Hydrodynamic on-rail droplet pass filter for fully passive sorting of droplet-phase samples

A hydrodynamic droplet pass filter for droplet-phase sample sorting was developed in this study. Using only groove rails, without additional components or external controls, droplets were sorted based on their physical properties. This is the first report of a droplet pass filter used for effective sorting, and the sorting structure provides a novel fluidic component for fluidic circuits for many applications. Depending on the number of rails, we obtained high-pass, low-pass, band-pass, and multi band-pass filters for sorting droplet samples, and their filtration performance was controlled by varying the dimensions of the rail structures. We evaluated in detail the effect of the rail width on sorting, threshold size of droplets sorted into each rail, and capillary number-dependent sample sorting. Furthermore, band-pass droplet sorting, useful for sample quantification, was provided, and multi-step rail ways allowed multi band-pass droplet sorting that was independent of flow conditions. The proposed sorting method does not require any external systems or skilled operation, and thus, it is expected that the device can contribute to on-site sample treatment and analysis in various fields such as medical care or the military.