S. Berliner, III's berliner-ultrasonics.org Ultrasonics Page 2 keywords = "Berliner III Berlin ultrasonic processing cavitate cavitating cavitation cleaning fluid filtration home.att.net"
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S. Berliner, III
Consultant in Ultrasonic Processing
"changing materials with high-intensity sound"

SONOCHEMISTRY * REACTION ACCELERATION * DISRUPTION
HOMOGENIZATION * EMULSIFICATION * POLLUTION ABATEMENT
DISSOLUTION * DEGASSING * FINE PARTICLE DISPERSION
BENEFICIATION OF ORES AND MINERALS
CLEANING OF SURFACES AND POROUS MATERIALS

also see
Keywords (Applications) Index

[consultation is on a fee basis]

Specializing in brainstorming and devil's disciplery for new products and
reverse engineering and product improvement for existing products.

{"Imagineering"}

Technical and Historical Writer, Oral Historian
Popularizer of Science and Technology  


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Ultrasonics Page 2



INDEX

PLEASE NOTE:  If some of the internal links on this page refuse to work,
please click on Back and scroll down.

On the main Ultrasonics Page:
    Applications List.
    Probe-type Ultrasonic Processing Equipment.
    Brain Storming - bright ideas, pipe dreams, pie-in-the-sky?

On Ultrasonics Page A:
    AL-1C - "CONDENSED GUIDE TO ULTRASONIC PROCESSING"
        (A Layperson's Explanation of a Complex Letterhead)
    AL-1P - "A POPULARIZED GUIDE TO ULTRASONIC PROCESSING"
        (A Non-Technical Explanation of a Complicated Letterhead.

On Ultrasonics Page 1 (the second page):
    AL-1V - "A POPULARIZED GUIDE TO ULTRASONIC CAVITATION"
        (A Non-Technical Explanation of "Cold Boiling").
    ULTRASONIC DEGASSING.
    TUBULAR HORNS (Radial Radiators).
    CARE of TIPS (Radiating Faces).

On Ultrasonics Page 1A (the preceding page):
    AL-4 - AMPLITUDE MEASUREMENT.
    Call for Contributions for Book

On Ultrasonics Page 2 (this page):
    More on Cavitation
    AL-2 - "ULTRASONICS AND FINE PARTICLES -
      BENEFICIATION OF SLURRIES AND FINE-PARTICLE SUSPENSIONS
      [CERAMICS, COAL & ORES, COATINGS, COLUMN PACKINGS,
        SINTERING, SLIPS]

On Ultrasonics Page 3:
    AM-1 - "ULTRASONIC STERILIZATION and DISINFECTION"
    UM-1 - "ULTRASONICS, HEARING, and HEALTH"
    Ultrasonics and Living Organisms
    Keywords (Applications) Index.
    What's New?

On Ultrasonics Page 4:
    Foaming and Aerosoling - moved 28 May 02 from Page 1A.
    Ultrasonic Propulsion (Propulsive Force) - Moving Material.
    Ultrasonic Fountains - Atomization, Nebulization, Humidification,
        Misting, Particle Creation and Sizing.
    Ultrasonics and Nuclear Fusion.
    Quick Links for Ultrasonic Probe Manufacturers (moved 10 Jul 2002).

On the Ultrasonic Cleaning page:
    ULTRASONIC CLEANING {in process}.
    Immersible Transducers.
    What's New?

On the Ultrasonics Glossary page:
    ULTRASONICS GLOSSARY {in process}.

ULTRASONICS BIBLIOGRAPHY
    Ultrasonic Bibliography Page 1 - Reference Books on Acoustics,
        Vibration, and Sound.
    Ultrasonic Bibliography Page 2 - Sonochemistry.
    Ultrasonic Bibliography Page 3 - Selected Articles.


You are invited to visit the ULTRASONIC INDUSTRY ASSOCIATION home page.


CALL FOR CONTRIBUTIONS:  I am writing a book on "High-Intensity Ultrasonic Technology and Applications", on the practical application of power (high intensity) ultrasonics, the use of ultrasonic energy to change materials.  Contributions are welcome.


