Scientific publications

This research is the first of its kind to study the comparison between spherical and flat probe adhesion behavior as a function of viscoelasticity. Viscoelastic properties were tailored through the use of acrylate networks synthesized from tert-butyl acrylate and poly(ethylene glycol) dimethacrylate (PEGDMA) solutions. The molecular weight and the weight fraction of PEGDMA crosslinker was altered to maintain a constant glass transition temperature of approximately 57 °C, but systematically vary the viscoelastic properties and the rubbery moduli (1-62 MPa). Dynamic mechanical analysis was performed to characterize the low-strain thermo-mechanical behavior of the materials. Viscoelastic behavior of the materials was characterized by creep testing and was observed to inversely correlate with crosslinking density. The samples tested with the spherical probe exhibited low pull-off forces at temperatures well above and well below the glass transition temperature of the material. A maximum in pull-off force was observed in the vicinity of the glass transition temperature owing to the viscoelastic energy dissipative processes. The peak in pull-off force was observed to decrease with an increase in crosslinking density and modulus. Adhesion measurements using the flat probe demonstrated a strong dependence of pull-off force on the modulus of the material above the glass transition temperature. It is concluded that viscoelasticity is a dominating factor in increasing the pull-off force values in the vicinity of the glass transition, while it plays a little or no role for temperatures +/-20 °C away from transition region, opening the possibility of thermally switchable adhesives.

Four acrylate-based networks were developed such that they possessed similar glass transition temperature (~-37 °C) but varied in material stiffness at room temperature by an order of magnitude (2-12 MPa). Thermo-mechanical and adhesion testing were performed to investigate the effect of elastic modulus on adhesion profiles of the developed samples. Adhesion experiments with a spherical probe revealed no dependency of the pull-off force on material modulus as predicted by the Johnson, Kendall, and Roberts theory. Results obtained using a flat probe showed that the pull-off force increases linearly with an increase in the material modulus, which matches very well with Kendall's theory.

Adhesion and buckling of single PDMS micropillars were investigated as a function of compressive preload. The micropillars had diameters of 10, 12, 14, and 20 µm and aspect ratios of 1 to 3.3. Adhesion generally increased with a decrease in the aspect ratio. A dependence of pull-off strength on the compressive preload stress was found for micropillars that underwent buckling. When buckling was reversible, tip contact recovered upon unbuckling, which resulted in only a slight reduction of adhesion. In situ observation studies identified irreversible buckling, i.e., lack of tip-contact re-formation, resulting in adhesion loss. It is concluded that the edge radius of the tip, which acts as a circumferential crack, controls adhesion. Fibril buckling is found to be broadly consistent with the predictions of Euler buckling theory.

Crystalline monolayers of polymer particles are useful templates for surface microstructuring. Here, the authors discuss the use of oxygen plasma to tune interparticle distances in such films. A systematic evaluation of the etch process depending on particle size, plasma power, etching time and particle density was performed. The size evolution of individual particles was analyzed using scanning electron microscopy and compared with different models of the etching process. The authors conclude that none of the existing etch models fit the data very well. Analysis of the particle shape throughout the etching process indicates that changes in particle geometry occur depending on their original size and density. In dense films, bridges form between the particles’ original contact points. Particles increasingly deviate from a spherical geometry. Such shape changes are not captured by current models of the etching process. The authors propose a mechanism to explain the formation of bridges between the particles and their role in the preservation of long-range order. This paper is supplemented by supporting information. The associated files are available online for published issues or can be obtained directly from the managing editor (sohini.banerjee@icepublishing.com) for Ahead of Print articles.

The effect of the addition of trioxadecanoic acid (TODS) on the performance of Dye sensitized solar cells (DSSCs) was studied. An increase of the overall light to current conversion efficiency was observed, due to a significant increase of the photocurrent density. The increase of the photocurrent density referred to the more homogeneous porous structure and the relevant increase of the surface area, which allowed higher amount of dye loading. This resulted in a positive impact on the enhancement of the photo current density. The impedance spectroscopy showed that the addition of TODS resulted in a decrease of the charge transfer resistance of the cell, which explained by increasing the charge regeneration.

