Grain size effects on the wettability of as-grown graphene and dropwise condensation

Abstract

Understanding the wettability of graphene is both fundamentally important and crucial for development of graphene based electronic/optical devices. However, graphene wettability characterized by the static contact angle (CA) has been inconsistent and even contradictory in literature. After systematically characterizing the wettability of chemical vapor deposited polycrystalline graphene, we learn that the wettability of polycrystalline graphene on copper (Cu-Gr) varies significantly with the average graphene grain areas. With the average graphene grain areas increasing from 0.47 μm2 to 22.16 μm2, the static CA on polycrystalline graphene surfaces increases from 47.9° to 68.8°, while the hysteresis CA decreases from 45.9° to 32.7°. Pinning contact lines of nucleated micro-scale droplets along the graphene grain boundary (GB) were identified, indicating inhomogeneous wetting. To further verify the impact of graphene GB on the graphene wettability and manipulating droplet dynamics, we conducted water vapor condensation experiments on the Cu-Gr surfaces. We found that a larger graphene grain size (less graphene GBs per unit area) would lead to a higher droplet departure frequency, resulting in an additional 45% enhancement of dropwise condensation (DWC) heat transfer rates. This study successfully quantifies the role of graphene grain size in determing CAs and reveals that the graphene surface wettability and DWC performance can be effectively engineered by varying graphene grain size.