NOTE:  This page "evaporated" from my server (perhaps aliens abducted it?) on 13 Sep 99 and has been recreated from a back-up copy - please accept my apologies if I have inadvertently overlooked any typographical errors which might have crept in in the process of recreating the document.

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 (the first page):

On Ultrasonics Page A

On Ultrasonics Page 1:

On Ultrasonics Page 1A:

On Ultrasonics Page 2 (the preceding page):

On Ultrasonics Page 3:

On Ultrasonics Page 8 (this page):   (24 May 07)

{Page index truncated to save space - see the Ultrasonic Index Page}

On the Ultrasonic Cleaning page:

    ULTRASONIC CLEANING {in process}.

On the Ultrasonics Glossary page:

    ULTRASONICS GLOSSARY {in process}.

ULTRASONICS BIBLIOGRAPHY

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

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

---

Continuous Flow Cells

Here is a means of using a horn in a continuous flow system; one or two (or more) liquids are pumped into the "CONTINUOUS FLOW CELL", mix and are processed in the annulus, controlled by an adjustable orifice which can be moved in and out axially and with interchangeable orifice plates, thus giving full control over the process parameters:

[Image by and © 2007 S. Berliner, III - all rights reserved]
(Thumbnailed image - click on picture for larger image)
Continuous Flow Cell   (24 May 07)

Taking the flow cell a step further, the author combined the features of a flow cell with a so-called FLOW-THROUGH HORN, one which is cross and axially drilled and came up with a novel (the U. S. Patent Office agreed) means of getting guaranteed uniform mixing of two components by passing one through the horn so that the liquid (Fluid "A") exits the horn through the tip in the center of the cavitational field where it meets the other liquid coming radially inward from the body of the cell.  Because the second liquid (Fluid "B") travels radially inward, all aliquots pass along a radius of equal length, thus assuring equal sonication and mix with Fluid "A" directly in the cavitation field at the end of the horn:

[Image by and © 2007 S. Berliner, III - all rights reserved]
(Thumbnailed image - click on picture for larger image)
Ultrasonic Processing Cell   (24 May 07)

In one of the many other embodiments in both patents, the two fluids are injected coaxially against the radiating face of a standard (not Flow Through) horn, as shown in this sketch:   (24 May 07)

[Image by and © 2007 S. Berliner, III - all rights reserved]
(Thumbnailed image - click on picture for larger image)
Alternate Ultrasonic Processing Cell

Note that the direction of flow is chosen to avoid entrapment of unprocessed or non-homogenous aliquots in the cells.

AM-9    - -    APPLICATIONS MONOGRAPH     - -    03 June 2007

THE USE OF ULTRASONIC PROBES
IN FUEL RESEARCH

Revised and updated by
S. Berliner, III
27 April 2004 and 03 June 2007

The potential of ultrasonic energy has often not been appreciated in such diverse applications as emulsifying, extracting, and dispersing.

Instruments that perform these functions, like the SONICATOR® Disruptor from Misonix, have long been used in biological research and are now finding wider application in physics, chemistry, and industry.  The availability of much larger power outputs and the use of continuous flow attachments have made this transition possible.  Better understanding of ultrasonic theory has helped too.

When applied to fuel research the SONICATOR probe has several interesting applications: emulsifying fuel and water, breaking long chain polymers in fuel as anti-mist agents, dispersing coal particles in oil, dissolving coal in solvents, extracting oil from shale, and aiding the determination of chitin in contaminated fuels.

Some Theory

When an A.C. voltage is applied to a crystal, it changes shape in phase with the electric field.  This is the piezoelectric effect.  These continuous changes in shape are the pulsations or sound waves which travel through the probe into the liquid.  Alternate compressions and rarefactions of this wave produce a phenomenon known as cavitation --- the making and collapsing of microscopic bubbles.  These bubbles or cavities take many cycles to grow to "resonant" size.  At this point they collapse violently in one compression cycle, producing on the order of 20,000 atmospheres of pressure.  This mechanical shock is felt at a distance of a few microns from the cavitation bubble collapse.

Although the SONICATOR is an interesting instrument and, also, relatively simple to use, a knowledge of ultrasonic theory as well as techniques recently developed are important for best possible results.

Almost any variable that tends to suppress cavitation --- such as higher ambient pressure, higher surface tension or tensile strength, degassing, lower temperature --- will increase intensity if there is enough power to produce cavitation.

