was, however, shown in the blog Ultrasonics – Degassing Using a Pulse Degas Cycle that a slow pulse cycle can, indeed, improve the degassing effect achieved using ultrasonics.  Interestingly, there are also benefits to ultrasonic cleaning which can be derived from using pulse at higher pulse rates.

Ultrasonic cleaning is all about forming bubbles which implode to produce the cleaning effect.  Anything that prevents the formation of bubbles will reduce the cleaning effect.  As it turns out, cavitation produced by ultrasonics is a self-limiting phenomenon.  When the ultrasonic energy is first turned on, provided that nothing else gets in the way, the sound wave has a clear and unimpeded path to travel through the liquid generating cavitation bubbles as it goes along.  Although we haven’t talked about it before now, not all cavitation bubbles grow and implode during the passage of a single sound wave.  In fact, a cavitation bubble more typically oscillates through several sound wave cycles growing a little larger during the passage of each sound wave until it finally achieves an unstable size and implodes.  The cavitation bubbles that are in the “growth” stage can, in fact, attenuate the intensity (amplitude) of the sound wave as it travels through a liquid.  It makes sense that the ideal situation would be one in which the bubbles in the growing stage were not present.  This leads us to a “Catch 22” situation in which more bubbles in the growth stage means less ultrasonic energy to help them grow.

Using the proper pulse rate and duty cycle, cavitation bubbles in the growth stage can be allowed to collapse (without actually “imploding”) during the off cycle.  This allows maximum amplitude sound waves to again travel through the liquid creating more, larger, “first generation” bubbles once the ultrasonic energy is turned on.  The result is an increase in the ultrasonic cleaning effect.  The following illustration may be helpful in understanding this phenomenon.

Illustration showing the effect of bubble count on ultrasonic cavitation and implosion.

When ultrasonic energy is initiated, cavitation bubbles are created. These bubbles usually require several cycles of sound waves to grow to a size which will result in an implosion on collapse. The population of bubbles in the growth stage interferes with the transmission of sound waves thereby producing a diminished number of cavitation bubble implosions. Pulsing the ultrasonic energy allows growing bubbles to collapse resulting in a burst of higher intensity sound waves which produce a higher intensity cavitation effect for a short period when the ultrasound is again turned on. The result is better ultrasonic cleaning overall.

Although most ultrasonic generators today have an “optimized” pulse built in.  The “optimized” pulse rate and duration is based on the operating frequency and the intended application.  More sophisticated ultrasonic generators allow manual adjustment of both the pulse rate and the pulse duration (sometimes as “duty cycle”).  These adjustments, although not always well understood, are critical in many ultrasonic cleaning applications.  Still more flexibility is provided by ultrasonic generators with the capability to sweep or randomly vary the pulse rate.  More details of this feature will be discussed in an upcoming blog.

–  FJF  –

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