Evaporation of sessile saline droplet with surfactant on silicon wafer substrate

• Main Author: Yap, Ming Sheng
• Other Authors: Fei Duan
• Format: Final Year Project
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/74544
• Institution: Nanyang Technological University

The initial wettability, evaporation characteristic and crystallisation pattern of sodium chloride (NaCl) droplets with Cetyltrimethylammonium Bromide (CTAB) below critical micelle concentration (CMC) on silicon wafer substrate are studied by varying the concentrations of NaCl and CTAB. Circular initial wetting area are formed on the substrate where the droplet settled. The initial wettability of NaCl droplets with 0.8 millimolar (mM) CTAB decreases with increasing NaCl concentration and 10% NaCl droplets with CTAB decreases with increasing CTAB concentration up to 0.2mM. The evaporation process of NaCl droplets with 0.8mM CTAB only demonstrated the constant contact radius (CCR) mode. Also, the evaporation rate increases with higher NaCl concentration. However, 10% NaCl droplets with CTAB demonstrated different individual evaporation transitions with different ranges of CTAB concentrations. From 0.2mM to 0.8mM CTAB concentration, the evaporation process demonstrated CCR mode with the contact diameter at CCR and evaporation rate increases with higher CTAB concentration. On the other hand, at very low CTAB concentration, the evaporation process for 0.04mM CTAB undergoes a transition from mixed mode to constant contact angle (CCA) mode. For the crystallisation process, various number of NaCl crystals are formed near the edge of the droplet during the later stage of evaporation process and move towards the centre as it grow due to the increase in crystal size which becomes confined between the solid substrate and free surface. The NaCl and CTAB concentration have insignificant effect on the number of NaCl crystals formed and CTAB concentration has insignificant effect to the distance away from the droplet centre where it first formed. However, at very low CTAB concentration of 0.04mM, the variation of crystal distance is wide but get narrower with increasing CTAB concentration. In addition, the crystal distance also increases with higher CTAB concentration from 0.04mM to 0.8mM. The final size of NaCl crystals increases with higher NaCl concentration. In addition, due to presence of CTAB, crystallised NaCl dendrites are observed spreading over the wetted area that grow coarser with CTAB concentration up 0.8mM and grow finer with higher NaCl concentration.