An emulsion consists of two immiscible liquids, which are finely dispersed in each other. For oil-in water emulsions, the oily phase (dispersed phase) is evenly mixed into the aqueous phase (continuous phase). Ultrasonic emulsifiers are well-established to prepare oil-in-water emulsions and nano-emulsions. Ultrasonic emulsions excel by uniform droplet dispersion and long-term stability therefore being widely used in laboratories and industry.
Ultrasonic Oil-in-Water Emulsions
Oil-in-water (O/W) emulsions are widely used in many consumer products such as food, beverages, cosmetics, pharmaceuticals etc. as well as in industry (e.g. material science, polymers, paints, coatings, fine chemistry etc.).
Definition of Oil-in-Water Emulsion
An oil-in-water emulsion is a mixture in which an oily phase is dispersed in water or another aqueous liquid. Small droplets of oil are finely dispersed in the water phase (continuous phase) to create this oil-in-water emulsion. If oil (dispersed phase) is dispersed in water (continuous phase), it is an oil-in-water emulsion; if water droplets are added into an oily continuous phase, it is called a water-in-oil emulsion.
For oil-in-water emulsions, the resulting emulsion quality depends on the formulation (oil:water ratio, emulsifying agent) and the emulsification technique. Ultrasonic emulsifiers use acoustic cavitation, which is characterized by intense high-shear forces and turbulences in the liquid. These highly intense forces disrupt the two immiscible liquid phases into minute droplets and mixes them uniformly with each other. Ultrasonic emulsification can easily generate nano-sized particles with droplet sizes of down to 10nm. The very small droplet size and the homogeneous, even droplet size distribution of ultrasonically prepared emulsions turns sonication into the preferred emulsification technique.
Benefits of Ultrasonic Emulsification
Ultrasonic emulsification excels other emulsifying techniques such as high-pressure homogenization, high-shear mixing and microfluidization by efficiency, consistent production of nano-droplets, high-emulsion stability and capability to treat any volume from small tubes and beakers to large streams.
For instance, Li and Xiang (2019) showed in their comparative study of high-pressure homogenization and ultrasonic emulsification that ultrasonication yields in more uniform, stable emulsions. Aggregation in the coconut oil-in-water emulsions appeared only after being subjected to high-pressure homogenization, while the emulsion made by ultrasonic emulsification remained stable during 30 days storage.
When compared to mechanial agitation, emulsions prepared by ultrasonic technique were found to be more stable for longer duration of time when compared to emulsions prepared by mechanical agitation which can be attributed to the small droplet size which is thermodynamically stabilized. Ultrasonic technique give more stable emulsions than the conventional mechanical agitation method. (cf. Ramisetty and Shyamsunder, 2011)
O’Sullivan et al. (2015) show in their study that ultrasonic emulsification consistently yielded submicron emulsions (less than 200 nm). However, nano-emulsions with uniform droplet size of 10nm can be easily and reliably achieved under optimized conditions. The droplet size of ultrasonically produced emulsions is a function of processing time and ultrasonic amplitude for both batch and continuous processing.
- highly efficient process
- sub-micron and nano-droplets
- uniform droplet size distribution
- reproducible results
- for any volume
- precisely controllable
- linear scalable
- safe and easy to operate
The Working Principle of Ultrasonic Emulsification
The ultrasonic production of emulsions and nano-emulsions (such as oil-in water and water-in-oil emulsions) is based on the working principle of acoustic cavitation. In order to create acoustic cavitation, high-intensity, low-frequency ultrasound is generated by an ultrasound transducer and transmitted via ultrasonic horn and probe (sonotrode) into a liquid. High-power ultrasound is considered ultrasound in the range of 16-30kHz. The ultrasound probe expands and contracts e.g., at 20kHz, thereby transmitting respectively 20,000 vibrations per second into the medium. When the ultrasonic waves travel through the liquid, alternating high-pressure (compression) / low-pressure (rarefaction / expansion) cycles create minute cavities (vacuum bubbles), which grow over several pressure cycles. During the compression phase of the liquid and bubbles, the pressure is positive, while the rarefaction phase produces a vacuum (negative pressure). During the compression-expansion cycles, the cavities in the liquid grow until they reach a size, at which they cannot absorb further energy. At this point, they implode violently. The implosion of those cavities results in various highly energetic effects, which are known as the phenomenon of acoustic / ultrasonic cavitation. Acoustic cavitation is characterized by manifold highly energetic effects, which impact liquid-liquid, solid-liquid and gas-liquid systems. The energy-dense zone or cavitational zone is known as so-called hot-spot zone, which is most energy-dense in the close vicinity of the ultrasonic probe and declines with increasing distance from the sonotrode. The main characteristics of ultrasonic cavitation include locally occurring very high temperatures and pressures and respective differentials, turbulences, and liquid streaming. During the implosion of ultrasonic cavities in ultrasonic hot-spots, temperatures of up to 5000 Kelvin, pressures of up to 200 atmospheres and liquid jets with up to 1000km/h can be measured. These outstanding energy-intense conditions contribute to sonomechanical effects that intensify processes by highly effective mixing, dispersing and emulsifying. The ultrasonic shear forces, liquid streaming and turbulences disrupts the droplets and mixes them uniformly with each other. High-performance ultrasonication produces reliably nano-droplets with a uniform size distribution. This makes ultrasonic emulsification the preferred method to prepare oil-in-water emulsions / nano-emulsions with high emulsion stability and shelf-life.
