Hans-Georg Braun

Wetting Resistance at Its Topographical Limit: The Benefit of Mushroom and Serif T Structures

Springtails (Collembola) are wingless arthropods adapted to cutaneous respiration in temporarily rain-flooded habitats. They immediately form a plastron, protecting them against suffocation upon immersion into water and even low-surface-tension liquids such as alkanes. Recent experimental studies revealed a high-pressure resistance of such plastrons against collapse. In this work, skin sections of Orthonychiurus stachianus are studied by transmission electron microscopy. The micrographs reveal cavity side-wall profiles with characteristic overhangs. These were fitted by polynomials to allow access for analytical and numerical calculations of the breakthrough pressure, that is, the barrier against plastron collapse. Furthermore, model profiles with well-defined geometries were used to set the obtained results into context and to develop a general design principle for the most robust surface structures. Our results indicate the decisive role of the sectional profile of overhanging structures to form a robust heterogeneous wetting state for low-surface-tension liquids that enables the omniphobicity. Furthermore, the design principles of mushroom and serif T structures pave the way for omniphobic surfaces with a high-pressure resistance irrespective of solid surface chemistry.

Controlled dewetting processes on microstructured surfaces : a new procedure for thin film microstructuring

Numerous high tech applications of polymer materialsbase on their thin film properties. These applications arerelated to microelectronic devices, optics layers includingphotographic material or diagnostic systems based onbiochip technology. Especially bioanalytical applicationsvery often require polymers in order to bind biofunctionalmolecules by chemical modification within the films.The perfectness of thin and ultrathin films prepared byspin-or dip-coating is strongly dependent on the wettingand dewetting behaviour of the polymer solutions on thesubstrate. In particular the preparation of ultrathin films(thickness a30 nm) is very sensitive regarding the dewet-ting behaviour. A typical scenario that appears by dewet-ting on homogeneous surfaces during the dip-coating pro-cess and the resulting film morphologies is illustrated inFig. 1.These typical features can be also observed duringdewetting of thin molten polymer films.Full Paper: We describe a new experimental attempt togenerate defined ultrathin microstructured polymer films.Microcontact printing of 11-undecanoic acid has beenused to modify the wetting properties of bare gold sur-faces to prepare poor wettable surface domains downwardto 4 lm size. Film formation of polystyrene films thinnerthan 20 nm was achieved by dip-coatingfrom a solutionof polystyrene in chloroform. The nucleation of holes andthe dewetting process was completely controlled by theheterogenity of the surface. Large areas with well definedhole patterns were achieved by the controlled dewettingduring the dip-coating process. The remaining filmshowed no indications for dewetting on the bare gold sur-face. Especially at higher polymer concentrations Ray-leigh instabilities within coalescent rims of adjacent holesare shown to influence the film morphology. Some criteriawhich are relevant for the design of surface film patternsby controlled dewetting are discussed.Low voltage scanning electron microscopy (LVSEM)which is used appears to be an excellent method for thecharacterisation of ultrathin films and organic layersdownward to 5 nm thickness.

Biologically Inspired Omniphobic Surfaces by Reverse Imprint Lithography

Springtail skin morphology is translated into robust omniphobic polymer membranes by reverse imprint lithography. The combination of overhanging cross-sections and their arrangement in a self-supporting comblike pattern are crucial for mechanically stable coatings that can be even applied to curved surfaces.

Crystalline structures in ultrathin poly(ethylene oxide)/poly(methyl methacrylate) blend films

Amorphous poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blend films in extremely constrained states are meta-stable and phase separation of fractal-like branched patterns happens in them due to heterogeneously nucleated PEO crystallization by diffusionlimited aggregation. The crystalline branches are viewed flat-on with PEO chains oriented normal to the substrate surface, upon increasing PMMA content the branch width remains invariant but thickness increases. It is revealed that PMMA imposes different effects on PEO crystallization, i.e. the length and thickness of branches, depending on the film composition. q 2003 Elsevier Science Ltd. All rights reserved.

Ultrathin Microstructured Polypeptide Layers by Surface‐initiated Polymerization on Microprinted Surfaces

Science and Technology: New Series, Vol. 17b: Semiconductors, Springer, Berlin 1982. [24] J. P. Faurie, A. Million, J. Cryst. Growth 1981, 54, 582. [25] K. Nishitani, R. Ohkata, T. Murotani, J. Electron. Mater. 1983, 12, 619. [26] J. P. Faurie, A. Million, J. Piaguet, J. Cryst. Growth 1982, 59, 10 [27] As mentioned in e.g. J. Rockenberger, L. Troger, A. L. Rogach, M. Tischer, M. Grundmann, H. Weller, A. Eychmuller, Ber. Bunsenges. Phys. Chem. 1998, 102, 1561, the size of nanoparticles depends on the method used to determine this quantity. This problem is still under investigation.

