Emulsifying by Ultrasonic Cavitation

A wide range of intermediate and consumer products – such as cosmetics, skin lotions, pharmaceutical ointments, varnishes, paints, lubricants, and fuels – are based wholly or in part on emulsions.
Hielscher manufactures the world’s largest industrial ultrasonic liquid processors for the efficient emulsification of large-volume streams in production plants.

How Ultrasonic Emulsification Works

Laboratory Applications: In laboratory settings, the emulsification power of ultrasound has been known and applied for a long time due to the various benefits tied to ultrasonic homogenization and emulsification.

The Technology

Reliable ultrasonic emulsification is based on the use of ultrasonic probes, also known as sonotrodes. The process works as follows:

1. Ultrasound Coupling: Via the ultrasonic probe, high-intensity ultrasound is coupled into liquids, creating acoustic cavitation.
2. Cavitation Effect: Ultrasonic or acoustic cavitation generates high shear forces, which provide the required energy to disrupt large droplets down to nano-size droplets.
3. Emulsion Formation: Two or more liquid phases are mixed into a uniform submicron- or nano-emulsion.

 
Industrial Scale-Up via Flow-Through Technology: Using ultrasonic flow cells allows for linear scale-up to industrial production of nanoemulsions, processing large volume streams in continuous flow-through.

The Advantages of Ultrasonic Emulsification

Ultrasonic emulsification using a probe-type ultrasonicator offers several advantages over other emulsifying techniques:

1. 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- and nano-sized droplets can be reliably produced using power ultrasound.
2. Energy efficiency: Ultrasonic emulsification requires less energy than other emulsification methods, making it a more energy-efficient process.
3. 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.
4. Time-saving: Ultrasonic emulsification can be a very rapid process, with emulsions forming in seconds to minutes, depending on the liquids, volume and equipment.
5. 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.
6. 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.

The advantages of ultrasonic emulsification using a probe-type ultrasonicator make it an excellent choice for emulsification in a variety of fields, including food and beverage, pharmaceuticals, cosmetics, fine chemicals and fuels.
Read more about ultrasonic mayonnaise emulsification!
Read more about the production of paraffin wax emulsions using sonication!
Read more about Water-in-Diesel Emulsions produced using ultrasonics!

The DLS measurement shows the uniform droplet size distribution of an ultrasonically produced rose oil-in-water emulsion.

What is an Emulsion?
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 – The Power Application for Sonicators

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.

 
The video below shows the emulsification process of oil (yellow) into water (red) by using the UP400S lab ultrasonicator.

Working Principle of Ultrasonic Emulsification

Acoustic Cavitation: The Driving Force Behind Ultrasonic Emulsification and Nano-Emulsification
Ultrasonic emulsification relies on the powerful effects of acoustic cavitation, a phenomenon that occurs when high-intensity ultrasound waves pass through a liquid. During this process, microscopic bubbles form, grow, and then collapse violently. The implosive collapse of these bubbles generates extreme localized conditions, including intense pressure and temperature gradients, high shear forces, shock waves, and liquid micro-jets. These forces effectively break down large particles, droplets, and agglomerates into much smaller structures.
The image on the left illustrates acoustic cavitation generated by the UIP1000hdT ultrasonic processor (1000 W) operating in a liquid-filled glass column.

How Acoustic Cavitation Improves Emulsification

In both emulsification and nano-emulsification, cavitation intensity is a key factor in determining droplet size. As cavitation bubbles collapse, the resulting shear forces fragment larger droplets into increasingly smaller ones. At the same time, the localized pressure and temperature changes promote the formation of new droplets while helping to stabilize the emulsion.
This combination of droplet disruption and stabilization enables ultrasonic technology to produce highly uniform emulsions with exceptionally fine droplet size distributions.
 

Droplet size distribution of water-olive oil emulsions prepared by (a) classic homogenization method (b) ultrasonic homogenization (using the UP400S) method with MD, WPI and their mixture, having 40% dry matter with 9% oil (w/w) content. Ultrasonic emulsification results in significantly smaller droplets, less creaming and a better overall emulsion stability.
(study and graphs: Zungur et al., 2015)

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.

MultiPhaseCavitator (MPC48)
The MultiPhaseCavitator is a powerful accessory compatible with Hielscher ultrasonic flow cell reactors: Using the insert MPC48, the dispersed phase is injected via 48 cannulas as thin liquid strands into the ultrasonic hot zone, where the dispersed phase and the continuous phase are mixed as minute droplets into a nanoemulsion.
Discover how the MultiPhaseCavitator improves emulsification!

Literature / References