Towards Droplet on Demand for Microfluidic Networks

Two-phase flow microfluidics, where the droplets flow in closed microchannels, is a promising platform for the realization of Labs-on-Chips (LoCs). In order to achieve specific functionality of LoC, it is crucial to be able to control the droplets’ path throughout the microfluidic chip. Two approaches have been developed to guide the droplets towards selective locations on the chip: Active Droplet Control (ADC) and Passive Droplet Control (PDC). ADC uses integrated microvalves or externally applies electric forces, magnetic fields or surface acoustic waves. These means of manipulation can provide precise droplet control, but suffer from complex and costly fabrication as well as biocompatibility for some biological settings [1]. In contrast, PDC exploits only hydrodynamic principles to control the path of the droplet, and, thus, can be fabricated at low cost and offer a higher degree of chemical compatibility. For these systems, channel geometries and applied hydrodynamic forces are critical design parameters. Recently, microfluidic networks for two-phase flow microfluidics (cf. [2], [3] and references therein), as well as corresponding design methods [4]–[6] have been introduced, which aims at realizing programmable and flexible LoC devices using PDC. In particular, microfluidic networking targets to dynamically assign the droplets’ path through a microfluidic network in order to perform specific analyses. The key element in microfluidic networks are microfluidic switches, which are able to control the path of a single or multiple droplets, based on the distance between the individual droplets [2], [3]. However, the current microfluidic network research only includes little experimental work [7], [8], considering only single switches and trains of droplets with a given volume and distance. In order to fully exploit the potential of microfluidic networks it is crucial to create individual droplets of desired volumes (microliter to nanoliter in volume) at a prescribed time and with precisely controlled droplet distances – motivating Droplet on Demand (DoD) solutions for microfluidic networks. Most of the reported DoD systems use integrated microvalves to control the flow of the fluids required for the droplet generation (e.g., [9]). However, these systems require complex fabrication and may introduce limitations in chemical compatibility. Only a few DoD systems externally controlling the fluids flow are proposed [10], [11]. In [10] a microfluidic flow-focusing geometry with a negative pressure induced at its outlet is used. However, this approach is not applicable for microfluidic networks, since the negative pressure would affect the functionality of the network. In [11], the flow of the continuous and dispersed phase in a T-junction is controlled through two external valves that are mutually switched on and off. In this work we propose an alternative DoD system, which only needs to control the flow of the dispersed phase while keeping the flow of the continuous phase undisturbed1. We derive a mathematical model for the equilibrium state, which is an important step towards the practical realization of the proposed DoD system – a crucial accomplishment in order to exploit the potential of microfluidic networks for practical purposes.