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.”