A hybrid flow focusing nozzle design to produce micron and sub-micron capillary jets

•We designed a nebulizer based on the flow focusing principle.•The nebulizer produces droplets much smaller than those obtained with flow focusing.•We analyze the stability and outcome of the nebulizer.

Flow focusing is providing new tools in Analytical Chemistry for its unique combination of simplicity, droplet size control just above the micrometer scale, and aerosol beam collimation. However, the most interesting droplet size range lies at the stability limit of the standard flow focusing operating mode. Nevertheless, that stability limit can be forced down to produce jets on the submicrometer scale by underpinning the cone-like meniscus at the tip of a hypodermic needle. Using a nebulizer design based on that concept, we show that the flow focusing steady jetting regime extends down to the limit imposed by the instability of jet itself, even for Reynolds numbers as low as 10−2. This stability enhancement leads to droplet sizes well below those previously obtained with flow focusing. In this work, we experimentally determine the influence on the jet's radius of the liquid flow rate and viscosity, as well as the focusing orifice. We analyze the instability mechanisms that impose the minimum values of the issued flow rate and jet's size. Finally, the jet's breakup and the size of the resulting micron-sized droplets are examined from high-speed optical imaging.

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