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We demonstrate a novel approach for controlling the line defect formation in microscopic wrinkling structures by patterned plasma treatment of elastomeric surfaces. Wrinkles were formed on polydimethylsiloxane (PDMS) surfaces exposed to low-pressure plasma under uniaxial stretching and subsequent relaxation. The wrinkling wavelength λ can be regulated via the treatment time and choice of plasma process gases (H2, N2). Sequential masking allows for changing these parameters on micron-scale dimensions. Thus, abrupt changes of the wrinkling wavelength become feasible and result in line defects located at the boundary zone between areas of different wavelengths. Wavelengths, morphology, and mechanical properties of the respective areas are investigated by Atomic Force Microscopy and agree quantitatively with predictions of analytical models for wrinkle formation. Notably, the approach allows for the first time the realization of a dramatic wavelength change up to a factor of 7 to control the location of the branching zone. This allows structures with a fixed but also with a strictly alternating branching behavior. The morphology inside the branching zone is compared with finite element methods and shows semi-quantitative agreement. Thus our finding opens new perspectives for “programming” hierarchical wrinkling patterns with potential applications in optics, tribology, and biomimetic structuring of surfaces.

A simple, novel, and completely lithography-free assembly strategy is reported for directed colloidal crystal assembly on optically transparent substrates. The templates are stable, well-controlled relaxation-wrinkles of ultrathin multilayer films in a non-stretched state, which are fabricated by layer-by-layer self-assembly of polymeric films on soft elastomeric substrates followed by uniaxial plastic deformations. The results show that the wrinkles can be used efficiently to topographically direct colloidal crystal assembly in dip coating. Remarkably highly regular 1- and 2-dimensional patterned colloidal crystals with controlled structures have been obtained. Furthermore, the concept is rather universal and applicable to various particle types provided surface interactions between particles and template are suitable.

We report here the first AFM single capsule mechanical measurements on hydrogen-bonded polymeric multilayer microcapsules made of poly(vinylpyrrolidone)/poly(methacrylic acid) (PVPON/PMAA) and of poly(-vinylpyrrolidone-co-NH-20)/poly(methacrylic acid) (PVPON-co-NH-20/PMAA), as well as of capsules derived from these systems chemical crosslinking. The stiffness of the non-crosslinked hydrogen-bonded capsules was found to be proportional to the square of the wall thickness which is in agreement with previous observations on other multilayer capsules and continuum mechanical theory. The found elastic modulus of 610 MPa for low pH (= 2) is typical for a highly stable, glass-like structure similar to electrostatically bound multilayers. At pH > 6, (PMAA) capsules obtained through chemical crosslinking of hydrogen-bonded (PVPON/PMAA) multilayers, or crosslinked (PVPON-co-NH-20/PMAA) capsules showed a sharp hundreds-fold stiffness decrease to ∼1 mN/m which was orders of magnitude lower than those reported earlier for polymeric multilayer systems. pH-Triggered softening was reversible and highly reproducible. Softening of both (PMAA) and (PVPON-co-NH-20/PMAA) crosslinked capsules resulted from increased PMAA ionization, and additional dissociation of intermolecular hydrogen bonds occurred in the case of (PVPON-co-NH-20/PMAA) crosslinked system.

Alignment of nanoparticles to hierarchical periodic structures is an emerging field in the development of patterned surfaces. Common alignment methods are based on templates that guide particle self-assembly. These can be formed using lithographic methods offering an almost free choice of the motif, while being expensive and time-consuming for large-scale production. Alternatively, template formation by controlled wrinkling offers a low-cost formation, but often suffers from the formation of defect structures like line-defects and cracks. Here, we show a preparation technique for nanoparticle alignment substrates that is based on the inscription of holographic surface relief gratings with a periodic sinusoidal wave pattern on the surface of azobenzene films. As interference patterns are employed for structure formation, very uniform and defect-free gratings with tunable grating height and grating period can be prepared. These substrates were successfully replicated to poly(dimethyl siloxane) and the replicas used for the alignment of polystyrene latex particles. Accordingly produced substrates exhibiting gratings with a variation in grating height allow for efficient screening of nanoparticle alignment in a geometrical confinement in one single experiment. We anticipate our studies as a promising tool for the development of sensors, tunable gratings and metamaterials.

