Ultrasonic Mist
Channel: Aaron Schwarz
Date: 2020-06-28

Turning electricity from 120 vac to 24 vdc, a class 2 switching plug in adapter powers an ultrasonic 113-kHz (±3 kHz) resonance frequency transducer located inside a resonant chamber, atomising water with drops of essential oil into a fine mist of uniform droplets by converting the sound energy into standing resonant waves that translate that energy into mechanical energy that breaks up the liquids into energetic droplets that are forced out the nozzle of this ultrasonic humidifier or vaporizer by air pressure from a fan. The machine integrates the transducer into the bottom of a water tight container, where energy translated into the water making a fine mist. A gentle breeze from the fan pushes the mist out of the nozzle. 

A driver circuit inside the machine a relatively complicated AC pulse output device that sends high frequency pulses of current at a metal suspended ceramic dris that oscillates rapidly. Fluids in contact with the resonating ceramic mass are energized as the high frequency sound energy directly impinges on the fluid interface thereby translating sounds into mechanical energy that breaks up the fluid into uniform sized droplets. Geometry of the apparatus fluid chamber optimized to translate the transducers energy into the fluid so a stream of air from a fan can propel the droplets out the top nozzle into the room where the water evaporates into a gas humidifying the air & leaving higher boiling point essential oils in the air as tiny droplets, making a fine oil vapor mist of the essential oil that make the room smell like the essential oil drops added to the water. Water acts to cool the ultrasonic transducer located on the floor of the emitter chamber. 

Think of it like this, Ultrasonic humidifier uses ultrasonic waves to create a fine mist through cavitation in the water with a broken capillary wave at the water surface. The humidifier consists of a piezoelectric ceramic disc which is provided with current through two nickel electrodes. Since the ceramic is piezoelectric, it oscillates in the presence of electric current at ultrasonic frequency (So you do not hear the sound waves generated in water)

As the frequency of oscillation of the plate increases, water tries to follow the frequency oscillating with the plate. But water fails to match the frequency due to its inertia and weight and causes water hammers. So as the water wave lags behind the wave of the disc, an area of low pressure is created between them which forms a cavity. This is called cavitation. The cavity can be a void or filled with air since there is air dissolved in water and with the formation of low pressure, air moves into the cavity. When this cavity implodes it creates a lot of energy and the imploding jet as shown in the figure generally has a lot of energy. The vibrating disc also causes capillary waves at the surface of water (ripples). The capillary wave keeps oscillating up and down due to surface tension and gravity. With the cavity imploding, a crossed capillary wave is formed at the surface and as a result at the crest of the wave, very minute droplets of water have enough energy to break off the surface tension and get out of water. As soon as these minute drops of water exit the surface they are absorbed into the stream of air being blown by a fan and leave the humidifier as mist. The size of these aerosols would be smaller with increasing frequency. Moreover if the capillary waves are oscillating at around half of the driving frequency, the atomization threshold will be approximately linear with the viscosity of water. 

One interesting application of ultrasonic mist to make a water mist to cool intercoolers in turbocharged engine applications. Also swamp coolers or evaporative cooling systems can benefit from integration of ultrasonic mist makers. This technology can also be used to humidify room air in building in arid regions to make the air more lung & eye friendly. Exhaust emissions control systems can also use vaporized urea NOX trap systems to clean up diesel engine exhaust of oxides of nitrogen. Another interesting application in cleaning machines used to clean jewelry & machine parts & medical parts & tools & other instruments, components or peices etc