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Water droplets on top of different surfaces: a surface fouled with oleic acid (a), a clean, smooth and hydrophilic surface (b), a clean and hydrophobic surface (c) and a clean and rough surface (d). For rough surfaces, the water contact angle cannot be measured, and for the hydrophobic surface, the standard cannot be used, which is indicated by the red color and crosses. The green color and checks indicate that hydrophilic, smooth surfaces are suitable for this standard.
