Emulsifying by Ultrasonic Cavitation
A wide range of intermediate and consumer products, such as cosmetics and skin lotions, pharmaceutical ointments, varnishes, paints and lubricants and fuels are based wholly or in part on emulsions. Hielscher manufactures the world’s largest industrial ultrasonic liquid processors for the efficient emulsifying of large volume streams in production plants.
In the lab, the emulsification power of ultrasound has been known and applied for long due to various benefits that are tied to ultrasonic homogenisation and emulsification. Reliable ultrasonic emulsification is based on the use of ultrasonic probes, so-called sonotrodes. Via the ultrasonic probe, high-intensity ultrasound is coupled into liquids and creates acoustic cavitation. Ultrasonic or acoustic cavitation generates high shear forces, which provide the required energy to disrupt large droplets down to nano-size droplets. Thereby, two or more liquid phases are mixed into a uniform submicron- or nano-emulsion.
Advantages of Ultrasonic Emulsification
Ultrasonic emulsification using a probe-type ultrasonicator offers several advantages over other emulsifying techniques:
- Improved emulsion stability: Ultrasonic emulsification creates smaller droplet sizes and more uniform droplet distribution, resulting in improved emulsion stability and a longer shelf life. Submicron- andn nano-sized droplets can be reliably produced using power ultrasound.
- Energy efficiency: Ultrasonic emulsification requires less energy than other emulsification methods, making it a more energy-efficient process.
- Scalability: Ultrasonic emulsification can be easily scaled up or down depending on the required volume, making it a versatile process for both laboratory and industrial applications.
- Time-saving: Ultrasonic emulsification can be a very rapid process, with emulsions forming in seconds to minutes, depending on the liquids, volume and equipment.
- Reduced need for surfactants: Ultrasonic emulsification can reduce the need for surfactants, which are often required to stabilize emulsions. However, with a reduced droplet size, the surface area of the particle is increased and more area must be covered by a surfactant. Ultrasonication is compatible with almost any kind of surfactant including alternative and novel emulsifiers.
- Minimal and controllable heat generation: Ultrasonic emulsification is a non-thermal process and heat generation during processing can be avoided or reduced to a small degree. Thereby, the risk of thermal degradation of sensitive compounds or ingredients is reduced.
Overall, the advantages of ultrasonic emulsification using a probe-type ultrasonicator make it a popular choice for emulsification in a variety of fields, including food and beverage, pharmaceuticals, cosmetics, fine chemicals and fuels.
The video below shows the emulsification process of oil (yellow) into water (red) by using the UP400S lab ultrasonicator.
Emulsions are dispersions of two or more immiscible liquids. Highly intensive ultrasound supplies the power needed to disperse a liquid phase (dispersed phase) in small droplets in a second phase (continuous phase). In the dispersing zone, imploding cavitation bubbles cause intensive shock waves in the surrounding liquid and result in the formation of liquid jets of high liquid velocity.
Nano-Emulsions – A Power Application for Ultrasonicators
Nanoemulsions are emulsions with droplets that are typically less than 100 nanometers in size. Nanoemulsions offer several advantages over conventional emulsions, including unique functional properties, higher stability, transparency, etc.
Ultrasonication outcompetes traditional emulsification technologies especially when it comes to the formation of nanoemulsions. This is due to the highly efficient and energy-intense working principle of ultrasound.
Working Principle of Ultrasonic Emulsification
Ultrasonic emulsification processes use the forces of acoustic cavitation. Acoustic cavitation refers to the phenomenon of the formation, growth, and implosive collapse of small bubbles in a liquid medium subjected to high-intensity ultrasound waves. The implosion of these bubbles generates intense local pressure and temperature gradients, which can create high-shear forces, shock waves, and micro-jets that can break down large particles and agglomerates into smaller ones. The picture left demonstrates ultrasonic cavitation generated at the probe of the ultrasonicator UIP1000hdT (1000 watts) in a liquid-filled glass column.
In emulsification and nano-emulsification, the intensity of acoustic cavitation plays a critical role in reducing the size of droplets in the emulsion. The implosive collapse of the cavitation bubbles can create strong shear forces that break down larger droplets into smaller ones. Moreover, the local pressure and temperature gradients generated by the cavitation can also promote the formation of new droplets and stabilize the emulsion.
The unique aspect of acoustic cavitation is its ability to provide localized and intense energy input to the liquid medium, without the need for high mechanical or thermal stresses. This makes it an attractive technique for nano-emulsification, as it can reduce the energy input required for the emulsification process while achieving a smaller droplet size and narrower droplet size distribution.
Due to these precisely controllable ultrasonic forces, acoustic cavitation is a powerful tool for nano-emulsification. Its ability to generate localized and intense energy input allows to break down larger droplets forming submicron- and nano-sized ones at a very high efficiency.
