Bridging Volume Levels in Droplet-based Microfluidics – Automated Electro-coalescence in Series of Droplet Sets
in: Sensors and Actuators B-Chemical (2017)
Digital microfluidics make use of a wide range of different droplet volumes. Droplet volumes in the range from pL to µL are applied in two different principles: emulsion based droplet processing and channel confined droplet processing (segmented flow). To bridge the gap between these different technologies, droplet transfer devices for volumetric operations are required. Hence, the defined dilution of an initial droplet by volume addition to a larger droplet is a quite essential process. Here, we describe a polymeric microfluidic device which enables the volumetric transfer of droplets of about 40 nL to variable volumes of 500 to about 2500 nL by two different fluidic principles: a) the unification of one or more small droplets with an in-situ generated larger droplet and b) the coalescence of a defined number of small droplets to a unified larger droplet. The coalescence of surfactant stabilized droplets was facilitated by application of the electro-coalescence principle. Therefore, the device was equipped with surface mounted electrodes. The fluidic operation unit works in a self-controlled volumetric way and releases droplets of about 500 nL volume, even if no small droplets are added by coalescence. The system was designed for repetitive cyclic operations and is a part of a complex microfluidic network for bacterial screening purposes. The layout was chosen on the basis of microfluidic simulations beforehand. Materials, layouts and processing steps were adapted to the polycarbonate based digital-video-disc (DVD) fabrication-technology in order to use this efficient technique for a high-quality mass production of the device. The experimental investigation confirmed a high tolerance of the developed device against the variation of key parameters of the aqueous phase like salt content, pH value and fluid viscosity. The control of the electro-coalescence pulses enable two different operation modes: a) permanent pulsing facilitates the immediate coalescence of neighboring droplets inside the chamber and b) controlled pulsing enables the defined coalescence of a defined number of droplets which were collected in the chamber before. By application of the second mode the automated coalescence of about 12 droplets of about 40 nL volume produced a highly ordered output sequence with individual droplet volumes of about 500 nL volume.