Surface Acoustic Wave Atomizer Research Articles

Simple, and highly efficient edge-effect surface acoustic wave atomizer

Conventional surface acoustic wave (SAW) atomizers require a direct water supply on the surface, which can be complex and cumbersome. This paper presents a novel SAW atomizer that uses lateral acoustic wetting to achieve atomization without a direct water supply. The device works by simply pressing a piece of wetted paper strip against the bottom of an excited piezoelectric transducer. The liquid then flows along the side to the unmodified surface edge, where it is atomized into a well-converging mist in a stable and sustainable manner. We identified this phenomenon as the edge effect, using numerical simulation results of surface displacement mode. The feasibility of the prototype design was demonstrated by observing and investigating the integrated process of liquid extraction, transport, and atomization. We further explored the hydrodynamic principles of the change and breakup in liquid film geometry under different input powers. Experiments demonstrate that our atomizer is capable of generating high-quality fine liquid particles stably and rapidly even at very high input power. Compared to conventional SAW atomizer, the dispersion of mist width can be scaled down by 70%, while the atomization rate can be increased by 37.5%. Combined with the advantages of easy installation and robustness, the edge effect-based atomizer offers an attractive alternative to current counterparts for applications requiring high efficiency and miniaturization, such as simultaneous synthesis and encapsulation of nanoparticles, pulmonary drug delivery and portable inhalation therapy.

Atomization characteristics of 9.6 MHz directional surface acoustic wave for 1-micron spray system

Surface acoustic wave atomization is also effective for generating microparticles and nanoparticles, because of the small wavelength with the high frequency. In this work, a novel directional surface acoustic wave (SAW) atomizer via a novel 9.6 MHz unidirectional interdigital transducer (U-IDT), a driving interdigital transducer (D-IDT) and a reflector interdigital transducer (R-IDT), is proposed and studied for the 1-micron spray system, such as the pulmonary drug delivery system and high-precision film deposition system. The atomization characteristics of a water drop are evaluated with atomized droplet size, atomizing particle velocity, and atomizing rate. Around 1 µm atomized droplet diameter, 585 mm/s atomizing particle velocity, and 4.2 ml/min atomizing rate are achieved and investigated. Based on the 1-micron atomized droplet diameter and high diffusion velocity, a novel pulmonary drug delivery system with the SAW spray can be excepted as an effective non-intrusive treatment method. It is also suitable for microparticle and nanoparticle spray without the pump and fan systems in the fields of industry, cosmetics, medicine, and medical devices.

Thermal Stress Analysis and Control Method for Surface Acoustic Wave Atomizer

To prevent the potential failure of the surface acoustic wave (SAW) atomizer caused by the concentration of thermal stresses, this study investigates the thermal elevation process inherent to the operation of the surface wave atomizer. Subsequently, a method for temperature regulation is proposed. By collecting the temperature rise data of SAW atomizers with water, olive oil, and glycerol at 5/6/7 Watts (W) of power, the temperature curves of the atomizer surface under different conditions are obtained, and the stress changes in the working process are simulated additionally. The results indicate that although the stress generated by surface acoustic wave atomizers varies for different media, there is always a problem of rapid heating during the initial working stage in all cases. To address the above issues, this study analyzed the time when the maximum stress occurred and proposed control methods based on experimental data. The simulation results show that by controlling the driving power within 4 s after the start of atomization, the problem of excessive stress during the heating stage can be avoided. Finally, the feasibility of the control method was verified through a simple power control method (limiting the driving power to 3 W in the first 2 s), proving that this method can effectively reduce the thermal stress during the working process of the atomizer and prevent the atomizer from cracking.

Combination of pulse signal modulation and hydrophilic treatment of a substrate for controlling the thermal distribution in surface acoustic wave atomization

Owing to the temperature limitations of biochemical or biomedical atomization applications, temperature control within a surface acoustic wave (SAW) atomizer is critical. This paper presents a method for controlling the thermal distribution of SAW atomization. This method combines pulse signal modulation and hydrophilic treatment of an SAW substrate. By setting the duty factor to 50 % and contact angle to 30°, the temperature reduces to about 99.3 °C for water and approximately 68.4 °C for a 50 wt% alcohol/water binary mixture during SAW atomization. Therefore, the proposed method effectively controls the thermal distribution during SAW-based atomization. In addition, the particle diameters of the different liquids used for SAW atomization are explored using this method. The results reveal that the general method increases the particle diameters of tiny droplets from 5 μm to 10 μm. Notably, this is within an acceptable range, hence the method has lower effects on the atomization particle diameter.

