Droplet bistability and its application to droplet control

Novel methods for controlling droplets precisely in the microchannels are presented, which employ microfluidic bifurcation channels with outlet restrictions based on droplet bistability, utilizing the Laplace pressure caused by interfacial tension that arises when a droplet encounters a narrow restriction. The bistable geometry possesses two symmetric branches and restrictions that operate as capillary valves allowing a droplet to be trapped in front of a restriction and released through it when the next droplet arrives at the other restriction. This trap-and-release occurs repeatedly and regularly by the succeeding droplets. Furthermore, a critical flow rate is found to exist, which is necessary for achieving droplet bistability. This occurs only when the apparent Laplace pressure surpasses the pressure drop across the droplet. By adopting a simplified hydrodynamic resistance model, the droplet bistable mechanism is clearly explained, and droplet bistability is shown to enable the simple and precise control of droplets at a bifurcation channel. Thus, precise droplet traffic control is achieved at a bifurcation channel connected with a single inlet channel and two outlet channels using an appropriate channel design that induces droplet bistability. In particular, droplets are distributed at a junction in a manner of perfect alternation or perfect switching between the two outlet channels. This article proposes that bistable components can be used as elaborately embedded droplet traffic signals for red light (trap), green light (release), and turn light (switching) in complex microfluidic devices, where droplets provide both the chemical or biological materials and the processing signal.