Amir Farajian (Committee Member), Hong Huang (Committee Member), Sharmila Mukhopadhyay (Advisor)
Master of Science in Engineering (MSEgr)
Carbon nanotubes (CNTs) have gained great attention due to their excellent mechanical, electrical, and thermal properties. Carbon nanotube (CNT) arrays attached to solid substrates resemble a nano-carpet and have potentials in a variety of devices. In this study, the focus is to investigate the surface modification approaches of CNT carpets on flat (compressed) graphite. Wettability, or permeation of a liquid such as water into the CNT strands, may play very significant roles in many applications where a CNT-coated device may be used in aqueous environment. Short-term microwave plasma treatments were used in this project to functionalize CNT carpets for wettability. Hydrophilic/ hydrophobic behavior, measured by the contact angle of water, could be well tuned by controlling the exposure time. A simple method to reverse the changes caused by plasma is to anneal in air at a temperature of 110 °C. The CNT carpets could be cycled between hydrophobic and hydrophilic by repeated treatments of plasma and air annealing. The extent and ease of cycling depended on the morphology of the CNT carpets. We investigated the influence of surface nanostructure, morphology and surface chemistry on wettability. Nanostructure and morphology of CNT arrays was studied using field emission scanning electron microscopy (FESEM). The chemical composition of the surface was investigated by X-ray photoelectron spectroscopy (XPS). Pure CNT carpets are superhydrophobic (contact angle > 160°) as long as they are well aligned and do not contain covalently bonded oxygen groups. Superhydrophobicity may be lost, or contact angle decreased, if either alignment of CNTs is disturbed or covalent oxygen-containing groups bind to its surface. Young's, Wenzel's and Cassie-Baxter regimes are three models of contact angle that are used to explain these observations. The wetting state and contact angle of hydrophobic CNT arrays were compared with the values predicted by Cassie-Baxter equation on pristine CNT carpets. The morphological parameter was obtained by measuring multiscale dimensions of CNT carpets from SEM images. The chemical parameter represented by inherent contact angle was obtained by measuring contact angle on the graphitic materials, and reviewing the literature values reported on graphite and graphene surfaces. As an example of how wettability may influence an engineering property such as water flow, CNT arrays were synthesized on carbon fiber cloth. The flow of water through this porous fabric was tested. It was seen that water flow was enhanced by CNT attachment, which may be attributed to drag reduction on super-hydrophobic surface. However, making the CNT carpets fully hydrophilic through plasma treatment seemed to further increase overall water flow rates, which may raise the possibility of capillary transport through hydrophilic nano-carpet. These effects need to be investigated more carefully in future for better understanding.
Department or Program
Department of Mechanical and Materials Engineering
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