We have repeated the friction measurements for droplets of different sizes R = 1.0-2.9 mm and contact-line speeds U # = 5-2400 µm s -1 for the two surfaces. In Figure , we chose to normalize F fric by the contact radius r to get a non-dimensional friction F fric /2rγ, which is also equivalent to the contact angle hysteresis ∆ cos θ = cos θ rcos θ a (Furmidge's law ), where θ a and θ r are the advancing and receding contact angles. We found that Black Si with F fric /2rγ = (1.7 ± 0.6) ×10 -3 is significantly more water-repellent than Glaco with F fric /2rγ = (4.6 ± 1.4) ×10 -2 . We also found that F fric is independent of contact-line speeds for superhydrophobic surfaces (not true for other classes of liquid-repellent surfaces )-an important result we will use when deriving the restitution coefficient e in the next section.