## 3 Results and Discussion
A faster heterogeneous nucleation rate is also promoted, due to the fact that interface temperature becomes almost as cold as substrate temperature which dramatically changes the receding dynamics contact angle. In fact, complete superhydrophobicity break down was observed at 5 ms (see Supplementary Materials) due to the aforementioned factors. Reduction of the dynamic receding contact angle and rapid increases in viscosity, especially at the contact line up to 8-fold, results in a significant reduction in capillary force which is a function of receding contact angle and liquid drop surface tension, \(\pi\gamma D_{\text{max}}(1-cos\theta_{rec})\). Experimental observation shows that (see Figure 6A) the effect of increased viscosity slightly changes the maximum spreading diameter of an impacting drop (i.e., within the current study) while significantly changing the droplet contact time through reduction of receding velocity. Therefore, by reducing the receding velocity, the chance of heat transfer cooling and the heterogeneous nucleation process is even further increased due to the larger interfacial area. Quantitative analysis of an impacting water drop on superhydrophobic surface in still air, as illustrated in Figure 6B, shows that the capillary force become zero, corresponding to a receding velocity of about 11 ms (i.e., corresponding dimensionless time is 7). Figure 2:1 Comparison of dimensionless contact time of supercooled water droplet at various air speeds from 0 (i.e., still air) to 10 m/s. Results are also compared with water-glycerol (i.e., red triangle) solution with same viscosity to that of supercooled water drop (i.e., sky blue and green error bars represent supercooled water at \(-2\)\({}^{\circ}\)C and \(-3.5\)\({}^{\circ}\)C, respectively). On the other hand, when droplet is under the effect of cold air flow it becomes an even further complicated process due to the presence of another type of nucleation mechanism such as homogeneous nucleation and crystallization induced nucleation from inside-out [52]. Due to the presence of forced convective evaporation cooling, which results in the presence of aforementioned nucleation mechanism, the gas-liquid interface temperature becomes even lower than air temperature, which is a function of air velocity and air humidity [16]. It was observed that, when relative humidity is negligible and the chance of superhydrophobicity degradation is small [53], which is the case in the present study, the effect of evaporation cooling becomes significant and homogeneous nucleation happens at the air-liquid interface. This phenomenon has a further adverse effect on wetting dynamics of an impacting droplet by reducing surface tension force. Likewise, heterogeneous ice nucleation, which changes wetting dynamics by reducing receding dynamic contact angle, the effect of homogeneous nucleation can affect the wetting dynamics by changing the droplet surface tension value. In fact, surface tension of the mixture of ice-water is about 20 mN/m [17] which shows a reduction in liquid surface tension of up to 3.5-fold that of the room temperature droplet. Therefore, after formation of ice nuclei on the gas-liquid interface, it gradually changes the surface tension or capillary force of impacting water drop, especially at the time of maximum spreading diameter where the droplet is exposed to utmost evaporation cooling due to the larger interfacial area. To this end, the capillary force experience largely decreases (i.e., due to combination of both surface tension and apparent receding contact angle reduction, \(\pi\gamma D_{\text{max}}(1-\cos\theta_{rec})\)) where cooled stagnation flow is present. As illustrated in Figure 6B, aforementioned factors result in droplet pinning at 4.5 ms which is slightly after the initiation of the retraction phase. Furthermore, the solidified wetting diameter becomes considerably larger compared to the still air case.