Microfluidic encoding: Generation of arbitrary droplet sequences by electrical switching in microchannels

Switching of aqueous nanoliter fluid segments in a branched micro channel filled with tetradecane was realized using a non-galvanic electrical actuation. Therefore, copper electrodes were integrated into moulded polycarbonate chips. Precondition for the droplet switch analysis was the online imaging of the droplet motion during the change of the electric field. Reproducible switching operations were realized by the help of an image based trigger. In this way the direction of the segment transport could be chosen with very high reliability. This gives the possibility for the generation of an arbitrary chosen segment pattern in an outlet channel. It was possible to generate error-free segment sequences with thousands of segments and to use this technique for microfluidic encoding. The motion of a droplet could neither be initiated by a constant electric field nor by switching an ac field. Model experiments demonstrate that the motion of droplets is initiated only by switching a static electrical field, not by the field itself. This observation supports the assumption that the fast field-induced stretching of a droplet followed by a slower shape relaxation is responsible for the switching effect.