A rise in temperature may allow more bubbles to nucleate and grow, enhancing the scrubbing action of a tank type ultrasonic cleaner.  But the increased vapor pressure within a cavitation bubble reduces the violence of collapse; a violence which is necessary for the probe SONICATOR's more difficult work.  As fuels are reduced in temperature, then they can be emulsified with water more easily.  Similarly, applying static pressure during treatment compresses and dissolves air bubbles in the fluid.  This increases intensity by removing the air which will cushion bubble collapse.  It is possible to completely eliminate cavitation in organic solvents by saturating with certain gases.  These gases fill the cavitation bubble and prevent collapse.

Raising the frequency also lowers the intensity.  This by reducing the size of the cavitation bubble, which then collapses with less force.  Cavitation is more difficult to produce as the frequency is raised and cavitation can not be initiated at any power level at frequencies over about 2.5 MHz.

Tensile strength is not a term normally applied to liquids.  However, the forces necessary to separate water molecules (probably van DerWaal's), like those of metals, are quite high.  In fact, if water were absolutely pure, it would take about 15,000 psi to cause fracture or cavitation.  Since water is never pure --- distilled water has many thousands of impurities per cc, air and particles ---, far less pull, or negative pressure, is necessary to produce cavitation.  The greatest strength achieved in a liquid is -277 atmospheres; this was produced in a highly purified water which was "fractured" in a spinning tube by centrifugal force.

Solvents or fuel oil have lower tensile strengths than water and cavitation bubbles produced in them collapse with less force.  Ordinary degassed kerosene requires about one atmosphere of negative pressure for cavitation, water 7 atmospheres.  When comparing most organic fluids, higher viscosity and lower vapor pressure may be used as a rough indicator of tensile strength --- or expected cavitation intensity.  Although other liquid properties such as density, surface tension, and speed of sound are also involved, the former two are the most useful or practical predictors.  Adding a polymer to a particular fluid increases viscosity, but this will not increase intensity of bubble collapse.  The added viscosity is due to friction, not greater molecular attraction, and the bubble would simply collapse more sluggishly.

The addition of wetting agents also reduces tensile strength, or its close cousin, surface tension.  The use of solid combustible powders like lampblack as emulsifying agents rather than the soap type, might be preferable since this will not affect surface tension and intensity as drastically.  One reason oil-in-water emulsions are easier to form ultrasonically than water-in-oil, is possibly due to the "stronger" disperse phase.  Cavitation in a continuous water phase is more intense for a given generator (output) setting than for a continuous oil phase.

As a matter of interest, the height of Redwood trees is limited by the tensile strength of the water rising to the top branches by capillary action.  If the trees grew higher, the liquid would "fracture" and the uppermost leaves would not be fed.

Making Emulsions

The use of water/oil emulsions is recognized as having several advantages in combustion processes and engine fuel:

    The challenge facing us now:

Here, ultrasonic techniques hold great promise.  The SONICATOR will produce an oil and water emulsion of 1 micron size in a few seconds; and in a minute, 1/10 micron.  The distribution curve is sharp compared to that of mechanical homogenizers.

As sonication proceeds, the particle size gets smaller and the size distribution narrows. For any particular intensity there is a size and disperse phase concentration limit.  At some point in the sonication process, the particle size will start growing rather than reducing, because flocculation or coagulation have become more predominant than ultrasonic dispersing.

It takes less intensity to initiate an oil-in-water emulsion than its reverse and the particle size is smaller.  Oil droplets in and oil/water emulsion are electrically charged which tends to stabilize the mixture.

As might be expected, the addition of stabilizers increases the concentration of the emulsion as well as the rate of formation.  Highly dispersed emulsions of 35% concentration can be obtained without surface active agents, however.  Conceivably, if the disperse phase is broken down fine enough, say, approaching 1/100 micron, the dispersion might be stable without emulsifiers, like a colloid.  Certainly there is less tendency to "cream out" due to sedimentation.  A water/oil ratio of up to 45% (water/water + fuel) can be burned.  With just natural surfactants in fuel, the ultrasonic emulsifying capacity may approach this higher ratio.  Oil/water/oil or multiple emulsions, might be an interesting addition to fuel research.  These are frequently formed when inverting the emulsion.  Possibly the phases go on, ad infinitum, although this phenomenon is not easily seen in a microscope or otherwise detected.

