Inter-droplet frosting propagation mechanism on micropillar patterned surfaces under various supersaturation conditions

To address the burgeoning demand for anti-icing applications in various environments, an effective means of retarding inter-droplet frosting with a high degree of environmental adaptability is urgently required. In this study, frosting experiments were performed on Si micropillar arrays with varying pitches under controlled environmental conditions, especially at low temperatures and high supersaturation, which indicate more severe icing possibilities. The results showed that droplet behavior was highly dependent on the environment and surface geometry. The droplet numbers at the bottom sites increased with humidity, whereas those at the top sites decreased and the transition in droplet diameter proved to be opposite. Dimensionless quantity eta(b) was proposed to describe droplet distribution under each working condition, and six frost propagation regimes were categorized into two groups: 0 < eta(b) < 0.5 and 0 . 5 < eta(b) < 1. For each condition, six pillar pitches resulted in two or three different regimes, and the lowest frost propagation velocity was observed at the turning point of the last regime (optimal ratio of the normalized pillar pitch to the pillar diameter [ L / D ]). With the reduction of supersaturation, the optimal L/D was 2 at 10 degrees C/90 %RH and 50 %RH, optimal L/D was 3 at 10 degrees C/20 %RH, and optimal L/D was 4 at- 5 degrees C/90 %RH and 50 %RH. This significantly differed from the room-temperature conditions, as the optimal L/D was found to be larger than 4 in previous studies. These findings provide further understanding of the application of patterned surfaces in anti-icing in harsh and variable environments.