Liquid-liquid droplet reactors have garnered significant interest in biochemical applications with the obvious benefits of reduced reagent consumption, well controlled droplet size and confinement of biochemical reactions away from external interference. This Tutorial Review provides a succinct overview of widely employed liquid-liquid droplet reactors, namely single emulsions, multiple emulsions and all-aqueous emulsions, under the scope of thermodynamics, with a particular emphasis on how their intrinsic interfacial properties may endow mass transport for a variety of demands. Beyond spatially compartmentalizing a thermodynamic system, the artificial interface of droplet reactors has shown initial promising for multi-step or complex reactions. Moving forward, the artificial interface shall be tailored further towards "functional" to imitate the "intelligent" interface surrounding natural vesicles or cells.