This paper presents novel methods for precisely controlling water droplets by use of a microfluidic bifurcation channel with outlet restrictions, based on droplet bistability which utilizes the Laplace pressure due to interfacial tension arising when a droplet encounters a narrow restriction. We implement droplet bistable geometry, which has two symmetric branches and restrictions, to operate as capillary valves, so that a droplet can be trapped in front of a restriction and released through it when the next droplet arrives at the other restriction. It is observed that this trap-and-release occurs repeatedly and regularly by the succeeding droplets. It is also found that there is a critical flow rate to achieve droplet bistability which occurs only when the apparent Laplace pressure surpasses the pressure drop across the droplet. By adopting a simplified hydrodynamic resistance model, droplet bistable mechanism is clearly explained. Droplet bistability enables simple and precise control of droplets at a bifurcation channel. Thus, by an appropriate channel design to induce droplet bistability, precise control of droplet traffic is achieved at a bifurcation channel connected with a single inlet channel and two outlet channels. In particular, we are able to distribute droplets at a junction in a manner of perfect alternation between the two outlet channels. Bistable components can be used as an elaborately embedded droplet traffic signal for red light (trap) and green light (release) in complex microfluidic devices, where droplets provide both the chemical or biological materials and the processing signal.

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