(a) Inlet manifold simulation velocity field (in m/s) at mid-plane. The width of the manifold is 9.3 cm. (b) A photo of the stacked system. The stack consists of 20 rectangular toroidal channels (500 μm high, 1:6 aspect ratio). The manifold described in Fig. 5-a is inserted at the centre of the stack (white block) to ensure an homogenous fluid distribution between the different layers. Two aluminium box sections are sealed at the top of the system to collect the liquid coming out from the focused and unfocused outlets respectively, with gravity equalising the back pressure at the outlets. Two populations of particles (as described in Table S6) are diluted in 7.05 L of tap water and processed through the stack at 1 L/min over 1.7 recirculations. (c) Corresponding particle size distributions at the inlet (purple curve), focused (red curve) and unfocused (green curve) outlets.