Footnotes

• Contributors AS is the guarantor for this project. MAS conceived the project. MAS and AS developed the theory and performed the measurements. JEP analysed the data. All authors helped shape the research, analysis and manuscript.

• Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Provenance and peer review Not commissioned; externally peer reviewed.