Multiscale Hierarchical Surface Patterns by Coupling Optical Patterning and Thermal Shrinkage

Abstract

Herein, a simple hierarchical surface patterning method is presented by effectively combining buckling instability and azopolymer-based surface relief grating inscription. In this technique, submicron patterns are achieved using azopolymers, whereas the microscale patterns are fabricated by subsequent thermal shrinkage. The wetting characterization of various topographically patterned surfaces confirms that the method permits tuning of contact angles and choosing between isotropic and anisotropic wetting. Altogether, this method allows efficient fabrication of hierarchical surfaces over several length scales in relatively large areas, overcoming some limitations of fabricating multiscale roughness in lithography and also methods of creating merely random patterns, such as black silicon processing or wet etching of metals. The demonstrated fine-tuning of the surface patterns may be useful in optimizing surface-related material properties, such as wetting and adhesion, producing substrates that are of potential interest in mechanobiology and tissue engineering.

Keywords: azopolymers; hierarchical surfaces; surface relief gratings; tunable wetting; wrinkling instability.

Figure 1

(a) Chemical structure of the supramolecular polymer–azobenzene complex and (b) schematic of the fabrication process.

Figure 2

(a) Diffraction efficiency curve for the inscription of an azopolymer (SRG) with ca. 1 μm period. (b) Digital holographic microscopy (DHM) phase image of the same SRG and the average surface profile.

Figure 3

Digital holography (DH) and SEM images of samples with (a, b) 1D and (c, d) 2D optical patterns and after inducing (e–g) 1D and (f–h) 2D thermal wrinkles. (Small-scale grating pattern periodicity otherwise 1 μm, for (c), 1.5 μm).

Figure 4

Optical images of droplets on the 2D–1D sample when the contact line is advancing (a) along the grooves and (b) across the grooves. The scale bar (bottom right) is 1 mm.

Figure 5

Schematic representation of the selective spinning technique, as well as (a–e) SEM images of the resulting gradient surface with ca. 0.4 μm distances, demonstrating a gradual transition between 1D and 2D larger-scale wrinkles, and (f) the wavelength periodicity of 2D wrinkling as a function of deviation from the 1D region, derived from FFT analyses.