The physical properties of liquids have a significant effect on their ability to cavitate and produce imploding cavitation bubbles useful for ultrasonic cleaning. Since other factors including temperature and chemistry can have an effect on the physical properties of liquids, these, too, play a significant role in ultrasonic cleaning. This blog explores the effect that a number of liquid properties have on ultrasonic performance. Since the following discussion benefits from a general understanding of ultrasonic cavitation, the reader may want to first review the following – –
SURFACE TENSION –
Surface tension is the property present in the form of tension at the unsupported surface skin of a liquid. Surface tension would be analogous to the stretch of the rubber in a balloon filled with water. Although we usually think of surface tension in terms of the outer perimeter of a liquid surface (air/water interface or the surface of a drop of water) the same effect applies but in the reverse when a cavitation bubble is produced within a liquid. Surface tension opposes the creation of a cavitation bubble within a liquid and assists in its collapse once it has been formed. Lower Surface Tension makes it easier for cavitation bubbles to form within a liquid under vacuum conditions. In the case of ultrasonics, this means that more cavitation bubbles will be formed and that they will grow to a larger size under otherwise similar conditions. Also, due to the fact that the formation of each cavitation bubble requires less energy, more cavities are formed for a given energy input. Once a cavity is no longer supported by other forces, in the case of ultrasonics, the negative pressure portion of a sound wave, surface tension contributes to the collapse of the cavity. Higher Surface Tension contributes to the energy released in a cavitation bubble implosion.
Related – What is Surface Tension?
TENSILE STRENGTH –
Although we usually relate tensile strength with solids, liquids also exhibit tensile strength. Tensile strength is the force required to separate molecules of the liquid from one another. Forming a cavitation bubble in a liquid requires enough force to separate the molecules at the beginning of cavity formation. A liquid with Lower Tensile Strength makes it easier to form cavitation bubbles. However, since cavitation formation is easier, more cavities will be formed for a given power input. The resulting cavities, however, will not grow as large and, because the cavities are smaller, they will collapse and implode with less force.
Viscosity is the resistance to motion within a liquid. Since forming cavities in a liquid requires motion of the molecules relative to one another, Higher Viscosity is generally a bad actor in ultrasonic processes with energy lost to internal friction in the liquid. Lower Viscosity enhances both the formation and collapse of cavitation bubbles. Extremely high viscosity liquids including heavy oil, syrups, tar, etc. may be impossible to cavitate at all using typical ultrasonic cleaning equipment.
DISSOLVED GAS –
Most liquids contain dissolved gas. Microscopic bubbles formed by dissolved gas act as “seeds” for the growth of cavitation bubbles. More Dissolved Gas results in the formation of more cavitation bubbles. As additional dissolved gas migrates into the growing cavitation bubble it grows larger than if dissolved gas was not present. Also, cavitation bubbles containing gas may not implode at all or not with enough force to produce the desired effect due to the resistance of the contained gas. Less Dissolved Gas results in cavitation bubbles the implode releasing significant energy. The elimination of dissolved gas is critical to the success of ultrasonic cleaning.
Related – Degassing – What Gas? and Why?
VAPOR PRESSURE –
Vapor pressure is the pressure that develops over a confined liquid due to spontaneous vaporization of the liquid. More simply, it is the measure of how readily a liquid changes to vapor. High Vapor Pressure results in growing cavitation bubbles filling with liquid vapor. This is similar to the case with dissolved gas except in this case the gas is the vapor of the liquid. Cavitation bubbles containing liquid vapor may not readily collapse with enough force to produce the desired implosion effect due to the cushioning effect of vapor confined in in them. Lower Vapor Pressure is beneficial to ultrasonic cleaning. Liquids including alcohol and many other solvents with high vapor pressure may not produce cavitation bubbles that implode releasing sufficient energy.
Density is the mass of a given measure of liquid. The density of water is 1. Cavitating a liquid of High Density is more difficult due to the requirement to overcome the inertia of the heavier liquid. Having said that, it should be noted that some extremely dense liquids including molten aluminum, zinc and lead have been successfully cavitated with resulting energy release on implosion. This did, however, require extremely high input power.
The above are generalizations based on observation and hypothesis. In fact, the interaction of properties is extraordinarily complex with the magnitude of each effect balancing against the other difficult to predict or measure. In some cases, there are conflicting effects which counteract one another within a single variable. In the next blog we will discuss how some important ultrasonic cleaning parameters impact the above and the reasons for their effect on cleaning.
– FJF –