Fig. 4

Storage and loss moduli of the ultramarine layer at 2 \({\upmu }m\) and 5 \({\upmu }m\) indentation depths, measured along a frequency sweep profile of 5 measurements from 1 Hz to 10 Hz at an amplitude of A = 0.05 \({\upmu }m\); the increase of the moduli with the frequency indicates the viscoelastic behaviour and the decrease of the moduli with indentation depth illustrates the non-linearity of the properties (with lines as guide to the eye)