Michael Röhrig

3D Direct Laser Writing of Nano‐ and Microstructured Hierarchical Gecko‐Mimicking Surfaces

Applying 3D direct laser writing, artificial hierarchical gecko-type structures are designed and fabricated down to nanometer dimensions. In this way, the elastic modulus and the length scale of the geckos setae are very closely matched. Direct laser writing is a very flexible rapid prototyping method allowing the fabrication of arbitrary nanostructures. Since the parameters of the structures can be easily changed, this technique is perfect for design studies of dry adhesives. Measuring the adhesional forces by atomic force microscopy, the influence of several design parameters like density, aspect ratio, and tip-shape on dry adhesion performance are systematically examined. In this way, it is revealed that hierarchy is favorable for artificial gecko-inspired dry adhesives made of stiff materials on the nanometer scale.

Staying sticky: contact self-cleaning of gecko-inspired adhesives.

The exceptionally adhesive foot of the gecko remains clean in dirty environments by shedding contaminants with each step. Synthetic gecko-inspired adhesives have achieved similar attachment strengths to the gecko on smooth surfaces, but the process of contact self-cleaning has yet to be effectively demonstrated. Here, we present the first gecko-inspired adhesive that has matched both the attachment strength and the contact self-cleaning performance of the geckos foot on a smooth surface. Contact self-cleaning experiments were performed with three different sizes of mushroom-shaped elastomer microfibres and five different sizes of spherical silica contaminants. Using a load–drag–unload dry contact cleaning process similar to the loads acting on the gecko foot during locomotion, our fully contaminated synthetic gecko adhesives could recover lost adhesion at a rate comparable to that of the gecko. We observed that the relative size of contaminants to the characteristic size of the microfibres in the synthetic adhesive strongly determined how and to what degree the adhesive recovered from contamination. Our approximate model and experimental results show that the dominant mechanism of contact self-cleaning is particle rolling during the drag process. Embedding of particles between adjacent fibres was observed for particles with diameter smaller than the fibre tips, and further studied as a temporary cleaning mechanism. By incorporating contact self-cleaning capabilities, real-world applications of synthetic gecko adhesives, such as reusable tapes, clothing closures and medical adhesives, would become feasible.

Bioinspired air-retaining nanofur for drag reduction.

Bioinspired nanofur, covered by a dense layer of randomly distributed high aspect ratio nano- and microhairs, possesses superhydrophobic and air-retaining properties. Nanofur is fabricated using a highly scalable hot pulling method in which softened polymer is elongated with a heated sandblasted plate. Here we investigate the stability of the underwater air layer retained by the irregular nanofur topography by applying hydraulic pressure to the nanofur kept underwater, and evaluate the gradual changes in the air-covered area. Furthermore, the drag reduction resulting from the nanofur air retention is characterized by measuring the pressure drop across channels with and without nanofur.

Hot pulling and embossing of hierarchical nano- and micro-structures

Hot embossing and pulling techniques are presented allowing the fabrication of hierarchical micro- and nano-structures. By utilizing demolding forces we replicate nanopillars with high aspect ratio (AR of 10) and small diameters (200 nm). Furthermore, we draw a dense nanofur either with defined or random design. Introducing a electromechanical sensor system we subsequently fabricated a threefold hierarchical structure. Using these replication techniques tiny hairs for bio-inspired designs can be realized with short cycle times and high scalability.

Tunable Diffractive Optical Elements Based on Shape-Memory Polymers Fabricated via Hot Embossing

We introduce actively tunable diffractive optical elements fabricated from shape-memory polymers (SMPs). By utilizing the shape-memory effect of the polymer, at least one crucial attribute of the diffractive optical element (DOE) is tunable and adjustable subsequent to the completed fabrication process. A thermoplastic, transparent, thermoresponsive polyurethane SMP was structured with diverse diffractive microstructures via hot embossing. The tunability was enabled by programming a second, temporary shape into the diffractive optical element by mechanical deformation, either by stretching or a second embossing cycle at low temperatures. Upon exposure to the stimulus heat, the structures change continuously and controllable in a predefined way. We establish the novel concept of shape-memory diffractive optical elements by illustrating their capabilities, with regard to tunability, by displaying the morphing diffractive pattern of a height tunable and a period tunable structure, respectively. A sample where an arbitrary structure is transformed to a second, disparate one is illustrated as well. To prove the applicability of our tunable shape-memory diffractive optical elements, we verified their long-term stability and demonstrated the precise adjustability with a detailed analysis of the recovery dynamics, in terms of temperature dependence and spatially resolved, time-dependent recovery.

Topographic cues guide the attachment of diatom cells and algal zoospores

Abstract Surface topography plays a key role in the colonization of substrata by the colonizing stages of marine fouling organisms. For the innovation of marine antifouling coatings, it is essential to understand how topographic cues affect the settlement of these organisms. In this study, tapered, spiked microstructures and discrete honeycombs of varying feature dimensions were designed and fabricated in order to examine the influence of topography on the attachment of zoospores of the green macroalga Ulva linza and cells of the diatom (microalga) Navicula incerta. Contrasting results were obtained with these two species of algae. Indeed, the preferred location of cells of N. incerta was dominated by attachment point theory, which suggested a positive correlation between the density of cells adhering and the amount of available attachment points, while the settlement of spores of U. linza was mainly regulated by both Wenzel roughness and local binding geometry.