Blanketing

(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.

    See also Ultrasonic Whistles (Nozzles, Atomizers, Nebulizers).

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):

U. S. Patent #4,337,896, "Ultrasonic fuel atomizer".}

[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!]

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More about Probe-type Ultrasonic Processing Equipment.

    (continued from the main Ultrasonics Page, Probe-type Ultrasonic Processing Equipment.)

Horn Terminology

Horn Types

STEP HORNS provide maximum amplification (high gain) with high stress at the step and are used primarily in liquid processing.  EXPONENTIAL HORNS provide moderate gain with moderate stress.  CATENOIDAL HORNS provide low gain with low stress and are used primarily in joining.

[Note also that horns are commonly called "tools" in joining (welding, bonding, etc.) applications.]

For a brilliant (literally and figuratively) dissertation on horn design, see Don Culp's Krell Engineering site, replete with horn performance (FEA) animations.

Cup Horns

(Click on picture for a larger image)

CUP HORNS are high-intensity baths used in ultrasonic liquid processing to prevent cross-contamination of samples and tips, in which the sample is placed in a test tube or small beaker placed in a transmission liquid in the cup and irradiated through the base and walls of the vessel.