The entire setup was placed on an optical table and enclosed to minimise vibration and draught. By minimising environmental noise, we can measure forces as small as a 20 nN. Figure show the friction force measurements F fric = 5 and 0.15 µN for two water droplets of similar sizes (R = 2.9 and 2.6 mm) moving on Glaco and Black Si surfaces, respectively. The Black Si surface is significantly more waterrepellent than Glaco.