Construction, categories, and part numbers
The dispensers manufactured by MicroFab are actuated piezoelectrically. An annular piezoelectric (PZT) element, poled radially, is bonded to a glass tube with an integrated nozzle and orifice. The glass tube is mounted in a protective housing and, at the supply end, bonded to a fitting.
The annular actuator has electrodes on the outer and on the inner surfaces. The inner electrode wraps around on the outer surface for easier electrical connection; a small region on the outer surface has the metallization removed to separate the two electrodes. Small gauge wires are soldered to the outer electrode and the (wrap around portion of the) inner electrode; the wires are loosely twisted and placed together in a connecter that matches the output cable of MicroFab’s JetDrive™ electronics box.
The configuration of the dispensers or micro jets (MJ) is differentiated first by the type of fitting: AB – barbed (or slip on), AT – threaded (Minstac connection – small compression fitting by the Lee Company), ABL – barbed large, AL – Luer fitting. The AB and AT devices cab be manufactured with a protective feature (ABP and ATP respectively). More details on the various constructions can be found at Products/Microdispensing Devices/Low Temp. Devices on MicroFab’s web site.
An example of the numbering for such a micro dispenser or micro jet is MJ-ATP-xxx, where the xxx is the orifice diameter in micrometers. The standard range of orifice diameter is 20-80um for all devices except ABL devices where the range is extended up to 120um. The part number MJ-ABP-075 refers to a micro dispenser with a barbed fluid connector, protected tip and an orifice with a 75um diameter.
Principle of operation
When applying a voltage differential, the electrical field is generated between the inner and outer electrodes causing the piezoelectric actuator to expand radially (and contract axially) or, depending on the voltage polarity and poling, contract radially (and expand axially). The deformation occurs only along the portion where both electrodes are present, as the electric field is not generated in the region without electrodes and the wrap around region of the inner electrode.
The simplest actuation signal consists of a trapezoidal waveform that is applied, whenever a drop is desired, to one electrode while the other electrode is electrically grounded. Deformation occurs during the transition periods (rise and fall) and ceases during the constant voltage (dwell) period.
In MicroFab’s configuration the outer electrode is grounded (blue wire) while the inner electrode (red wire) receives the actuation voltage.
During the rise time, the tubular PZT expands its circumference while becoming thinner and shorter. This fast deformation is transmitted through the epoxy bond to the glass tube and results in an outwards motion of the inner glass surface which produces a negative pressure (with respect to the equilibrium). The negative pressure travels in the fluid at the speed of sound along the glass tube in the form of an expansion acoustic wave to both the orifice and the supply end. The expansion wave is reflected as a compression wave (higher pressure than the equilibrium pressure in the glass tube) at the supply end and travels back towards the orifice. If the dwell time is selected to start when the positive pressure wave matches the piezoelectric actuator, the inwards motion of the inner glass surface reinforces it resulting in a faster and larger droplet.
The sequence of events at the orifice leading to the droplet formation starting with the equilibrium condition is shown in the figure: fluid flush at the orifice (first image). In the second image, the fluid interface is withdrawn from the equilibrium position indicating the arrival of the expansion wave at the orifice. The third image is after the compression wave reaches the orifice causing the fluid to emerge. Another expansion wave reaching the orifice causes the fluid to pull back (images four and five) and to break off and leave the orifice (image six). The ejected fluid is pulled in a spherical drop by surface tension forces (image seven).
The images are obtained by a short pulse of light from an LED that is synchronized with the pulse generating the drop. By adjusting the delay between the actuation pulse and the pulse applied to the LED, the droplets are captured at different locations along the flight path.
Requirements for operation and observation:
- Proper operation requires that, when the dispenser is not actuated, the fluid is flush with the orifice. If the forces acting on the fluid at the orifice are not balanced, the fluid will either drip or withdraw in the nozzle. Both conditions result in a drop generation failure. The Backpressure control section discusses two possible methods to maintain the fluid flush with the orifice.
- The operation is based on acoustics and wave propagation. The length scales and the speed of sound values result in timing in the tens of microseconds. Thus, the drive electronics should allow the adjustment of the timing in one microsecond increments or less.
- Observation of the drop formation is very important, especially when developing new solution formulations. The cameras, the LED and its driver are required to observe the droplets. To ensure sharp images, the pulse applied to the LED for illumination has to be very short (4-6us) and, at the same time provide sufficient light at low frequencies.
Precautions using & cleaning the dispensers
- Do not immerse the body of the MicroJet device into the Micro-90 Cleaning Solution or other solvent, as these fluids can damage the epoxy and nickel coating of the PZT. Water and/or IPA can be used to clean the MicroJet casing.
- The low temperature MicroJet devices (MJ-AB, MJ-AT, MJ-ABL, MJ-AL) can be operated safely at temperatures of up to 70°C. Operating at higher temperatures for extended periods of time could lead to a loss of the piezoelectric effect.
- Avoid the use of high power output ultrasonic cleaners, as they can damage the edge of orifice. Also, minimize the amount of time the MicroJet device tip is exposed to ultrasonic cleaning.