Detailed contact angle measurements are performed for protein drops on phenylcarbamide (PC), polymethyl methacrylate (PMMA), silicon nitride, and glass substrates by goniometer-sessile drop technique. Drop sizes are ranged from 0.1 to 6 w l with radius of curvature less than 1 mm. The bovine serum albumin (BSA) protein concentrations are 0.0, 0.1, 1.0, and 10 mg/ml. The results show weak dependence of the drop size for deionized water drops on all substrates and increased dependence of the drop size for protein drops with different protein concentrations. The significance of drop size effect varies from substrate to substrate; the effect is strongest for PC substrate, which is a nearly hydrophobic surface. The drop size dependence can be partly explained by the modified Young's equation.
