Liquid bridges: A novel approach for dispensing biofluids, characterisation and correlations

Control of fluids at the microscale represents an important point of interest in the widely studied field of Microfluidics. In fact, most of the biological and medical research undergone would benefit from Microfluidic solutions. One of the engineering challenges brought about by this technologic evolution involves the dispensing of fluids at these scales. The sludy presented in this paper concerns the development of a novel dispenser of biofluids, which would find its first application in the measurement of multi-gene expression levels as part of cancer diagnosis. The studied geometry is termed "two-way liquid bridge" and consists of injecting a continuous fluid to be segmented via an inlet PFE tubing in a microgravity environment until an isothermal mass of liquid is held by surface tension between the inlet and outlet tubings, parallel and opposite. Due to constant pressurisation of the microgravity environment, this mass eventually ruptures delivering a segmented volume of biofluids on which an analysis such as PCR can be performed. Experimental investigations were conducted in a backlighted transparent PMMA device in which fluids were injected using Harvard Apparatus syringe pumps. A CMOS colour camera recorded the images which were automatically analysed using a Canny edge detection algorithm. A dimensional analysis was conducted highlighting the main dimensionless groups for a complete understanding of the occurring phenomena. Experimental observations showed good repeatability and consistency in the dispensing process. It was also shown that fluids flowrates, tubings sizes and length of separation between inlet and outlet tubings have a direct impact on the size and frequency of the produced droplets. The present paper addresses the complete characterisation of the geometry as well as the establishment of correlations in order to provide a useful engineering design tool.