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 -The hairs of a painting brush withdrawn from a wetting liquid self-assemble into clumps whose sizes rely on a balance between liquid surface tension and hairs bending rigidity.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".Introduction. -Everyday's life experience teaches us that wet hairs assemble into bundles. This phenomenon is however amplified at the scale of Micro-Electro-MechanicalSystems (MEMS) since surface forces tend to dominate over bulk forces when the scale is reduced. Indeed, if L is the typical size of a structure, surface forces are proportional to L, while elastic or gravity forces scale as L 2 and L 3 , respectively. Controlling 'stiction' is then a challenging issue in micro-engineering technologies since it often leads to the fatal collapse of microstructures [6][7][8]. Nevertheless, the self-assembly of micro-structures through capillary forces can also be viewed as a useful tool to build complex shapes [1][2][3][4][5]. Beyond engineering applications, surface forces may also have a strong effect on living structures. For instance, filamentous fungi living in aqueous environment have difficulty in growing their hypha through the water interface into the air. Indeed some species have to produce surfactant molecules that reduce capillary forces in order to develop the aerial structures necessary for dissemination [24].In the case of slender structures, the interaction between elasticity and interfacial forces can be defined by a typical elastocapillary length scale, L EC = B/γ ∼ Eh 3 /γ, where E is the Young modulus of the material, h and B are the thickness and the bending stiffness per unit width of the structure, respectively, and γ the liquid surface tension or the solid adhesion energy [25][26][27][28][29]. The validity of this macroscopic length scale has recently been confirmed at the scale of graphene sheets through atomistic simulations [30].In this paper we study the case of a carpet-like structure immersed in a drying liq-

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