Filmwise-to-Dropwise Condensation Transition Enabled by Patterned High Wetting Contrast

Abstract

Recent advances in condensing surfaces with hybrid architectures of superhydrophobic/hydrophilic patterns allow us to decrease the nucleation energy barrier and spatially control the water condensation. However, the condensed water is susceptible to the large pinning force of the hydrophilic area, leading to an ultimate flooding. Here, we demonstrate a hierarchical nanostructured surface with patterned high wetting contrast to achieve a natural transition from filmwise-to-dropwise condensation, which reconciles the existing problems. The energy-dispersive X-ray spectroscopy (EDX) indicates that the fluorinated hydrophobic coating conformably covers the nanostructures except for the tops of micropillars, which are covered by hydrophilic silicon dioxide (FIG 1), resulting in an extreme wetting contrast. Condensation on the hybrid surface was observed in the environmental scanning electron microscope (ESEM) and ambient conditions with controlled humidity. Water preferentially nucleates on the top of micropillars and exhibits a rapid droplet growth (FIG 2). The enhancement is attributed to the filmwise-to-dropwise transition induced by the unique architectures and wetting features of the hybrid surface (FIG 3). The water embryos initially nucleate on the hydrophilic tops and quickly grow to a liquid film covering the whole top area. Since the superhydrophobic surrounding confines the spreading of condensed water, the localized liquid film gradually transits to an isolated spherical droplet as it grows. Remarkably, the condensate morphology transition activates an unusual droplet self-propelling despite the presence of abundant hydrophilic patches. It is important to note that such coalescence-induced jumping is dependent on the size of hydrophilic patches, that is, for larger hydrophilic patches, the energy released by coalescence may not overcome the increased droplet pinning, resulting in an immobile coalescence (FIG 4). The droplet departure ensures the recurrence of filmwise-to-dropwise transition, thus prevents the water accumulation in continuous condensation. These visualizations reveal the undiscovered impact of heterogeneous wettability and architectures on the morphology transition of the condensed water, and provide important insights into the surface design and optimization for enhanced condensation.