I have defined, on Page 1 "ULTRASONICS" as the application of sound at extremely high intensity and high frequency (normally above human hearing, 20kHz - 20,000 cycles per second - and above) to change materials.  Changing materials with ultrasonics, such as to clean, homogenize, and accelerate both physical and chemical reactions, among many other things, is accomplished largely due to the action of cavitation.  I have described in lay terms on Page 1 how the cavitation bubble is formed, even in a cold liquid ("cold boiling"), by shearing molecules just as in a solid,and the shock wave generated by the implosion which results when the bubble collapses.  However, there are more actions inherent in bubble collapse which are of significance.

First, it should be noted that the term "bubble,in itself, could be misleading.  A bubble, by definition, contains a gas or vapor.  After that gas or vapor condenses, there is still a void or cavity in place until the implosion occurs.  Unless otherwise stated, I shall use the term "bubble" for all three forms (bubble, void, cavity).

From Prof. Lawrence Crum of the Applied Physics Laboratory at the University of Washington in Seattle, writing about lithotriptry, in which a kidney stone is broken with ultrasonic energy, "When pressure surrounding a bubble falls below the vapor pressure of the liquid, the bubble fills with vapor and grows explosively.  The bubble collapses violently when pressure returns.  If the collapse occurs near a boundary, such as {a} targeted kidney stone, a high velocity liquid jet is formed that impacts the boundary with great force.  These extremely violent processes are thought to play a major role in stone destruction and associated tissue damage."  Going beyond lithotriptry, that same jet, a focused stream of liquid forced with extreme pressure and velocity and very similar in nature to the molten-metal jet formed in a "shaped charge" in explosive ordnance, causes a number of violent and useful effects.  The jet occurs due to the non-symmetric geometry at the surface of the substrate and can be seen in formation in Professor Crum's most dramatic stop-motion microphotograph seen above.  The substrate is at the bottom of the photo and the top surface of the bubble can be seen dimpling downward as the jet (remember that the background of the picture is clear liquid) begins to form and progress downward through the middle of the bubble.

When the liquid molecules at the forward (down in the photo) edge of the jet impinge on solid material (the substrate, i.e: bone, stone, ceramic, or metal), the collision breaks off the least-tightly bound surface molecules of the substrate.  In the author's work, cavitation erosion of metal has been seen to propagate inward along grain boundaries and in dendritic fashion.  As a point of fact of long standing, one of the earliest tests for activity in an ultrasonic cleaning tank was to immerse aluminum foil, energize the tank, and observe the perforation of the foil by cavitation erosion.  Even materials as hard as aluminum oxide (sapphire) and tungsten carbide are eroded by cavitation.  The reference in AL-1V to the discovery of cavitation through the investigation of why ship/boat propellors wear away was done in the mid-19th Century by John William Strutt, Lord Rayleigh, then not yet the Nobel laureate, who wrote one of the first definitive texts on acoustics, covering this subject, "Theory of Sound" in 1877-78.

In addition to erosion or ablation of surfaces by the jet, cavitation causes many other actions noted at the top of this page, on Page 1, and elsewhere.  Notable among these in a purely physical sense is the action of intense shock fronts generated by imploding cavitation bubbles against kidney stones (lithotriptry), gall stones, tumors, and other intrusions in the body.  Some of this action can also be accomplished by direct impact of a vibrating ultrasonic tool tip, but no (or minimal) cavitation is involved.  In work by Dr. Frank E. Barber of the Radiology Department at Harvard Medical School and by Prof. Crum at the University of Washington, and others, remotely-generated shock waves are focused inside the body to produce intense cavitation alongside the stone or other object, shattering it for easier removal via catheter.  The source of remote energy can be from a phased array of transducers (Barber) or from an array of spark-gaps; in the latter case, a steam bubble results each time a spark passes across a gap under water.

Similar arrangements are being used to destroy cataracts, excise tumors and diseased tissue, heal wounds, close torn blood vessels, relieve benign prostatic hyperplasia, and improve absorption and transfection of drugs.

