Effects of anisotropic liquid spreading on liquid transport in arrow-like micropillar arrays

Directional Water Wicking on a Metal Surface Patterned by Microchannels

This work focuses on the simulation and experimental study of directional wicking of water on a surface structured by open microchannels. Stainless steel was chosen as the material for the structure motivated by industrial applications as fuel cells. Inspired by nature and literature, we designed a fin type structure. Using Selective Laser Melting (SLM) the fin type structure was manufactured additively with a resolution down to about 30 μm. The geometry was manufactured with three different scalings and both the experiments and the simulation show that the efficiency of the water transport depends on dimensionless numbers such as Reynolds and Capillary numbers. Full 3D numerical simulations of the multiphase Navier-Stokes equations using Volume of Fluid (VOF) and Lattice-Boltzmann (LBM) methods reproduce qualitatively the experimental results and provide new insight into the details of dynamics at small space and time scales. The influence of the static contact angle on the directional wicking was also studied. The simulation enabled estimation of the contact angle threshold beyond which transport vanishes in addition to the optimal contact angle for transport.

Morphology and Surface Properties of Roach Water Transport Arrays

We report on morphological studies of wharf roaches, Ligia exotica, which can passively absorb and transport water through the microscopic protrusions on their legs. We systematically investigated the geometrical variables of the protrusions on each podite of legs to reveal a particularized structural complexity. For the morphological analysis, each podite was split into nine different zones by grouping the protrusions with similar shapes and organization. The protrusions are shown to possess three different types of shapes located on each specific zone of the podite. In addition, the nanoscale surface morphologies of the protrusions on the wharf roach legs were probed by using atomic force microscopy, and the surface properties of the hairy arrays were determined for identifying the localized hydrophobicity distribution. The protrusion surface possessed a nanoscale periodic patterned texture, and both the valley and ridges of a periodic pattern on the protrusion surface exhibited an identical low surface energy. We suggest that the structural morphologies and distinct hydrophobicity of the protrusions can be critical in determining the directional wettability of an entire leg and important for designing a sturdy water transport and passive water-absorbing system without external energy consumption.