It was noted, that on top of volume of the exchanged liquid discussed above, also orientation of the cavity versus the flow direction could potentially be a factor influencing the effectiveness of the exchange. Thus, we have performed a set of numerical analysis aiming to identify the impact of the cavity placement in th e microfluidic system. The results are shown in Fig. and S3 of the Supplementary Information and indicate negligible influence of the cavity orientation vs direction of the flow on the exchange of liquid within the timescale of the experiments. The flow rate has a large influence on the final concentration in the channel. For flow rates above 100 µl/min the ultimate concentration is less than the one in the sample loop (Fig. ), therefore experiments were performed using this flow rate to limit the time needed for the measurements. Next, oxidized and reduced forms of a common redox probe in KCl, as well as KCl as a base electrolyte, were used as liquids flushing the cavity in the microsystem. It can be seen in Fig. (A) that when any of the probes is introduced to the supporting electrolyte, the wavelength corresponding to the transmission minimum decreases, but it decreases more (by about 2 nm) when K 4 Fe(CN) 6 is applied. K 4 Fe(CN) 6 has a higher RI than the one of K3Fe(CN)6. A slight increase in the drop with exchange sessions can be seen for K4Fe(CN)6. The exchange between the two forms shown in Fig. (B) correspond well with these results and here also wavelength shift reaching about 2 nm can be noticed.