Spider webs have inspired a whole range of materials used for diverse application including parachutes, durable yet lightweight clothing and bulletproof vests. Spider silk is lightweight, yet exceptionally strong. In some cases, it is even stronger than steel. It is these combinations of properties that has ultimately led to the materials mentioned being developed.
A team of researchers from the UK, Italy and France and are however the first to explore the acoustic properties of spider webs. Based on their findings, the team have created an acoustic metamaterial based on the web architecture of the Nephila spider, also known as the golden silk orb-weaver. This spider’s web is unusually intricate. Metamaterial is a material made of frequently repeating structures.
In an interview with Phys.org, Federico Bosia, a physicist at the University of Torino in Italy and coauthor of the paper, notes that in recent years many metamaterials have been developed. The quest is to find the most efficient arrangements of wave attenuation and manipulation. The study has shown that the variable elastic properties of circumferential and radial silk, coupled with the spider web architecture, is able to absorb and attenuate vibrations over a wide frequency range, in spite of it weighing so little.
The researchers experimented with various versions of the new spider web inspired acoustic metamaterial. They were able to show that the new prototypes are more easily tuned to different frequencies than other sound controlling materials. It was also found to be more effective at inhibiting low-frequency sound.
When both the heterogeneity stiffening and the mechanical properties of spider silk are considered, the tunable acoustic materials developed could be used for a new range of applications needed to control vibrations. These include acoustic cloaking, earthquake protection for suspended bridges and buildings, sub-wavelength imaging and noise reduction.
Part of the acoustic advantages of the spider web is because of the concentric circles, or rings, of the web. When subjected to vibrations, the rings resonate at a specific frequency. Using this natural architecture as an example, the researchers designed the acoustic metamaterial to have square units. These contain resonating rings supported by ligaments radiating outward from the center of the rings. The same design could be assimilated into many diverse synthetic structures.
Bosia explained that the same type of structure could be used for wave reduction in the acoustic range. Practical examples would include sound reduction originating from road or rail infrastructures. On a larger scale, suspended bridges or tensile structures could possibly be made earthquake resilient through vibration isolation. All of these solutions would utilize a periodic repetition of the spider web like units integrated among the main and suspender cables.
Traditional acoustic materials’ geometry is defined by less than the five parameters that can be tuned with the new metamaterial. As each parameter can be controlled individually, it results in a huge number of variations that respond to different acoustic frequencies. The band gap of materials is defined by the frequency range that is inhibited. Apart from large ranges of tenability, the band gap of spider web inspired acoustic metamaterials is much wider than that of traditional materials.
Future research is planned to study the unusual vibration reducing properties of spider webs. The study will also investigate how these properties can be used in practical applications.
Bosia would also like to understand if a single spider web’s structure facilitates vibration attenuation and if it focuses its effects on different locations within the web that somehow helps to fulfill the spider’s needs. The team is also keen to study other hierarchical architectures in nature to determine if these can also be used in the design of different metamaterials. One application of interest is seismic shields – these would require attenuation at multiple frequency scales.
Study has been published in the journal Applied Physics Letters.