Ultrasonic Degassing and Defoaming of Liquids
Degassing and defoaming of liquids is an interesting application of ultrasonic devices. In this case the ultrasound removes small suspended gas-bubbles from the liquid and reduces the level of dissolved gas below the natural equilibrium level.
The degassing and defoaming of liquids is required for many purposes, such as:
- sample preparation before particle size measurement to avoid measurement errors
- oil and lubricant degassing before pumping to reduce pump wear due to cavitation
- degassing of liquid foods, e.g. juice, sauce or wine, to reduce microbial growth and increase shelf life
- degassing of polymers and varnishes before application and curing
When sonicating liquids, the sound waves that propagate from the radiating surface into the liquid media result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. During the low-pressure cycle, the ultrasonic waves can create small vacuum bubbles or voids in the liquid. The large number of small bubbles generates a high total bubble surface area. The bubbles are also well distributed in the liquid. Dissolved gas migrates into these vacuum (low pressure) bubbles via the large surface area and increases the size of the bubbles.
The acoustic waves support the touching and coalescence of adjacent bubbles leading to an accelerated growth of the bubbles. The sonication waves will also help to shake bubbles off vessel surfaces and will force smaller bubbles resting below the liquid surface to rise through and release the entrapped gas to the environment.
Put It to The Test
The process of the degassing and defoaming of liquids can be made visible easily. In a glass beaker of freshly-poured tap water, ultrasonication will force the small suspended bubbles (cloudiness) to coalesce and to move up rapidly. You can see this effect at the progress image below. Please click at the image below to enlarge the pictures.
Agitated oil contains a high number of suspended bubbles (foam). In particular in coolants, this is a problem, as the bubbles promote caviation induced wear in pumps and nozzles. The progress image below shows the ultrasonic defoaming effect. Click at the image below to enlarge the pictures.
Even in still water, e.g. after 24 hours, sonication will generate small bubbles in the clear water. These bubbles fill with dissolved gas, that migrates into the bubbles. Consequently the bubbles grow and move up. The degassing effect is well visible in any translucent liquid.
As the ultrasound improves the rising of small suspended bubbles to the liquid surface, it reduces the contact time between the bubble and the liquid, too. For this reason, it limits the re-dissolving of gas from the bubble to the liquid, too. This is of particular interest for higher viscosity liquids, such as oil or resin. Since the bubbles have to move to the liquid surface, the ultrasonic degassing works better, if the container is shallow so that the time to the surface is shorter.
Beyond the Visible Effects
While the visible determination of the degassing effects is limited in accuracy, gas content measurements, e.g. by neutron radiography, are a more accurate way to tell about ultrasonic degassing efficiency.
Fluids contain a certain amount of dissolved gas. The concentration of gas depends on factors, such as temperature, ambient pressure, agitation of the liquid. Under constant conditions, the gas concentration will approach an equilibrium. Ultrasonic degassing will change the conditions, because the liquid is exposed to low pressure bubbles and agitation. Therefore, ultrasonication will lower the gas concentration in the liquid beneath the former equilibrium level.
When the sonication stops and the initial conditions are re-established, the gas concentration will slowly approach the initial equilibrium level again, unless the liquid is not exposed to any gas, e.g. in a closed bottle. Because the re-dissolving of gas into the liquid is fairly slow, it is possible to work with the low-gas liquid after sonication. The graph below illustrates this effect. (Click to enlarge.)
Degassing before Emulsifying and Dispersing
Ultrasonic degassing can contribute significantly to the quality of dispersions and emulsions.
Emulsions and dispersions often contain surfactants in order to increase stability. The surfactants will inhibit the touching and coalescence or agglomeration of the dispersed material in the liquid phase. For this, the surfactants will form a layer around each particle. The same surfactants can also encapsulate gas bubbles that were suspended in the liquid phase. Such stabilized bubbles can prove to be very robust. It consumes surfactant, reduces the quality of the emulsion or dispersion, and it can generate erratic readings when measuring the particle size.
In order to reduce the problem of stabilized gas bubbles, liquids can be simply degassed by sonication. Before adding the disperse phase, such as oil or powder, sonicate the liquid until the number of generated bubbles reduces. When mixing other material in, avoid generating new bubbles or a vortex while stirring. This would increase the gas content quickly.
Forcing Carbon Dioxide Out
The effect of degassing is being used in the leak-testing of cans and bottles containing carbonated drinks, such as cola, soda or beer. Please click here to get more information.
Ultrasonic Degassing in Brief
The ultrasonic degassing of liquids works better if you:
- apply low to moderate amplitudes
- use sonotrodes with larger surface area
- provide a low pressure or vacuum above the liquid surface
- heat the liquid
- incorporate a shallow container
- avoid turbulent agitation
Ultrasonic degassing can be used in batch- or flow mode. In the case of a flow operation, a stand-pipe for the discharge of the gas should be installed and a gas pump should be applied.