Ultrasonics – Cavitation 101

Cavitation of liquid due to high amplitude ultrasonic vibration within the liquid is the backbone of ultrasonic cleaning.  Liquids have the unique ability to cavitate.  In order to cavitate, a material must exhibit three properties –

1. It must be relatively inextensible and uncompressible.  It can’t be able to stretch or expand or be compressed to significantly change it’s volume.
2. It must have limited tensile strength.  If subjected to enough tensile force it will break.
3. It must be able to “heal.”  If divided into two or more pieces, the pieces must be able to re-combine to form a whole that is indistinguishable from the original.

Of solids, liquids and gasses, only liquids meet all of the above requirements.  Gasses fail on points 1 and 2 while solids fail on point 3.  I am often reminded that some solids can be “healed” by welding.  In fact, welding requires that the solid be converted into a liquid and then behaves as a liquid, not a solid.  Once “healed” the material re-solidifies back to a solid.

Interesting Point – Cavitation has been created in many molten metals and other materials that are solid under normal conditions.

We are familiar with the idea that liquids are not compressible based on the laws of hydraulics.  It may, however, be a little difficult to imagine liquids having a limited tensile strength since we seldom see them under conditons of tension.  It’s really difficult to grab hold of a piece of water, for example, and pull on it.  There is, though, one common “liquid” can be used to easily demonstrate the effect of tensile force on liquids.  “Silly Putty^®,” a material readily avialable in toy stores anywhere (usually in little egg-shaped plastic containers), is really a sluggish liquid.  It can easily change shape, but not volume.  Although (based on information found at Wikipedia.com) the purist will say that Silly Putty^® isn’t exactly “slow water,” its characteristics will suffice for our purposes here.  If you leave a ball of Silly Putty^® on a flat table, it will (eventually) “run” and even drip off the edge of the table (bring your lunch and a sleeping bag).  If you pull slowly on a ball of Silly Putty^®, it will stretch and deform but will maintain its original volume.  If you pull very fast, however, the Silly Putty^® will “break” as its tensile strength is exceeded.  If you then put the two pieces back together again, the recovered whole will become indistinguishable from the original.

Preceding blogs explained how acceleration forces are increased in vibrating materials as frequency and amplitude are increased.  Acceleration creates tension as the vibration conducting media actually pulls against itself to transmit the vibrations.  The following illustration demonstrates this effect.

This phenomenon is called “cavitation” and results in a void within the liquid called a “cavitation bubble.”  Depending on the liquid and its properties, this void may become completely or partially filled with air that was dissolved in the liquid to begin with or with vapor of the liquid itself as the reduced pressure causes the liquid to flash into vapor.  For the moment, we will consider the void an area of vacuum created within the liquid.  This void is unstable and supported only by the transient (passing) negative pressure or rarefaction portion of the sound wave.  We’ll talk about the consequences of the pressure wave that follows in an upcoming blog.

–  FJF  –