Non-contact optical sensor to detect free flying droplets in the nanolitre range

This paper reports on a non-contact optical sensor for the detection of single droplets in flight. The sensor allows for online process control of non-contact dispensing systems delivering droplets in the nanolitre range. A dispensed liquid droplet, which passes through the optical transducer, leads to a change of the light intensity caused by absorption, reflection and diffraction. The change in light intensity measured by a photo transistor provides information about, e.g. droplet size, velocity and shape. This information is encoded in the time dependent signal shape that yields a characteristic “fingerprint”-signal for each droplet. The fabrication of the sensor is achieved by standard printing circuit board (PCB) technology. Therefore, an easy adaption of the sensor to different kinds of contactless dispensing system and furthermore a very cost efficient production is granted.