Icephobic materials have wide applications for economic reasons, as well as for risk reduction of ice accretion on an airframe. Inspired by the doubly reentrant overhanging edge skin structure in micro scale of a spring tail that can survive in olive oil, the superomniphobic surface integrating doubly reentrant overhanging edge with microcavity is proposed for simultaneously increasing static and dynamic liquid repellency. With our improved fabrication techniques, pitch distance of the micro cavity achieved was 15 μm, which is the smallest in the world on silicon wafer. Under this scale size, applications including dynamic hydrophobicity and icephobicity become possible.

First, to compare our new surface with conventional designs, static and dynamic wetting experiments were conducted on...[ Read more ]

Icephobic materials have wide applications for economic reasons, as well as for risk reduction of ice accretion on an airframe. Inspired by the doubly reentrant overhanging edge skin structure in micro scale of a spring tail that can survive in olive oil, the superomniphobic surface integrating doubly reentrant overhanging edge with microcavity is proposed for simultaneously increasing static and dynamic liquid repellency. With our improved fabrication techniques, pitch distance of the micro cavity achieved was 15 μm, which is the smallest in the world on silicon wafer. Under this scale size, applications including dynamic hydrophobicity and icephobicity become possible.

First, to compare our new surface with conventional designs, static and dynamic wetting experiments were conducted on straight wall pillar, doubly reentrant pillar, straight wall cavity and the newly developed surface under different surface temperatures. The new surface demonstrated excellent omniphobicity for low surface tension liquids and dynamic icephobicity. The mechanism behind is analyzed utilizing ‘air spring effect’, and micro wetting behavior of Laplace Breakthrough on nanostructures. Breakthrough pressure on the novel surface is three times larger than that of the straight wall cavity, which was regarded as the best conventional design. It was found that the ratio of pitch distance and microstructure height is the most important parameter to control droplet dynamics and heat transfer. The surface with a small ratio (P/H ˂1) of pitch distance (P) and microstructure height (H) has the best performance, which can successfully repel the droplet even when We=1000 under -20°C.

Then, to have a deeper understanding of how surface morphology of doubly reentrant cavity can affect icephobicity, systematical comparison of parameters including line width, micro structure height and solid fraction are studied. It is noticeable that during the impact the temperature of the compressed air inside the cavity is largely increased and can restrain ice nucleation rate and reduce the viscosity of the water to achieve icephobicity. When the compressed air temperature is high enough, a new state named ‘doubly recoil’ state for a bubble generated at the center and the liquid recoiled from both center and outside that can largely reduce the contact time of droplet impact is observed.

To reveal the mechanism behind doubly recoil state, a set of samples that varied the structure size, including line width, solid fraction, microstructure height and overhanging edge were provided to study the droplet rebound state. By analyzing the liquid penetration in micro scale based on mass-spring-damper model, the requirements for doubly recoil state to happen is generalized as high Weber number of droplet impact that can provide high enough compressed air temperature inside the cavity to make liquid evaporate fast and let the bubble generate and expand at center. In addition, more horizontal space for overhanging edge can make the liquid harder to touch the side wall that can avoid energy loss due to friction, and the doubly recoil state can then be achieved for a relatively low Weber number of droplet impact.

Finally, future work of applying the new surface to reduce the flow drag force and the process flow for fabricating nano pillar on the new cavity is discussed by briefly introducing the conventional studies and proposing possible experiment plans.

Key words: Doubly reentrant cavity, Droplet impact, Omniphobic, Icephobic, Contact time, Doubly recoil