Abstract: A system and a method are described for separating particles suspended in a fluid stream according to their weight, size or density, based on their viscoelastic interaction with a suspending fluid. The system may include a particle manipulation stage for manipulating the particles in the sample fluid, an input channel for inputting a sample fluid containing target particles and non-target material, a viscoelastic region that separates particles according to their viscoelastic behavior in the sample fluid upstream of the particle manipulation stage and at least one pickoff channel that removes a subset of the separated particles.