Impact dynamics of supercooled microdroplets on water-repellent coatings

Understanding the impact dynamics of micrometric supercooled water droplets (SWD) on surfaces during ice accretion is essential to the development of new icephobic coatings. In this work, we explore the impact dynamics of SWD on four distinct surfaces with wettabilities ranging from near superhydrophilic to superhydrophobic in an icing wind-tunnel, generating clouds of microdroplets (tens of micrometers in size) with 10 m/s air speed and subzero temperatures down to -18 degrees C. Both hydrophilic and hydrophobic surfaces showed strong pinning of droplets and rapid ice growth, while the superhydrophobic coating demonstrated excellent droplet repellency even at -18 degrees C. In the latter case, neither the contact time nor the restitution coefficient showed any correlation with temperature. This suggests that, for micron-size SWD, surface tension effectively dominates the impact dynamics, contrary to what can be observed in the case of millimetric droplets. The dense nano-texture of the superhydrophobic surface was critical for the repellency. Nonetheless, ice growth was able to spread by the pinning of microdroplets on micro-size defects present in the coating. Interestingly, droplet fragmentation was not observed in the experiments. This study highlights the challenges of ice growth from SWD, while particularly demonstrating the icephobic potential of superhydrophobic coatings for aerospace applications.