Experimental research on surface acoustic wave microfluidic atomization for drug delivery

This paper demonstrates that surface acoustic wave (SAW) atomization can produce suitable aerosol concentration and size distribution for efficient inhaled lung drug delivery and is a potential atomization device for asthma treatment. Using the SAW device, we present comprehensive experimental results exploring the complexity of the acoustic atomization process and the influence of input power, device frequency, and liquid flow rate on aerosol size distribution. It is hoped that these studies will explain the mechanism of SAW atomization aerosol generation and how they can be controlled. The insights from the high-speed flow visualization studies reveal that it is possible by setting the input power above 4.17 W, thus allowing atomization to occur from a relatively thin film, forming dense, monodisperse aerosols. Moreover, we found that the aerosol droplet size can be effectively changed by adjusting the input power and liquid flow rate to change the film conditions. In this work, we proposed a method to realize drug atomization by a microfluidic channel. A SU-8 flow channel was prepared on the surface of a piezoelectric substrate by photolithography technology. Combined with the silicon dioxide coating process and PDMS process closed microfluidic channel was prepared, and continuous drug atomization was provided to improve the deposition efficiency of drug atomization by microfluidic.

Slippage effect on interfacial destabilization driven by standing surface acoustic waves under hydrophilic conditions

The influence of the slippage phenomenon over the interfacial dynamics of a millimeter-order fluid drop exposed to surface acoustic wave atomization is numerically studied under hydrophilic conditions. Implementation of the Navier-slip model into the governing hydrodynamic equations yields an interfacial evolution equation. The solution suggests that slippage at the wall constitutes a valuable phenomenon to manipulate the parent drop geometric aspect ratio and consequently the characteristic aerosol size during the atomization process.

Thermal effect in the process of surface acoustic wave atomization

This paper describes an experiment conducted to investigate thermal distribution during liquid atomization. An infrared camera was used to measure liquid temperature changes over time during atomization. Effects of device frequency, input power, and liquid viscosity on thermal distribution during surface acoustic wave (SAW) atomization were investigated, both experimentally and through simulation. We found that the temperature value of the atomizing area was higher than other areas. These results indicate that liquid heating was due to viscous dissipation of irradiated longitudinal wave energy; it was not caused by inelastic effects in the piezoelectric substrate. In the low-frequency atomization process, the increase of atomization rate was closely related to the thermal effect, but the atomization energy and the efficiency value were very low. Note that for a 30-MHz device with input power of 5.26 W, the atomization energy was 0.397 mW, and the efficiency was 0.00769%. Moreover, the viscosity had a remarkable influence on the thermal effect, not only on temperature changes, but also on the heat transfer mechanism in the liquid. To explain the thermal effect caused by radiated acoustic waves, we conducted a numerical study using finite element software. Based on the corresponding simulation results and experimental comparison with experimental results, we concluded that the energy radiated into the liquid by SAW was mainly dissipated by viscous dissipation, which caused the temperature of liquid to rise, while the energy consumed by atomization constituted a very small fraction. Finally, through the study of different adhesive layer materials, we found that the main reason for the SAW device cracking. The higher the input power, the greater the temperature difference taking place simultaneously. A SAW device using the silver paste can achieve continuous water atomization under an input RF power of 10.49 W for more than 1 h without cracking after many tests. At this input RF power, atomization rate can be achieved 0.57 ml/min. This has important practical significance for commercialization of SAW atomizers.

