Squeezing Drops: Force Measurements of the Cassie-to-Wenzel Transition

Superhydrophobic surfaces have long been the center of attention of many researchers due to their unique liquid repellency and self-cleaning properties. However, these unique properties rely on the stability of the so-called Cassie state, which is a metastable state with air-filled microstructures. This state tends to transit to the stable Wenzel state, where the inside of the microstructures eventually wets. For potential industrial applications, it is therefore critical to maintain the Cassie state. We investigate the Cassie-to-Wenzel transition on superhydrophobic micropillar surfaces by squeezing a water drop between the surface and a transparent superhydrophobic force probe. The probe's transparency allows the use of top-view optics to monitor the area of the drop as it is squeezed against a micropillared surface. The impalement, or Cassie-to-Wenzel transition, is identified as a sharp decrease in force accompanied by an abrupt change in the drop's contact area. We compare the force measured by the sensor with the capillary pressure force calculated from the observed drop shape and find a good agreement between both quantities. We also study the force and pressure at impalement as a function of the pillar's slenderness ratio. Finally, we compare the impalement pressure with three literature predictions and find that our experimental values are consistently lower than the theoretical values. We find that a possible cause of this earlier Cassie-to-Wenzel transition may be the coalescence of the squeezed drop with microdroplets that nucleate around the base of the micropillars.