One local Long Island (New York) firm making devices for such work is Misonix Incorporated in Farmingdale (formerly Heat Systems, for which the author was Director of Technical Services - a blatant plug!).

Let me repeat here, yet again, what was stated before:

AL-2		    ULTRASONICS AND FINE PARTICLES - 		  Jan 98

BENEFICIATION OF SLURRIES AND FINE-PARTICLE SUSPENSIONS

[CERAMICS, COAL & ORES, COATINGS, COLUMN PACKINGS, SINTERING, SLIPS]

{Also including suggestions for best mixing of batch samples -
"sifting solids and swirling solutions"}

Sonication of suspensions of ultrafine particles provides a number of significant benefits, not the least of which is better dispersion.  Ultrasonics substantially reduces particle size of ultrafine suspensions in one tenth the time of traditional ball milling methods.  In addition, one can expect disaggregation and deagglomeration of clumps (particle size reduction), degassing of the carrier liquid, increased slurry flow properties, higher homogeneity, and denser castings, sinterings, or packings.

Cavitation, the formation and implosion of microbubbles in a high-intensity ultrasonic field, propagates shock waves through the liquid.  This intense energy accelerates both physical and chemical reactions, enhancing surface chemistry and causing violent particle motion and generating high-velocity interparticle collisions.

Bubble formation occurs in the liquid between particles.  Cavitation can not occur in air, gas, or vapor.  Thus, no action is found in unwetted, gas-filled voids in a particulate mass.  For an insoluble material suspended in an inert liquid, the effective viscosity of the parent liquor (that property of the suspending liquid affecting cavitation) is just that of the basic liquid, which can be quite low, and not the apparent viscosity of the suspension, which can be quite high.  For this reason, it is possible to sonicate extremely dense suspensions or slurries in water or light solvents.  Such thick slurries might have apparent viscosities far above the range of 5,000 to 10,000 centipoise (5 to 10 Pa.s), which is the threshold of cavitation for most simple liquids.

Wetted beds of particles can be fluidized with probes or in cup horns or even in ultrasonic cleaners for laboratory-scale experimentation.

Field experience has borne out these ideas.  Several practical examples follow:

In addition, ultrasonic processing of suspensions in chemically-reactive liquids provides greater yields through acceleration of surface chemistry.  A new field, sonochemistry, has, in part, resulted from the realization of the ability of cavitation to both expose fresh surface and enhance reactions.

2.  SIFTING SOLIDS AND SWIRLING SOLUTIONS

  The ability of an ultrasonic liquid processor to effectively stir, mix, or agitate a batch depends to a large degree on the sample volume being appropriate for the horn and tip being used.  Such sample volumes are usually indicated in manufacturers' catalogs.  On occasion, however, it becomes necessary to process volumes larger than recommended.  While inefficient, wasteful of tips, and time consuming, the procedure can be improved if appropriate steps are taken:

3.  MISCELLANY

In addition, there are two major areas in ultrasonic processing of fine particles which require careful attention in order to achieve best results.  These have to do with the horn/tip interface and with continuous flow cells:

In spite of the preceding caveat regarding entry of fine particulates into the horn/tip joint, it has been found repeatedly that sonication in suspensions of ultrafine abrasive materials retards cavitation erosion of the radiating surface.  No definitive studies on this phenomenon are known to the author but it appears that the mechanism of significance is peening.  Normal tip erosion proceeds as those most-loosely-bound molecules on the surface are broken loose and the form of erosion appears to follow the dendritic structure of the tip material (usually titanium alloy).  Sonication in a fine abrasive slurry or suspension, such as diamond dust, clay, or tungsten carbide powder, seems to peen the surface, closing up the dendritic pores and polishing the tip as fast as it would otherwise erode.  Long-term sonication will result in classic erosion patterns (concentric rings of lost material with an uneroded circumferential edge) but with a highly polished microfinish.

Contact the author for more information on the above-noted applications or other areas in which sonication might prove advantageous.

©  S. Berliner, III  1995/1993  (all rights reserved)    AL-2    ULTRASONICS AND FINE PARTICLES    Jan 98