Fabrication of bionic hydrophobic micropillar arrays by femtosecond lasers for droplet manipulation

Abstract

<p indent="0mm">Nature is the source of inspiration for human design and manufacture. The preparation of biomimetic functional surfaces is a hot research topic, especially in the preparation of biomimetic hydrophobic surfaces. Common preparation methods include plasma etching, lithography, and so on. But those preparation methods are complex. In this article, a simple strategy for the preparation of hydrophobic micropillar arrays is proposed based on femtosecond laser micro-nano fabrication and replica-mold technology, and the relationship between structural parameters, surface chemical modification, and wettability is systematically investigated. It was found that structural parameters, such as spacing, diameter, and height, had a large effect on the wettability of the chemically modified micropillar arrays. When the spacing of the micropillars increases from 400 to <sc>600 μm,</sc> the droplet sliding angle increases from 31° to 76°. This is because as the spacing increases, the droplets gradually penetrate the gaps between the micropillars. As the droplet slides across the surface, it creates a larger energy potential barrier, so the larger the spacing, the greater the sliding angle, which means the greater adhesion force on the surface. Similarly, when the diameter increases from 100 to <sc>300 μm,</sc> the droplet sliding angle increases from 40° to 80° accordingly. The contact area between the droplet and the micropillar array increases with increasing diameter, that is, the adhesion force between the droplet and the micropillar array increases gradually. Therefore, the sliding angle of the droplet is increased on a larger diameter micropillar array, i.e., the adhesion force is relatively high. The height of the micropillar also affects wettability. When the height is below <sc>100 μm,</sc> the droplet will fall into the structure under the action of its gravity, and contact with the substrate, resulting in a larger adhesion force. Even when the sample is turned over 90°, the droplets do not slip off. When the height is greater than <sc>200 μm,</sc> there is an air layer between the droplet and the substrate, which becomes increasingly visible as the height increases. The sliding angle of the droplet on the micropillar array maintains at ~50° and does not change significantly with increasing height. This is because the contact state between droplets and microcolumns will not be changed after the height increases to a certain extent (≥200 μm). Furthermore, the prepared samples exhibit excellent fatigue resistance. After testing, the maximum tensile rate of the sample can reach 400%, and the micropillar array can still recover to the original morphology. Based on the above experimental results, the surface adhesion force of the micropillar array can be controlled by adjusting the structural parameters, such as diameter and spacing. We designed a micropillar array with different spacings (400 and <sc>600 μm)</sc> to realize the control of droplet sliding behavior by using this property. In addition, the micropillar arrays with different spacings can be combined into specific patterns to realize the micro-reaction of droplets. This preparation method of biomimetic hydrophobic micropillar arrays is not only simple but also widely applicable. The prepared hydrophobic micropillar arrays have potential applications in microfluidic chips, biomedicine, and chemical micro-reactions.