Objective quantification of surface roughness parameters affecting superhydrophobicity

Abstract

This study proposes new optical roughness parameters that can be objectively quantified using image processing techniques, and presents an analysis of how these parameters are correlated with the degree of superhydrophobicity. To this end, photolithography and dry etching processes were used to form regular square pillars with different heights and spacings with a length of tens of micro-meters on silicon wafers. Optical roughness parameters of the specimens were obtained using image processing, and surface wettability was characterized using static contact angle and sliding angle measurements for water droplets of volume V _D = 3.5 μl or 12 μl. As a result, seven optical roughness parameters were derived to describe the surface roughness topography in a multi-faceted way. Between the Cassie-Baxter state and the Wenzel state, two distinct wetting states intermediate state I, and intermediate state II were observed. Multiple linear regression of optical roughness parameters and superhydrophobicity demonstrated that in the stable Cassie-Baxter state, the contact angle can be increased or sliding angle decreased more effectively by adjusting the spacing between pillars than by just tuning the solid area fraction. However, in the metastable state where the Cassie-Baxter state can be changed to intermediate state I and vice versa by adjusting V _D or surface geometry, reducing the solid area fraction is a priority to ensure a stable Cassie-Baxter state. Horizontal-perspective roughness parameters had a great effect on dynamic wettability in the Cassie-Baxter state. The results confirmed that the proposed optical roughness parameters may be useful for quantitative analysis of the complex effects of roughness on superhydrophobic surfaces.

Fig. 1. Illustrative two basic wetting models: (A) Wenzel model; (B) Cassie–Baxter model.

Fig. 2. Schematic illustration of developed program flow chart built with OpenCV-Python API.

Fig. 3. Static contact angle (red lines, left axis) and sliding angle (blue lines, right axis) of samples with pillars with height = 50 μm. Orange line: prediction of Cassie–Baxter model.

Fig. 4. Static contact angle (red lines, left axis) and sliding angle (blue lines, right axis) of samples with pillars height = 10 μm. Orange line: prediction of Cassie–Baxter model.

Fig. 5. Side view of contact between the water drop and pillars at four different wetting states.

Fig. 6. Shapes of water droplets (left: 3.5 μl droplet, right: 12 μl droplet).

Fig. 7. Simple schematic illustration of reflection of light rays: (A) pillars height H = 50 μm; (B) pillars height H = 10 μm.