Fabrication and Application of a Chemical Resistant Low-Cost Microdrop Generator

This paper introduces a chemical resistant piezoelectrically driven microdrop generator which can be fabricated in a cost and time saving manner by using rapid prototyping techniques. Thus it is especially suitable as an experimentation platform. For the adaption of microdrop generators to various fluids, an experimentation platform is needed which allows the rapid change of geometry, dimensions, and material parameters of the microdrop generator. The size of the nozzle, the geometry of the pumping chamber, and the thickness of the used piezo-transducer have to be adaptable to various fluids to achieve drops of the size, speed, and uniformity that are needed. This microdrop generator uses a sandwich structure which consists of a silicon wafer, a Pyrex diaphragm, and a PZT transducer. A pumping chamber is milled into the silicon by laser micromachining; and the Pyrex is anodically bonded on top of the silicon plate to seal off the pumping chamber. The piezo-transducer is then glued to the diaphragm with an epoxy adhesive to obtain a bimorph actuator. When electrically driven, the actuator bends inwards into the pumping chamber which in turn creates a pressure wave inside the chamber that finally leads to the ejection of a drop out of the lateral nozzle. Since only the Pyrex and the silicon are in contact with the fluid the assembly is very resistant to aggressive media like solvents, adhesives, or acids. The thickness of the piezo-actuator can be varied according to the intended application. Depending on the piezoceramic used, the operating temperature is up to 250 °C. Single- and multi-nozzle arrays as well as the integration of a heated fluid reservoir can be realized. The drop volume is set by proper dimensioning of the microdrop generator. Manufacturing, assembly, and interconnection technology of the droplet generator will be described later in this paper. The electro-mechanical behaviour of the droplet generator is analyzed by determining the step response function and by measuring the frequency-dependant impedance. For the first fluidic validation of the experimentation platform, isopropanol is used because of its well known properties. The relationship between drop velocity and drive voltage on the PZT transducer is established. Special attention is paid to the calculation of the microdrop generator material cost which only amounts to $25 for a multi-nozzle array. By using rapid prototyping techniques the microdrop generator is manufactured within 180 min. This shows the potential for a low-cost and rapidly producible experimentation platform.