Out-of-Plane Biphilic Surface Structuring for Enhanced Capillary-Driven Dropwise Condensation

TL;DR

This paper introduces a novel out-of-plane biphilic micro-structured surface that enhances droplet ejection during condensation, significantly improving heat transfer efficiency through a capillarity-driven slingshot mechanism.

Contribution

It presents a new three-dimensional biphilic surface design with pyramidal micro-structures that promotes low-volume droplet ejection, surpassing existing superhydrophobic surfaces.

Findings

Droplet ejection volume reduced by up to 56%

Optimal micro-structure angles enhance droplet deformation and ejection

Capillarity-driven ejection mechanism improves condensation efficiency

Abstract

Rapid and sustained condensate droplet departure from a surface is key towards achieving high heat transfer rates in condensation, a physical process critical to a broad range of industrial and societal applications. Despite progress in enhancing condensation heat transfer through inducing its dropwise mode with hydrophobic materials, sophisticated surface engineering methods that can lead to further enhancement of heat transfer are still highly desirable. Here, by employing a three-dimensional, multiphase computational approach, we present an effective out-of-plane biphilic surface topography, that reveals an unexplored capillarity-driven departure mechanism of condensate droplets. This texture consists of biphilic diverging micro-cavities wherein a matrix of small hydrophilic spots is placed at their bottom, that is, amongst the pyramid-shaped, superhydrophobic micro-textures forming the cavities. We show that an optimal combination of the hydrophilic spots and the angles of the pyramidal structures can achieve high deformational stretching of the droplets, eventually realizing an impressive slingshot-like droplet ejection process from the texture. Such a droplet departure mechanism has the potential to reduce the droplet ejection volume and thus enhance the overall condensation efficiency, compared to coalescence-initiated droplet jumping from other state-of-the-art surfaces. Simulations have shown that optimal pyramid-shaped biphilic micro-structures can provoke droplet self-ejection at low volumes, up to 56% lower compared to superhydrophobic straight pillars, revealing a promising new surface micro-texture design strategy towards enhancing condensation heat transfer efficiency and water harvesting capabilities.