Fabrication of Reentrant Micromesh Suspended by Shape Memory Polymer for Liquid-Repellency

Omniphobic surfaces, which are referred as having both hydrophobic and oleophobic behaviors under environments involving interaction with liquids, have been vigorously studied due to its robust liquid-repellency. By its non-wetting properties, the liquid-repellent surfaces have potential to be utilized in various applications such as self-cleaning, oil/water separation, droplet manipulation, and antifouling. Accordingly, scientists are developing the liquid-repellent surface by forming microscale structures with a specific functional morphology. Among the representations of the morphology, reentrant topography inspired by springtail in nature have enormous influences on microstructure engineering fields for its exceptional wetting resistance even under liquids with low surface tension than water. Reentrant structures show an overhanging profile, by which a surface can uphold liquids so as to have liquid-repellency. Surface engineers have been consistently trying to imitate the reentrant topography by 3D printing, capillary interaction, self-assembly, photolithography, and imprint lithography ever since the studies of omniphobic surfaces have manifested. Among the aforementioned methods, imprint lithography by the UV light is one of the most commonly employed techniques due to its ability to achieve micro/nanoscale resolution affordably and rapidly. However, imprinting has limited feasibility in complex topography due to the difficulties in demolding process, for which 3-D structures with a semi-freestanding feature that below the surface is excavated and penetrated can be hard to be achieved. This shortcoming limits the practical realization of functional structures present in nature such as springtails, gecko, and so forth. Shape memory polymer (SMP) is a switchable material that has double states which are defined as a permanent shape and a temporary shape. Through manipulation of the shape under external stimuli such as heat or light, the temporary shape can be newly formed and fixed. Then SMP switches back to the permanent shape if triggered again by a stimulus. Recently, scientists have been utilizing SMP’s unique property in developing functional surfaces including wetting controllable structures, diffractive optical elements, and dry adhesive surfaces. Herein, we fabricated suspended micromesh structures along with three different periods of the meshes. The meshes were imprinted on temporary-shaped flattened SMP and recovered to its permanent pillar shape when heat was applied above the glass transition temperature. Accordingly, the meshes were suspended as SMP recovered to its initial pillar shape and reentrant topography was formed consequently. In comparison with simple meshes, suspended micromeshes can realize the Cassie-Baxter state even with a liquid that has the intrinsic contact angle under 90 degrees by the geometry of the reentrant structure. Also, the apparent contact angle of suspended micromeshes increased compared to the simple meshes because of the created air fraction of the surfaces. Considering the reentrant geometry, we calculated the breakthrough pressure of suspended micromeshes based on the Laplace pressure and compared its values to the measured breakthrough pressure. In addition, drag reducing effect of suspended micromeshes were also evaluated to prove its robustness in liquid-repellency. In this work, we proposed a new method to fabricate the reentrant topography with the meshes by a combination of imprint lithography and SMP’s own property. Wettability and hydrostatic robustness of fabricated suspended micromeshes were investigated via measurements of the contact angle, and breakthrough pressure, by which it is demonstrated that the reentrant topography was successfully established based on the comparison between the calculated values and the measured values. In addition, slip lengths were measured to further present the liquid-repellency of the structures owing to its created air layers.