In the usual case, lower temperature produces better emulsions.  Oil/water is more stable under the effects of temperature than water/oil, and probably more stable with time in general.  If there is a change in the relative temperature of the two phases, emulsification will be reduced.  A little water, or a little oil, is easier to emulsify.

The process becomes more difficult as the starting ratios of oil and water approach 50/50.  It is also more difficult if one phase is, or becomes, more viscous than the other.

Research on emulsions and their practical use in fuel oil has been hampered by a lack of good means for measuring water droplet size, the disperse phase.  Indication of size distribution by an in-line detecting system would be especially valuable.

Although conventional light microscopes are not too useful for size estimations much below a micron, new "interference" methods might be applicable.  There are also several other possibilities.  For example, since the viscosity of a given phase ratio is influenced by mean droplet size as well as size distribution, measurement of this parameter might provide a continuous flow measurement system.

Electrical conductance has been used as a qualitative test for phase type, and high voltage AC is known to break emulsions.  Instinct would suggest that there is an electrical approach to this problem.  High and low voltages or currents, AC superimposed on DC, or capacitance of a thin emulsion film, might be tried.

Still another possibility is the coherent light source of a laser.  By monitoring both the absorption and scattered radiation from a laser beam, the particle size and distribution may be determined.  The highest frequency available, 25 MHz, produces a wavelength in fuel oil of 70 microns.  Only a small amount of sound energy will therefore by scattered by one micron water droplets.  However, if the sound receiver is positioned to pick up scattered radiation only, and the source is a multi-frequency transmitter, a suitable emulsion detecting device is possible.

Making Anti-Mist

Polymers are now being added to fuel oil as anti-mist agents.  If the polymers can be broken sufficiently when needed, for example, ... to start an engine, their use becomes a practical solution to the fire hazard problem.  Researchers have used ultrasonics to break long-chain polymers since the early thirties.  These polymers are usually dissolved in organic solvents.  Ultrasonic degradation or depolymerizing of many molecules have been studied, including polystyrene, methyl methacrylate, nitrocellulose, proteins, rubber, and starch.

The larger the molecule and more intense the ultrasonics, the faster the degradation.  An increase of intensity also reduces the ultimate particle size.  The molecule is usually broken in half at the C-C bond, and its fragments are in turn broken in half, producing finally a narrow dispersion of molecular weights.  As a contrast, depolymerization due to heat or oxidation produces fragments of varying molecular weights.

Polystyrene can be reduced to 1/6 its original molecular weight.  The disruption of these molecules is not due to frictional forces produced by the sound wave or by heat.  Neither is it due to a resonant effect in the long chain. --- although the largest polymers may reach a length of 1 micron, the sound wavelength in liquid even at 1MHz is far longer.  Cavitation produces this breaking effect; and suppression of cavitation by degassing or high static pressures usually prevents it.  There is evidence that vibrating bubbles without collapse can cause breakage.  This phenomenon produces large local pressure changes also and is the probable cause of cavitation erosion damage to ship propellers rather than actual bubble collapse.

Dissolved gases within the organic solvent may have a significant effect.  Highly soluble gases such as sulfur dioxide, ammonia or carbon dioxide dissolved in benzene can suppress cavitation activity by penetrating into the cavitation bubbles in great amount, cushioning or preventing collapse.

Depolymerization may produce free radicals or ions concomitant with the splitting of the C-C bond.  These radicals can be detected by their ability to oxidize iodide to iodine.

THE SONICATOR

As a research tool or production device the SONICATOR disruptor/emulsifier is ideal in many respects.  Periodic maintenance is obviated since there are no "moving" parts and nothing to be cleaned.  Smoothly variable output intensities provide much needed flexibility for pilot development.  The Continuous Flow Cell permits adjustment of static pressure as well as fuel flow rate.  This Cell is made of LEXAN® polycarbonate resin so that emulsification or other process can be viewed; and the outlet may be placed directly in series with the combustion nozzle or measuring device (cells of other materials are available).

The standard 600 watt, 20KHz, SONICATOR will emulsify about 1/2 gal/min down to 1 micron droplet size.  Lower frequency 10KHz devices, which once were available in the 8000 watt range, were more effective emulsifiers because of the larger cavitation bubble, and were more efficient for a given wattage input.  10KHz processors were, however, quite noisy and physically much larger than the equivalent 20KHz devices and are no longer in production.