Open Access

The gecko is of high interest for scientists due to its ability to attach and to move on different surfaces with various roughnesses. To date, research groups worldwide aim to study adhesion mechanisms of gecko-like structures and to mimic gecko adhesion. However, most investigations have been performed in controlled environments and under near to ideal conditions, which present a significant constraint for transferring the results to applications. Therefore, two important parameters have been the subject of investigations in the present work, the surface roughness and elevated temperatures. For the first time, the impact of roughness on the adhesion of gecko-like, micropatterned structures was systematically studied. Two adhesive regimes, which are dependent on the pillar geometry and the roughness parameters, were discovered: an adhesive and a non –adhesive regime. The influence of the temperature on adhesion was studied on micropatterned samples fabricated out of three materials, which are interesting candidates for industrial applications. Promising correlations were determined between the temperature dependent mechanical properties and the adhesion values: the glass transition temperature was identified as the temperature of maximum adhesion. These results can support the improvement of bioinspired adhesives for industrial applications.

Colloidal crystals can exhibit novel properties arising from the combination of particle properties and collective phenomena of particle packings. Particular colloidal crystals composed of nanoparticles are interesting, because of the unique properties of nanoscale objects, and because of the formation of three-dimensional structures on scales that can be manufactured using established methods only with great technical effort. The aim of this work was to develop appropriate ways to produce the crystals. Two approaches were chosen. In the first approach, colloid particles were deposited on surfaces in a process similar to dip coating. Large-area crystalline particle films with low defect density were obtained by an optimized deposition geometry. In the second approach attractive interactions between particles were used. Reducing the thermal energy induced agglomeration of the particles. This approach allowed production of a variety of particle structures. Besides the expected result, formation of hexagonal particle packings, unexpected results were obtained. In the first approach a superposition of two crystallization mechanisms ensured a robust formation of hexagonal particle packings. In the second approach crystallization among the particles was suppressed in a pure thermally induced agglomeration.

2D and 3D photonic crystals active in the visible wave range are highly interesting for applications, such as waveguiding elements, sensors, or counterfeiting features. However, the tuneable production of such crystals with the current processes is challenging. Two-photon lithography (TPL), which is mainly used to manufacture microstructures, offers this versatility but currently suffers from insufficient structure resolution and mechanical stability for sub-micrometre structures. In the course of this work several novel approaches including an improved development and standing wave enhanced two-photon lithography are presented. These approaches improve the structure resolution and quality, and thus, allow stable features sizes down to 120 nm in horizontal and 45 nm in vertical direction. The new capabilities were used to fabricate distinct photonic crystals inspired by the 2D-pillar grating found on the moth eye and the 3D ‘Christmas tree’-like structures covering the wings of the Morpho-butterflies. Resulting structures were analysed in detail regarding their sizes and optical properties, showing highly effective diffraction, promising anti-reflection properties, and outstanding angle independent iridescence. The experimental work is supported by correlated simulations investigating the optical properties, structures sizes, and the influences of different experimental parameter settings relevant for the fabrication.

In this work, a new process of generating nanobubbles* in transparent polymers was developed. The azo-chemical initiators AIBN (Azobisisobutyronitrile) and ABVN (2,2’-Azobis(2,4-dimethylvaleronitrile)) were used as chemical blowing agents (CBA) that deliver nitrogen gas to form the bubbles. Specifically, the photo initiator Irgacure 819, which could be activated by a longer wavelength of UV (405 nm) than that of AIBN (345 nm), was added to the solution in order to minimize the decomposition of the azo chemicals during the first stage of sample preparation by UV-pre-curing. As a result, the CBAs remained unreacted, thus they could decompose and be used merely as a blowing agent. In addition, the post-foaming processes were conducted under the different foaming conditions to investigate critical factors affecting the nucleation and growth of bubbles. The results were discussed based on thermodynamics, mainly by conventional nucleation theory (CNT). Finally, stepwise optimization of those factors led to the generation of nanobubbles. This technique could be applicable to UV curable polymer systems in general. This novel approach was used to make prototype optical devices, such as security mark coating and light out-coupling in optical waveguide. This study also gives important information about the nucleation and growth of bubbles in meta-stable state of polymers, which could help to verify the background theories.

Nickel-titanium (NiTi) shape memory alloys are functional materials that are capable of undergoing a reversible temperature-induced shape change. Specifically in martensitic NiTi alloys, a reversible two-way shape memory effect can be induced using indentation techniques enabling a temperature-induced change in topography. Combining switchable topographies with nano- or microstructures could expand the properties of functional surfaces, and in addition make the surfaces responsive to their environment. For example, it would be possible to change the adhesive properties of surfaces with switchable dry adhesive microstructures or to control celladhesion on implant materials with specific nano- and micro-switchable structures. In this study, the indentation induced two-way shape memory effect was investigated in different NiTi alloys. In particular, the effects of alloy microstructure, deformation parameters (training) and thermal treatments on switchability were explored. In an austenitic NiTi alloy a specific thermal treatment led to the formation of coherent precipitates, which were shown to be crucial for the two-way shape memory behavior; exceeding the phase transformation temperature considerably decreased the switchability of the topography. At higher temperatures the stabilized martensite, which is required for an oriented phase transformation and consequently for the two-way shape memory behavior, transforms to austenite. An embossing and an electrochemical forming process were developed to prepare switchable topographies on larger areas. Both methods led to surface arrays on NiTi with two-way shape memory topographies. Finally, two approaches were presented, which use the switchable topographies to enable switching of a formerly passive surface function. In combination with bioinspired dry adhesive structures, the switchable NiTi topography led to a reversible, temperature-induced change of the adhesive properties of the surface. Secondly, the two-way shape memory effect was transferred to an alloy system with a phase transformation temperature near body temperature and a small width of hysteresis. By this, a switchable topography was induced, which is controllable within a physiological temperature range. The only issue impeding the use of this switchable surface for experiments on cell-surface interactions is an increased leakage of harmful copper ions. Therefore, surface passivation through oxidation is presented as a method to reduce the ion leakage.

