Low Velocity Pressureless Spray
One feature that distinguishes pressureless, ultrasonic atomizing nozzles from most other spray nozzles is their soft, low-velocity fine mist spray, typically on the order of 3-5 inches per second. Liquid is atomized into a fine mist spray using high frequency sound vibrations. Piezoelectric transducers convert electrical input into mechanical energy in the form of vibrations, which create capillary waves in the liquid when introduced into the nozzle.
Capillary waves forming on
ultrasonic nozzle tip.
In coating applications, the unpressurized, low-velocity spray significantly reduces the amount of overspray since the drops tend to settle on the substrate, rather than bouncing off it. This translates into substantial material savings and reduction in emissions into the environment.
The spray can be controlled and shaped precisely by entraining the slow-moving spray in an ancillary air stream. Spray patterns from as small as 0.070 inches wide to as much as 1-2 feet wide can be generated using specialized types of spray-shaping equipment.
Ultra-low Flow Rate Capabilities
Since the ultrasonic atomization process does not rely on pressure, the amount of liquid atomized by a nozzle per unit time is primarily controlled by the liquid delivery system used in conjunction with a nozzle.
The flow rate range for the entire family of Sono-Tek ultrasonic nozzles is from as low as a few microliters per second to up to about 6 gallons per hour.
Depending on the specific nozzle and the type of liquid delivery system employed (gear pump, syringe pump, pressurized reservoir, peristaltic pump, gravity feed, etc.), the technology is capable of providing an extraordinary variety of flow/spray possibilities.
Drop-size Range Selectivity
In general, the drops produced by ultrasonic atomization have a relatively narrow size distribution. Median drop sizes range from 18-68 microns, depending on the operating frequency of the specific type of nozzle. As an example, for a nozzle with a median drop size diameter of approximately 40 microns, 99.9% of the drops will fall in the 5-200 micron diameter range.
The phenomenon referred to as ultrasonic atomization has its roots in late 19th century acoustical physics, specifically in the works of Lord Rayleigh, author of the definitive work The Theory of Sound.
Simply stated, when a liquid film is placed on a smooth surface that is set into vibrating motion such that the direction of vibration is perpendicular to the surface, the liquid absorbs some of the vibrational energy, which is transformed into standing waves. These waves, known as capillary waves, form a rectangular grid pattern in the liquid on the surface with regularly alternating crests and troughs extending in both directions.
When the amplitude of the underlying vibration is increased, the amplitude of the waves increases correspondingly; that is, the crest become taller and troughs deeper. A critical amplitude is ultimately reached at which the height of the capillary waves exceeds that required to maintain their stability. The result is that the waves collapse and tiny drops of liquid are ejected from the tops of the degenerating waves normal to the atomizing surface. A useful analogy that helps visualize this process comes from our everyday experience. Ocean waves coming into shore go through a transition from stability on the open water to instability as they approach shore. The instability is evident as the waves form foamy breakers.
The reason for instability in this type of wave is that as it approaches shore, the bottom of the wave contacts the ocean floor and is slowed down by frictional forces. The wave top, on the other hand, continues to move ahead unimpeded. The net result is that the wave topples over. In this process of breaking up, a spray of tiny drops is ejected from the wave surface. Although the mechanisms governing the creation of a spray from capillary and ocean waves differ, the results are similar.
Ultrasonic Spray Nozzles
As their name implies, ultrasonic nozzles employ high frequency sound waves, those beyond the range of human hearing. Disc-shaped ceramic piezoelectric transducers convert electrical energy into mechanical energy. The transducers receive electrical input in the form of a high frequency signal from a power generator and convert that into vibratory motion at the same frequency. Two titanium cylinders magnify the motion and increase the vibration amplitude at the atomizing surface.
Nozzles are configured such that excitation of the piezoelectric crystals creates a transverse standing wave along the length of the nozzle. The ultrasonic energy originating from the crystals located in the large diameter of the nozzle body undergoes a step transition and amplification as the standing wave as it traverses the length of the nozzle.
Since wavelength is dependent upon operating frequency, nozzle dimensions are governed by frequency. In general, high frequency nozzles are smaller, create smaller drops, and consequently have smaller maximum flow capacity than nozzles that operate at lower frequencies.
The nozzle body is fabricated from titanium because of its good acoustical properties, high tensile strength, and excellent corrosion resistance. Liquid introduced onto the atomizing surface through a large, non-clogging feed tube running the length of the nozzle absorbs some of the vibrational energy, setting up wave motion in the liquid on the surface. For the liquid to atomize, the vibrational amplitude of the atomizing surface must be carefully controlled. Below the so-called critical amplitude, the energy is insufficient to produce atomized drops. If the amplitude is excessively high, the liquid is literally ripped apart, and large “chunks” of fluid are ejected, a condition known as cavitation. Only within a narrow band of input power is the amplitude ideal for producing the nozzle’s characteristic fine, low velocity mist.
The fine control of input energy is what distinguishes ultrasonic atomizing nozzles from other ultrasonic devices such as welders, emulsifiers, and ultrasonic cleaners; these other devices rely on cavitation with input power of the order of hundreds to thousands of watts. For ultrasonic atomization, power levels are generally under 15 watts. Power is controlled by adjusting the output level on the power supply.
Since the ultrasonic atomization mechanism relies only on liquid being introduced onto the atomizing surface, the rate at which liquid is atomized depends solely on the rate at which it is delivered to the surface. Therefore, every ultrasonic nozzle has an inherently wide flow rate range. In theory, the “turn down” ratio (ratio of maximum to minimum flow rate possible) is infinite. In practice, this ratio is limited to approximately 5:1 as result of design constraints.