Microscopic Wetting at Microstructured Surface of Porous Silicon

Abstract

Surface wettability is an important factor for micro/nanoscale thermal fluidic systems and it has attracted much interest for both fundamental research and practical applications. As one of the most attractive materials with controllable wettability, porous silicon is easy to be produced by the electrochemical etching. In this study, the effects of the microstructures of porous silicon on the wetting behavior of a pure water droplet were investigated experimentally. The solid-liquid contacting surface of the porous silicon substrate was prepared by varying both the geometrical microstructure and the chemical composition. The anodic etching was applied to the n type silicon substrate of orientation (100) and the geometrical microstructures of porous silicon were controlled by varying the fabrication conditions of the electrochemical etching. The pores of diameter ranging from 1–6 micrometers and the porosity up to 0.8 were obtained. Also, the surface chemical composition was controlled by coating the SiO2 layer or the CYTOP fluoropolymer layer directly on the porous silicon surface. The contact angle of the pure water droplet was measured at the prepared porous silicon surface in a room with constant temperature and humidity. The effects of the microstructures on the contact angle were discussed and the results were compared to both the classical theoretical models and a modified model based on the molecular dynamics simulations.

Contact angle of ethanol–water solutions on crystalline and mesoporous silicon

“…Contact angles were also measured for ethanol-water solutions on both HF-dipped and untreated crystalline silicon. Figure 3 shows that contact angle decreases from ∼80 • for water (surface tension ∼72 mN m −1 ) on HF-dipped silicon, consistent with other measurements on crystalline silicon [5,19] (measured approximately 24 hours after dipping), to ∼ 27 • at an ethanol concentration of 80% (surface tension ∼24 mN m −1 ). At higher ethanol concentrations the contact angle was found to approach zero.…”

“…This relatively low value of contact angle indicates the presence of a native oxide coating and is consistent with previous studies. For example, Nagayama et al [5] measured 47.2 • on crystalline silicon coated with a 100 nm PVCD SiO 2 film for drops of volume 4 mm 3 . Also, Muñoz-Noval et al [19] measured an angle of approximately 55 • on untreated crystalline silicon.…”

“…Contact angles on rough surfaces were studied by Wenzel [20]. This model (figure 1(a)) relates contact angle on a rough surface, θ W , to the Young contact angle, θ Y , and is given by cos where r W is the area ratio of structured surface to planar surface [5]. Cassie and Baxter expanded upon Wenzel's model to allow for measurements of porous surfaces [28].…”

“…Contact angle studies have been performed on the crystalline and porous forms of silicon [8][9][10] and on related nanostructured silicon surfaces [11][12][13][14][15][16]. Most studies on the porous variety have focused on measurement of water contact angle on microporous silicon [6,8,17] or macroporous silicon [1,5,10]. Contact angle measurements have also been performed on mesoporous silicon films.…”

Polydopamine-based hybrid materials with tailored surface wettability for immiscible liquid separation

“…Wenzel's state and Cassie-Baxter's state. 77 The Cassie-Baxter's state, thus, is called the meta-stable state. With the introduction of air pocket in the porous structure, the Cassie-Baxter's equation can be written as.…”