Abstract: The invention relates to a microfluidic system comprising a microfluidic device having a first and a second opposite surfaces and an optical detector, the microfluidic device comprises a flow channel with a detection channel section having a length of at least about 1 mm, the microfluidic device comprises at least one aperture section comprising at least a part of said detection channel section and a transparent window into said detection channel section, the optical detector is arranged to be in optical communication with said aperture section to determining at least one optical property of said aperture section as a function of time. The flow channel may have capillary dimensions and/or it may wholly or fully be arranged to drive a fluid flow by applying external forces.