Spontaneous Laser-Induced Micropatterning on Pre-Strained Elastomeric Surfaces

Functional surface patterns are versatile in engineering technologies such as heat transfer, surface chemistry, and wettability controls. Modulation of surface structures, in micro/nanoscale hierarchies, endow novel characteristics beyond innate material properties via interactions between interfaces. Especially, water contact angle (WCA) variation is a manifest observation of controlled wettability by mediating surface tensions among the solid, liquid, and gas phases. However, general methods of surface patterning, represented by photolithography and mold/die fabrication process for soft lithography technique, are expensive and time-consuming due to their complex procedures and invariable frames with rigid designs. To address these issues, we introduce a straightforward laser direct patterning (LDP) scheme to generate a tight distribution of trenches or periodic wrinkle patterns on pre-strained monolithic elastomer surfaces during stress relaxation. This novel approach transforms a homogeneous bulk material into a mechanical bilayer system by thermal degradation of the polymer, which alters the intrinsic elastic modulus and Poisson’s ratio.<br/><br/>As a conceptual demonstration, stretched Ecoflex samples are explored with laser-induced pyrolysis (LIP) in diverse laser scanning pitches and strain gauge (<i>ε</i>) conditions. After extracting the pyrolytic byproduct using 3M tape, trenches are formed in spacious intervals with unaffected regions remained on the surface at the laser scanning pitches over 50 μm. Overlapping laser scanning pitches result in sinusoidal waveform structures at 25 μm and equivalently rough surface at 10 μm. When the tensile stress was released, dimensional reduction at 100% pre-strain causes moderate adjoining of the trenches while the excessive compressive stress over structural strength generates closing of the gaps and wrinkles at 200% pre-strain. In particular, the surface buckling phenomenon originates from the differences in the elastic modulus and Poisson’s ratio between the diffusion-limited oxidation (DLO) layer and pristine Ecoflex. This bilayer system is induced by the instant heat penetration of laser exposure, producing the stiff oxidized thin layer via thermal degradation at a limited depth. The proportional increments in DLO layer thickness (<i>t</i>) to the laser power, which is a decisive parameter of the resultant morphology, enlarges the amplitude (<i>A</i>) and wavelength (<i>λ</i>) of the wrinkle.<br/><br/>In this study, we present two individual features, densely arranged trenches and repetitive hierarchical wrinkles, by characterizing the effect of surface structures on the WCA. The closer grooving of channels and micro-wrinkles with nanoscale roughness provides considerable underlying air gaps to the water droplet which enhances the hydrophobicity. Typically, oxidized polymers are known to be hydrophilic owing to the enhanced surface energy, however, the sufficient air gaps along the contact area overcome molecular attractions and support the water droplet. As a consequence, the WCA approximately increased by 30% on average (from 105° to 136°) compared with the flat Ecoflex surface. We claim that the proposed LIP micropatterning method for a stretched elastomeric polymer substrate is an adaptable technique for manufacturing hydrophobic polymer surfaces.