Plasma processing of cellulose surfaces and their interactions with fluids
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Cellulose is a biodegradable, renewable, flexible, inexpensive, biopolymer which is abundantly present in nature. In spite of these inherent advantages, cellulose fibers cannot be used directly in a number of potential industrial applications because of their hydrophilic nature; a surface modification is often required to alter the surface properties of cellulose. This thesis work reports a fabrication method that results in superhydrophobic properties (contact angle (CA) > 150°) on cellulose (paper) surfaces. Superhydrophobicity was obtained by domain-selective etching of amorphous portions of the cellulose fiber in an oxygen plasma, and by subsequently coating the etched surface with a thin fluorocarbon film deposited via plasma enhanced chemical vapor deposition from a pentafluoroethane precursor. Two forms of superhydrophobicity with vastly different degrees of adhesion were obtained by varying the plasma treatment conditions, in particular the duration of oxygen etching: "roll-off" (contact angle (CA): 166.7° ± 0.9° and CA hysteresis: 3.4° ± 0.1°) and "sticky" (CA: 153.4° ± 4.7° and CA hysteresis: 149.8±5.8°) superhydrophobicity. The CA hysteresis could be tuned between the two extremes by adjusting the oxygen etching time to control the formation of nano-scale features on the cellulose fibers. The effects of fiber types (soft vs. hard wood) and paper making parameters on fabricating superhydrophobic paper were also investigated. There were no significant differences in the formation of the nano-scale features created via oxygen etching on paper substrates obtained from different fiber types and paper making parameters. Because "roll-off" superhydrophobicity is primarily determined by the nano-scale roughness, this property is therefore not significantly affected by the fiber types or paper making parameters. While the fiber type does not affect "roll-off" or "sticky" superhydrophobicity, paper making process parameters affect the structure of the paper web on the micro-scale and thus lead to variations in "sticky" superhydrophobicity. Superhydrophobic paper substrates were patterned with high surface energy ink deposited using a commercial desktop printer. The patterns could be used to manipulate the drag and extensional adhesion of water drops on the substrates. Classic 'drag' and 'extensional' adhesion expressions were used to model the behavior of water drops on basic dot and line patterns of variable dimensions. A fundamental understanding of the adhesive forces of water drops as a function of pattern shape and size was thus obtained. Based on this knowledge, patterned paper substrates were then designed and fabricated to perform simple unit operations, such as storage, transfer, mixing and merging of water drops. These basic functionalities were combined in the design of a simple two-dimensional lab-on-paper (LOP) device. Further studies of more complicated pattern shapes led to the generation of patterns that allowed directional mobility and tunable adhesion of water drops. These developments are critical for designing novel components for two-dimensional LOP devices such as flow paths, gates/diodes, junctions and drop size filters.