Analysis of free surface oscillations of a droplet due to ultrasonic wave impingement

An analytical approach based on the linear potential theory is employed to enlighten the fundamental physics of atomization of droplets with an impinging sound wave, with a particular application in surface acoustic wave (SAW) atomization. When a plane sound wave, originated from the gas or the liquid side (resembling SAW), impinges on a liquid droplet, capillary waves are generated. It is shown that, for both cases, spatial phase-locking between the sound spherical modes and the free surface oscillations occurs. Hence, capillary waves will have the same spatial modes of the sound wave. The frequency spectrum analysis shows that the phase-locking causes two types of waves: the natural capillary waves with a wide range of frequencies, two to five orders of magnitude smaller than the impinging sound wave, and the forced wave, with a frequency equal to that of the sound wave. Since the instability of these surface waves leads to separation of droplets from the surface and the size of these droplets is correlated with the wavelength of the surface waves, this well explains the previous observations that droplets with a wide range of sizes are generated in the SAW experiments. Finally, a correlation is also proposed for predicting the atomized droplet size, which gives the size order for the generated droplets in SAW with good accuracy. The correlation could also suggest the possible size for remote atomization of the droplets by sound wave propagated in gas.

Influence of Waterproof Films on the Atomization Behavior of Surface Acoustic Waves.

One of the reasons why commercial application of surface acoustic wave (SAW) atomization is not possible is due to the condensation of aerosol droplets generated during atomization, which drip on the interdigitated transducer (IDT), thereby causing electrodes to short-circuit. In order to solve this problem, a SU-8-2002 film coating on an IDT is proposed in this paper. The waterproof performance of the film coating was tested on a surface acoustic wave (SAW) device several times. The experimental results reveal that the film coating was robust. The experiment also investigated the effects of the SU-8-2002 film on atomization behavior and heating.

Enhancement Method of Surface Acoustic Wave-Atomizer Efficiency for Olfactory Display

Since olfaction is an important sense in human interfaces, we have developed an olfactory display using a surface acoustic wave (SAW) atomizer and micro-dispensers. In this olfactory display, the efficiency of atomization is important in order to avoid smell persistence problems often encountered in human olfactory interfaces. Thus, the SAW device is coated with amorphous Teflon film to change the substrate nature from hydrophilic to hydrophobic. It is also necessary to silanize the piezoelectric substrate surface prior to Teflon coating to enhance the adhesion of the film. A dip coating method was adopted to obtain uniform coating on the substrate. The high-speed solenoid valve was used as micro-dispenser to spout a liquid droplet to the SAW device surface since its accuracy and reproducibility were high. Then, the atomization became easier on the hydrophobic substrate. In this study, the amorphous Teflon coating for minimizing the remaining liquid on the substrate after atomization was studied. The goal of the protocol described here is to show the methods for coating a SAW device surface with amorphous Teflon film and generating the smell using the SAW atomizer and a micro-dispenser, followed by a sensory test.

Enhancement Method of Surface Acoustic Wave-Atomizer Efficiency for Olfactory Display.

Since olfaction is an important sense in human interfaces, we have developed an olfactory display using a surface acoustic wave (SAW) atomizer and micro-dispensers. In this olfactory display, the efficiency of atomization is important in order to avoid smell persistence problems often encountered in human olfactory interfaces. Thus, the SAW device is coated with amorphous Teflon film to change the substrate nature from hydrophilic to hydrophobic. It is also necessary to silanize the piezoelectric substrate surface prior to Teflon coating to enhance the adhesion of the film. A dip coating method was adopted to obtain uniform coating on the substrate. The high-speed solenoid valve was used as micro-dispenser to spout a liquid droplet to the SAW device surface since its accuracy and reproducibility were high. Then, the atomization became easier on the hydrophobic substrate. In this study, the amorphous Teflon coating for minimizing the remaining liquid on the substrate after atomization was studied. The goal of the protocol described here is to show the methods for coating a SAW device surface with amorphous Teflon film and generating the smell using the SAW atomizer and a micro-dispenser, followed by a sensory test.

Multicomponent Olfactory Display Using Solenoid Valves and SAW Atomizer and its Blending-Capability Evaluation

An olfactory display for blending many ingredients at any recipe was developed. It consists of microdispensers based upon solenoid valves and surface acoustic wave atomizer. The accuracy and stability were much improved compared with previous olfactory display using micropumps. Then, its blending capability was tested using sensory test such as triangle test. It was found that the scent blended in the proposed olfactory display was identical to the one blended in advance in the liquid phase. Finally, the durability of the amorphous Teflon coating used for enhancing atomization capability was investigated. The durability of the amorphous Teflon coating was much improved when the concentration of silane-coupling agent was modified.

