3D Printed Bioinspired Hierarchical Surface Structure With Tunable Wettability

Abstract

Abstract Nature has examples of impressive surfaces and interfaces with diverse wettability stemming from superhydrophilicity to superhydrophobicity. The multiscale surface structures found in biological systems generally have high geometric complexity, which makes it challenging to replicate their characteristics, especially using traditional fabrication techniques. It is even more challenging to fabricate such complex microstructures with tunable wettability. In this paper, we propose a method to tune the wettability of a micro-scale surface by changing the geometrical parameters of embedded micro-structures in the surface. By taking inspiration from an insect (springtails), we designed micro pillar arrays with different roughness by adjusting geometric parameters such as reentrant angle, pitch distance, and the number of spikes and pillars. This study shows that, by changing geometrical parameters in micro-scale, the apparent contact angle, and hence the surface wettability can be calibrated. The micro-scale pillars were fabricated using a precise micro direct light processing (μDLP) three-dimensional (3D) printer. Different printing parameters were studied to optimize the geometric parameters to fabricate 3D hierarchical structures with high accuracy and resolution. The relationship between the wettability of the surface and the geometrical parameters was predicted using a modified Cassie’s model and experimentally validated. The largest apparent contact angle in our experiments is up to 160°, with pillars of 0.17mm height and 0.5 mm diameter, 55° reentrant angle, and a spacing of 0.36 mm between pillars. The lowest contact angle is ∼35° by reducing the pillar size and spacing. By controlling the size of different features of the pillar, pillar number, and layout of the mushroom shaped micropillars, the wettability of the surface is possible to be tuned from a highly nonwetting liquid/material combination to highly wetting material. Such wettability tuning capability expands the design space for many biomedical and thermofluidic applications.