Experimental and numerical investigation of droplet transport in a diffuser/nozzle structure

This paper investigates the droplet behavior in serially connected diffuser/nozzle structures using experimental and simulation methods. The serial structures act as diffusers in the expanding flow direction, and as nozzles in the converging flow direction. The opening angle of the structures is 45 degree. The test devices were fabricated in polydimethylsiloxane (PDMS) using soft lithography technique. The droplets under investigation were formed using T-junctions. Mineral oil with 2%w/w surfactant Span 80 and de-ionized (DI) water with fluorescent dye work as the carrier phase and the dispersed phase, respectively. The droplet behavior was captured using a CCD camera. The corresponding pressure drops across the test section were measured in both diffuser and nozzle flow direction using an external pressure sensor connected to the pressure port integrated on the device. The pressure drops across the test structure of two cases, with and without droplet, were analyzed and compared. The experimental results showed a linear relationship between the pressure drop and the oil flow rate. Furthermore, rectification effect was observed in the test device. The pressure drop in the diffuser configuration is higher than that of the nozzle configuration. Numerical models were employed to study the dynamics of the pressure drop across the diffuser/nozzle structures as well as the deformation of the droplet, which is modelled using level-set method. Corresponding to the experiments, two cases with and without were investigated and compared.