Fabrication of Superhydrophobic Cellulose Surfaces via Plasma Processing
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In 1805, Young proposed a relationship between the forces acting at an interface between a liquid and solid: “…for each combination of a solid and a fluid, there is an appropriate angle of contact between the surfaces of the fluid, exposed to the air, and to the solid…” However, most real substrates exhibit a variety of contact angles, depending on whether the liquid-air interface is advancing or receding on the solid surface, rather than a unique contact angle. The range of contact angles, usually defined as the difference between the maximum and minimum contact angles observed at the advancing and receding fronts of the liquid drop, is termed contact angle (CA) hysteresis. For classifying the interaction between substrates and liquids, it is critical to specify both the CA and CA hysteresis values. As will be shown in this presentation, even if a surface is superhydrophobic (according to the common definition of a static or advancing water CA > 150°), it can be strongly adhesive to water drops. Contact angle hysteresis most closely correlates with the magnitude of these adhesive forces and by tuning the hysteresis, the dynamics of drops on superhydrophobic surfaces can be controlled, making possible numerous new applications. Superhydrophobicity has been achieved on cellulose surfaces by domain selective etching of amorphous portions of the cellulose, followed by coating of the surface structures generated with a fluorocarbon film deposited via plasma enhanced chemical vapor deposition (PECVD). The hysteresis of these superhydrophobic surfaces can be tuned between 149.8±5.8° and 3.5±1.1° through the controlled fabrication of nano-scale features on the cellulose fibers. This process takes advantage of the inherent nano-meter length scales of the amorphous and crystalline domains of cellulose fibers and the non-conformal film deposition property of PECVD process. Superhydrophobic cellulosic surfaces with tunable hysteresis (adhesion) provide control of aqueous drop mobility and thus of the transfer characteristics of water drops. Moreover, the fact that these substrates are based on cellulose fibers, a biodegradable, inexpensive, flexible, biopolymer, widens potential commercial opportunities for these materials.