Fig. 4

Effect of humidity at the contact interface. SFG spectra for pristine aggregate glue/sapphire at low (red empty squares) and high (blue filled squares) humidity collected in PPP (a) and SSP (b) polarizations. Similar spectra are collected for washed aggregate glue/sapphire in PPP (c) and SSP (d) polarizations at low (black empty circles) and high (blue filled circles) humidity. The solid lines are a fit to the Lorentzian equation (Supplementary Tables 3 and 4)