Combining gradient and inclined cilia for enhanced directional liquid transport

Nature has served as a source of inspiration for scientists looking to speed up directional liquid transport, resulting in functional surfaces that mimic tropical pitcher plants, cacti, and spider silk. In such cases, the directional driving force comes from anisotropic surface structures, and previous work has tested surfaces with either an inclined cilia array or gradient-distributed cilia.

In an effort to combine these two strategies, Liu et al. developed a magnetized micro-cilia array surface with a gradient distribution and investigated its effects on directional liquid transport. The researchers were inspired by Ligia exotica, a semi-terrestrial isopod whose legs have a hairy array micropattern in order to drag water upward for imbibition.

The smart surface was fabricated using soft lithography with a magnetically responsive material. A conical stainless steel needle punched holes on a flat paraffin surface to form a micro-pit array, which was then filled in with a polymer mixed with magnetic micro-particles. The resulting flexible magnetic gradient micro-cilia structure had cilia with spacing that varied from 150 to 350 microns along its length, and the tilt angle of the cilia could be controlled by an external magnetic field.

With the gradient and inclined cilia combined in a single surface, an even higher liquid spreading speed with enhanced anisotropic directional spreading was achieved compared to a surface with only one feature.

“The combined strategies for liquid fast spreading and directional guiding could be used in cell diagnoses microfluidic devices, self-supplied heat dissipation, and self-supplied mechanical lubrication,” said author Liwen Zhang.

Source: “Liquid transport with direction guidance and speed enhancement from gradient and magnetized micro-cilia surface” by Guang Liu, Liwen Zhang, Yang Gan, Yan Wang, Dengke Chen, Yuguo Dai, Lin Feng, Pengfei Zhang, and Huawei Chen, Applied Physics Letters (2022). The article can be accessed at https://doi.org/10.1063/5.0089149 .