Abstract:

Bioinspired superhydrophobic materials have important applications in the fields， like self-cleaning， anti-fogging， anti-icing， water-oil separation and water collection. The shifting between different hydrophobic states will greatly promote the application in the emerging smart technologies. Here， polyurethane （PU） films with microstructures identical to the rose petal surface were prepared by soft-lithography. The surface microstructure can be dynamically regulated by mechanical stress， which reversibly shifts the surface microstructure between isotropic and anisotropic states. Monitoring the position of capillary's projection（MPCP） was used to quantitatively characterize the friction force of water droplets on the surface of PU films. The influences of the stretching （strain and direction）， the volume and the moving speed of the droplet on the liquid-solid friction were investigated in detail. PU films very-well replicate the microstructures on rose petals that they have the similar static contact angle and sliding angle as rose petals. The PU films thus are superhydrophobic and isotropic， and show strong adhesion to water droplets. The size and spacing of the microstructures along the stretching direction （D_S） and the vertical direction （D_V） change upon the mechanical stretching； however， the water contact angles and sliding angles along the two directions remain same on the surfaces with various elongations. In contact， the friction forces， revealed by CPS， show clear difference along the directions of D_S and D_V， and show clear different dependences on the droplet volume. Increasing the stretching （or in another word the elongation of the sample）， the static friction force （F_S） and dynamic friction force （F_K） of water droplets on the PU film increase significantly， which also shows a clear direction dependence. Along the D_V direction， the frictional resistances decrease monotonically with the increase of the moving speed of the droplet， while in the D_S direction， only when the moving speed of the droplet is higher than 1.55 mm/s， the frictional resistances decrease with the increase of the moving speed. In summary， we achieve the transition between the isotropic and anisotropic states on the highly-adhesive superhydrophobic surface and the regulation of liquid-solid friction by mechanical stress， which paves the way for the design， characterization and application of intelligent surfaces with special wettabilities.

Key words: Surface wettability, Contact angle, Sliding angle, Friction force, Capillary-projection sensing technology, Bioinspired material