(198am) Influence of Micro and Nanoscale Surface Roughness on the Wetting Characteristics of Flat Surface

Unique wetting characteristics of natural surfaces inspired researchers to study the wetting behavior of rough surfaces. In most of the artificial flat surfaces, the surface roughness is tailored to enhance solid-liquid adhesion, self-cleaning, anti-bacterial and directional controllability. The wettability (superhydrophobicity and superhydrophilicity) of a surface is mainly influenced by the geometric parameters of micro and nano-size pillared surface i.e. height, width and pitch of the pillar, and their relationship. In this work, the effect of varying height, width, and pitch of the pillars on the wetting state of the micro-patterned surface is investigated. Micro-structures are fabricated in a controlled manner on Silicon wafer (100) surface, with pillars height (50 µm, 70 µm, and 90 µm), width (30 µm, 45 µm, and 60 µm), and pitch (30 µm, 60 µm, and 90 µm) using photolithography. Scanning Electron Microscope (SEM) and Optical Profilometer confirm the different feature sizes on silicon wafers. These patterned surfaces are categorized into three types: 30 µm width, 40 µm pitch and height varies in the range 50 to 90 µm of the pillars are set as Type I, 70 µm height, 30 µm pitch and different width values from 30 to 60 µm of the pillars are set as Type II, and 70 µm height, 45 µm width and pitch varies in the range 30 to 90 µm are set as Type III. The wettability of the patterned surface is obtained from the contact angle data using sessile drop method. It is observed that increasing pillar height and decreasing the pitch of the pillars allow the droplet to rest in the Cassie Baxter regime with a contact angle greater than 150ÌŠ, and on the contrary, a Wenzel wetting state formed with strong solid-liquid adhesion at the interface. The apparent contact angle is strongly influenced by the contact angle hysteresis (CAH) caused by the feature size of the micro-structures. The transition in the wetting state is also studied with the droplet evolution during evaporation. Critical value of pillar height and pitch is determined to discuss the stability and transition of a droplet from Cassie Baxter to Wenzel state. The present work is also compared with the randomly designed micro/nano- scale surface roughness on the glass surface. The results demonstrated the extent and nature of surface engineering required to design a superhydrophobic surface.