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Hysteresis measurements on Super-hydrophobic Surfaces with Periodic Textures

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J. Teisseire - E. Barthel

Surface textures are used to impart advanced wetting properties (such as superhydrophobicity) to surfaces. However understanding the surface response in relation to the nature of the texture is still a challenge. Here we have measured advancing and receding contact angles on model hydrophobic surfaces with cylindrical pillars as a function of the pillar spacing (figure 1).

Figure 1: SEM view of a surface textured by silica sol-gel nanoimprint lithography. Here the pillar spacing is equals to 10 μm. Various surfaces were generated where the pillar geometry is kept constant and the spacings vary from 10 to 50 μm.

The advancing and receding contact angle measured on super-hydrophobic surfaces not always imply small hysteresis. For example we have observed on our surfaces that advancing contact angles remain constant for all geometry whereas the receding contact angles increase with the distance between pillars. Similar results have been already observed in literature and different models are proposed to explain such evolution.

Figure 2: Measured advancing (right triangle) and receding (left triangle) normalized work of adhesion w/= 1+cosθ as a function of pillar spacing. Error bars are mostly smaller than symbol size. A typical Cassie model (small dash) cannot account for the results. The pinning model (dash) simultaneously accounts for advancing and receding contact angles.

We have developed a model based on the local elastic instabilities of the triple line [1] which jumping on and off strong pinning site gives a good explanation of these evolutions. For an advancing triple line, the elastic energy of the line when it jumps to the next pinning site is controlled by the pillar spacing. As a result the advancing contact angle is nearly constant because the increasing elastic energy approximately cancels the decreasing number density. In contrast, the receding contact angle is primarily controlled by the pillar size because during the instability the triple line is peeled off the top of the pillar. Hence the receding contact angle decreases roughly as the density. Figure 2 shows a good correlation between experimental results (triangles) and pining model (dash).

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