That is, r is relatively insensitive to δ and F , and only depends on the droplet radius R and the contact angle θ, which is a material property given by the Young's relation (Equation ). As shown in Figure , the numerical and experimental values of r remain relatively constant on the approach curve: r num increases from 2.80 to 3.20 µm and r exp increases from 1.70 to 2.25 µm (an increase of between 15 and 30%), when increasing δ from 0 to 1 µm or equivalently F from -1500 nN to 10 nN (position 1 to 2). We found that r num is systematically higher than r exp , probably because our simplified analysis ignores the mechanical properties of the solid oxide layer which exerts an additional repulsive force during the indentation process. We repeated the force spectroscopy measurements 18 times for droplets of different sizes R = 3-15 µm and found that r exp /R = 0.46 ± 0.09 or equivalently θ exp = 152 ± 6 • (Equation ) for different alkanethiol surfaces and maximum applied force F N, max = 5-50 nN (See Supporting Table ), where the error is the standard error from the 18 measurements.