Studies at oil in water (water phase) and water in oil (oil phase) emulsions have shown the correlation between the energy density and droplet size (e.g. Sauter diameter). There is a clear tendency for smaller droplet size at increasing energy density (click at right graphic). At appropriate energy density levels, ultrasound can easy and reliably achieve mean droplet sizes in the nano-range.
Ultrasonic Probes for Efficient Emulsification
Hielscher offers a broad range of probe-type ultrasonicators and accessories for the efficient emulsification and dispersing of liquids in batch and flow-through mode.
Systems consisting of several ultrasonic processors of up to 16,000 watts each, provide the capacity needed to translate this lab application into an efficient production method to obtain finely dispersed emulsions in continuous flow or in a batch – achieving results comparable to that of today’s best high-pressure homogenizers available, such as the new orifice valve. In addition to this high efficiency in the continuous emulsification, Hielscher ultrasonic devices require very low maintenance and are very easy to operate and to clean. The ultrasound does actually support the cleaning and rinsing. The ultrasonic power is adjustable and can be adapted to particular products and emulsification requirements. Special flow cell reactors meeting the advanced CIP (clean-in-place) and SIP (sterilize-in-place) requirements are available, too.
|Batch Volume||Flow Rate||Recommended Devices|
|0.5 to 1.5mL||n.a.||VialTweeter||1 to 500mL||10 to 200mL/min||UP100H|
|10 to 2000mL||20 to 400mL/min||UP200Ht, UP400St|
|0.1 to 20L||0.2 to 4L/min||UIP2000hdT|
|10 to 100L||2 to 10L/min||UIP4000hdT|
|15 to 150L||3 to 15L/min||UIP6000hdT|
|n.a.||10 to 100L/min||UIP16000|
|n.a.||larger||cluster of UIP16000|
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Literature / References
- Ahmed Taha, Eman Ahmed, Amr Ismaiel, Muthupandian Ashokkumar, Xiaoyun Xu, Siyi Pan, Hao Hu (2020): Ultrasonic emulsification: An overview on the preparation of different emulsifiers-stabilized emulsions. Trends in Food Science & Technology Vol. 105, 2020. 363-377.
- Seyed Mohammad Mohsen Modarres-Gheisari, Roghayeh Gavagsaz-Ghoachani, Massoud Malaki, Pedram Safarpour, Majid Zandi (2019): Ultrasonic nano-emulsification – A review. Ultrasonics Sonochemistry Vol. 52, 2019. 88-105.
- Behrend, O., Schubert, H. (2000): Influence of continuous phase viscosity on emulsification by ultrasound, in: Ultrasonics Sonochemistry 7 (2000) 77-85.
- Behrend, O., Schubert, H. (2001): Influence of hydrostatic pressure and gas content on continuous ultrasound emulsification, in: Ultrasonics Sonochemistry 8 (2001) 271-276.
- F. Joseph Schork; Yingwu Luo; Wilfred Smulders; James P. Russum; Alessandro Butté; Kevin Fontenot (2005): Miniemulsion Polymerization. Adv Polym Sci (2005) 175: 129–255.
Facts Worth Knowing
Definition of the Term “Emulsion”
An emulsion is a mixture of two or more immiscible liquids, such as oil and water.
Emulsions can be either oil-in-water (where oil droplets are dispersed in water) or water-in-oil (where water droplets are dispersed in oil). Emulsions are used in a variety of applications, including food products (such as salad dressings and mayonnaise), cosmetics (such as lotions and creams), and pharmaceuticals (such as vaccines).
An emulsifier works by reducing the surface tension between the two immiscible substances (such as oil and water) in an emulsion. This reduces the tendency of the two substances to separate and allows them to form a stable mixture.
In general, emulsions require stabilization using an emulsifying agent or surfactant. Emulsifiers are amphiphilic – they attract both water and fatty substances. This means they have hydrophilic (water-loving) and hydrophobic (oil-loving) properties, which allows them to interact with both the oil and water phases of the emulsion. The hydrophilic part of the emulsifier molecule attaches to the water molecules, while the hydrophobic part attaches to the oil molecules.
By surrounding the oil droplets with emulsifier molecules, the emulsifier creates a protective layer around the droplets that prevents them from coming into contact with each other and coalescing (joining together) to form larger droplets. This helps to keep the emulsion stable and prevents separation.
As coalescence of the droplets after disruption influences the final droplet size distribution, efficiently stabilizing emulsifiers are used to maintain the final droplet size distribution at a level that is equal to the distribution immediately after the droplet disruption in the ultrasonic dispersing zone. Stabilizers actually lead to improved droplet disruption at constant energy density.
Examples of commonly used emulsifiers include lecithin (which is found in egg yolks and soybeans), mono- and diglycerides, polysorbate 80, and sodium stearoyl lactylate.