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

Nikolaj K Mandsberg,Stefania Boi,Daniel A Bunckenburg,Ming Zhang,Laura Pastorino,Anja Boisen

doi:10.1016/j.surfin.2021.101443

Nikolaj K Mandsberg, Stefania Boi + Show 4 more

Open Access

https://doi.org/10.1016/j.surfin.2021.101443

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Abstract

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 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.

Highlights

Whenever multiple phases are accessible, there exists a transi tion between them; either a stepwise or a smooth one
Hydrophobic particles at an air-water interface can act as tracer particles to visualize the Marangoni effect caused by local surfactant addition
We find that the onset of the star regime coincides with the dense packing of the hydrophobic particles

Summary

Introduction

Whenever multiple phases are accessible, there exists a transi tion between them; either a stepwise or a smooth one. Investigating the transition gives information about the phases, their differences, and how to traverse from one to the other. We seek to explore a smooth transition related to the Marangoni effect. The Marangoni effect is observed when a gradient in interfacial tension induces a mass transfer along the interface [1]. A simple demonstration of this is done by sprinkling pepper flakes on a plate with water and subsequently adding soap (Fig. 1a). The pepper acts as tracer particles that are transported with the moving liquid to visualize this flow. This region is circular (Fig. 1b-top) due to the force being radial and the negligible interaction between tracer particles

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