In situ experiments to reveal the role of surface feature sidewalls in the Cassie-Wenzel transition.

Waterproof and self-cleaning surfaces continue to attract much attention as they can be instrumental in various different technologies. Such surfaces are typically rough, allowing liquids to contact only the outermost tops of their asperities, with air being entrapped underneath. The formed solid–liquid–air interface is metastable and, hence, can be forced into a completely wetted solid surface. A detailed understanding of the wetting barrier and the dynamics of this transition is critically important for the practical use of the related surfaces. Toward this aim, wetting transitions were studied in situ at a set of patterned perfluoropolyether dimethacrylate (PFPEdma) polymer surfaces exhibiting surface features with different types of sidewall profiles. PFPEdma is intrinsically hydrophobic and exhibits a refractive index very similar to water. Upon immersion of the patterned surfaces into water, incident light was differently scattered at the solid–liquid–air and solid–liquid interface, which allows for distinguishing between both wetting states by dark-field microscopy. The wetting transition observed with this methodology was found to be determined by the sidewall profiles of the patterned structures. Partial recovery of the wetting was demonstrated to be induced by abrupt and continuous pressure reductions. A theoretical model based on Laplace’s law was developed and applied, allowing for the analytical calculation of the transition barrier and the potential to revert the wetting upon pressure reduction.

Branched Crystalline Patterns Formed Around Poly(ethylene oxide) Dots in Humidity

Branched patterns reminiscent of diffusion-limited aggregation form around poly(ethylene oxide) dots on Au surfaces in humidity by crystallization of polymer chains in water-assisted diffusion, involving fractal-like structure and dense branching-like morphology (DBM). Higher humidity leads to a slower growth of crystal lamellae. Crystal growth promotes outward diffusion. Fractal-like lamellae are viewed flat-on. Due to the reduction of chain availability in the diffusion field, the fractal-like structure turns into DBM.

Film formation of crystallizable polymers on microheterogeneous surfaces

Dewetting and crystallization of thin polyethyleneoxide (PEO) films obtained by dip-coating on microheterogeneous surfaces are investigated. Formation of thin polymer films and crystallization are characterized as sequential processes. Film topography and morphology are influenced by surface pattern geometry and polymer solution properties. Under appropriate experimental conditions in which heterogeneous nucleation is avoided, ultrathin non-crystalline PEO films can be prepared which are stable with respect to crystallization over a long time. The experimental procedure established generates films by dewetting on microheterogenous surfaces in which isolated amorphous micrometre-sized areas surrounded from non-wetting barriers are formed. Within these amorphous PEO areas, crystallization can be initiated on request with respect to starting time and location. The crystallization in ultrathin PEO films results in highly branched lamella morphology arising from a diffusion limited aggregation processes (DLA). As time and location for onset of diffusion limited crystallization can be chosen, the morphological features characteristic for DLA growth processes such as correlation width and growth direction of branches can be tuned. In addition, influences of limited material reservoirs in confined areas on film morphology are discussed.

Free-floating hydrogel-based rafts supporting a microarray of functional entities at fluid interfaces

In the present paper, we report a method for fabricating a macroscopic, free-floating device that supports a microarray of molecular functional entities. Therefore, the fabrication process developed by us combines bottom-up and top-down microfabrication strategies for a spatially controllable integration of molecular entities into the macroscopic device. For application, the generated device is transferred to fluid interfaces. Through a combined experimental and theoretical study, we demonstrate that the microscopic cavities of the intrinsically hydrophilic comb-shaped mesh structure enable the flotation of the device at a water–air interface of a sessile droplet due to surface tension effects. The design of the functionalized and free-floating device affords a long-term stable approach with respect to a chemical patterning of this fluid interface without loss of the lateral arrangement of the functional entities over time.

Preparation of evaporation-resistant aqueous microdroplet arrays as a model system for the study of molecular order at the liquid/air interface.

Aqueous arrays of microdroplets typically sized between 2 and 10 microm were generated by microfluid contact printing and stabilized with respect to evaporation by incorporation of poly(ethylene oxide). The arrays are used as a model system for the study of structure formation at liquid/air or liquid/liquid interfaces. In particular, we demonstrated the self-organization of fatty acids with photopolymerizable diacetylene units (10,12-pentacosadiynoic acid) at the liquid/air interface of the microdroplets. Topochemical polymerization behavior of this compound and the autofluorescence property of the resulting polyconjugated polymer are appropriate features to prove the molecular order of the amphiphilic molecules at the interface.