Passive removal of immiscible spacers from segmented flows in a microfluidic probe

Microfluidic probes (MFPs) are a class of non-contact, scanning microfluidic devices that hydrodynamically confine nanoliter volumes of a processing liquid on a surface immersed in another liquid. So far only chemical processes using a single processing liquid have been implemented using MFPs. In this letter, we present the design and implementation of a probe head that allows segmented two-phase flows to be used, which will enable different chemical species to be sequentially delivered to a surface in defined volumes and concentrations. Central to this probe head is a spacer-removal module comprising blocking pillars in the injection channel, a bypass and an orifice leading to the aspiration channel. We present a capillarity-based analytical model that provides insight into the functionality of the module based on geometrical parameters. In addition, we study the difference between two- and three-channel modules and predict a 30% reduction in fluctuation of the footprint of the confined liquid for the th...