Materials can be seen as either viscous or elastic. Viscous materials, when put under stress, shear and flow as often their bonds break but reform in new positions. Elastic materials stretch as energy is put into them and generally return to their original shape so long as the extending force was not too great. There are also materials that share the qualities of both viscous and elastic materials, called uncreatively, viscoelastic. When a force is applied to viscoelastic materials the molecules rearrange to be straighter and this is seen as a creep of the material, the viscous part. The further the molecules straighten the greater the counter stress they exert until eventually the stretching force is balanced. When the force is removed the viscoelastic material will return to its original length, the elastic property. These materials have similar thermodynamics to that of polymer rubbers and so act in a semi similar way.
To investigate these materials further research has been performed on thin layers (between 2 to 14 nanometers thick) by using acoustic waves to trigger the mechanical changes in the layer. For thicknesses below 9nm the viscoelastic properties were independent of the thickness but for ranges above this the phonons and sound speed were observed to change with a clear relation to thickness. Also it was found that sound absorption at these GHz frequencies was based on the square root of the frequency. This contradicts previous findings implying a quadratic or linear relation between a sounds frequency and its absorption coefficient. The movement and propagation of the waves in the viscoelastic material show mathematical analogues for a complex wave with equal real and imaginary parts for k the wavenumber. Biological materials like tendons are often viscoelastic so understanding how vibrations and stresses move through them will allow us to design better safety and phisiopherapy equipment for people who may need it.