Abstract: A tunable inertial sheathing (TIS) system and methods for particle-size-insensitive high-throughput single-stream focusing of particles suspended in a particle-carrying fluid are provided. The TIS conditions particles to distribute locally within one of compartments of inertial force field, followed by an inertial focusing to migrate it to a single foci. For the particle localization, the TIS system introduces an arbitrary form of peripheral sheathing by generating and accumulating sheath fluid from particle-carrying fluid through a combination of inertial focusing, channel bifurcation and channel confluence. Multiple forms of the TIS system are also provided, each including one main channel and at least one bypass channel. The main channel includes and cascades at least three segments, at least one bifurcating junction and at least one confluence junction.