Design and FE analysis of integrated sensing using gas compressibility for microdroplet generation

As trends in biology, chemistry, medicine and manufacturing have pushed macroscopic processes onto the microscale, robust technologies have become necessary to encapsulate liquids into microdroplets for further manipulation and use. In order to most effectively utilize these microdroplets, real-time sensing is needed during the generation process to monitor the size of the droplet generated, or if generation failed to occur. Current droplet generating technologies operate either in open-loop, with no direct feedback available to the control system to monitor the process, or in closed loop with external sensing, using photography or droplet weight measurement to measure droplet size. By utilizing internal system-based sensing to close the loop, corrections in the dispensing process could be made in real-time in response to malfunctions as they occur. Furthermore, the generator’s operator could be more quickly alerted when a systemic error, such as a clog, continues to occur. One candidate solution to provide system-based sensing is to monitor the pressure of a reservoir of compressible gas kept adjacent to the reservoir of droplet liquid, both within a constant volume fluid chamber. The gas reservoir pressure during the actuation sequence can be related both analytically and empirically to the volume of the droplet ejected from the device, including instances where generation fails and a droplet is not ejected. This paper describes the designs of potential systems to realize this design concept, and the development of a finite element simulation for one of the concepts capable of generating droplets while simultaneously monitoring the pressure of the gas reservoir. A linear relationship between this calculated pressure and the volume of the dispensed droplet is found, validating the sensible property as workable for implementation in a physical system.