Self-swallowing Droplets

Microdroplets attract huge interest as it provides prototype for many industrial and biological systems such as emulsions, porous media, and blood-flow. Specifically, controlling droplets shape is crucial for both fundamental and practical values. Here, we report a novel self-swallowing droplet behavior in a microfluidic channel. It arises when water droplet moves through a partially miscible oil containing large amount of colloidal particles. The rapid mass transfer causes reduction of interfacial area, which leads to particles jamming and phase transition. As a result, an elastic film is generated and soon undergoes a buckling instability in response to the compressive stress due to volume shrinkage of the droplets. Therefore, a re-entrant cavity forms at the rear interface through which a jet of the outer fluid penetrates the droplet. The degree of the swallowing behavior and thus the shape of the droplets could be well adjusted by changing the experimental parameters, and the whole swallowing process could be reversibly regulated. This work not only provides indications for intriguing fundamental questions concerning hydrodynamic and interfacial effects, but also inspires technical innovations including in microfluidics, printing, soft materials processing, etc.