Abstract: A method and device for separating particles in a fluid according to size is disclosed. The method includes the step of generating at least one recirculating flow by transporting a fluid containing the particles across a profiled surface carrying at least two adjacent regions of different depth which form a surface level step, where the fluid is transported by mechanically moving a flat first surface over the profiled surface. The adjacent regions of different depth are arranged such that the depth of the regions decreases in the net direction of a forward displacement of the first surface. A normal load is applied to at least one surface. The particles are allowed to separate by at least one recirculating flow generated by moving the first surface past the profiled surface.