Anisotropic, adhesion-switchable, and thermal-responsive superhydrophobicity on the femtosecond laser-structured shape-memory polymer for droplet manipulation

Abstract

Smart superhydrophobic surfaces with reversible wettability are attracting increasing attention. However, most of the reported smart superhydrophobic surfaces switch their wettability through reversibly changing surface chemistry rather than surface microstructure. In this paper, femtosecond laser direct writing was exploited to create microgroove array structure on the shape-memory polymer (SMP) substrates. The groove-structured surface exhibited superhydrophobicity and anisotropic wettability after fluoroalkylsilane modification. Due to the excellent shape-memory property of the SMP substrate, the morphology of the microgroove array could be transformed between original stand shape and deformed shape in response to heat. Correspondingly, water droplet on the surface was reversibly changed between sliding and sticky states through alternant pressing and heating treatments, indicating switchable adhesion of the laser-induced microgroove array. Meanwhile, the anisotropic wettability could also recover with the restoration of the grooved microstructure. Such wettability conversion on the resultant surface could be cycled 10 times without decline of the superhydrophobicity and anisotropic wettability. The anisotropic superhydrophobic SMP surface with both ultralow and ultrahigh water adhesion was successfully applied in rewritable liquid pattern, droplet-based microreactor, and gas sensing. By taking advantage of femtosecond laser, reversible microstructures and surface wettability can be obtained on various SMP substrates. We believe that such surfaces with thermal-responsive superhydrophobicity, anisotropic wettability, and switchable adhesion will have enormous potential applications in non-loss droplet transfer, biological detection, and lab-on-chip devices.