Conventionally, the contact angle hystersis ∆θ = θ aθ r is measured optically using a contact angle goniometer. However, because of the difficulty in establishing the position of the droplet's base, the error in ∆θ measurement can be as large as 10 • for superhydrophobic surfaces. Since ∆ cos θ ≈ ∆θ 2 /2 (θ in radian, derived using Taylor series), this translates to error of 10 -2 in ∆ cos θ much larger than our measurement values in Figure . In other words, conventional contact angle goniometry (and unlike our approach here) would not have been able to differentiate between the friction properties of two surfaces. Now that we have characterized the wetting properties of the two surfaces, we will proceed to derive e as a function of θ, cos θ, and U I (and equivalently Ũ ). Full dataset can be found in a publicly available database. Energy loss in droplet rebound. We measured e for droplets of different sizes R = 0.03-1.5 mm impacting both Black Si and Glaco surfaces over a wide range of speeds U I = 4-700 cm s -1 . As reported elsewhere, there is a non-monotonic dependence of e on U I . More importantly, the data for millimetric and sub-millimetric droplets collapse into one master curve when U I is non-dimensionalized as Ũ = U I / ! γ/ρR = 0.007-6 for each surface (Fig. ), though the critical Ũcrit for which there is no rebound e = 0 is higher for Glaco than Black Si (Fig. ). For Glaco surface, we have also included the dataset from Wang et al. (2022) for comparison. In this section, we will propose a model to explain the behavior of e in the limit of Ũ ≪ 1 and Ũ ≫ 1.