(59d) Surface Engineering for Designing Superhydrophobic and Superhydrophilic Particulate Solids

Micro-fabrication of flat surfaces for superhydrophobic (contact angle ≥ 150⁰) and superhydrophilic surfaces (contact angle ≤ 5⁰) is very common in many advanced applications. However, the technique cannot be directly employed to particulate solids. In this work, superhydrophobic and superhydrophilic surfaces are designed on particulate solids using chemical etching of 120 µm glass beads and Ar plasma treatment of 17 µm corn starch particles. While the SEM and AFM analysis show that both the surface fabrication techniques created randomly distributed nano-scale surface roughness, the XPS and surface energy (SE) data show that these fabrication techniques are also changed the surface chemistry of the particle. The contact angle (CA) data using Sessile drop method shows that the hydrophilic glass surface (CA ~ 80⁰) is transformed into a superhydrophobic surface (CA~150⁰) and the hydrophilic corn starch surface (CA ~ 48⁰) is converted into more hydrophilic surface with CA~ 20⁰ (towards the superhydrophilic surface). The change in wetting property of the glass surface and corn starch surface is implicated to change in surface groups, surface roughness including its distribution and aspect ratio (width to height ratio) of asperities on the particle surface. A comparison between the experimentally determined contact angle and that of theoretically calculated data shows that the adhesion between the water droplet and the glass surface is in Cassie impregnation regime. In contrast, the plasma treated corn starch (in the pellet form) is in the Wenzel wetting regime even after the increase in the surface roughness. The analysis of individual contribution of the surface chemistry and surface roughness shows that wetting behaviour of glass beads CA > 120⁰ is mostly controlled by nano-scale surface roughness. However, surface chemistry plays greater role for controlling the wetting behaviour of plasma treated corn starch particles. The present work demonstrated the extent and nature of surface engineering required to design a particle surface which is superhydrophobic or superhydrophilic. The work has immense implications in many advanced applications where wetting of particle surface is critical.