Marangoni-Induced Reversal of Meniscus-Climbing Microdroplets

TL;DR

This study reveals a size-dependent reversal of droplet movement on an oil meniscus driven by Marangoni convection, influenced by temperature and geometry, enabling new droplet manipulation techniques.

Contribution

The paper introduces a novel size-dependent reversal of droplet motion on a meniscus caused by Marangoni convection, with a predictive modified Marangoni number.

Findings

Larger droplets ascend the meniscus due to capillary forces.

Smaller droplets are carried downward by surface flow.

A modified Marangoni number predicts convection strength.

Abstract

Small water droplets or particles located at an oil meniscus typically climb the meniscus due to unbalanced capillary forces. Here, we introduce a size-dependent reversal of this meniscus-climbing behavior, where upon cooling of the underlying substrate, droplets of different sizes concurrently ascend and descend the meniscus. We show that microscopic Marangoni convection cells within the oil meniscus are responsible for this phenomenon. While dynamics of relatively larger water microdroplets are still dominated by unbalanced capillary forces and hence ascend the meniscus, smaller droplets are carried by the surface flow and consequently descend the meniscus. We further demonstrate that the magnitude and direction of the convection cells depend on the meniscus geometry and the substrate temperature and introduce a modified Marangoni number that well predicts their strength. Our findings provide a new approach to manipulating droplets on a liquid meniscus that could have applications in material self-assembly, biological sensitive sensing and testing, or phase change heat transfer.