Experimental investigation of surface acoustic wave atomization

Surface acoustic wave (SAW) atomization is a promising and relatively new method in the atomization field. However, the exact process is obscure due to the difficulty of observing such a small-scale series of complicated hydrodynamic processes. Lacking enough observations also makes it difficult in predicting the atomized droplet size distribution. Therefore, this study employed fundamental experimental methods to investigate the mechanism for SAW atomization. Firstly, a 20MHz SAW device was fabricated by UV photolithography technology. The whole atomization process was then recorded by using a high-speed camera and classified each reaction stage according to the applied power. The experiment reveals three main peaks in the size distribution and corresponding mechanisms are employed to provide a detailed explanation for each. Experimental results also show that Eckart streaming and Schlichting streaming dominate different stages in generating different scale droplets. Two methods of predicting the mean diameter of atomized droplets are compared in this work.

Rapid and Sustainable Surface Acoustic Wave Atomizer

Surface acoustic waves have considerable potential to rapidly generate micron to submicron sized aerosols due to the capillary wave at the liquid–air interfacial boundary. The atomization process, however, is often found to be unstable and discontinuous due to the inconsistency of the droplet position and the lack of accuracy of resonance frequency, resulting in less efficient atomization performance. We developed a rapid and sustainable surface acoustic wave (SAW) atomizer to generate aerosols in a robust, straightforward, and stable mechanism for a continuous, consistent, and reliable deposition of a functional material (i.e., poly(3,4-ethylenedioxythiophene) polystyrene sulfonate [PEDOT:PSS]). Two identical progressive focused surface acoustic wave devices were arranged in angles and in opposite directions with 1 mm wide opening to regulate appropriate amount of liquid sample to be atomized. Several parametrical studies (i.e., on the design of the SAW device, liquid flow rate, applied voltage, and depo...

Characterization of graphene oxide nanofilms obtained by the SAW atomization

Due to its ability to absorb water molecules, graphene oxide (GO) is considered a promising material for sensitive coatings in fast surface acoustic wave (SAW) humidity sensors. In this work, we characterize GO films obtained by the SAW atomization technique. It is shown that the atomized submicroliter droplets of aqueous suspension of GO can be deposited onto the surface of Si, LiNbO3 or quartz substrates forming discrete or continuous films of nanometer thickness. The deposited films were examined using AFM and electron microscopy. We discuss the dependence of thickness and structure of the obtained GO films on the parameters of deposition: the number of atomized droplets, a volume of the initial droplet, a distance between the atomizer and the sample, etc. To evaluate the adsorption characteristics of the obtained GO films, we used them as sensitive coatings of the SAW humidity sensors. We found that the adsorption characteristics of the GO films are determined by fast adsorption on the surface of GO sheets and slow adsorption, attributed to limited penetration of water molecules between the sheets, and depend on the number of deposited layers.

Influence of the deposition parameters of graphene oxide nanofilms on the kinetic characteristics of the SAW humidity sensor

Symmetric surface acoustic wave (SAW) atomizer was used to deposit graphene oxide (GO) nanofilms from microdroplets of the aqueous GO dispersion onto the surface of the SAW humidity sensors. The sensors with the deposited GO films were tested in a wide range of relative humidity and showed the amplitude of the response in the range of 1–40kHz sufficient for the majority of applications. The kinetics of the adsorption of water molecules was modeled using the assumption that two independent first order adsorption processes take place. Saturation amplitudes and rate coefficients for both processes were experimentally evaluated using LSQ fitting of the sensor response in the time domain for the films obtained by different number of depositions. The mechanism of the adsorption of water molecules to the GO flakes is proposed and discussed.

