The discovery that van der Waals forces are responsible for the remarkable sticking properties of gecko feet on dry surfaces1 has inspired the nanofabrication of biomimetic adhesive materials.
Fundamentally, van der Waals forces are caused by quantum charge-density fluctuations and arise whenever two neutral, but polarizable, objects are brought in proximity. Unlike chemical binding forces, they do not require immediate contact to ensure notable attraction.
However, their strong power-law decay with increasing distance makes them most effective in micro- to nanoranges. They have not only been measured with high accuracy, but have also been found responsible for stiction in nanofabrication.
In their work, the scientists show that chemically inert polymer films such as ECTFE can be used to achieve high van der Waals adhesion forces on dielectric or metallic substrates.
They predicted these forces by combining quantum-chemistry simulations of the polymers microscopic properties with a macroscopic quantum electrodynamics calculation of the Casimir force between polymer film and substrate.
A simple model of one-dimensional corrugations implies that substrate roughness will diminish the adhesion force. This model operates in a new regime of distances being comparable to or even smaller than the length scales characterizing the corrugations.
The predicted forces are based on the assumption that the polymer film is sufficiently flexible to follow the substrate surface profile and achieve contact distances equal to those found for monomers.
They thus have to be regarded as an upper bound to the forces found on practical implementations.
The team concludes that, to get a more precise estimate of the latter, further simulations of the microscopic structure of the polymer film will be necessary.