Control and tuning of surface properties is indispensable for the programmed and rational design of materials. Particularly, polymeric brush-modified colloids can be used as carrier materials for enzyme immobilization. Although it is of prime importance to control the brush architecture, there is still a lack of systematic investigations concerning the impact of grafting density on the properties of the designed interface, as well as on the immobilization of biomolecules. In this work, we investigate the surface properties of polymer brushes with different grafting densities prepared using a “grafting from” approach on flat and on colloidal particle substrates by varying the density of initiator groups. In this way, we control and tune interfacial properties of the carrier material such as swelling, charge, adhesion as well as adsorption of laccase from Trametes versicolor on the grafted polyelectrolyte layer. We show that there is no direct transferability of the results received from planar to curved substrates regarding the swelling behavior in dependence on the grafting density. The maximum of swelling degree of PDMAEMA layers is achieved at 0.34 nm−2 and at 0.1 nm−2 grafting density for planar and curved particle substrates, respectively. The adhesion properties of the polymeric layer on both substrates are also strongly influenced by the grafting density, i.e. a decrease of the grafting density causes a transition from the adhesive to non-adhesive state. As proven by the cryo-TEM and AFM force distance measurements, an immobilization of laccase from Trametes versicolor causes a decrease of the polymer swelling and therefore leads to the changes in the surface morphology, charge and adhesion performance of final polymer–enzyme layer. Moreover, the higher effectiveness and activity of laccase were observed for the intermediate grafting densities which seem to be preferable over the maximum brush densities.

Controlled surface wrinkling is widely applied for structuring surfaces in the micro- and nano-range. The formation of cracks in the wrinkling process is however limiting applications, and developing approaches towards crack-free wrinkles is therefore vital. To understand crack-formation, we systematically characterized the thickness and mechanics of thin layers formed by O2-plasma-oxidation of polydimethyl siloxane (PDMS) as a function of plasma power and pressure using Atomic Force Microscopy Quantitative Nano-mechanical Mapping (AFM-QNM). We found a nearly constant layer thickness with simultaneously changing Young's moduli for both power and pressure screenings. We determined the respective crack densities, revealing conditions for crack-free wrinkling. Thus we could identify correlations between the intensity of plasma treatment and the cracking behavior. The primary cause for crack-suppression is a continuous elasticity gradient starting within the soft bulk PDMS, and rising up to several hundred MPa at the oxidized layer's surface. With mechanical simulations via the Finite Elements Method (FEM) we were able to demonstrate a noticeable difference in maximal stress intensity σmax between a comparable, but theoretical single layer and a gradient interface. A threshold in tensile stress of σcrit = 14 MPa distinguishes between intact and cracked layers.

We demonstrate a novel approach for controlling the formation of line-defects in wrinkling patterns by introducing step-like changes in the Young's modulus of elastomeric substrates supporting thin, stiff layers. Wrinkles are formed upon treating the poly(dimethylsiloxane) (PDMS) substrates by UV/Ozone (UVO) exposure in a uniaxially stretched state and subsequent relaxation. Line defects such as minutiae known from fingerprints are a typical feature in wrinkling patterns. The position where these defects occur is random for homogenous substrate elasticity and film thickness. However, we show that they can be predetermined by using PDMS substrates consisting of areas with different cross-linking densities. While changing the cross-linking density is well known to influence the wrinkling wavelength, we use this parameter in this study to force defect formation. The defect formation is monitored in situ using light microscopy and the mechanical parameters/film thicknesses are determined using imaging AFM indentation measurements. Thus the observed wrinkle-wavelengths can be compared to theoretical predictions. We study the density and morphology of defects for different changes in elasticity and compare our findings with theoretical considerations based on a generalized Swift-Hohenberg-equation to simply emulate the observed pattern-formation process, finding good agreement. The fact that for suitable changes in elasticity, well-ordered defect patterns are observed is discussed with respect to formation of hierarchical structures for applications in optics and nanotechnology.

We present key factors that influence the mechanical stability of polyelectrolyte/nanoparticle composite microcontainers and their encapsulation behavior by thermal shrinkage. Poly(diallyldimethylammonium chloride) (PDADMAC), poly(styrenesulfonate) (PSS) microshells and citrate-stabilized gold nanoparticles are used. The presence of nanoparticles in the microshell renders the encapsulation process by heat-shrinking more difficult. The encapsulation efficiency is found to decrease as the concentration of material to be encapsulated increases. Increasing nanoparticle content in the microshell or the concentration of dextran increases the likelihood of getting fused and damaged capsules during encapsulation. On the other hand, mechanical studies show that doping microshells with gold nanoparticles significantly increases their stiffness and resistance to deformation. Internalization of capsules by cells supports that the incorporation of metal nanoparticles makes the shells more resistant to deformation. This work provides information of significant interest for the potential biomedical applications of polymeric microshells such as intracellular storage and delivery.

