Scientific publications

Highly enhanced and selective adhesion can be achieved between surfaces patterned with charges even when each one has no net charge. In this and a companion paper [C. Jin, Y. Bai, A. Jagota, and C.-Y. Hui, J. Appl. Phys. 110, 054903 (2011)] we analyze the possibility of adhesion selectivity between two flat surfaces patterned with stripes of surface charge chosen such that each surface initially carries no net charge. A few combinations, with appropriately matching strip widths, are predicted to adhere to each other. We also find that the deformability of the materials plays a critical role in defining the range of patterns that recognize each other, i.e., their selectivity. With increasing compliance, a significant enhancement of adhesion can be achieved by deformation that allows better matching between charge patterns.

The effect of viscoelasticity on adhesion was investigated for micropatterned epoxy surfaces and compared to nonpatterned surfaces. A two-component epoxy system was used to produce epoxy compositions with different viscoelastic properties. Pillar arrays with flat punch tip geometries were fabricated with a two-step soft lithography process. Adhesion properties were measured with a home-built adhesion tester using a spherical sapphire probe as a counter-surface. Compared to flat controls, micropatterned epoxy samples with low viscoelasticity (i.e., low damping factors) showed at least a 20-fold reduction in pull-off force per actual contact area for both low (E′ = 2.3 MPa) and high (E′ = 2.3 GPa) storage moduli. This antiadhesive behavior may result from poor contact formation and indicates that the adhesion performance of commonly used elastomers for dry adhesives (e.g., polydimethylsiloxane) is governed by the interfacial viscoelasticity. Adhesion significantly increased with increasing viscoelasticity. Micropatterned samples with high viscoelasticity showed a 4-fold reduction in adhesion for aspect ratio (AR) 1.1 patterns but a 2-fold enhancement in adhesion for AR 2.2 patterns. These results indicate that viscoelasticity can enhance the effect of surface patterning on adhesion and should be considered as a significant parameter in the design of artificial patterned adhesives.

Morphological and crystallographic structures of multilayered W/Cu nanocomposite thin films elaborated by physical vapor deposition were studied by varying copper and tungsten thicknesses. Sample examinations were performed by x-ray diffraction (XRD), grazing incidence small-angle x-ray scattering and transmission electron microscopy (TEM). Samples were found to be composed of copper nanoparticles, homogeneously dispersed in planes parallel to the film-substrate interface and periodically separated by tungsten layers along the growth direction. Our observations revealed an original texture development of the tungsten matrix from a mixture of unexpected α-W<111> and α-W<110> components to unique α-W<110> component as the copper coverage passes a thickness threshold of 0.6 nm. Local TEM texture stereology investigations revealed simultaneous columnar growth of both preferential orientations posterior to polycrystalline development while XRD reveals strong compressive residual stresses in both texture components. Physical origins of the preferential crystallographic orientation evolution are discussed. Copper mono layers adsorption on W surfaces promotes surface energy anisotropy and diminution which is effective over the threshold. Below, the presence of a W(Cu) solid solution which does not affect substantially the texture is revealed by the stress-free lattice parameter value extracted from XRD.

The remarkable mechanical properties of natural contact surfaces have inspired a great deal of interest and research in recent years. The underlying driver of this interest is the surprising range of surface mechanical properties such as adhesion, friction, and compliance that can be attained primarily by design of near-surface architecture using generic materials properties. A considerable literature has developed spanning the range from biological studies of structure and properties, through models to understand these properties, to development of bio-mimetic and bio-inspired structures, along with theory to understand how structure leads to development of surface mechanical properties. The literature has matured sufficiently that common architectures and principles have emerged, for which we attempt here to present a unified view. The field remains vibrant so we hope that this review can at the same time help in its further progress. Our goal in this paper is to review the field from the point of view of scientists and engineers interested to learn about the architecture, properties, and mechanisms of contacting surfaces in nature and how these might be mimicked to create new materials with unique and interesting properties. We begin with a brief description of natural systems, their architectures and properties, and follow by a discussion of the main bio-mimetic and bio-inspired materials that have been developed recently. We then discuss surface mechanical properties – adhesion, friction, and compliance – how these are related to materials and architectural parameters, and how these properties are measured. Where possible, we provide quantitative models for the relationship between structure and properties. We conclude the paper with a discussion of outlook and future possibilities in this field.

In Part I of this work, using a one-dimensional (1D) representation of the two-dimensional (2D) electrostatic and ion concentration fields, we showed that highly selective adhesion can be achieved between two charged surfaces separated by an electrolyte. In Part II of this work, we provide a detailed theoretical justification of our 1D model by solving the electrostatic problem in a 2D configuration. We establish the conditions under which a 1D approximation is accurate in terms of material and geometrical parameters. We provide the full 2D solution that can be used to analyze other physical phenomena involving migration and separation of particles under the influence of fixed surface charges.

We studied the effects of pillar dimensions and stiffness of biocompatible and biodegradable micropatterned surfaces on adhesion on different compliant substrates. The micropatterned adhesives were based on biocompatible polydimethylsiloxane (PDMS) and biodegradable poly(lactic-co-glycolic) acid (PLGA) polymer systems. Micropatterned and non-patterned compliant PDMS did not show significant differences in adhesion on compliant mice ear skin or on gelatin-glycerin model substrates. However, adhesion measurements for micropatterned stiff PLGA on compliant gelatin-glycerin model substrates showed significant enhancement in pull-off strengths compared to non-patterned controls.

Adhesion measurements on poly(dimethyl)siloxane samples were performed, for the first time, with flat glass probes under controlled tilt angle and the results were compared with measurements from spherical probes of two different radii. Experiments were made on both flat and patterned samples with structure diameters of 4.7 μm and heights of 0.82 μm and 1.95 μm, respectively. Pull-off forces measured with spherical probes showed the usual preload dependence and were independent of misalignment angle. On the other hand, pull-off forces measured with aligned flat probes were preload-independent, but dropped significantly and became preload-dependent with increasing misalignment. This effect was more pronounced for structured samples, where a misalignment by 0.2° resulted in a drop of adhesion by more than 30%. The comparison indicates that measurements from spherical probes underestimate adhesive forces for structured surfaces if compared with aligned flat probes. Finally, we propose a simple model which allows the prediction of angle-dependent plateau values of pull-off forces for measurements with flat probes on flat samples.

We developed a dry synthetic adhesive system inspired by gecko feet adhesion that can switch reversibly from adhesion to non-adhesion with applied pressure as external stimulus. Micropatterned polydimethylsiloxane (PDMS) surfaces with pillars of 30 µm length and 10 µm diameter were fabricated using photolithography and moulding. Adhesion properties were determined with a flat probe as a function of preload. For low and moderate applied compressive preloads, measured adhesion was 7.5 times greater than on flat controls whereas for high applied preloads adhesion dropped to very low values. In situ imaging shows that the increased preload caused the pillars to deform by bending and/or buckling and to lose their adhesive contact. The elasticity of PDMS aids the pillar recovery to the upright position upon removal of preload enabling repeatability of the switch.

This manuscript studies the formation of nanopatterns onto PET films under different experimental conditions. Homogeneous patterns with randomly distributed bumps or nanofibrils were observed depending on the plasma parameters. The influence of the power, oxygen pressure and electrode charge in the etching rate and final nanotopography is studied and discussed. The superhydrophobic properties of the nanostructured films are analysed.