Hydrophobicity on graphite surface by various temperatures and applied electric field

• Main Author: Myint Hlaing, Sein.
• Other Authors: Sun Changqing
• Format: Final Year Project
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Materials::Nanostructured materials
• Online Access: http://hdl.handle.net/10356/54461
• Institution: Nanyang Technological University

The contact angle of water droplets on graphite surface was investigated under various temperature and electric field to investigate the mechanism of the stimulated hydrophobicity. Water droplet on graphite surface resembles water droplet on lotus leaf. What graphite surface makes difference from lotus leaf is that graphite is a good conductor of heat and electricity. Without temperature or electric field, water behaves hydrophobic on them. However, as temperature goes higher or greater electric field is induced, the droplet becomes hydrophilic and wet the graphite surface. Contact angles of water droplet are key reasons for wetting and dewetting. High tension of water surface by strong hydrogen bond makes contact angle higher and insects can walk on it’s surface. In this report, stimulated water droplet’s electrical, chemical and physical mechanisms were investigated under various temperatures and electric fields. There are many applications ranging between hydrophobic to hydrophilic states of water. Examples of hydrophobic are anti-sticking, anti-contamination, self-cleaning technologies. Examples of hydrophilic are fluid transportation like administering medicines, electrowetting printing, wicking material in heat pipes for enhanced boiling heat transfer. Water contact angle was researched including Wenzel and Cassie’s law of the way water droplet in contact with the roughened surface with and without air pocket in micro-meter scale. Further research yielded in nanometer scale was explained that electric charge between water droplet’s outermost skin’s and surface’s monopole or dipole attraction was the hidden key factor. Coulomb repulsion also known as electric repulsion at the nanometer-sized contact by the water droplet’s locked monopole or dipole made it impossible to attract to the contacting surface and thus water or fluid can slip through between nanometer sized channels. Surface tension and capillary action of water was added to enhance the understanding of water’s nature although water is made up of just Oxygen and Hydrogen molecules. Graphite, graphene and Carbon Nano Tubes were widely explained on their structure and physical properties featuring contact angle on graphite surface by water droplet simulated under different temperatures. As temperature was increased, the molecules in the water droplet moved faster and attractive force becomes smaller causing less viscosity and lowered its surface tension. The polarized arrays of electric charges no longer repel one another and the droplet started to wet the surface. Electrowetting of water droplet was explained about the changes in the contact angles of water droplet with and without voltage applied onto the droplet. The physical changes on water droplet was observed under electric fleld directions both in parallel and in perpendicular. When normal (perpendicular) to the surface, water droplet’s contact angle is higher and more hydrophobic. However, when parallel to the surface, water droplet followed the direction stretching to larger base area and resulted to more hydrophilic.