The values of asymmetry factor γ is shown on the right side of Fig. . Despite that the absolute values of C H are dramatically different between metal-oxide and graphene-like surfaces, the asymmetry factor γ shows similar values. This indicates that the relationship between E ads and γ can be rather general across different types of electrode materials. Furthermore, it is worth noting that the water adsorption energy at graphene surface as determined from highly accurate electronic structure calculations is about 0.1 eV , which corresponds to a high value in the asymmetry factor γ of 0.25 in our quantification. This can be the physical reason behind the observed asymmetric response of the contact angle in the electrowetting experiment as recently performed on this type of materials .