Blanketing   (09 Jul 08)

(18 Mar 2002 image by and © 2008 S. Berliner, III - all rights reserved)

Note also that the horn or microtip might well break from over-extension when blanketing occurs at very high amplitude since the tip "unloads" when the resistence of the liquid is replaced by the lack of resistance of the gas or resultant foam (see below).

Foaming and Aerosoling

When a foam is generated in a lab sample, it interposes bubbles between the radiating surface and the body of the liquid to be treated or in which treatment is to occur.  This is somewhat akin to "blanketing" but is the result of gas bubbles, not cavitation bubbles interfering with free radiation of acoustic energy into the bath.  It is a self-limiting process.

Once a foam has been created, especially in viscous liquids, it becomes necessary to stop sonication and degas the liquid.  In some cases, at low viscosity, bubbles may rise against gravity and escape through the liquid surface.  If, however, they persist in the bath, short bursts of energy (pulsing), with long rest times between, may be sufficient to break the foam.  A fine mist of the parent liquid can be sprayed against the foam to break it; ultrasonic nozzles excel at this.  In extreme cases, centrifuging and/or vacuum must be applied or the sample may even have to be discarded.

Similarly, on the reverse stroke, molecules of liquid adhering to the surface of a vibrating object may be dragged above the interface (liquid surface) and released, or even ultrasonically nebulized and driven off ballistically, into the atmosphere ("aerosoling").  Obviously, this could pose a significant risk if the liquid is toxic or contains biohazards.  Various techniques beyond the scope of this monograph are available to minimize aerosoling or prevent the escape of the aerosol.

(18 Mar 2002 image by and © 2008 S. Berliner, III - all rights reserved)

As noted above, foam may be broken by applying ultrasonic energy from below the gas-liquid interface but another technique exists which is drawing more and more attention in which acoustical energy is driven through a gas onto the surface of the foam, much as in the '50s and '60s, but with far more effective devices.

Based largely on the work of Prof. Juan A. Gallego-Juárez, Research Professor of the Higher Council for Scientific Research (CSIC), and working at the Instituto de Acústica, Serrano, 144, 28006 Madrid, Spain, this technique utilizes large diameter "plate radiators" which are, in effect, oversized radiating faces driven by relatively "standard" horns.  A graphic representation of such a device is shown:

Plate Radiator
[Image by and © 2002 S. Berliner, III - all rights reserved.]

The radiator drives acoustic vibrations through the gas (air) into the foam bubbles, which are stressed beyond their elastic limit and break (pop), thus exposing the next layer of foam bubbles; the process is quite efficient and rapid:

Ultrasonic Defoaming
[Image by and © 2002 S. Berliner, III - all rights reserved.]

This phenomenon can most easily be seen by placing a horn tip just above a liquid and observing the waves that occur on the surface of the liquid; however, this may well cause the formation of standing waves.  To visualize the propulsive force, simply turn the horn system at a slight angle and you will see the liquid being pushed away from the tip; increasing the angle will increase the propulsive effect until at over 45° there is more lateral motion than vertical motion:

(14 Aug 2002 image by and © S. Berliner, III 2002 - all rights reserved)

(14 Aug 2002 image by and © S. Berliner, III 2002 - all rights reserved)

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Ultrasonic Fountains

Atomization, Nebulization, Humidification,
Misting, Particle Creation and Sizing
(moved on 10 Oct 04 from Page 4)

{09 Jul 2002 - preliminary}

When cavitation is induced in a film of liquid, or at least near the surface of a body of liquid, the bubbles bursting at or very near the gas-liquid interface cause fine droplets to be driven off to some distance:

(09 Jul 2002 image by and © S. Berliner, III 2002 - all rights reserved)

If either the process is inclined so that gravity has a vector at right angles to the axis of motion of the radiating face, or a gas stream is applied perpendicular to that axis, droplets will act ballistically and fall away dependent on their size.  In this way, fine particulates can be produced, such as from molten metals or super-saturated solutions of precipitates, and they can be sorted or characterized by their ballistic trajectories.

I should note here that two of the oldest and least-cost-effective applications of high-intensity ultrasonic (cavitation) atomization/nebulization are the ULTRASONIC OIL BURNER and the ULTRASONIC CARBURETOR.  While both of these applications were eminently successful, technically, they have always failed commercially (to date) because the equipment required has been too costly, and has required too much maintenance, compared to simple mechanical devices used in home and commercial oil burners and in automotive carburetion.

