Effects of anisotropic sub-millimeter microstructures on condensation and frosting characteristics of superhydrophobic surfaces

The growth, coalescence and freezing behaviors of condensed droplets during the initial frosting stage on superhydrophobic surfaces significantly influence subsequent growth rate of frost layer height. Developing efficient regulation methods for condensed droplet behaviors is crucial for enhancing the anti-frosting performance of superhydrophobic surfaces. This work fabricated aluminum surfaces featuring anisotropic microstructural arrays at sub-millimeter scale using nanosecond laser ablation, followed by global superhydrophobic modification of the microstructural metal substrates via an immersion method. By establishing a visualization experimental platform, conducting condensation-frosting experiments on superhydrophobic surfaces with typical topologies, including cylinders, triangular prisms and cuboids. The characteristics of droplet condensation and freezing behaviors, freezing front propagation velocity and subsequent frost layer growth properties on these diverse surfaces were obtained. This revealed the multifaceted influence mechanisms of microstructural topology on multi-scale droplet dynamics and surface frosting process. Results demonstrate that sub-millimeter scale microstructures influence frequency of coalescence, coalescence-induced bouncing and multi-droplet coalescence of small-scale droplets during the initial stage of condensed droplet clusters growth, thereby affecting droplet clusters size distribution. Its anisotropy subsequently influences the spatial distribution of large-scale droplets during the later growth stage. Together, these factors influence the droplet freezing time and freezing front propagation velocity. Among the three typical microstructural topologies, the triangular prism microstructure exhibited the most effective anti-frosting performance. Compared with a ordinary superhydrophobic surface, it prolonged the completely freeze time of surface by 106.87 % and reduced growth rate of frost layer height by 27.59 %.