The remarkable adherence of geckos is attributed to the hierarchical structure on their feet pads. Although significant progress has been made, inspired by nature, in fabrication of dry adhesive materials on smooth surfaces, materials with similar adherence against rough surfaces are yet to be found. To better understand the effect of hierarchy on adherence we fabricated macroscopic models made of polydimethylsiloxane with different levels of hierarchy that were brought into contact with glass and with variously rough aluminum substrates. It was shown that adhesion decreases with higher micro- and macroroughness of the substrate. Further no benefits were found for the introduction of hierarchy levels. Another approach to fabricate biomimetic patterns was to exploit polystyrene (PS) µm-particles self-assembled into monolayers on a silicon surface. By treating them with oxygen plasma, nonclose-packed particle arrays with defined spacing were generated. The size and shape evolution of the PS particle layers during etching was analyzed and compared with different etch models. The etch mechanism is more complex than reported in the literature. The resulting patterns were used to fabricate silicon master templates that yield the finest hierarchical level via replica molding. Adhesion measurements were carried out to assess the performance of the softmoldings based on PS particle arrays. The results may help to design new adhesive structures.

The controlled clustering of nanoparticles into defined geometric arrangements has opened a new research area for the design of novel materials with advanced functionalities. This thesis describes a route that exploits liquid droplets as confined templates within which nanoparticles are assembled. Upon removal of the dispersed solvent from the emulsion droplets, the particles formed cluster-like structures. The particles did not arrange into small pieces of dense packings, but resembled clusters predicted for Lennard-Jones interaction potentials. In-situ observation of the assembly process via surface plasmon spectroscopy and small-angle x-ray scattering suggested that assembly occured rapidly, shortly before complete evaporation. The resulting Lennard-Jones geometries represent minimum-energy arrangements of particles. The distribution of the gold nanoparticles in the emulsions was studied for different surfactants. Good surfactants (e.g. Triton X-100) blocked the interface and confined particles in the droplet, whereas others (e.g. Tween 85) formed synergistic mixtures with the nanoparticles at the interfaces. Supraparticles with Lennard-Jones geometries only formed for surfactants that block the interface. The assembly of nanoparticles into minimum-energy clusters was sensitive to interactions between ligands bound to the nanoparticles surfaces, too. The ageing of gold nanoparticles with dodecanethiol ligands was studied for different storage conditions. Surprisingly, fractionation of particles appeared upon ageing. Desorption of ligand was the major process responsible for sedimentation and changes in polarity upon ageing and led to changes in the structure of the supraparticles.

The remarkable properties of bio-inspired microstructures make them extensively accessible for various applications, including industrial, medical, and space applications. However, their implementation especially as grippers for pick-and-place robotics can be compromised by multiple factors. The most common ones are alignment imperfections with the target object, unbalanced stress distribution, contamination, defects, and roughness at the gripping interface. In the present work, three different approaches to assess the contact phenomena between patterned structures and the target object are presented. First, in-situ observation and machine learning are combined to realize accurate real-time predictions of adhesion performance. The trained supervised learning models successfully predict the adhesion performance from the contact signature. Second, two newly developed optical systems are compared to observe the correct grasping of various target objects (rough or transparent) by looking through the microstructures. And last, model experiments are provided for a direct comparison with simulation efforts aiming at a prediction of the contact signature and an analysis of the rate and preload-dependency of the adhesion strength of a soft polymer film in contact with roughness-like surface topography. The results of this thesis open new perspectives for improving the reliability of handling systems using bioinspired microstructures.

Non-covalent adhesion produced by the gecko is attributed to the structured surface of its toes. The synthetic adhesives mimicking this principle have now been around for a decade. However, the characteristic features of reversibility and self-cleaning ability of the gecko adhesive system have not yet been successfully integrated. The present work focuses on developing a switchable adhesive system responsive to an external stimulus. Elastomeric polydimethylsiloxane surfaces are structured with fibrillar arrays. Mechanical instability of the fibrils is recognized and utilized to produce a reversible switch between adhesion and non-adhesion. Normal compression caused the fibrils to buckle inducing a contact transition from their tips to the sides. When the contact transition occurred under moderate compressive loads, tip contact re-formed upon reversal of buckling and adhesion was reversible. However, when reversible buckling occurred under large compressive loads or when fibril side peeled without unbuckling, contact re-formation was impaired. Drastic change incontact area in the re-formed state resulted in a low adhesion state. The role of fibril contact shape, radius, aspect ratio, orientation and the applied compressive load in the adhesion switchability was examined. In situ visualization was employed to study the contact mechanisms. Contact shape, fibril orientation and preload were identified as the key parameters for controlling switchable adhesion.