A new method to monitor the relative flow rates of two different liquids in a microchannel is presented. By using a simple out-of-plane interferometer setup, shifts in the position of the interface between the two liquids can be determined. This label-free and real-time approach to observe the interface position yields the ratio of the flow rates. Applying an out-of-plane geometry allows the use of a flexible setup of chip-external optical components such as a laser and an image sensor, which is an advantage over fixed in-plane optics. With this measurement method we are able to monitor a change of about 1% of the relative flow rate between two fluidic inlets.
