where e is dilatation, i ∈ {1, 2, 3} are indices of Cartesian coordinates, λ and µ are the Lame elastic constants, and β is the thermo-elastic constant. The AFM-IR signal is proportional to the surface displacement. Depending on the type of transducer either the amplitude of displacement at a specific frequency 4 or the time dependent surface displacement 10 is recorded in AFM-IR. Considering the insignificance of the photoacoustic signal in AFM-IR measurements compared to the photothermal signal, along with its tendency to introduce artifacts , we have made the decision not to include it in the current model.