Understanding the effects of pressure on the contact angle of water on a silicon surface in nitrogen gas environment: Contrasts between low- and high-temperature regimes

Temperature Dependence of the Dynamic Contact Angles of Water on a Smooth Stainless-Steel Surface under Elevated Pressures

The temperature dependence of the dynamic contact angles (DCAs) of water on a metallic surface remains unclear, especially under elevated pressures. Here in this work, the advancing and receding contact angles (RCAs), as well as the contact angle hysteresis (CAH), of water on stainless-steel 316 (SS316) surfaces were studied using the dynamic sessile drop method for temperatures up to 300 °C and pressures up to 10 MPa. It was found that the temperature dependence of the DCAs exhibits a different pattern as compared to the piecewise linear decline of static contact angles. The advancing contact angle (ACA) remains nearly constant and does not decrease until the temperature becomes close to the saturated temperature. The decrease in ACA is attributed to evaporation, which reduces the advancement of energy barrier. The RCA linearly declines below 120 °C and remains stable above 120 °C. The increasing temperature enhances the pinning effect and changes the droplet receding mode. Under all pressures tested, the CAH demonstrates a "increase-constant-decrease" trilinear relationship with temperature. Furthermore, the mean solid surface entropy and solid-gas interfacial tension of SS316 were estimated to be 0.1152 mJ/(m2·°C) and 61.49 mJ/m2, respectively.

Contact angle of ethanol, water, and their mixtures on stainless steel surfaces in dense carbon dioxide

HYPOTHESIS

Contact angle can be a key parameter in chemical engineering. However, the development and the optimization of numerous processes using supercritical CO2, considered as environmentally friendly, require new measurements under dense CO2 atmosphere. Besides, the influence of the roughness or the wetting regime on the contact angle is known at ambient conditions but remains to be discussed for systems under high pressure.

EXPERIMENTAL

Contact angle measurements of ethanol, water, and their mixtures, with ethanol mass fractions ranging from 0.25 to 0.75, on two stainless steels in saturated CO2 at pressures ranging from 0.1 MPa to 15.1 MPa, and at313 K and 333 K were carried out in a set-up improving mass transfer between the studied liquid and the continuous fluid phase. Stainless steel surfaces have been characterized by atomic force and scanning electron microscopies allowing the application of the Wenzel equation.

FINDINGS

Ethanol wetted totally both stainless steels while contact angles of all other liquids were increased by the rise of pressure, with contact angles up to 128° for water at 15.1 MPa. Trapped bubbles were observed at the solid/liquid interface and the bubble formation is discussed. Furthermore, the potential influence of bubble presence on the wetting regime is prospected through the question: could the pressure rise modify the wetting regime?

Fabrication of Multiscale 1-Octadecene Monolayer Patterned Arrays Based on a Chemomechanical Method

A controlled and self-assembled micromachining system was built to fabricate a mico/nanoscale monolayer patterned array on a silicon surface using a diamond tip. The process was as follows: (1) we preprocessed a silicon wafer to obtain a hydrogen-terminated silicon surface; (2) we scratched three rectangular arrays of 10 μm × 3 μm with a spacing of 2 μm on the silicon surface with a diamond tip in 1-octadecene solution; the Si-H bonds were broken, and silicon free radicals were formed; (3) the 1-octadecene molecules were connected with silicon atoms based on Si-C covalent bonds, and the 1-octadecene nano monolayer was self-assembled on the patterned arrays of the silicon surface. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Sessile water contact angles were used to detect and characterize the self-assembled monolayers (SAMs). The XPS results showed that the Si2p peak and the O1s peak were significantly decreased after self-assembly; however, the C1s peak was successively significantly increased. Sessile water contact angles showed that the hydrophilicity was weakened after the formation of 1-octenecene SAMs on the silicon substrate. The nanofriction of the sample was measured with AFM. The change in nanofriction also demonstrated that the SAMs were formed in accordance with the patterned array. We demonstrated that, by using this method, self-assembled multiscale structures on silicon substrate can be formed quickly and conveniently.