Figure 3: Propagation of a step profile. The concentration profiles along the midline of the left channel is shown, shortly after opening the valve the first time. The top row shows simulations for a Péclet number Pe ≈ 4100, and the bottom row for Pe ≈ 800 after decreasing the fluid speed to Uphys = 1000[µm/s]. In the second column, the temporal resolution is tenfold increased as compared to the first columns. The chosen parameters are summarized in Table 3a For low temporal resolution and high Pe number, high numerical instabilities are triggered, which do not decrease as time passes. (b) Increased temporal resolution does not mitigate the instabilities. (c) At a five-fold lower Pe number, numerical instabilities can be initially found close to the high gradient, but they diminish as the step profile propagates and smoothens. (d) Higher temporal resolution does not lead to significantly different results.