Marangoni-induced pepper-patterns: Transition from circle to star shape

Hydrophobic particles (such as pepper) at an air-water interface can act as tracer particles to visualize the Marangoni effect caused by local surfactant addition. Light packing of the particles is necessary for them to be independent and faithfully represent this radially directed interfacial mass flow. Contrary, excessive addition of particles can result in their interaction and cause a dramatic change to the appearance. In fact, the typical circular opening in the particle layer can change to a star-like region. We investigate the transition between these two regimes by optically visualizing the evolution of the shape under different packing conditions. By defining a shape factor, we quantify the transition from a circle to a star shape. We find that the onset of the star regime coincides with the dense packing of the hydrophobic particles. The presented work is a simple, but thorough, demonstration of the transition between two regimes. It also shows how a system's sensitivity to the phasechange-inducing parameter (here, the pepper density) exhibits a local maximum. This is highly relevant for sensing applications and we believe that actively thinking about 'transitions for sensing' can inspire the development of novel sensors based on atypical transition-induced sensitivity such as the one presented here.