Predator-Prey Behavior of Droplets Propelling Through Self-Generated Channels in Crystalline Surfactant Layers.

Motile droplets provide an attractive platform for liquid matter-based applications and protocell analogues displaying life-like features. The functionality of collectively operating droplets increases by the advance of well-designed (physico)chemical systems directing droplet-droplet interactions. Here, we report a strategy based on crystalline surfactant layers at air/water interfaces, which sustain the propulsion of floating droplets and at the same time shape the paths for other droplets attracted by them. First, we show how decylamine forms a closed, crystalline layer that remains at the air/water interface. Second, we demonstrate how aldehyde-based oil droplets react to decylamine in the crystalline layer by forming an imine, causing the droplets to move through the layer while leaving behind an open channel (comparable to "Pac-Man"). Third, we introduce tri(ethylene glycol) monododecylether (CE) droplets in the crystalline layer. The crystalline layer suppresses the motion of the CE droplets, however, the aldehyde droplets create surface tension gradients upon depletion of surfactants from the air/water interface, thereby driving Marangoni flows that attract the CE droplets as well as the myelin filaments they grow: Causing the CE droplets to chase, and ultimately catch, the aldehyde droplets along the channels they have created, featuring a predator-prey analogy established at an air/water interface.