An earlier blog discussed the mechanics of ultrasonic removal of soluble contaminants. In many cases, however, it is not the soluble contaminants but, rather, particles that are the primary target of ultrasonic cleaning. Particles of concern range from a fraction of a micron to several hundreds of microns in size. Something larger than a BB would probably be better removed using means other than ultrasonics. When it comes to removing particles, there is no universal solution due to the variations in size and the variety of mechanisms that hold them attached to whatever substrate they are clinging to. The challenge is further complicated by the fact that particles don’t only adhere to substrates but also interact with and attach to each other.
The first step in physical displacement of a particle is to establish contact with it. As ultrasonic vibration is delivered through a liquid, this means that the liquid being used must make physical contact with or “wet” the particle. Wetting and is primarily related to the surface tension of the liquid. High surface tension may prevent the required coupling of energy to the particle. See this blog for more detail.
The next step in particle removal is to deliver sufficient energy to physically displace the particle enough to break the bold holding it in place. The amount of energy required depends on particle size – displacement of a small particle requires less energy than displacement of a larger particle. The amount of energy released by the implosion of a cavitation bubble depends on its size which, in turn, is related to the ultrasonic frequency. Higher ultrasonic frequency results in smaller cavitation bubbles which implode releasing less energy. One might then logically say, “Why not use low frequency to assure the maximum energy release from implosion to remove both large and small particles?” The problem is that due to a phenomenon called the barrier layer, larger cavitation bubbles can not form extremely near surfaces. If a particle is smaller than the thickness of the barrier layer, it will not be displaced by the implosion of a cavitation bubble that is further away from the substrate surface. In short, higher frequencies are better at removing smaller particles while lower frequencies are required to remove larger particles.
Particle Removal Using Ultrasonics
As a final step, liquid flow removes remaining particles to prevent re-deposition.