Effects of Ultrasonic Intensity on Emulsion Quality
Emulsion quality is mainly characterized by its stability. In turn, emulsion stability is a factor of droplet size distribution and emulsion formulation.
Ultrasonic intensity, i.e. amplitude and sonication duration, are important factors that influence the quality of the emulsion obtained.
In research, ultrasonic emulsification is divided into two phases: During the the first step, a combination of interfacial waves and Rayleigh-Taylor instability occur, causing the eruption of dispersed phase droplets into the continuous phase.
During the second step, droplets are broken up through acoustic cavitation near the droplet boundary phases. The intense effects of droplet disruption and mixing caused by cavitational shock waves generate very small droplet sizes. Ultrasonic intensity, amplitude, and sonication time are responsible for the obtained impact of cavitation and thereby the most influential parameters regarding fine-size emulsification. Hielscher Ultrasonics industrial ultrasonic processors can deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available.
High-Performance Ultrasonic Emulsifiers
Ultrasonic emulsification is a reliable processing technology, which facilitates and accelerates the production of high-quality emulsions with submicron and nano-sized droplets. Hielscher Ultrasonics portfolio covers the full range from compact lab ultrasonicators to industrial systems for emulsification applications. This allow us at Hielscher to offer you the most suitable ultrasonicator for your envisaged emulsion quality and process capacity. Our long-time experienced staff will assist you from feasibility tests and process optimisation to the installation of your ultrasonic system on final production level.
The small foot-print of our ultrasonic extractors as well as their versatility in installation options make them fit even into small-space processing facilities. Ultrasonic processors are installed worldwide in food, pharma and nutritional supplement production facilities.
Hielscher Ultrasonics – Sophisticated Emulsification Systems
Hielscher Ultrasonics product portfolio covers the full range of high-performance ultrasonic emulsifiers from small to large scale. Additional accessories allow for the easy assembly of the most suitable ultrasonic device configuration for your emulsification process. The optimal ultrasonic setup depends on the envisaged capacity, volume, raw materials, batch or inline process and timeline.
Batch and Inline
Hielscher ultrasonic emulsifiers can be used for batch and continuous flow-through processing. Ultrasonic batch processing is ideal for process testing, optimisation and small to mid-size production level. For a producing large volumes of emulsions, inline processing might be more advantageous. A continuous inline mixing process requires a sophisticated setup – consisting in a pump, hoses or pipes and tanks -, but it is highly efficient, rapid and requires significantly less labour. Hielscher Ultrasonics has the most suitable emulsification setup for your emulsion volume and process goals.
Ultrasonic Probes and Reactors for Emulsification at Any Size
Hielscher Ultrasonics product range covers the full spectrum of ultrasonic processors from compact lab ultrasonicators over bench-top and pilot systems to fully-industrial ultrasonic processors with the capacity to process truckloads per hour. The full product range allows us to offer you the most suitable ultrasonic emulsifier for your process capacity and production targets.
