Some lab-on-a-chip applications require to establish a controlled spatial concentration gradient of (chemical) species, for example for iso-electrical focusing or to study chemotactic properties of cells. We show that covering a microchannel floor with special grooves or ridges, well-controlled concentration gradients can be created, depending on the geometrical design of the grooves or ridges. In our case, the pattern consists of ridges that are slanted with respect to the main channel direction. Similar patterns have been applied in the past to achieve mixing by introducing chaotic advection. We present experimental and numerical results that prove the mixing effectiveness of the ridges. In addition, making use of the local mixing capabilities of the ridge patterns, we show, using numerical simulations, how to achieve a concentration gradient across a microfluidic channel.