  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.
