Solute self-assembly on hydrophobic surface
Date of Issue2016
School of Mechanical and Aerospace Engineering
The study of the evaporation of sessile droplets on solid substrates and its resultant deposition pattern is essential in important technological applications such as inkjet printing, nanostructure self-assembly and functional particle coating. While many studies have been conducted to investigate the commonly observed coffee-ring effect and the means to control it, there is a lack of consolidated study on the specific effects of interactions on this phenomenon. The aim of this project is to investigate the effects of two important parameters, salt concentration and hydrophobicity of substrate, on the evaporation characteristics and final crystallization pattern of a NaCl solution droplet on Si wafer, polytetrafluoroethylene and methyl methacrylate monomer. Using data on contact diameter and contact angle, variations in pinning time, lifetime of droplet and final crystallization pattern were studied. It was observed that constant contact radius (CCR) mode tends to dominate over constant contact angle (CCA) mode for more hydrophilic substrate while the opposite was true for the more hydrophobic MMA substrate. The evaporation rate of solution droplet on the hydrophobic surface (MMA) was up to 3.5 times slower than the evaporation rate of droplet on the more hydrophilic surface. On the same substrate itself, increasing the concentration can reduce the evaporation rate by at most 1.15 times. It was also noted that the thickness profile of crystals increased with increasing initial concentration of solution droplet. For future work, fluorescent dye and high-speed imaging equipment, like TEM and SEM, could be used to examine the precise movement and packing of suspended particles so as to understand the internal flow affecting the deposition pattern. Solute added can also be varied to include nanoparticles or particles of different shapes because such particles are also able to influence the evaporation characteristics and deposition patterns of drying droplets.
Final Year Project (FYP)
Nanyang Technological University