Ultrasonic Processing of Particles: Mixing, Emulsifying, Dispersing, Milling0 pages

Ultrasonic Milling and Dispersing Technology for Nano-Particles
Kathrin Hielscher, Hielscher Ultrasonics GmbH, Germany
kathrin@hielscher.com; www.hielscher.com

1. Ultrasonic Milling and Dispersing in the Nano Technology Sector
The dispersing and deagglomeration of solids into liquids is an important application of
ultrasonic devices. If powders are wetted, the individual particles build agglomerates and are
held together by attraction forces of various physical and chemical nature, including van der
Waals forces and liquid surface tension. This effect is stronger for higher viscosity liquids,
such as polymers or resins. The attraction forces must be overcome on order to
deagglomerate and disperse the particles into liquid media. An even dispersion and
deagglomeration is important to use the full potential of the particles. Especially nano
particles offer extraordinary characteristics, which can only be exploited in highly even
dispersed state.
The application of mechanical stress – e.g. generated by ultrasonic cavitation - breaks the
particle agglomerates apart. Also, liquid is pressed between the particles. Different
technologies are commonly used for the dispersing of powders into liquids. This includes
high pressure homogenizers, agitator bead mills, impinging jet mills and rotor-stator-mixers.
High intensity ultrasonication is an interesting alternative to these technologies and
particularly for the particle treatment in the nano-size range the only effectual method to
achieve the required results.
2. Ultrasonic Cavitation
By high-power/ low-frequency ultrasound high amplitudes can be generated. Thereby, highpower/ low-frequency ultrasound can be used for the processing of liquids such as mixing,
emulsifying, dispersing and deagglomeration, or milling. When sonicating liquids at high
intensities, the sound waves that propagate into the liquid media result in alternating highpressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the
frequency. During the low-pressure cycle, high-intensity ultrasonic waves create small
vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can
no longer absorb energy, they collapse violently during a high pressure cycle. This
phenomenon is termed cavitation. Cavitation, that is "the formation, growth, and implosive
collapse of bubbles in a liquid. Cavitational collapse produces intense local heating (~5000 K),
high pressures (~1000 atm), and enormous heating and cooling rates (>109 K/sec)" and
liquid jet streams (~400 km/h)”. (Suslick 1998)
There are different means to create cavitation, such as by high-pressure nozzles, rotor-stator
mixers, or ultrasonic processors. In all those systems the input energy is transformed into
friction, turbulences, waves and cavitation. The fraction of the input energy that is
transformed into cavitation depends on several factors describing the movement of the
cavitation generating equipment in the liquid. The intensity of acceleration is one of the
most important factors influencing the efficient transformation of energy into cavitation.
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