A two-dimensional thermodynamic model based on Helmholtz free energy for wettability regulation of textured metal surfaces

To provide theoretical support for the wettability regulation of textured metal surfaces, a two-dimensional (2-D) thermodynamic model based on Helmholtz free energy is proposed to reveal the nature of the infiltration phenomenon. The relationship between the apparent contact angle and the free energy of the infiltration system is established. Micropyramid arrays (MPAs) with different pitches are textured on a high-phosphorus electroless nickel-phosphorus (Ni-P) alloy plating by ultraprecision machining for the verification of the theoretical model. The results show that the minimum free energy corresponds to the apparent contact angle of the water droplet when it is stable on the textured surface. MPA-textured surfaces with smaller pitches have a wider variation range of apparent contact angle and are expected to achieve better hydrophobicity. Periodic energy barriers created by the MPAs promote the free energy consumption. By employing ultrasonic assistance, secondary microstructures are generated on the inclined sidewalls of the micropyramids, thus enhancing the energy barrier effect. A maximum apparent contact angle of 124.4° is obtained for the MPA-textured surface machined with ultrasonic vibration when the pitch is 20 μm. The experimental apparent contact angle values are all within the theoretical range, indicating the feasibility of the theoretical model.