THE CAVITATION BUBBLE

Larry Crum's Cavitation Bubble

[image from University of Washington, Applied Physics Laboratory (Lawrence Crum, Ph.D.)
- bubble diameter approximately 1mm]


MORE ON CAVITATION

[Continuing from AL-1V - "A POPULARIZED GUIDE TO ULTRASONIC CAVITATION"
(A Non-Technical Explanation of "Cold Boiling")]

Cavitation initiates most readily at, and proceeds radially outward from, discontinuities (voids, contaminant particles, and such) in the liquid, where bonds between adjacent particles are weakest.  Theoretically, a completely pure liquid (an unlikely happenstance) would be virtually inmpossible to cavitate.  However, somewhat conversely, once cavitation initiates, any gas bubbles in the liquid absorb energy to no avail and must be removed before effective processing can proceed; this is normally done by running the device (degassing it) for a few minutes until free bubbling ceases.  This applies primarily to bath (cleaning tank) sonication.  Probe sonication is at so much higher an energy intensity that this procedure is not normally necessary in that process.  In addition, any surface with a concavity which could trap air or other gases and prevent full wetting of the surface will prevent activity on the that surface.  Not only must the surface be wetted, it must be wholly submerged in the liquid, not merely wet.  To effect such, the object to be cleaned (or otherwise treated) must be rotated, completely under the surface, if necessary, to discharge any pockets of air or gas such that the gas rises out of the liquid bath.

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 was Misonix Incorporated in Farmingdale (formerly Heat Systems, for which the author was Director of Technical Services); that product line, Misonix SONICATOR Ultrasonic Liquid Processors, is now made and sold by Qsonica, LLC - a blatant plug!   rev.gif (06 Mar 10)

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

CAVITATION ACCELERATES BOTH CHEMICAL AND PHYSICAL REACTIONS.

More to follow.


AL-2 ULTRASONICS AND FINE PARTICLES Jan 98

ULTRASONICS AND FINE PARTICLES -
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"}

1.  GENERAL -

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:

  a.  Ceramic insulation for resistors and capacitors benefits greatly from the homogeneity and degassing provided by sonication.  Probes are used in conjunction with continuous flow cells to give on-line production capability in a number of electronics applications.

  b.  Iron oxides and similar disk and tape coatings are dispersed and degassed immediately prior to application in slurry form.  Freedom from voids and smears provides a far superior product for manufacturers of magnetic media.

  c.  Clays, limes, and other fines can be compacted (dewatered) ultrasonically.  Significant development work has been done in this area.  Compacting of soil samples and selectively increasing or decreasing permeability are also practicable.  In addition, a phenomenon of gelling of clays, especially in the presence of petroleum- based oils, has been noted by the author.

  d.  Coal beneficiation and ore refining obtain greater yields and coal slurries can be made denser for better transport and improved combustion properties.

  e.  Pharmaceutical preparations and tablet pressing operations are also areas in which sonication of fine particle suspensions has been applied.  Reduction of tablet size is a very attractive application.  [Tablet producers should note that ultrasonic liquid processors are also used for dissolution of samples for faster QC analysis.]

  f.  Glass beads (microspheres) are commonly used as column packings for filtration and for HPLC (High Pressure Liquid Chromatography) and similar applications.  Sonication of the suspended beads immediately prior to packing the column has repeatedly resulted in 20% denser packing.  This allows 20% shorter equivalent columns or a 20% higher performance in the same column length.

  g.  Industrial ceramics and even fine china table service can be improved by sonicating the slip prior to pouring the mold.  Better slip homogeneity, fewer voids from bubbles, smoother surface finishes, and less cold jointing result.  Probes can be fitted directly to slip pouring nozzles at molding stations.

  h.  Sintered carbide tool bits are made by suspending fine particles of tungsten or other carbides in light fluorocarbon liquid, pouring the suspension in a die cavity, evaporating (and recovering) the solvent, and pressing the particles in the cavity under extreme pressure and temperature.  By sonicating the particles either immediately before pouring, or even directly in the cavity, a 20% improvement in density was found.  Resultant bits are stronger, hold an edge longer, cut cleaner, and remain cooler.  Thinner bits can be made, thus providing cost savings.

  i.  Photographic emulsion grains can be sonicated prior to coating to degas and homogenize the material and to prevent formation of voids and discontinuities.

