Recently it was found that at moderate Reynolds numbers, Re > 1, the displacement behavior of suspended particles in deterministic ratchets changes with increasing flow rates, resulting in superior separation at higher flow rates. This change was attributed to the inertial flow regime where the flow pattern strongly depends on Re. The objective of this study was to visualize the fluid streamlines of inertial flow in deterministic ratchets, using 2D flow field simulations and high speed camera images. Experimental investigation and numerical simulation of the flow field in two deterministic ratchet designs provided more insight in the improved separation efficiency. The most remarkable observation was the formation of vortices, which increased in size at higher flow rates. For cylindrical-shaped obstacles a pair of vortices developed at Re = 9 and grew to 70% of the gap size between two consecutive obstacle rows at Re = 30. For quadrilateral-shaped obstacles vortices developed at Re = 2 and grew to 75% at Re = 26. The expected effect of vortex formation on particle movement is preventing the 'zigzag' motion of particles by narrowing the gap between two consecutive obstacle rows. Another observation is the increase in the number of flow lanes at higher Re. This is explained by the asymmetry of the fluid velocity profile in the gaps, caused by the anisotropy of the obstacle arrays in combination with the development of vortices. The increase in number of flow lanes reduced the critical particle size for displacement by 14% for the design with cylindrical-shaped obstacles and 24% for the design with quadrilateral-shaped obstacles. This gives opportunities for new designs with larger gap sizes, which reduces the chance for clogging and lowers the pressure drop. The observations underline that anisotropy or asymmetry of the obstacles and the flow regime are both important parameters for the separation efficiency of deterministic ratchets. (c) 2013 Elsevier B.V. All rights reserved.