A Great Instrument To Atomize Liquid: Ultrasonic Vibrator

        Now, in nanotechnology era, the smallest sized thing is regarded. In domain of about 10^-9 m or 10 Å size, material becomes unique. We have already known the implementation of SPIO (small particle iron oxides which have size distribution up to 10 nm) in biomedical application such magnetic hyperthermia, because they have unique size effect called superparamagnetism. Since size becomes important parameter in nanomaterial design, the researchers are urged to find best method to obtain finest result of nanomaterial synthesis, throughout chemical approaches, mechanical approaches even acoustical approaches. One of the best methods is ultrasonic atomization. 

         The applications of ultrasonic atomization for the generation of aerosol sprays are growing due to its capability for producing very small droplets with very narrow size distributions. The droplet size depends on the ultrasonic frequency, and hence it is possible to generate droplets in the micron range by using MHz-order driving frequencies. With higher frequencies, capillary waves are excited on the free surface primary drop at shorter wavelengths; destabilization of these waves then results in smaller atomized droplets. Nevertheless, ultrasonic atomizers are limited by a maximum frequency of around 1 MHz, due to the limitations of the piezoelectric material.

 

        Ultrasonic atomizing is carried out by focusing ultrasonic energy on a liquid surface and by scattering liquid particles by the energy. As shown in figure, when an ultrasonic vibrator is placed toward a liquid surface, a high intensity ultrasound is omitted. Waves arise at the center of the area with higher acoustic pressure and successive water columns are formed by focused ultrasonic energy. At this moment, surface waves are generated on the surface of the water columns. A stationary wave of the surface waves is very fine wave called a capillary wave. From the end of the wave or wave head, liquid particles are scattered away. This is the principle of the phenomenon of atomization.

         In atomization by an ultrasound particle size and volume can be controlled independently. Practically, an electrical input intensity can control the volume of atomization, while an ultrasonic frequency can control particle size. Generally an ultrasound of a higher frequency makes a smaller size of particles. The mean diameter, D of particles generated by using ultrasonic vibrators can be estimated from Lang’s equation wherein y and p are the surface tension and the density of the liquid, respectively, and f is the applied frequency.

 

         It’s already said above that ultrasonic vibrator has limitation in generated frequency. It is said up to 1 MHz in maximum. It is however possible to circumvent this limitation by employing surface acoustic waves (SAWs), which are nanometer order amplitude elastic waves that propagate along the surface of a piezoelectric substrate, wherein it is possible to obtain working frequencies in the MHz-GHz range. The SAW atomization device essentially comprises a single inter-digital transducer (IDT) consisting of pairs of straight aluminum electrodes sputter-deposited onto a 127.68° yx-cut lithium niobate (LiNbO3) single crystal piezoelectric substrate. When a sinusoidal electrical signal matching the operating frequency is applied to the IDT, x-propagating Rayleigh waves are generated. A Rayleigh wave has an elliptical displacement on the surface due to the combination of its two mechanical acoustic components: a longitudinal component along the direction of propagation, and a transverse one perpendicular to the surface.