Basics of Ultrasonics

By Controlled Environments

Created 2010-12-28 01:41

Barbara Kanegsberg

Ed Kanegsberg

You don’t clean without energy. It takes energy to overcome the forces binding contaminants to the substrate. In most cleaning systems, a liquid cleaning agent is used; and energy, beyond the innate solvency properties of the cleaning agent, is required. This energy can come from the motion of atoms and molecules, such as from the kinetic energy associated with high temperatures. The motion associated with liquid spray is another source of kinetic energy widely used in critical cleaning. Another method for providing this motion is from sound waves in the ultrasonic frequency range.
Ultrasonics have proven to be an effective tool for many critical cleaning applications, ranging from initial cleaning after machining to final assembly in controlled environments. The forces associated with ultrasonics are very powerful; the local atoms have kinetic motions equivalent to temperatures as hot as the surface of the sun. The phenomenon is instantaneous and transient, so that successful cleaning with ultrasonics can be achieved without damage to fragile surfaces. Ultrasonics include omni-directional action. In contrast with line-of-sight processes, this allows the cleaning energy to reach complex surfaces, in some cases including blind holes. Both particulates and thin films can be removed from surfaces by ultrasonic action.
HOW ULTRASONICS WORKS
Generation of sound waves in a liquid in an ultrasonic tank is analogous to generation of sound waves in air by an audio system. A transducer converts electrical signals to mechanical vibrations that generate sinusoidal sound waves in the liquid. The sine wave has a positive or compressive phase during which liquid molecules move toward one another, and a negative or rarefaction phase during which the molecules move away from each other. The instantaneous pressure, P, in the fluid at time, t, can be expressed as

When Ps > Po, the pressure during the rarefaction phase is reduced to less than the vapor pressure of the liquid. During this time of “negative” pressure, a tear or vacuum “bubble” will form and grow. During the subsequent compression, the bubbles suddenly collapse, creating shock waves and microjets of fluid (Figure 1).¹ It is these shock waves or microjets, not the transducer generated sound, that provides the energy to dislodge unwanted soils. The creation and collapse of vapor bubbles is called “cavitation.”

MORE IS NOT NECESSARILY BETTER
The frequency and amplitude of the ultrasonic sound waves determine the energy created by the collapsing cavitation bubbles. The energy increases with increasing amplitude but decreases with increasing frequency. As the frequency increases, the positive and negative phases of the sine wave become shorter. As a result, smaller, “gentler” bubbles are produced.
Any force, including ultrasonic forces used in cleaning, has the potential for both positive and negative effects. The shock waves and jets that dislodge soil can also dislodge (erode) the underlying surface, the substrate that is being cleaned. On a macro level, cavitation also occurs with ship propellers creating vacuum tears in the liquid. This causes erosion that is a major cause of ship and boat propeller failure. Therefore a balancing act, a compromise, must be reached—sufficient energy to dislodge the soils but not so much as to damage the substrate. Substrates from softer metals, like aluminum and copper, are more easily damaged. This does not mean that one can not use ultrasonics to clean them, but rather that the process must be appropriately controlled. In fact, it is important to consider potential damage issues when considering any cleaning force. For example, many critical cleaning processes use high-pressure spray in air. When miniature components are cleaned using inline systems, longer exposure to high-pressure spray may be recommended. Excessively high impingement spray can damage delicate components; just as, on a macro level, wind-driven rain can damage property.
Commercially available ultrasonic systems have frequencies that range from 20kHz to over 400kHz. The lower end (20kHz-40kHz) are effective for hard metals, such as steel or titanium, and for removal of larger particles. Higher frequency units, because the duration of negative pressure is shorter, create cavitation with smaller bubbles and “gentler” implosions and are less likely to damage the surface. The higher frequencies can also be more effective at reaching small particles. This is because, as cavitation energy decreases at high frequency, a fluid flow effect called “acoustic streaming” dominates and can penetrate a fluid surface boundary layer to reach the smaller particles. At even higher frequencies, above about 500kHz, the cleaning process, essentially entirely from acoustic streaming, is referred to as megasonics. Acoustic streaming is unidirectional as are classic megasonic systems; and megasonics is traditionally used in microelectronics where surfaces are flat.
Some ultrasonics systems feature multiple frequencies in the same tank to address both large and small particles.² There are many other parameters that affect the efficacy of an ultrasonic system.³ One of those parameters is the liquid medium in the tank. Bubble collapse is a function of viscosity, surface tension, and temperature. For instance, adding a surfactant to water lowers its surface tension, and makes the creation of cavitation bubbles easier. At low temperatures, viscosity impedes cavitation; at temperatures close to the boiling point, a high vapor pressure causes vapor filled or “squishy” bubbles, reducing the impact of the collapse. For water, there is an optimal temperature at about 55°C.⁴
References

1. J. Fuchs, “The Fundamental Theory and Application of Ultrasonics for Cleaning,” Handbook for Critical Cleaning, B. Kanegsberg and E. Kanegsberg, editors; CRC Press (2001).
2. K. Gopi & S. Awad, “Ultrasonic Cleaning with Two Frequencies,” Handbook for Critical Cleaning, Second Edition, B. Kanegsberg and E. Kanegsberg, editors; CRC Press (expected 2011).
3. B. Kanegsberg & E. Kanegsberg, “Parameters in Ultrasonic Cleaning for Implants and other Critical Devices,” Journal of ASTM International, April 2006, Vol. 3, No.
4. 4. L.D. Rosenberg, “On the Physics of Ultrasonic Cleaning,” Ultrasonic News, 4, p. 16 (1960).