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 its 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 shows how a system's sensitivity to the phase-change-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.

空气-水界面处的疏水性颗粒（如胡椒）可作为示踪剂颗粒，以显示由局部表面活性剂添加引起的 Marangoni 效应。粒子的轻堆积对于它们独立和忠实地代表这种径向定向的界面质量流是必要的。相反，过量添加粒子会导致它们的相互作用并导致其外观发生巨大变化。事实上，粒子层中典型的圆形开口可以变成星状区域。我们通过光学可视化不同包装条件下形状的演变来研究这两种制度之间的转变。通过定义形状因子，我们量化了从圆形到星形的过渡。我们发现星区的开始与疏水颗粒的密集堆积相吻合。所呈现的工作是对两种制度之间过渡的简单但彻底的演示。它显示了系统对相变诱导参数（此处为胡椒密度）的敏感性如何呈现局部最大值。这与传感应用高度相关，我们相信积极思考“传感转换可以激发基于非典型转换感应灵敏度的新型传感器的开发，例如此处介绍的那种。