DOE OSTI2019
Condensation heat transfer is a vital process for a plethora of industrial application. Dropwise condensation of steam on hydrophobic polymers, typified by the formation of discrete water droplets which shed and clear the surface for re-nucleation, has a 10X higher heat transfer coefficient when compared to filmwise condensation on bare metallic surfaces. However, 8 decades of research have conclusively demonstrated a lack of durability of the hydrophobic coatings during dropwise condensation to quantify industrial implementation. They key bottleneck is the coating conundrum, which states that thick hydrophobic coatings (> 10 µm) are required for higher durability, while thin hydrophobic coatings (< 1 µm) are required for low parasitic thermal resistance due to the low intrinsic thermal conductivity of polymeric materials (k ~ 0.1 W/m·K). Here, we attempt to solve the conundrum by developing rationally-designed metal-polymer composite coatings to increase the effective thermal conductivity of the hydrophobic coating, thereby enabling thicker coatings for higher durability. We study nanowire, inverse opal, and sintered sphere metallic structures infilled with hydrophobic polymers. To quantify the heat transfer performance, we analyze the vapor-to-coolant and vapor-to-surface heat transfer coefficients of composite coated flat surfaces having varying wire/sphere diameters (D = 100 nm, 1 μm, 10 μm) and coating thickness (10 nm < t < 100 μm) using three-dimensional (3D), steady state, Comsol simulations. We show that analytical models such as Maxwell or effective medium theory fail to predict the composite thermal conductivity. By conducting simulation with two independent sets of boundary conditions; isothermal surfaces and convection, as well as by varying the interfacial metal fractions, we demonstrate the importance of 3D heat spreading near the coating-vapor and coating-substrate interface. To predict the wetting characteristics to ensure stable dropwise condensation, we coupled our 3D heat transfer simulations to a unified model for contact angle hysteresis on heterogeneous surfaces. Our wetting predictions identify the key tradeoff between low spreading resistance (high k, filmwise condensation) and low contact angle hysteresis (low k, dropwise condensation) on high and low surface metal fractions, respectively, and identify the optimum metallic structure, volume fraction, interfacial solid fraction, and thickness for enhanced durability. Our work provides a novel approach to utilize rational metallic nanostructuring to create composite hydrophobic coatings which promise enhanced durability.