A method for determining surface energies of solids: Temperature-variant contact angle method

Abstract

The cosine of the contact angle of a sessile drop increases linearly with increasing temperature: cos θ = A + BT (°C). The sessile drop is an organic liquid on a polymer substrate, a liquid metal on a ceramic substrate or a molten glass on a metal substrate. By combining the linear relation with Young's equation, γ SV = γ LV cos θ + γ LS, an expression for the specific surface free energy of the substrate is obtained: γ SV=(γ o ̊ LV−αT) (A+BT)+αB T− 1 2 γ o ̊ LV α − A B + β αB 2 where ( γ LV o - αT) represents the specific surface free energy of the sessile drop and β the temperature coefficient of the specific surface free energy of the substrate. By using two kinds of sessile drops for a given substrate, the values for γ SV and β can be determined simultaneously; the specific surface free energy and surface entropy thus determined for polytetrafluoroethylene are in excellent agreement with literature values. If only one set of data is available, the above equation can be approximated: ∼ SV=(γ o ̊ LV−αT) (A+BT)+αB T− 1 2 γ o ̊ LV α − A B + β αB 2 This equation is applied to various ceramic and platinum substrates, and the values thus obtained are in good agreement with the values reported in the literature.