Estimation of Surface Energies from Contact Angles

Abstract

A RECENT communication by Gray¹ illustrates a possible pitfall in the use of the theories of Fowkes^2–5 and Good and Girifalco^6,7 to estimate surface energies, and the various components of surface energy, from contact angles. This source of error is the incorrect identification of the surface tension terms, and the equating of the contact angle in a contaminated, experimental system to that in a system composed of properly pure components. Thus, Gray wrote Fowkes's equation in the form and used his observed contact angle data for mercury on polyethylene, paraffin wax and polytetrafluoroothylene, together with Fowkes's estimates of γ^d_s for the solids and of γ^d_L for mercury, to calculate values for γ_L for mercury. The fact that the values of γ_L turned out to be very much larger than 485 dynes/cm was then taken to be an unexplained discrepancy in the theory. In his discussion, Gray apparently also misinterpreted a remark of Fowkes⁵ about the effect of a contaminant in the mercury on the observed contact angle.