title: Adsorption energy as a metric for wettability at the nanoscale.
abstract: Wettability is the affinity of a liquid for a solid surface. For energetic reasons, macroscopic drops of liquid form nearly spherical caps. The degree of wettability is then captured by the contact angle where the liquid-vapor interface meets the solid-liquid interface. As droplet volumes shrink to the scale of attoliters, however, surface interactions become significant, and droplets assume distorted shapes. In this regime, the contact angle becomes ambiguous, and a scalable metric for quantifying wettability is needed, especially given the emergence of technologies exploiting liquid-solid interactions at the nanoscale. Here we combine nanoscale experiments with molecular-level simulation to study the breakdown of spherical droplet shapes at small length scales. We demonstrate how measured droplet topographies increasingly reveal non-spherical features as volumes shrink. Ultimately, the nanoscale droplets flatten out to form layer-like molecular assemblies at the solid surface. For the lack of an identifiable contact angle at small scales, we introduce a droplet's adsorption energy density as a new metric for a liquid's affinity for a surface. We discover that extrapolating the macroscopic idealization of a drop to the nanoscale, though it does not geometrically resemble a realistic droplet, can nonetheless recover its adsorption energy if line tension is included.
authors.affiliation: IBM Research, Av. Pasteur 138/146, CEP 22290-240, Rio de Janeiro, RJ, Brazil.
authors.forename: Ronaldo
authors.initials: R
authors.lastname: Giro
authors.affiliation: IBM Research, Av. Pasteur 138/146, CEP 22290-240, Rio de Janeiro, RJ, Brazil.
authors.forename: Peter W
authors.initials: PW
authors.lastname: Bryant
authors.affiliation: IBM Research, Av. Pasteur 138/146, CEP 22290-240, Rio de Janeiro, RJ, Brazil.|IBM Research, 1101 Kitchawan Rd, Yorktown Heights, NY 10598, United States of America.
authors.forename: Michael
authors.initials: M
authors.lastname: Engel
authors.affiliation: IBM Research, Av. Pasteur 138/146, CEP 22290-240, Rio de Janeiro, RJ, Brazil.
authors.forename: Rodrigo F
authors.initials: RF
authors.lastname: Neumann
authors.affiliation: IBM Research, Av. Pasteur 138/146, CEP 22290-240, Rio de Janeiro, RJ, Brazil.|IBM Research, 1101 Kitchawan Rd, Yorktown Heights, NY 10598, United States of America.
authors.forename: Mathias B
authors.initials: MB
authors.lastname: Steiner