(355d) "Zipping Wetting": Filling Dynamics During The Cassie Baxter – Wenzel State Transition

The Cassie Baxter ¹ and the Wenzel ² states are well known in the field of small scale hydrodynamics. In the Cassie Baxter state, liquid is resting partly on the features of a solid material and bridging the air in between these features. It is a state with contact angles in the super-hydrophobic region (>150°) and low contact angle hysteresis. In the Wenzel state, the liquid contacts the whole solid surface and therefore has a lower contact angle and high hysteresis.

Many researchers have replicated nature's super-hydrophobic surfaces (like the lotus leaf ³) with micro-patterns and have described contact angles and contact lines of both the Cassie Baxter and the Wenzel state ^{4, 5, 6}. The work presented here and in literature ⁷, describes the conditions and kinetics for a transition between these two states. Theoretical calculations, experiments and numerical simulations have provided design criteria based on geometry and material properties. The filling dynamics (speed and pathway) are studied experimentally through a microscope and a high-speed camera and numerically with Lattice Boltzmann simulations.

Figure 1: (left) Scanning electron microscopy (SEM) picture of a micro-patterned polymer film with w = 5 μm, h = 10 μm and a = 5 μm (see sketch at the right).

The surface pattern was formed by casting a polymer solution on an etched silicon wafer (figure 1). Different gap sizes (a) and heights (h) in the micrometer range were compared as well as two different polymers (Kraton D 1102CS and PDMS). The film geometry and surface chemistry fully determines if a drop of liquid positioned on a patterned surface will switch from Cassie Baxter state to Wenzel state. When a transition does occur the fluid fills up the void between four pillars from the top and then the wetting front spreads in horizontal direction. The macroscopic wetting line can progress with either a square front or a more circular front. This is determined by the filling speed. A square area will be created when the front speed is slower than the zipping speed (see figure 2). The macroscopic wetted area will become circular when the zipping and front speeds are similar.

Figure 2: Four snapshots (left to right) showing the zipping effect: the front is moving from right to left in milliseconds, while the liquid fills up a row from top to bottom in microseconds.

References

(1)        Cassie, A. B. D.; Baxter, S. Transactions of the Faraday society 1944, 40, 546-551.

(2)        Wenzel, R. N. Industrial & Engineering Chemistry 1936, 28, 988-994.

(3)        Neinhuis, C.; Barthlott, W. Annals of Botany 1997, 79, 667-677.

(4)        de Gennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity and wetting phenomena: drops, bubbles, pearls, waves; 1st ed.; Springer Science+Business Media, Inc.: New York, 2003.

(5)        Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22, 3125-3130.

(6)        Gao, L.; McCarthy, T. J. Langmuir 2007, 23, 3762-3765.

(7)        Sbragaglia, M.; Peters, A. M.; Pirat, C.; Borkent, B. M.; Lammertink, R. G. H.; Wessling, M.; Lohse, D. submitted to Nature Materials 2007 (preprint).