Mechano-responsive wetting transitions on strain-dependent fs-laser-processed PDMS surfaces

Tuning the wettability of smart surfaces is of great interest for innovative lab-on-a-chip applications and microfluidic systems. However, conventional stimuli limit changes of the materials surface energy or morphology. We demonstrate an effective strategy for varying wettability by using surface modification based on strain-dependent fs-laser-processing and dynamic external mechanical forces. For this purpose, groove structures were introduced into unstretched and stretched PDMS surfaces, whereby the structures created under strain can be reversibly opened and closed in response to mechanical stress as a simple and inexpensive stimulus due to the elastomeric properties of the substrate material. These strain-dependent topography changes are accompanied by a change in the liquid–solid contact area and consequently in the wetting properties. Depending on the structural parameters of groove and plateau width, groove depth and period, wetting phenomena on anisotropic groove structures, such as the directional dependence of the contact angle as well as the contact angle hysteresis, have been confirmed on mechano-responsive grooves. Especially on hierarchically structured surfaces, strain-dependent groove-processing allows a dynamic control of the microstructure to reversibly switch between different adhesion states and to introduce isotropic lotus leaf-like or anisotropic rice leaf-like wetting properties.