Functionality of a ultrasonic homogenizer
An ultrasonic homogenizer consists essentially of three components: the HF (high frequency)- generator GM, the ultrasonic converter UW, and the functioning tip. The HF-generator first transforms the alternating mains supply voltage from 50-60 Hz into a HF-voltage of 20 kHz. If this voltage is applied to a suitable oscillator inside the ultrasonic converter, it will modify the mechanical size of this oscillator and shape in rhythm with the electric alternation of plus and minus. This is the piezoelectric effect which has been well known since 1880. By means of this effect, it is possible to transform electric oscillations into mechanical oscillations of same frequency and vice versa - for example, a conventional record player works on this principle. It converts mechanical impulses from the groove of the record into electric signals. The ultrasonic converter in a homogenizer utilizes the reverse effect: A sintered ceramic oscillator is stimulated electrically, thus producing large mechanical amplitudes. It must, of course, be composed of piezoelectric material and be precisely matched in size and shape to the frequency used. Natural piezoelectric materials such as quartz or tourmaline are not significant in the today's generation of ultrasonics. They are too expensive and their frequencies are too high for practical application. The lower the frequency, the larger the crystal which can be selected and the greater amount of energy can be transmitted. Therefore, sintered ceramics are primarily used today, such as lead zirconatetitanate (PZT), which has an efficiency of more than 90% and produces large amplitudes at low stimulation frequencies. Frequencies less than 20 kHz are already in the human hearing range, so working with too low frequencies is avoided because of noise stress. The mechanical oscillation is transmitted by the ultrasonic converter through sonotrodes and is transmitted into the sample through the horns connected in between. Depending on the application, the sonotrodes may have cone tips with a diameter of about 6 mm, flat plates with considerably larger area, or tiny micro-tips with a diameter of 2 or 3 mm which will even fit into test tubes. The larger the quantity of liquid to be treated the larger the sonotrode which should be selected. The working intensity transmitted into the medium increases in inverse proportion with the diameter of the sonotrode area. The smallest tips transmit the largest power per measure of area in maximum oscillation amplitudes of several tenths of a millimeter.
Theory of the ultrasonic effect
When a tip vibrates at a high frequency in a fluid, a special phenomenon occurs: cavitation. Due to the ultrasonic alternating pressure of high intensity, the fluid pulls apart in the expansion phase of the oscillation, because the cohesion forces between the molecules of the liquid are overcome by the alternating ultrasonic pressure. This effect is especially pronounced at boundary areas, at air bubbles or small particles (“cavitation seeds”). Through the pulling apart of the fluid, millions of microscopic voids can grow to visible dimensions of around 0.1 mm over several oscillation cycles. When a critical size is exceeded, however, they become unstable, collapse (implode), and produce pressure surges of considerable energy intensity. The exact values are difficult to determine, but the latest results assume that local temperatures of several thousand degrees Celsius and pressure peaks around 500 bar occur. However, the range of the homogenizer pressure waves is limited, the zone of maximum cavitation being several millimeters. For this reason also, working at the lowest possible frequencies is advantageous because the cavitation bubbles can become larger (expansion and compression phases last longer) and thus more energy can be contained and released upon implosion. The effect of cavitation also is dependent on still other factors. But one should not succumb to the illusion that a higher power of the homogenizer automatically means a higher intensity. The power is measured in watts and only indicates the energy output of the generator. For the user, on the other hand, only the intensity, i.e. the energy which is actually applied to the sample is significant. This in turn depends on the amplitude of the tip surface from which the sound is emitted. This means that it is important in the ultrasonic treatment to be able to monitor and control this amplitude, because as far as possible, the same reproducible results should be obtained when the homogenizer is used under differing loads. Above all for the scientific research, homogenizers must therefore always be equipped with automatic amplitude control. The nature of the fluid also plays a considerable role in the intensity of cavitation. Temperature, surface tension, vapor pressure, and viscosity are also important factors, and the values of these factors can mutually effect each other. The most important and most easily influenced value is the temperature. Higher temperatures result in higher vapor pressure and lower surface tension. This does increase the tendency for cavitation bubbles to form (due to the lower surface tension, the fluid pulls apart more easily), but the force of implosion is reduced because the higher vapor pressure from the gas-filled micro-bubbles essentially counteracts this effect. With an increase in temperature, it may be possible to break up sensitive cells more easily (since more cavitation bubbles form), but not be able to have any effect on more stable spores (since the power of the implosion is no longer sufficient). From this it follows that the intensity of ultrasonic exposure is increased if one works with the lowest possible temperatures, for example by using a cooling bath. Surface tension can also be selectively influenced by the addition of a wetting agent. As described above, the tendency for cavitation decreases with increasing surface tension, i.e. more homogenizer power is required, but at the same time the intensity increases, increasing the force of the implosion. Interestingly, almost all effects which influence the formation of cavitation follow this pattern. The more the formation of cavitation bubbles is made difficult through external conditions - through low temperatures, high surface tension, high hydrostatic pressure, degassing - the greater is the force of the cavitation - assuming there is enough power available to generate bubbles in the fluid in the first place.