Constrained droplet base in condensed water on carbon nanoparticle coating for delayed freezing

We studied the freezing of condensed water on carbon nanoparticles coated surfaces under high humidity. The coating is composed of assembled carbon nanoparticles with diameter of 20-30 nm and fabricated through a simple flame synthesis process followed by transfer-printing on polydimethylsiloxane (PDMS) substrates. Through in-situ observation of water condensation using environmental scanning electron microscopy (ESEM), we found that the water nucleation initiated from the underlying PDMS substrate rather than the coating due to the larger free energy barrier induced lower nucleation rate of nanoparticles. Unlike the growth of condensed micro-droplets with constant contact angle on bare PDMS surfaces, the condensed micro-drops on the coating were observed to grow with a constant base area. We showed that the small yet constant solid-liquid contact area during growth resulted in a low heat transfer rate between the condensed water droplets and the substrate, and thus an over twice longer freezing time of condensed water on coated PDMS surfaces than that on uncoated ones. Such a prolonged freezing performance is found to be comparable to that on superhydrophobic surfaces utilizing insulating air pockets to reduce the heat transfer rate, suggesting a complimentary strategy for design of anti-icing materials working under high humidity.