To address the poor mechanical stability and high interfacial thermal resistance of conventional polydimethylsiloxane (PDMS) coatings in condensation heat transfer, the work aims to propose a one-step, fluorine-free laser ablation strategy. A mixture of PDMS and copper powder was pre-coated onto an H62 brass substrate, followed by direct infrared nanosecond laser ablation in air. Orthogonal experiments were conducted to optimize laser parameters: scan spacing (100-300 μm), scan passes (5-40), and scan speed (50-250 mm/s). The PDMS-to-copper mass ratio was systematically changed from 2∶0 to 2∶6. The optimal parameters were determined as scan spacing = 200 μm, scan passes = 20, scan speed = 150 mm/s, and PDMS∶Cu = 2∶1. Under these conditions, the fabricated superhydrophobic surface (SHS-Cu) exhibited a hierarchical micro/nanostructure comprising coral-reef-like micro-scale skeletons decorated with nano-scale flocculent features. The static water contact angle (WCA) reached 158.4° and the sliding angle (WSA) was as low as 6°.
Mechanical stability was evaluated by sandpaper abrasion (800# grit, 90 g load, up to 60 cycles) and sand impact (30 g sand from 40 cm height, cumulative up to 240 g). After 45 abrasion cycles, WCA remained 149°. After 60 cycles, it was still 146.6°. After 240 g sand impact, WCA remained 147.7° with a WSA of 18.5°. Thermal stability tests showed that after heat treatment at 300 ℃ for 12 h, WCA stayed above 151.8° and WSA below 9°. In a continuous steam environment, the surface maintained superhydrophobicity (WCA >151.4°, WSA <9°) for over 9 h. After 12 h of steam exposure, WCA dropped to 124.2° and WSA increased to 78.8°, but a subsequent heat treatment at 100 ℃ for 2 h fully restored the superhydrophobic performance.
Condensation heat transfer performance was evaluated with a custom-built experimental system over a subcooling range of ΔT = 1-10 K. At ΔT = 1 K and the pure PDMS-coated surface achieved a heat transfer coefficient 2.12 times that of the smooth copper plate (OS-Cu), confirming the benefit of dropwise condensation. Remarkably, the composite coating with PDMS∶Cu = 2∶1 attained an HTC 5.85 times that of OS-Cu and 2.76 times that of the pure PDMS coating at ΔT = 1 K. This enhancement was attributed to the continuous three-dimensional thermal conductive network formed by copper powder within the PDMS matrix, which effectively reduced interfacial thermal resistance according to the Maxwell-Garnett effective medium model. As subcooling increased, the enhancement factor gradually decreased toward unity, consistent with classical dropwise condensation theory.
In conclusion, the one-step laser ablation of a PDMS/copper powder mixture successfully produces a superhydrophobic copper surface with exceptional mechanical durability, thermal stability, and condensation heat transfer performance. The synergistic mechanism involves: (1) the copper network providing efficient phonon transport to lower thermal resistance, and (2) the laser-induced hierarchical structure trapping air, minimizing solid-liquid contact, and protecting the low-surface-energy components. This fluorine-free, scalable approach offers a promising solution for high-performance condensation heat transfer in power generation, water harvesting, and electronics thermal management.
Key words
laser curing /
PDMS/Cu powder /
superhydrophobic surface /
wear-resistant /
high-efficiency condensation
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Funding
National Natural Science Foundation of China (12272153); Jiangsu University of Technology (XSJCX22_64)
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