Droplet merging on the heterogeneous surface

• Main Author: Kho, Chen Wei
• Other Authors: School of Mechanical and Aerospace Engineering
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/60879
• Institution: Nanyang Technological University

Wetting has received vast interest due to its application in many industrial processes. Multiple researches have been done in both fundamental and application perspective especially in recent years, super-hydrophobic surfaces gained the most interest due it potential applications such as self-cleaning window, nano-fludic and super-hydrophobic (repel dirt/water) fabric. Various surface modifications can be achieved with micro-fabrication technology making the different wetting characteristic on surfaces controllable. Part of this report consist of investigation of the modified solid surface conditions to determine the variation of the contact angle and the different spreading, hence understanding the behavior of the two droplet merging. The contact angles of the seven specimens designed were obtained experimentally by varying the droplet volumes on all the specimens and were categories by groups. Group A specimens which had an increasing pillar interval displayed a trend of increasing contact angle while Group B specimens that had an increasing pillar diameter displayed a trend of decreasing contact angle. The trend was explained with the theoretical results calculated from Cassie-Baxter equation. The experiment concluded that super-hydrophobic surface (contact angle exceeding 150) can be achieved if the modifications adopt the design of high pillar interval but low pillar diameter. The effect of the droplet volumes, droplet volume and diameter correlation on the different specimens were also investigated. The contact angle results on all the specimen surfaces were found not to be dependent on the variation of the droplet volumes. The contact angles remained almost constant after increasing the drop volume from 5uL to 19uL. In the area of the effect of the droplet volume on the diameter of the droplet related to the different specimens, it was proven to be insignificant in the context of the micro-litre drop region. In this project, capillary force was used as an actuating mechanism to induce movement which eventually led to the merger of two droplets. The effects of the different pillars contact angles and the droplet volumes on the capillary force were investigated. The results show that specimens with higher contact angle (more hydrophobic) and larger droplet volume will produce higher capillary forces. This trend was explained with the formula of the capillary force.