Experimental investigation of condensation and freezing phenomena on hydrophilic and hydrophobic graphene coating

Abstract

Graphene possesses exceptional electrical, mechanical, thermal, optical and chemical properties with potential for a wide range of applications. While most of the above properties are relatively well studied in past decade, it’s wetting phenomena and surface interactions with water molecules and droplets are still not understood. Graphene has potential for improving performance and saving energy in applications such as, frost delaying, aviation/transportation, and condensate harvesting. In the present study, surface engineering has been applied via graphene coatings to reveal new insight into condensation and freezing dynamics. The tests have been conducted in presence of non-condensable gases (air) at different relative humidity (RH), since RH significantly changes the nucleation energy barrier as well as the nucleation site density. Two sets of graphene coatings on quartz and silicon oxide surface were used to study the condensation and freezing dynamics. Droplet growth was followed by initial nucleation, direct growth, coalescence, and then sweeping off the surface. Later, the effect of ice bridging length, individual ice bridge velocity, and droplet diameter on frosting dynamics were explored for hydrophilic graphene surfaces. A ∼1.6× freezing delay was observed for the hydrophilic graphene coating at a temperature of 271 K and 40% RH compared to a temperature of 271 K and 60% RH. The hydrophobic graphene coating slows the inter-droplet wave propagation and delays frost formation at 60% RH compared to hydrophilic graphene coating. A maximum of ∼3.65× freezing delay was observed for InkJet-printed graphene (IPG) surface compared to hydrophilic graphene surface at the same freezing temperature due to fewer ice bridging instances and jumping droplet phenomenon. Both hydrophobic and hydrophilic graphene surfaces delay freezing compared to a plain silicon surface under same operating conditions. Moreover, the present study has found that surface energy play a more significant role in condensate harvesting compared to relative humidity. This result promises the applicability of engineered graphene surface for significant energy savings in frost delaying, thermal management, and condensate harvesting applications.