In this study, polyetheretherketone composites were compounded using a two-screw extruder followed by injection moulding. The effects of multi-fillers on the mechanical properties and crystallization performances were investigated. Differential scanning calorimetry results indicate that the addition of fillers slightly increases the crystallization temperature and crystallinity. Compared to neat polyetheretherketone, the incorporation of inorganic filler leads to a significant improvement in matrix hardness, matrix stiffness and a slight increase in tensile strength. However, the material ductility, the impact strength and the fracture toughness of polyetheretherketone composites decrease. Fractography analyses show that the addition of fillers restraints the ductile deformation of polymers, which is responsible for the reduction of material ductility, impact strength as well as fracture toughness of polyetheretherketone composites.

The tribological properties of Poly-ether-ether-ketone (PEEK) were studied systematically by multiple scratch tests in both unidirection and bidirection mode on the micro- and nano-length scale. The tip geometry has a strong influence on the scratch friction behavior, in particular on the scratch initiation and on the resulting damage patterns. Plowing contributions to friction are significant for nano-scale tips during the initial scratch cycles. Shear contributions dominate for both nano- and micro-scale once a groove has been established by multiple scratches. While the damaging mechanisms are the same for both micro- and nano-scratch tests, the resulting damaging patterns differ depending on the scratching mode (unidirection or bidirection) and the normal load. Patchy layers of material formed by scratching are torn into fracture by nano-scale tips, while they are stretched to flakes by the micro-scratch indenter.

Synthetic dry adhesive surfaces with mushroom-shaped pillars have been the subject of recent research investigation. This study is the first to systematically investigate the effect of peel angle, pillar diameter, and pillar aspect ratio on the force required for peeling. Explicit emphasis was placed on relatively large pillar structures to allow for in situ optical visualization in order to gain insights into fundamental mechanisms which dictate peeling. Traditional molding techniques were used to fabricate optical-scale mushroom terminated structures with pillar diameters of 1 mm and 400 µm and aspect ratios of 1, 3, and 5. Results were quantitatively compared to peel testing theory for conventional adhesives. It was convincingly demonstrated that the critical decohesion energy of a patterned surface changes as a function of angle and cannot be treated as a constant. Variability in the critical decohesion energy was linked to mechanistic differences in detachment through in situ observations and finite element analysis (FEA). Experimental results showed that smaller pillars do not necessarily lead to higher adhesion during peeling, and contact mechanics combined with optical observations were used to explain this phenomenon. Finally, unlike results from normal adhesion studies, aspect ratio was shown to play little role in peeling adhesive behavior due to the mechanics of peel testing. The results and conclusions from this study uncover the detachment mechanisms of mushroom-shape tipped dry adhesives under peel loading and serve as an outline for the design of these surfaces in peeling applications.

Previous research has shown that body-centred cubic (bcc) metals exhibit a smaller size dependence of strength than what is commonly observed in face-centred cubic (fcc) metals. This work investigates compression testing of focused ion beam-manufactured molybdenum pillars ranging in size from 300 nm to 5 μm, both above and below its critical temperature at 300 and 500 K. At 500 K the size effect is found to be consistent with what is observed in fcc metals, owing to the increased mobility of screw dislocations.

Background: Beside the promising application potential of nanotechnologies in engineering, the use of nanomaterials in medicine is growing. New therapies employing innovative nanocarrier systems to increase specificity and efficacy of drug delivery schemes are already in clinical trials. However the influence of the nanoparticles themselves is still unknown in medical applications, especially for complex interactions in neural systems. The aim of this study was to investigate in vitro effects of coated silver nanoparticles (cAgNP) on the excitability of single neuronal cells and to integrate those findings into an in silico model to predict possible effects on neuronal circuits. Methods: We first performed patch clamp measurements to investigate the effects of nanosized silver particles, surrounded by an organic coating, on excitability of single cells. We then determined which parameters were altered by exposure to those nanoparticles using the Hodgkin-Huxley model of the sodium current. As a third step, we integrated those findings into a well-defined neuronal circuit of thalamocortical interactions to predict possible changes in network signaling due to the applied cAgNP, in silico. Results: We observed rapid suppression of sodium currents after exposure to cAgNP in our in vitro recordings. In numerical simulations of sodium currents we identified the parameters likely affected by cAgNP. We then examined the effects of such changes on the activity of networks. In silico network modeling indicated effects of local cAgNP application on firing patterns in all neurons in the circuit.
Conclusion: Our sodium current simulation shows that suppression of sodium currents by cAgNP results primarily by a reduction in the amplitude of the current. The network simulation shows that locally cAgNP-induced changes result in changes in network activity in the entire network, indicating that local application of cAgNP may influence the activity throughout the network.