Abstract
In this paper, we introduce an innovative microinjector design, featuring a bubble valve, which entails superior droplet ejection characteristics and monolithic fabrication, which allows handling of a wide range of liquids. This new microinjector uses asymmetric bubbles to reduce crosstalk, increase frequency response and eliminate satellite droplets. During a firing, i.e., droplet ejection, the "virtual valve" closes, by growing a thermal bubble in the microchannel, to isolate the microchamber from the liquid supply and neighboring chambers. Between firings, however, the virtual valve opens, by collapsing the bubble, to reduce flow restriction for fast refilling of the microchamber. The use of bubble valves brings about fast and reliable device operation without imposing the significant complication fabrication of physical microvalves would call for. In addition, through a special heater configuration and chamber designs, bubbles surrounding the nozzle cutoff the tail of the droplets being ejected and completely eliminate satellite droplets. A simple one-dimensional model of the operation of the microinjector is used to estimate the bubble formation and liquid refilling. Simulation results show that the reported bubble valve can improve the frequency response at least three times compared with the passive chamber neck design commonly used for commercial devices.