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Dynamics of the liquid contact line on a superhydrophobic surface and its impact on the adhesion

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J. Teisseire- E. Barthel

The context

A solid surface is considered superhydrophobic if the contact angle of a water drop sitting on the surface is larger than 150°. It is commonly identified by the fact that a small droplet lying on it takes an almost spherical shape. This superhydrophobicity is intimately related to the micro- or nano-roughness of the surface. Indeed, the liquid sits on top of the asperities and hardly touches the surface at all. However, to wipe out the water from the surface one has to move the edge of the droplet, the so-called triple line, on the surface. This line interacts with the surface asperities, deforms and sometimes stays pinned. As a result, the force needed to remove the non-wetting droplet may be significant.

Nature offers beautiful examples of both slippery - the lotus leaf - and sticking - the red rose’s petal
- superhydrophobic surfaces. For technological applications such as self-cleaning surfaces, a strong pinning of the line is an undesirable behavior.

Our work

We have realized experiments on specially designed glass surfaces. We monitored the displacement of the contact line during the evaporation or the retraction of the drop, and we discovered that the contact line does not move as a whole: it is in fact a kink-like distortion of the line which controls the depinning.

The consequences of these observations are significant: this mechanism lowers the threshold for the depinning of the contact line, and so the adhesion of the drop to the surface. This result suggests that current ’line displacement’ models are missing a crucial point and may fail to predict the correct wetting properties of a textured surface. Actually, this mechanism is a reminiscence of the well-known dislocations theory introduced in the 40’s to explain the low threshold for plastic deformations in crystalline lattices. Moreover, the propagation of kinks allows the drop to maintain a circular shape even on strongly anisotropic surfaces. We have also proved that for a special design (a circular pattern) which avoids the formation of kinks the contact line does move as a whole and the adhesion is that expected by line displacement models. We also developped numerical simulations for the propagation of the kink, based on an energy minimization strategy using Surface Evolver. The simulation results have fully validated the experimental results.

Students

  • M. Rivetti (PostDoc 2012-2013)
  • J. Unger
  • A. Gauthier
  • A. Dubov

Recent Publications

  • Anaïs Gauthier, Marco Rivetti, Jérémie Teisseire & Etienne Barthel, Finite Size Effects on Textured Surfaces: Recovering Contact Angles from Vagarious Drop Edges, Langmuir, 30 (6), pp. 1544-1549 (2014) Hal
  • Anaïs Gauthier, Marco Rivetti, Jérémie Teisseire & Etienne Barthel, Role of kinks in the dynamics of contact lines receding on superhydrophobic surfaces, Phys. Rev. Lett. 110, 046101 (2013) - Journal Page, Hal