In this work, two investigation methods are demonstrated and compared of a wash-out phenomenon in a microfluidic structure. In the beginning of my thesis work a review of the current technologies is presented and two of those were selected to further investigation and implementation. The goal of my investigations was detecting the accurate time characteristics of the mixing process in a widening channel (in this case, in a cuvette). The wash-out process was observed by a CCD camera, attached to an optical microscope (in one case) and by an optical waveguide sensor system (in the second case). Possible measurement setups are introduced and investigated in details highlighting the advances in mixing detection and showing the difficulties concerning the unwanted diffusion in the actuators. The process of the optical detection was demonstrated through two functioning measurement arrangements. In the end I compare the two methods, present the revealed problems, summarize the experiences and make proposals for further work.