Nozzleless spray cooling using surface acoustic waves

Surface acoustic wave (SAW) atomization is an attractive approach for generating monodispersed microdroplets for a diversity of applications, from drug delivery to mass spectrometry, due to its reliability, miniaturizability, and portability. Here, we demonstrate a nozzleless spray cooling technique based on SAW atomization, with the key advantage of downward scalability: increasing the operating frequency facilitates the fabrication of a chip-sized atomizer to use in compact cooling of electronic devices. Using deionised water, cooling is improved by 15% when the atomization rate is increased by 40%; when the gap separating the SAW device and heat source is halved, the cooling is improved by 20%. By constructing the device such that the atomized droplets are easily deposited upstream of the flow circulation, the performance is improved further. The atomization of CuO nanoparticle suspensions (at 3%) increased the cooling performance by 30%. Merely increasing the nanoparticle mass concentration in the suspension from 1% to 3% leads to an improvement in the cooling by 10% due to the deposition and formation of nanoparticle clusters on the heated surface, thereby increasing the total surface area. Further increases in the nanoparticle concentration to 10% however results in a diminution in the cooling due to the increase in the suspension viscosity μ, that leads to a reduction in the atomization rate ṁ~μ−1/2 for a given input power. Finally, we demonstrate the concept of using tapered finger transducers to selectively enhance local cooling in a desired area by simply changing the excitation frequency, without requiring repositioning of the SAW device.

Odor Sensing System and Olfactory Display

In this review, an odor sensing system and an olfactory display are introduced into people in pharmacy. An odor sensing system consists of an array of sensors with partially overlapping specificities and pattern recognition technique. One of examples of odor sensing systems is a halitosis sensor which quantifies the mixture composition of three volatile sulfide compounds. A halitosis sensor was realized using a preconcentrator to raise sensitivity and an electrochemical sensor array to suppress the influence of humidity. Partial least squares (PLS) method was used to quantify the mixture composition. The experiment reveals that the sufficient accuracy was obtained. Moreover, the olfactory display, which present scents to human noses, is explained. A multi-component olfactory display enables the presentation of a variety of smells. The two types of multi-component olfactory display are described. The first one uses many solenoid valves with high speed switching. The valve ON frequency determines the concentration of the corresponding odor component. The latter one consists of miniaturized liquid pumps and a surface acoustic wave (SAW) atomizer. It enables the wearable olfactory display without smell persistence. Finally, the application of the olfactory display is demonstrated. Virtual ice cream shop with scents was made as a content of interactive art. People can enjoy harmony among vision, audition and olfaction. In conclusion, both odor sensing system and olfactory display can contribute to the field of human health care.

A self-converging atomized mist spray device using surface acoustic wave

We developed a surface acoustic wave (SAW) atomizer, which can produce a narrow mist spray of submillimeter width. The atomized mist spray converges spontaneously and thus requires no additional spray converging system. Key features in achieving the self-converging atomization are two-port interdigital transducers (IDTs) with a groove in between. The SAW is driven by applying an electric field on IDTs patterned on a piezoelectric substrate. The two-port IDTs drive SAWs in opposite directions toward the groove, which then separates the SAW-driven streaming on each side of the groove. Two types of self-converging atomizer were experimentally demonstrated, a line and a point atomizer. While the former uses parallel IDTs to atomize water from the groove into a thin-planar-shaped spray, the latter uses arc-shaped SAW lenses to focus the SAW into a certain point in the groove to atomize water into a narrow point spray. In addition, by adding a water reservoir, continuous operation of up to 30 s was achieved with the submillimeter narrow point atomizer.

Improvement of Odor Blender Using Electroosmotic Pumps and SAW Atomizer for Low-Volatile Scents

This paper reports an improvement of miniaturized six-component odor blender using electroosmotic (EO) pumps and a surface acoustic wave (SAW) device. Tiny droplets of liquid odor samples spouted from EO pumps were atomized by SAW atomizer into odorant vapors to generate smells. Even the scents of low volatile substances that typically require long time to be released can be presented rapidly. The device works soundlessly, does not produce heat to the nearby environment, and is more robust against bubble comparing with an olfactory display composed of the widely used inkjet devices. In this paper, a quartz crystal microbalance gas sensor was used to measure the temporal changes of generated odor intensities. Therefore, the capability to blend up to three odor components was confirmed.