Use of the Equilibrium Contact Angle as an Index of Contact Surface Cleanliness

Advancing contact angles formed by water and aqueous ethanol solutions were measured on both bare surfaces and film-covered surfaces of acrylic, glass, hydrophilic and hydrophobic silicon, polycarbonate, polyester, and #304 stainless steel. Each bare surface was initially characterized with respect to its hydrophilic/hydrophobic balance. Both homogeneous protein films and heterogenous milk films were prepared on each surface; the adsorbed mass comprising each film was measured with ellipsometry. Contact angle methods proved useful in detecting the presence of a protein film on sufficiently hydrophilic or hydrophobic surfaces, i.e., materials for which W_a^p _water (the polar component of the energy required to separate water from its surface) is greater than 55 mJ/m² or less than 37 mJ/m². Contact angle methods were found to be useful for detecting the presence of a milk film on solid surfaces as well. However, the detection limits were dependent not only on W_a^p _water but also on the ethanol concentration of the diagnostic liquid used.