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 Ultrasonic Cleaning

By Nathan Schiff, PhD

Associate Editor - Institutional

Have you ever thrown a small pebble into water, and noticed how the ripples spread out in all directions? If you could examine the waves at any instant, you would notice that they consist of equally spaced peaks and valleys. Sound waves, although invisible to the naked eye, travel through air in a similar manner. The peaks are referred to as points of compression and the valleys are referred to as points of rarefaction or cavitation. The number of peaks that occur each second is referred to as the wave's frequency, which is usually expressed in cycles per second or cps. This article examines the technology behind cleaning window coverings and other hard surfaces using ultrasonic sound waves.

What are Ultrasonic Sound Waves ?
Ultrasonic waves are sound waves with frequencies above the limits of human audibility. They range in frequency from 20,000 to 100,000 cycles per second. The most common frequencies used for cleaning are between 20,000 cps to 50,000 cps.

How do Sound Waves Clean Surfaces ?
Sound waves travel normally through water, as they do through air, provided that their power or loudness (amplitude) remains relatively low. Once their power is increased beyond 50 watts/gallon of solution, the liquid through which they pass, fractures into tiny bubbles. It does so at the millions of different sites where rarefaction or cavitations of the waves occur. Cavitation bubbles grow to unstable sizes, followed by their violent collapse or implosion. Individual implosions produce shock waves, resulting in microscopic jets of liquid traveling at speeds up to 400 km/hr. The intense pressures produced can locally superheat fluids up to 2,500^oC, for a split second. This combination of events produces a powerful scrubbing action, which can dislodge deeply embedded soil particles from hard surfaces.

Factors Which Affect Ultrasonic Cleaning
Ultrasonic cleaning is about implosion of air bubbles. However several other factors play a key role in determining the effectiveness of the cleaning process, including: surface tension, temperature, frequency and power of the ultrasonic waves.

1. Detergents lower the surface tension within the solution. In so doing, less energy is required to produce cavitation bubbles.
2. Cavitation bubbles form more readily as the solution temperature increases. However, at very high temperatures, close to the boiling point of the liquid, the cavitation bubbles grow too large and are unable to implode. Temperatures in the range of 55^oC -70^oC provide the best cleaning results.
3. Strange as it may seem, the higher the frequency of the ultrasonic waves, the less power the wave carries. This affects its ability to remove large soil particles. For this reason general cleaning is carried out at frequencies of 20,000 cps, whereas, the removal of extremely fine particles from electronic components, requires a frequency of 80 KHz range.

Turning Ultrasonic Blind Cleaning into a Business
For the past 8 years, Peter Rosteck, president of Ultrasonic Network has successfully applied this fast-growing technology for use in cleaning window blinds, and is now considering franchising his process across Canada. According to Peter, the drapery business once held the lion's share of the window covering market. Today, venetian blinds and verticals claim the bulk of the consumer and industrial market.

Window blinds tend to attract airborne particles, including: molds, yeasts, bacteria and dust. Some of these particles can become airborne during each opening or closing of the blinds - which creates a localized, high "dust" concentration area. If inhaled, the particles may lead to respiratory problems in sensitive individuals. Inhalation of dust residues and their associated microorganisms, creates a localized situation similar to the "sick building syndrome".

So how does Ultrasonic Network help solve the problems associated with dirty window coverings? The blinds are first immersed into the ultrasonic bath, which houses multiple transducer units; spaced throughout the tank. A mild, biodegradable detergent is added and the power turned on for 30 to 60 seconds. In this remarkable process, you can actually see soil as it is being dislodged from every hidden crevice. What is really amazing to watch is how heavily soiled blind cords are returned to their original white condition.

The blinds are then transferred for a few seconds to a rinse tank, then hung up to dry. Once dry, the blinds appear practically new and are ready for hanging.