How to Degas Liquids with Ultrasonic Energy

How to Degas Liquids with Ultrasonic Energy

Degassing liquids of various viscosities is undertaken across a broad spectrum of industries and for a variety of reasons. Before describing how ultrasonic energy achieves fast, efficient degassing we provide you with a few definitions.

Degassing Defined

Degassing, as the name suggests, is removing trapped air (deaeration) or other gases from liquids.  Most if not all liquids contain trapped air.  You can see that by allowing a glass of water to remain on the countertop for a period of time.  Bubbles will form on the walls of the glass. 

A more vivid demonstration is opening a bottle of seltzer, carbonated beverage, champagne or beer.  Bubbles fizz immediately after the pressure in the container is reduced.  In this case the gas being removed is carbon dioxide rather than air.

But degassing is also necessary to remove trapped air from oils, adhesives and paint among other products where the air can interfere with performance.  Note on cans of polyurethane coatings the admonition not to shake before brushing on surfaces.

As another example, degassing is also recommended as a preliminary step when preparing fresh ultrasonic cleaning solutions. Trapped air interferes with the ultrasonic cleaning action. 

Viscosity Defined

A posting by Princeton University defines viscosity as a measure of a fluid’s resistance to flow.  It describes the internal friction of a moving fluid. Viscosity can be measured in units of centipoise. A fluid with high viscosity resists motion because its molecular makeup gives it a lot of internal friction.  (Try pouring cold molasses.) A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion. 

Accurate viscosity measurements are critical for quality control and should be taken on samples that have been degassed.

How to Employ Ultrasonic Energy for Degassing

Shampoo Viscosity is an Indication of Quality

As you might imagine, the more viscous the liquid the more difficult it is to degas that liquid.  Accurate viscosity measurements on relatively viscous liquids such as shampoo necessitate removing trapped air from product samples before viscosity analysis can proceed.   

Without employing ultrasonic energy this can be a time-consuming operation.  But first, a quick rundown on how ultrasonic energy is created. 

To accomplish degassing we suggest employing ultrasonic cleaners such as offered by Tovatech.  They are called ultrasonic cleaners because their prime application is thoroughly cleaning any surface that can be safely immersed in a water-based cleaning solution.

These cleaners have ultrasonic transducers bonded to the bottom of the tank.  They are activated by a generator to vibrate at ultrasonic frequencies such as 37,000  cycles per second (37 kHz).  This causes the bottom of the tank to vibrate as a membrane, creating billions of microscopic vacuum bubbles.

For cleaning tasks, the bubbles implode violently on contact with immersed surfaces, blasting loose and carrying away contaminants.  The process is called ultrasonic cavitation.

The process also degasses fresh cleaning solutions.  Some models have a degas mode that speeds the process, and is particularly useful when using large capacity ultrasonic cleaning equipment.

But enough of this.  Let’s move to degassing samples using an ultrasonic cleaner.

Sample Degassing Using Ultrasonic Cleaners

Samples to be degassed, whether of high or low viscosity, are placed in glass containers such as beakers or Erlenmeyer flasks.

In brief, cavitation energy in the ultrasonic cleaner bath passes through the glass walls of immersed sample containers.  It causes trapped air in the samples to coalesce into bubbles and rise to the surface.  The process is similar to that used for sample preparation in pharmaceutical labs to homogenize, dissolve, disperse, and mix samples.

In this post we use shampoo as an example but the process should be suitable for many viscous products.  Some experimentation is required on your part to optimize the process for your applications.

Here is an example.

How to Degas Viscous Samples

Degassing multiple small samples

For this we use an  Elmasonic S series cleaner, specifically the Elmasonic S150 ultrasonic cleaner available from Tovatech. 

Developed for labs and healthcare facilities it holds approximately 4 gallons of water in its 20 x 12 x 4-inch deep tank and delivers exceptionally high 300 watts average ultrasonic power at 37 kHz ultrasonic frequency.

It has a timer displaying set and remaining time, a manually activated and an auto-degas mode and a sweep mode to evenly distribute ultrasonic energy throughout the bath.

Several glass sample containers can be placed on the unit’s mesh bottom tray for simultaneous processing.

Degassing larger samples for viscosity measurements

For degassing larger samples the Elma S50R 37 kHz ultrasonic cleaner is a candidate. 

Cylindrical in design with an inside diameter of 9.5 inches and a depth of 5.12 inches and with a capacity of 0.8 to 1.3 gallons it can support single sample containers and subject them to a special degas function that switches back and forth between maximum power and a mode that drives out the now macroscopic bubbles upwards via a lift-out effect.

Accessories include a support stand and clamp to hold sample flasks at the right depth in the tank.

How to Set Up a Representative Procedure for Ultrasonic Degassing

We caution you that this is a representative procedure.  Follow industry recommendations and your own experience to refine your processes.

  • Add water to the tank fill line and a surfactant to facilitate degassing.  Turn the unit on and activate the degas function for about 10 minutes.  This degassing step removes air from the bath for more efficient cavitation while mixing and warming it.
  • While this is happening pour sufficient shampoo or other liquid to be tested into glass containers.  Position these on the tray bottom.  If degassing larger samples in the S50R use the clamp to position the flask in the bath.  
  • Activate the ultrasound and lower the sample containers into the bath.  Note that the sample surface can be above the bath level. Warmth from the bath penetrates the containers and helps reduce sample viscosity to speed ultrasonic degassing.  Experiment with timing until you develop your processes. 
  • When gas bubbles stop rising to the surface of the liquid the degassing is complete.  Proceed with your viscosity analysis.

Some Suggestions on Degassing Viscous Liquids

  1. Water and other low viscosity liquids can be most rapidly degassed at frequencies higher than 100 kHz.  Degassing is effective at lower frequencies as well.  In some cases an ultrasonic cleaner such as the dual-frequency Elmasonic TI-H series offering a choice of 35 or 130 kHz may be considered. 
  2. As the viscosity of the liquid increases, the optimum frequency for degassing decreases.
  3. For liquids with a viscosity of 10,000 centipoise or above, degassing by ultrasound is not possible.
  4.  Heating may be used to reduce sample viscosity enhancing the degassing process. Highly viscous samples may require more heat than provided by cavitation in the S150.  Ask us about heater-equipped Elmasonic S units such as the 3.4 gallon S120H.
  5. To accelerate sample degassing pull a slight vacuum on the containers.

Need Help? Contact the ultrasonic professionals at Tovatech for help in selecting the proper equipment for degassing viscous liquids or for answers to your questions on ultrasonic cleaning in general.

About Rachel Kohn

So how did an MIT Ph.D. end up selling refrigerators? When I figured out that a lot more scientists buy lab refrigerators than innovative leading-edge instruments. I hope that my many years of lab experience will help you find the right equipment for your work. Before co-founding Tovatech I worked in business development and project management at Smiths Detection, Photon-X, Cardinal Health, and Hoechst Celanese. And before that I spent 12 years as an R&D chemist at Hoechst Celanese and Aventis working on advanced drug delivery systems, polymer films and membranes, optical disks, and polysaccharides. Some day, eventually, I’ll make enough money to develop an innovative technology that will change the world. Read More