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Fabrication and Characterization of Thickness Mode Piezoelectric Devices for Atomization and Acoustofluidics
Chapters
Summary August 5th, 2020
Fabrication of piezoelectric thickness mode transducers via direct current sputtering of plate electrodes on lithium niobate is described. Additionally, reliable operation is achieved with a transducer holder and fluid supply system and characterization is demonstrated via impedance analysis, laser doppler vibrometry, high-speed imaging, and droplet size distribution using laser scattering.
Transcript
These techniques can be used to answer questions about resonance frequency, vibrational mode excitement, vibration amplitude, and how transducers with these characteristics perform as atomizers. With information provided by these analyses, it's possible to rigorously quantify the effects of independent variables and experiments involving thickness mode transducers. This technique enables the development of devices that can be used to atomize drugs for the treatment of respiratory diseases such as pneumonia.
These methods are useful for characterizing atomization phenomena and can be applied to the study of capillary waves on the surface of a droplet. Because many competing factors must be balanced, it can be difficult to achieve continuous atomization. Adjust the power input, wick position and wick orientation and observe how the behavior changes.
Many of these techniques are simple to perform after demonstration, but requires some dexterity and spatial awareness that do not come across in text. To assemble a custom transducer holder, solder two surface mount spring contacts to each of two custom printed circuit boards and clip the outer contacts so they do not short the circuit. Press fit the spikes into the plated holes on the custom boards such that the spikes point away from each other.
Use board spacers and screws to connect the two custom circuit boards so the contacts are just in contact with each other. Adjust the spacing with plastic washers as necessary. Then slide a 3 x 10 millimeter transducer between the inner pair of contacts.
To identify the resonance frequency by impedance analysis, connect a transducer to the open port of the network analyzer and select the S11 reflection coefficient parameter via the user interface of the network analyzer. Select the frequency range of interest and perform the frequency sweep. Then select save recall and save trace data to export the data to an appropriate data processing software program to identify the precise minimal locations.
To characterize the vibration by LDV, place a transducer in pogo plate contact on the LDV stage and connect the pogo probe leads to the signal generator. Ensure that the correct objective is selected in the acquisition software and focus the microscope on the surface of the transducer. Select define scan points and settings.
A single point scan gives the user vibration amplitude at a single point. To determine the vibration mode and resonance, an area scan must be performed. Under the general tab, select the FFT or time option depending on whether the scan is being performed in frequency or time domain and set the number of averages.
In the channel tab, make sure that the active boxes are checked and adjust the reference and incident channels to select the maximum signal strength from the substrate. In the generator tab, if the measurement is carried out under single frequency signal, select Sine from the waveform pull-down list. If it's under a single band, select MultiCarrierCW.
Then in the frequency tab, change the bandwidth and FFT lines to adjust the scan resolution for a frequency domain scan. If the time domain measurements are being performed, change the sample frequency in the time tab. To create the fluid supply system, select a 25 millimeter long 2 millimeter diameter wick composed of a bundle of fibers of a hydrophilic polymer designed to transport acquiesce liquid.
Trim one end of the wick so that it forms an asymmetrical tip and then insert it into a Luer lock syringe with the desired capacity, allowing the wick to extend at 15 millimeters beyond the end. Lock a syringe tip onto the syringe providing a snug fit around the wick and mount the assembly such that the wick is 10 to 90 degrees from horizontal and the tip of the wick is just in contact with the edge of the transducer. Then fill the syringe with water.
Set the voltage to zero and apply a continuous voltage signal at the resonance frequency determined using the impedance analyzer. Increase the voltage until the liquid is atomized continuously without the device flooding or drying out. If the suggested adjustments fail, roughen the gold surface of the transducer near the wick contact point with fine sandpaper without removing the gold entirely.
To observe the device dynamics via high-speed imaging, rigidly mount a high speed camera horizontally on an optical table and place a transducer in pogo plate contact on an XYZ stage near the focal length of the camera. Position a diffuse light source at least one focal length on the opposite side of the transducer from the camera and use a pipette to place a sessile drop on the surface of the transducer. Adjust the camera focus and the XYZ position to bring the fluid sample into sharp focus and select a frame rate that is at least twice as large as this frequency according to the Nyquist rate to avoid aliasing.
Adjust the light intensity, camera shutter, or both to optimize the contrast between the fluid and the background. Then connect alligator clips from the amplified signal generator to the pogo probe leads and capture the phenomenon by simultaneously triggering the video in the camera software and applying the voltage signal. For laser scattering analysis of the droplet size, adjust the laser transmitting and laser receiving modules along the rail of the laser scattering system with a 20 to 25 centimeter gap between the two modules.
Rigidly mount a platform in this gap such that when the transducer and fluid supply assemblies are placed on it, atomized mist will be ejected into the laser beam path. To facilitate this alignment, turn on the laser beam and select tools, laser control, and laser on. Fix the transducer hole to the platform.
Fix the fluid supply assembly to an articulated arm. Position the fluid supply assembly so that the tip of the wick is just in contact with the edge of the transducer and use alligator clips to connect the signal source to the spike terminals on the transducer holder and click new standard operating procedure in the laser scattering system software. Set the template to default continuous and the sampling period to 1.
Under data handling, click spray profile to set the path length to 20 millimeters. Click alarms to uncheck use default values and set the minimum transmission to 5 and 1%and the minimum scattering to 50 and 10. When all of the parameters have been set, click start standard operating procedure and select the created procedure.
Fill the fluid supply reservoir with water up to the desired level and note the volume. Once the measurement has started, turn on the voltage signal and start the stopwatch as soon as atomization begins. Once the desired volume of fluid has been atomized, turn off the voltage signal while stopping the stopwatch and record the final volume.
In the resulting measurement histogram, select the portion of the data during which the atomization was occurring as expected and the signal at the receiver was strong enough to be statistically significant. Click average and okay to generate a distribution based on the selected data. Then copy and the data into a text file and save with an appropriate filename.
The characterization of these devices includes determination of the resonant frequency and harmonics using an impedance analyzer. In this representative analysis, the fundamental frequency of the devices was found to be close to seven megahertz as predicted by the thickness of the substrate. Further characterization using non-contact laser Doppler vibrometry can be used to determine the magnitude and displacement of the substrate, which is usually in the nanometer range.
In addition, the droplet vibration can be evaluated by high-speed imaging and the atomization dynamics can be determined by measuring the droplet size distribution. Remember that to achieve atomization, the transducer must be operating at a thickness mode resonance frequency. If the device is underperforming, then you may not be at the correct frequency.
Using this protocol as a foundation, many thickness mode parameters can be varied and compared such as electrode thickness or lateral dimensions. Having established this protocol with water, thickness mode transducers can now be used with other fluids for applications such as pulmonary drug delivery, cooling and coding.
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