Polygonal Droplets on Microstructured Surfaces

In this work, we demonstrate interesting polygonal Wenzel droplets on microstructured surfaces. Dynamic contact angle experiments on functionalized (𝜃􏰻􏰼􏰽􏰾,􏰽􏰿􏱀􏰽􏱁􏱂􏱃􏱁􏱄 = 87°, 𝜃􏰻􏰼􏰽􏰾,􏱅􏱆􏱂􏱆􏰿􏱃􏱁􏱄 = 70°) cylindrical silicon micropillar arrays in a square pattern (diameter D, pitch L, and height H) were performed using a microscopic contact angle meter (MCA-3, Kyowa Interface Science). Water was dispensed using a piezo ink head at a rate of 0.75 nL/sec. Liquid addition was stopped approximately at 20-25 seconds after which the droplet was allowed to evaporate to ambient at 38% relative humidity. Top and side views of the droplet profile were visualized during the advancing (a and c) and pinning stages (b and d). Visualization of the contact line dynamics illustrated the associated anisotropy in the de-pinning contact angles (side views) along the two axes of symmetry (axis along 0° in a and b and along 45° in c and d ). The ratio of pillar diameter D to pitch which is L along 0° (a and b) and √2𝐿 along 45° (c and d) axes governs the advancing contact angles that dictate the droplet contact line shape during growth (a and c). The difference between the de-pinning contact angles along these two directions is larger for high pillar density (𝐷, 𝐿, 𝐻 = 7, 10, 5 𝜇m) resulting in a square contact line which is only visible in the top view after the droplet contact angle along all directions decreases below 90° during the pinning stage (b and d). Conversely, a droplet with octagonal contact line is observed for sparse pillar arrays (𝐷, 𝐿, 𝐻 = 4, 10, 5 𝜇m). Interesting features such as sharp droplet edges (solid yellow arrows) due to a square contact line and magnified view of the contact area (dashed blue arrows) due to the liquid lens effect are also illustrated. The visualization provides key insights into complex droplet shapes during growth which are critical for condensation heat transfer on surfaces.