Suspension separation with deterministic ratchets at moderate Reynolds numbers

Deterministic ratchets were evaluated in this study as continuous separation devices for suspensions. Compared to conventional microfluidic ratchets (gap width approximate to 10 mu m), ratchet designs in this study were 70 times larger. Apart from the hydrodynamic regime (2 < Re < 34 versus Re < 1) no other changes were introduced. Three deterministic ratchet designs were constructed and evaluated for their separation efficiency with polystyrene particles having sizes between 309 and 532 mu m.The separation efficiency was defined as the ratio between the highest and the lowest outlet concentration. The separation efficiency increased with increasing flow rate from a ratio of 1 (Re = 2) at the lowest flow rates up to a ratio of 47 at the highest flow rates (Re = 18). An explanation for this strong dependence on flow rate may be the presence of inertial lift forces and a pair of vortices behind the obstacles (typically for Re > 1), which lead to additional displacement. Furthermore, there was a maximum concentration of particles in the device above which the separation decreased, e.g., 12 v/v% for a design with cylindrical-shaped obstacles. The high suspension concentrations did not lead to blockage of the device and were much higher compared to concentrations normally achieved for membrane separations. The up-scaled deterministic ratchets show potential for fractionation. Particles above a critical particle size were better separated than particles below that critical particle size. (C) 2012 Elsevier Ltd. All rights reserved.