The adsorption of ionic amphiphilic diblock copolymers comprising a polycationic block, polybutadiene- block-poly(2-(dimethylamino)ethyl methacrylate) (PB-b-PDMAEMA); and its quaternized derivative (PB-b- PDMAEMAq) from aqueous media onto graphite-based surfaces was examined. Both diblock copolymers in aqueous solution form star-like micelles with a hydrophobic PB core and a cationic corona built up from either strong cationic PDMAEMAq or pH-sensitive PDMAEMA. AFM experiments show that PB-b-PDMAEMAq micelles interact slightly with a graphite surface providing films with a low surface coverage. PB-b-PDMAEMA micelles adsorbed onto a graphite surface at pH 7 result in a more homogeneous coverage of the graphite surface by the diblock copolymer. The adsorption of two enzymes, tyrosinase (Tyr) and choline oxidase (ChO) on the graphite surface premodified with these diblock copolymers was also monitored by AFM and by electrochemical measurements of the enzymatic activities of PB-b-PDMAEMA--Tyr and PB-b-PDMAEMA--ChO films. A pronounced increase in the enzymatic activity of tyrosinase was observed with the increasing concentration of PB-b-PDMAEMA micelles in solution used for their depositions. Also, a pronounced increase in the enzymatic activities of both tyrosinase and choline oxidase was observed with the increasing pH of the deposition of the micelles from 2 to 10. The enzymatic activity increases with the coverage of the graphite surface with the preadsorbed copolymer. Finally, the polymer--enzyme films were tested as biosensors for phenol (when tyrosinase was adsorbed) and choline (when choline oxidase was adsorbed) and their activity and stability were compared to already existing setups.

We created hierarchically ordered structures of nanoparticles on smooth planar hydrophilic substrates by drying colloidal dispersions in confinement under macroscopic stamps with microscopically wrinkled surfaces. Experiments were carried out with model nanoparticle suspensions that possess high colloidal stability and monodispersity. The structures ranged from single parallel lines of particles to arrays of dense prismatic ridges. The type of observed structure could be controlled by the particle concentration in the initial dispersion. Confinement between two crossed stamps led to interconnected meshes of particles. The precise morphology could be predicted in all cases by Monte Carlo computer simulations of confined hard spheres. Our findings open up possibilities for versatile nanoparticle assembly on surfaces.

We present a novel method combining fluorescence microscopy with AFM force spectroscopy to investigate release of encapsulated materials from polyelectrolyte multilayer capsules upon mechanical deformations. The experimental approach allows independent measurement of release and plastic deformation thresholds. Our results reveal a well defined critical load upon which release takes place and let us correlate release with the onset of plastic deformations.

We present a method based on colloidal probe atomic force microscopy (AFM) to measure adhesion energies and to study other contact phenomena of surfaces. The method employs an elastomeric colloidal probe, rendering the contact area between probe and sample much larger as compared to standard atomic force microscopy techniques. The technique allows us to determine the contact area via microinterferometry and measure the applied forces at the same time. The adhesion properties can then be accessed by using the Johnson–Kendall–Roberts (JKR) approach, i.e. measuring (a) the contact area as a function of applied load, and (b) the elastic parameters and the thermodynamic work of adhesion. We test this method in ambient conditions as well as in aqueous media on well-known surface chemistries, and can clearly characterize the contributions of capillary in air, hydration forces and hydrophobic interactions in water. This novel method provides a means to study the contact behavior of soft colloids and enhanced sensitivity for adhesion measurements.

We report a novel approach to model the temperature dependence of the volume and elastic modulus of polymer gels with a cross-linking gradient based on the well-known Flory-Rehner (FR) theory. The proposed model consists of dividing the inhomogeneous polymer gel into an arbitrary number of independent and homogeneous layers that are treated independently by the FR approach. Experimental measurements of volume and elastic modulus with atomic force microscopy (AFM) on PNIPAM microgels, unexplained by the classical FR model, are in good agreement with the proposed model.

In this study we describe the first direct, real-space characterization of a novel type of poly(vinyl alcohol) (PVA) based microballoons in aqueous environment using scanning transmission X-ray microscopy (STXM). From the oxygen K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra taken from the microballoon interiors we could unambiguously distinguish between water- and air-filled particles. We also demonstrate that STXM imaging below and above the O K-edge (520 eV and 550 eV) can provide unique information on the composition of microballoons in water. It was found that the particular microballoon system investigated here has a remarkable high stability and is able to contain gases for more than 6 months, making it well suited for biomedical applications.

Microfibrillar cellulose has been organised into nanofilms with well-controlled features. This has been achieved with adsorption onto microcontact printed surfaces and with a novel microcontact lift-off technique.

We studied the deformation properties of single Pickering-emulsion droplets using a combination of Scanning Force Microscopy (SFM) in force spectroscopy mode and Reflection Interference Contrast Microscopy (RICM). The droplets were made from an oil-in-water emulsion system which was stabilized by cowpea mosaic virus (CPMV) particles; the interfacial particle layers were either native or crosslinked after interfacial assembly. A general problem in the interpretation of deformation measurements of such complex microcapsules is the fact that both surface tension and mechanical membrane tensions may contribute to the force response of the droplet against deformation. While of different nature, the two contributions are difficult to separate and their relative importance in Pickering droplet deformation is unknown. In our approach, the droplet shape can be reconstructed during deformation using the RICM data. This enables calculation of stress–strain relationships of the droplet surface and a comparison with a continuum mechanical shell theory. In these relations interfacial tensions are evidenced as an offset of membrane tension at zero deformation, while mechanical membrane-tensions increase for further deformation as expected by membrane theory. We find that for this particular system, interfacial tensions dominate the response to deformation.