    {Refs. (just for examples - there are many of each):

[Oil burners and carburetors are such perennial "better mousetraps" that I used to start my lectures to new engineers and salespeople by warning them NOT to get suckered into spending any time on such schemes as they were almost guaranteed to be approached by would-be Edisons for just such earth-shaking "inventions"-cum-"discoveries"!  That does not mean that no one can ever come up with a workable system, but I do notice that when I mention money (as in my getting paid to work on such schemes), the geniuses (genii?) instantly evaporate!]

Cup Horns

Boosters (Booster Horns) are sometimes used to enhance output amplitude.

Frequency - one problem that frequently (yes - I wrote that!) arises is the question of what frequency is best for a particular operation.  As noted elsewhere, higher frequencies result in smaller bubble size, penetrating further into crevices or inter-granular spaces but with lower shock-front intensity, while lower frequencies give bigger bubble sizes with higher shock-front intensity ("more bang for the buck") but less penetration.  In addition, frequencies lower than 20KHz are readily audible and can cause personal discomfort.  When an experimenter is unsure of which equipment to obtain, the obvious answer is to get a multi-frequency device; the problem with that idea is that there aren't any such on the market.  Bear in mind that the probe (horn/tool) is a resonant body (i.e.: it rings like a bell) and so has only one primary resonant frequency (usually ~20KHz or ~40KHz); while it is possible to design units (and I have had them made) to force vibration at higher and lower harmonics, such as 10KHz or 80KHz, coping with mechanical impedance in the probe prevents such machines from being economically viable.

Resonant Bodies (Bells) - talking about frequency (preceding), the materials used for resonant bodies have to conduct sound well; they are, as noted elesewhere on these pages, "bell" metals and the horns themselves are longitudinal "bells".  If one holds a horn (or Microtip ot extender) at the "sweet spot", the nodal point, and strikes it with a hard object, it will ring just like a bell.  In fact, if not damped, in fact, not only will it ring purely and sweetly for a very long while before the sound attenuates but such ringing can be used as a rough test for a cracked horn , which will not ring true.  In order to ring, the material must have the same acoustic properties as any bell and the best bell metals follow the same series as for electrical conductivity:  "SCAG" - i.e., Silver, Copper, Aluminum, and Gold.  Close behind these metals are Iron and Titanium.  'Way down the scale comes Stainless Steel, which is very "lossy" (a poor conductor).

Clearly, the least "lossy" materials, the best conductors, leave a lot to be desired.  Silver is soft and malleable, expensive, and very reactive.  Copper is also soft, expensive, and reactive, as well as being poisonous.  Aluminum, while relatively inexpensive, is soft and reactive.  Finally, Gold, while non-reactive, is soft and a wee bit expensive.  Iron, while an excellent bell metal, is VERY reactive (rust, anyone?).  Two good metals are Nickel and Monel, a nickel alloy, both of which are expensive and hard.  Other than being "lossy", Stainless Steel offers a lot; moderate cost, strong, machineable, inert, and readily available; in sheet form, as the diaphragm between the transducer and the liquid in ultrasonic cleaning tanks, it serves quite well.  However, it just does not ring well and so is not a good choice for horns.  Well, that leaves us with Titanium - also moderately priced, as strong as steel, machineable, inert, and readily available, almost as light as aluminum, AND an excellent bell metal.

Thus, Titanium has long been the material of choice for horns, Microtrips, and extenders.

Cooling Samples - one problem that also plagues researchers is that the energy imparted to liquid samples rapidly translates into heat, raising the temperature of the sample and degrading the components.  The most obvious solution (another pun?) is to reduce the intensity of sonication, often quite unacceptable; another is to place the sample vessel (test tube, beaker, etc.) in a cooling bath.  If the rate of temperature rise exceeds the rate of heat transfer through the vessel walls, increasing the surface area of the vessel can give better energy dissipation.  That can be rather hard to do with very small samples, so, in the earliest days of the technique, Dr. Theodore Rosett designed the ROSETT COOLING CELL:

(05 Aug 2006 image by and © S. Berliner, III 2006 - all rights reserved)

As with any glass vessel, avoid contact with the oscillating tip to prevent MICROTIP or vessel breakage.

Please note that a far-more detailed explanation of ultrasonic processing, as well as other technical literature, is available at no charge to consultation clients.  However, as what I believe to be a public service, I shall be adding more of my monographs on ultrasonics on this site; watch for them in the main index.