A computational study of wetting on chemically contaminated substrates

Abstract

The partial wetting of droplets on rigid, chemically contaminated substrates is investigated numerically for Bond numbers Bo≤5. To this end a recently developed numerical framework (Sauer [1] and Sauer et al. [2]) based on a finite element discretisation and a computational contact algorithm is used. Its applicability to wetting on chemically heterogeneous substrates is demonstrated. An analytical expression for the change in the apparent contact angle in the presence of an arbitrarily strong chemical contamination is derived based on a perturbed force equilibrium analogy at the triple phase contact line. The analytical prediction is compared to the theory by de Gennes [3] as well as to the modified Cassie's law [4] with perfect agreement in the limit of small perturbations, i.e. small chemical contaminations. The predicted change in the apparent contact angle is used to define a mapping between the droplet shape for wetting on homogeneous substrates and chemically heterogeneous substrates. Numerical results are presented for circular, patterned substrates (axial and radial patterns) as well as locally introduced perturbations on otherwise homogeneous substrates. The results for the droplet shape on chemically contaminated substrates show good agreement with the analytical solution for the droplet's height and wetting radius on homogeneous substrates when the presented mapping is applied. Moreover, the present results are in good quantitative agreement to results reported in literature.