Piercing an interface with a brush: collaborative stiffening

TL;DR

This paper investigates how flexible hairs or micro-structures can resist capillary forces during immersion and evaporation, revealing mechanisms of self-assembly and buckling that enhance their stability at interfaces.

Contribution

It introduces the concept of spontaneous hair bundling as a means to resist capillary buckling, providing new insights into micro-structure stability at liquid interfaces.

Findings

Hairs form self-assembled bundles under capillary forces.

Bundling enhances resistance to buckling and collapse.

Patterns resemble those in micro-structured surfaces like carbon nanotubes.

Abstract

The hairs of a painting brush withdrawn from a wetting liquid self-assemble into clumps whose sizes rely on a balance between liquid surface tension and hairs bending rigidity. Here we study the situation of an immersed carpet in an evaporating liquid bath : the free extremities of the hairs are forced to pierce the liquid interface. The compressive capillary force on the tip of flexible hairs leads to buckling and collapse. However we find that the spontaneous association of hairs into stronger bundles may allow them to resist capillary buckling. We explore in detail the different structures obtained and compare them with similar patterns observed in micro-structured surfaces such as carbon nanotubes "forests".