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:

  a.  In any batch sonication procedure, the shape and size of the vessel can be critical.  In general, beaker-shaped vessels are best, and round-bottomed vessels such as test tubes and boiling flasks are fair, while square-bottomed vessels or complex shapes are poor.  Vessels with no free space around the tip will experience much faster temperature rise than those with ample heat sink volume and large cooling area.  Approximately one to five tip diameters can be used as a first approximation for volume sizing.

  b.  Depth of immersion and free space under the tip are also significant factors.  There is virtually no sonication alongside the tip or above it.  Material floating on the surface will not be treated, nor will material taken up on foam bubble surfaces.  Insufficient tip immersion will cause spattering or foaming.  Empirical tests in actual samples or referee liquids are recommended.

  c.  Large volumes of glass beads or other particles can be suspended in liquids by direct sonication.  If the solids exceed about 20% of the liquid by volume, especially in oversized vessels, the solids may be too deep in the vessel to allow full depth sonic penetration of the mass of solids.  Sifting the particles in slowly while the liquid is being sonicated will greatly increase the proportion of solid to liquid, even to the extent that particles can touch all surrounding particles.  In this latter case, the effect is that of creating a fluidized bed.

  d.  If the bulk of solids is already in the liquid or is inconvenient to sift, a mechanical or magnetic stirrer may be used in conjunction with the ultrasonic probe.  For best results with a stirrer, the stir bar or impellor should be inserted far from the center of the beaker or other vessel, vertically along one side wall, with the probe/horn/tip offset 180° across the vessel along the opposite side (but NOT touching the wall).  It is especially important to avoid drawing a vortex under or alongside the probe to prevent foaming.

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:

  a.  For the interface (joint) between the horn and tip and, in fact, that between the front driver of the convertor and the horn (which should never be wetted) or between horns and boosters or extenders or microtips, present operators of ultrasonic processors are referred first to the paragraph(s) on the care of horns and tips in their processor instruction manual.  Normal sonication produces cavitation in liquids, the formation and collapse of microscopic vapor bubbles, generating shock waves which radiate outward and disrupt cells, homogenize and emulsify samples, degas liquids, etc.  Shock waves, being radially symmetrical, attack the acoustic radiating surface of the tip or horn as well as the sample.  The erosion which results reduces the efficiency of sonication in direct proportion to the degree of erosion.  Certain solvents have been found to wick under replaceable flat tips and cause deterioration and early failure.  Solid horns are thus recommended in lieu of tapped horns for such applications.  Considerations regarding intrusion of liquids carrying ultrafines into the horn/tip joint are also covered in greater detail in Applications Leaflet AL-5, Extenders and Sapphire Tips.

  b.  The standard polycarbonate and stainless steel continuous flow cells, and most other accessories fitted to the body of standard horns, are manufactured with extremely fine (32-pitch) threads which can bind up or gall if accidentally contaminated with micronic or sub-micronic particles.  Two solutions to this problem have been used.  Larger cells use sanitary clamps and loosely fitted removable parts to avoid binding; they are, however, much more expensive.  The alternative has been to make the smaller cells without threads, with customers devising a mechanical version of an optical bench, longitudinal ways in which the convertor and cell are supported laterally and moved axially (by screw jack or hydraulics) for adjustment and servicing.  In this latter case, some form of gymbal mounting of the convertor is required to avoid any side-thrust on the horn.

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

- - - * - - -

NOTE re sonication and fine particles - I had the honor (and the fun) of setting up Prof. Kenneth S. Suslick's first SONICATOR disruptor at his laboratory in the Noyes Lab of the Department of Chemistry at the University of Illinois at Urbana-Champaign.  Since then, Ken has become a leading light in the discipline now known as SONOCHEMISTRY, beginning with the production of iron carbonyls and other species that had not been amenable to creation by thermolysis, catalysis, photolysis, etc.  In the 24 Jun 98 Journal of the American Chemical Society (as reported in Vol. 154, Science News, 18 Jul 98, p. 47), Ken and his colleagues announced the development of a novel method of creating ultra-small crystals of molybdenum disulfide, known to me as a super lubricant but apparently also a major catalyst to remove sulfur from petroleum.  Ken, et al., uses sonication to create tiny crystals some ten times more active than the usual MoS2, even better at catalysis than ruthenium disulfide, currently the best available.  I KNEW that (well - I knew sonication could do it)!  Ah, but Ken is a chemist and actually did it, and more power to him!


[Corporate information given above has been updated as of 01 Aug 1998]


You may wish to visit the main Ultrasonics page, et seq., as well as the Ultrasonic Cleaning page {in process} and the Ultrasonics Glossary page {also in process}.



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To tour the Ultrasonics pages in sequence, the arrows take you from the main Ultrasonics Page (with full index) to Pages A, 1, 1A, 2, 3, and 4, Glossary Page, Cleaning Page, and Bibliography Pages 1, 2, and 3 (see Index, above).



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