Aerodynamically Levitated Droplets as Small‐Scale Chemical Reactors and Liquid Microprinters

A thin liquid film spread over the inner surface of a rapidly rotating vial creates an aerodynamic cushion on which one or multiple droplets of various liquids can levitate stably for days or even weeks. These levitating droplets can serve as wall‐less (“airware”) chemical reactors that can be merged without touching—by remote impulses—to initiate reactions or sequences of reactions at scales down to hundreds of nanomoles. Moreover, under external electric fields, the droplets can act as the world's smallest chemical printers, shedding regular trains of pL or even fL microdrops. In one modality, the levitating droplets operate as completely wireless aliquoting/titrating systems delivering pg quantities of reagents into the liquid in the rotating vial; in another modality, they print microdroplet arrays onto target surfaces. The “airware”, levitated reactors are inexpensive to set up, remarkably stable to external disturbances and, for printing applications, require operating voltages much lower than in electrospray, electrowetting, or ink jet systems. Droplets of various solvents can levitate for days to weeks over the same or another liquid spread over the walls of a rotating vial. Individual or multiple droplets can be levitated, and can be used as “airware” reactors or as “wireless” chemical printers requiring no external connections to deliver into a reaction mixture fL volumes and pg quantities of reagents.