Surfaces inspired by geckos can be switched between adhesive and non-adhesive states, study finds

Adhesives inspired by the gecko can be made to switch on and off reversibly and repeatedly. The key design parameters for these materials are identified in a study published in Journal of the Royal Society Interface today.

Geckos use thread-like fibres on their hands and feet to stick to surfaces. Synthetic gecko-inspired adhesives rely on the same fibrillar structures. In both cases nonchemical adhesion is created by concentrating the intermolecular forces between two bodies.

In 2007 researchers from the Leibniz Institute for New Materials, Germany created adhesive materials which could be switched on and off using differences in pressure. Now the same research group have shown precisely how to do this by adjusting the shape of the surface fibres.

Dr Paretkar and his team identified the key parameters that influence adhesion switchability; namely the fibrillar contact shape, radius, aspect ratio, orientation and the applied compressive load. They found that adding flap structures to the ends of the fibrils significantly enhanced how effectively adhesiveness could be switched on and off.

The synthetic adhesive materials are 'switched' on by pressing them against a surface and 'switched' off by increasing their pressure on the surface, which causes loss of adhesion.

The findings mean that new materials can be developed in which adhesiveness can be precisely controlled. This study was conducted using biocompatible material; if the same results can be repeated in biodegradable materials then they could be used during delicate medical procedures in which small objects have to be moved around. These adhesive materials could also be scaled-up and used as fillers in operations such as repairing a damaged ear drum without the use of stitches.

More information: Paretkar, D. et al. Preload responsive adhesion: effects of aspect ratio, tip shape, and alignment, Journal of the Royal Society Interface. dx.doi.org/10.1098/rsif.2013.0171

Journal reference: Journal of the Royal Society Interface

Provided by The Royal Society

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Computational study of ionic liquids illuminates detailed CO2 interactions

The ?2 and ?1-CT structures of the acetate-CO2 complex.

Ionic liquids (ILs), which can be thought of as salts that are molten at room temperature, are being studied for use as part of CO2 adsorption and/or separation technologies. These applications depend on having strong interactions between the CO2 and the ions of the IL. In order for significant advances to occur in this area of research, the interaction between the CO2 and each IL must be understood and described with accuracy. Computational methods are used to describe these interactions on a molecular level.

National Energy Technology Laboratory scientist Jan Steckel has used a variety of methods to elucidate the complex nature of the interactions between CO2 and acetate ion. The results of this study were published recently in the Journal of Physical Chemistry A. The acetate ion was chosen because it is representative of the anions used in many ILs currently under investigation as CO2 sorbents or as part of a separation technology.

Dr. Steckel has shown that the acetate-CO2 potential energy surface is very complex. Eight energy minima, representing the most stable configurations, were located and characterized using computational methods that apply first-principles molecular orbital calculations to obtain an accurate description of these interactions at the molecular level.

The most stable structure is denoted ?2, (eta 2) where the CO2 interacts with both oxygen atoms of the acetate. This complex structure is predicted to have a binding energy of -10.6 kcal/mol, a measure of stability. There are several other complexes with binding energies close to -8.5 kcal/mol, but of these, the ?1-CT complex (eta 1) is unique. This complex is notable because the CO2 is bent to about 140°, the C atoms of the CO2 are only 1.54 Å away from the O of acetate, and there is evidence of charge being transferred from the acetate to the CO2 upon complexation.

Using these interaction energies as benchmarks, it was possible to investigate the degree to which more affordable methods can describe these complexes. Unfortunately, many popular and affordable computational methods do not succeed in describing the ?1-CT complex accurately. This study helps to provide a clear understanding of the acetate-CO2 interaction and supplies previously missing energetic and structural benchmark data. However, another important contribution made by this work is the revelation that widely-used but less accurate methods fail to accurately describe this interaction.

Journal reference: Journal of Physical Chemistry A

Provided by US Department of Energy

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