Impalement of fakir drops

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By Sung Hoon Kang

Title: Impalement of fakir drops

Reference: M. Reyssat, J. M. Yeomans, and D. Quere, Europhys. Lett. 81, 26006 (2008).

Soft matter keywords

hydrophobicity, fakir state, impalement, surface energy, contact angle

Abstract from the original paper

Water drops deposited on hydrophobic materials decorated with dilute micro-posts generally form pearls. Owing to the hydrophobicity of the material, the drop sits on the top of the posts. However, this “fakir state” is often metastable: if the drop impales inside the texture, its surface energy is lowered. Here we discuss the transition between these two states, considering the drop size as a parameter for inducing this transition: remarkably, it is found that a drop impales when it becomes small, which is interpreted by considering its curvature. This interpretation allows us to propose different recipes for avoiding this detrimental effect.

Soft matter example

In this paper, the authors used hydrophobic surface with regular arrays of micropillars for studying the transition between "fakir state" and "impaled state" as a function of the drop size. They fabricated samples with controlled geometry and coated with a fluropolymer. The contact angle for water on a flat surface was between 100-110° while it was ~160° for a surface with texture. Figure 1 shows the images of a water drop evaporating on a sample with the micropillars of diameter = 3um, height = 4.8 um and mutual distance of 17 um.

Fig. 1. Evaporation of a water drop sitting on a hydrophobic surface decorated with pillars (visible in the photos) of diameter d = 3 μm, height h = 4.8 μm and distance l = 17 μm. The initial radius is 120 μm, and the time interval between two successive photos is 5 s. As the drop radius reaches 75 μm (fifth snapshot), the state of the drop abruptly changes, from a super-hydrophobic state to a hydrophilic-like one.