Tunable capillary-induced attraction between vertical cylinders

TL;DR

This study investigates fluid-mediated attractive forces between vertical cylinders at a fluid interface, demonstrating tunable cohesion based on cylinder height and separation, with experimental, numerical, and theoretical analysis.

Contribution

It introduces a model for tunable capillary forces between vertical cylinders, validated by experiments and simulations, revealing the dominant force regimes and control parameters.

Findings

Pressure dominates at small separations.

Surface tension dominates at large separations.

Force can be tuned by adjusting cylinder height and separation.

Abstract

Deformation of a fluid interface caused by the presence of objects at the interface can lead to large lateral forces between the objects. We explore these fluid-mediated attractive force between partially submerged vertical cylinders. Forces are experimentally measured by slowly separating cylinder pairs and cylinder triplets after capillary rise is initially established for cylinders in contact. For cylinder pairs, numerical computations and a theoretical model are found to be in good agreement with measurements. The model provides insight into the relative importance of the contributions to the total force. For small separations, the pressure term dominates, while at large separations, surface tension becomes more important. A cross-over between the two regimes occurs at a separation of around half of a capillary length. The experimentally measured forces between cylinder triplets are also in good agreement with numerical computations, and we show that pair-wise contributions account for nearly all of the attractive force between triplets. For cylinders with equilibrium capillary rise height greater than the height of the cylinder, we find that the attractive force depends on the height of the cylinders above the submersion level, which provides a means to create precisely-controlled tunable cohesive forces between objects deforming a fluid interface.