INTRODUCTION: Superhydrophobic coatings have non-wetting surfaces with water contact angle >150° and contact angle hysteresis of <10°. These surface characteristics are known as the Lotus Effect due to the early discoveries that high water repellency and self-cleaning behavior are due to micro-nanobinary structures (MNBS) on the surface of the lotus flower, Nelumbo nucifera (Figure 1). In fact, a wide variety of biological structures such as rice leaves, butterfly wings, moth eyes, mosquito eyes, cicada wings, red rose petals, gecko feet, desert beetle, spider silks and fish scales show similar superhydrophobic behavior.
Fig. 1: (a) Lotus leaf with high contact angle; (b) SEM image of Nelumbo nucifera showing micro-nanobinary structures (MBNS).
A number of models have been developed to explain the water repellent properties of superhydrophobic surfaces. Whereas Young’s model applies to a liquid drop on a flat surface and Wenzel’s model takes into account a roughened surface, the Cassie-Baxter wetting state considers a roughened surface where grooves under a droplet can fill with vapour instead of liquid (Figure 2). This interaction explains how a droplet can “sit” on top of a material supported by a cushion of air to create a droplet that easily rolls over a surface.
Fig. 2: (a) Young, (b) Wenzel and (c) Cassie-Baxter wetting models.
The term self-cleaning refers to the attraction of surface contamination to a freely rolling droplet. This condition requires extremely low surface energy such that attraction between a
contaminant and moving droplet is greater than with the surface itself. This behaviour is characterized by the contact angle hysteresis (CAH). Defined as the difference between advancing and receding contact angles of a droplet as it moves across a surface (Figure 3), when CAH is <10°, a surface is considered “slippery” and self-cleaning.
Fig. 3: Advancing and receding contact angles of droplet on tilting surface.
Superhydrophobic surfaces are of special interest because their structures give rise toanti-sticking, anti-contamination and self-cleaning behaviour. These properties have found important industrial and medical applications such as anti-fouling paints for boats, anti-sticking of snow to windshields, lab-on-a-chip microfluidics, stain resistant textiles, dust free coatings, corrosion resistant electronics, cell and protein resistance and so forth.
At PVA Tepla, we have created highly efficient and affordable methods to prepare superhydrophobic structures. Herein we discuss a few such strategies.
METHODS: Generally, a superhydrophobic surface only needs to be hydrophobic and rough on a scale much less than the capillary length of water (< 273 um). This leaves an extremely wide scope for the actual chemistry and topography. We have focused on 2 plasma techniques due to:
- flexibility and control of surface features
- rapid and cost effective synthesis
Diffusion Limited Growth: Plasma enhanced chemical vapour deposition (PECVD) allows for a natural growth pattern when deposition occurs with no surface transport. Low pressure chambers ranging in size from 40 – 335L are employed for batch processing using pulse plasma methods. Specialized system options which include direct auto-matching network and generator communication, vapour-phase mass flow controllers, heated gas lines and our monomer delivery system allow for many specialized liquids or solids to be vaporized for plasma activation and subsequent thin film deposition. Large irregular shaped items or long diffusion paths inside an object can often be coated using this method.
Pulse plasma treatments resulting in 3D architectures from diffusion limited growth often require long reaction times (> 30min) due to the low ton/ton+toff duty cycle ratio. The usual deposit looks much like a cauliflower head and has fractal morphology with very high surface area (Figure 4).