Ultrasonic benchtop systems are ideal for feasibility testing and process optimization. Linear scale-up based on established process parameters makes it very easy to increase the processing capacities from smaller lots to fully commercial production. Up-scaling can be done by either installing a more powerful ultrasonic extractor unit or clustering several ultrasonicators in parallel. With the UIP16000, Hielscher offers the most powerful ultrasonic emulsifier worldwide.
Precisely Controllable Amplitudes for Optimum Results
All Hielscher ultrasonicators are precisely controllable and thereby reliable work horses in production. The amplitude is one of the crucial process parameters that influence the efficiency and effectiveness of ultrasonic emulsification. Amplitude is a crucial factor, which influences the quality of ultrasonic emulsification decisively. All Hielscher Ultrasonics processors allow for the precise setting of the amplitude. Sonotrodes and booster horns are accessories that allow to modify the amplitude in an even wider range. Hielscher’s industrial ultrasonic processors can deliver very high amplitudes and deliver the required ultrasonic intensity for demanding applications. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation.
Precise amplitude settings and the permanent monitoring of the ultrasonic process parameters via smart software give you the possibility to treat your emulsion with the most effective ultrasonic conditions. Optimal sonication for best emulsification results!
The robustness of Hielscher’s ultrasonic equipment allows for 24/7 operation at heavy duty and in demanding environments. This makes Hielscher’s ultrasonic equipment a reliable work tool that fulfils your emulsification requirements.
Easy, Risk-free Testing
Ultrasonic processes can be completely linear scaled. This means every result that you have achieved using a lab or bench-top ultrasonicator, can be scaled to exactly the same output using the exactly same process parameters. This makes ultrasonication ideal for risk-free feasibility testing, process optimization and subsequent implementation into commercial manufacturing. Contact us to learn how sonication can improve your emulsion production.
Highest Quality – Designed and Manufactured in Germany
As a family-owned and family-run business, Hielscher prioritizes highest quality standards for its ultrasonic processors. All ultrasonicators are designed, manufactured and thoroughly tested in our headquarter in Teltow near Berlin, Germany. Robustness and reliability of Hielscher’s ultrasonic equipment make it a work horse in your production. 24/7 operation under full load and in demanding environments is a natural characteristic of Hielscher’s high-performance mixers.
The table below gives you an indication of the approximate processing capacity of our ultrasonicators:
|1 to 500mL
|10 to 200mL/min
|10 to 2000mL
|20 to 400mL/min
|0.1 to 20L
|0.2 to 4L/min
|10 to 100L
|2 to 10L/min
|10 to 100L/min
|cluster of UIP16000
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Literature / References
- O’Sullivan, Jonathan; Murrayc, Brian; Flynn, Cal; Norton, Ian (2015): Comparison of batch and continuous ultrasonic emulsification processes. Journal of Food Engineering Volume 167, Part B, Dec. 2015. 114-121.
- Li, Yujie; Xiang, Dong (2019): Stability of oil-in-water emulsions performed by ultrasound power or high-pressure homogenization. PlosOne March 8, 2019.
- Ramisetty, Kiran A.; Shyamsunder R. (2011): Effect of Ultrasonication on Stability of Oil in Water Emulsions. International Journal of Drug Delivery 3, 2011. 133-142.
Facts Worth Knowing
What is the Difference between Oil-in-Water and Water-in-Oil Emulsions?
The primary difference between an oil-in-water and water-in-oil emulsion is the composition of the suspension. Emulsions can be either oil-in-water or water-in-oil. Technically, when generating oil-in-water vs. water-in-oil emulsions, one phase (known as the dispersed phase) is mixed into the other (the continuous phase). In other words, one liquid serves as a sort of base into which another liquid is added. When an emulsion is “oil-in-water,” oil is the dispersed phase that is distributed into the continuous phase, water. In a water-in-oil emulsion, the roles are switched. Milk is an example of an oil-in-water emulsion, while butter is water-in-oil.
The choice of emulsifying agents is important factor, which impact the quality and stability of both, oil-in-water and water-in-oil emulsions. Although some nano-sized emulsions can be naturally stable, most emulsion are not naturally stable over longer periods of time. Therefore, the formulation must be carefully designed to create dispersions with an enhanced shelf life. Not every emulsifying agent is compatible with any type of emulsion. An emulsifying agent with good solubility in water may facilitate an oil-in-water emulsion better than water-in-oil, whilst an emulsifier with good solubility in oil may work best for water-in-oil emulsions.