Physics

Biography

Biography: Paul E Wagner

Abstract

The wetting behaviour of liquids on solid surfaces depends on the interaction of molecules in the solid, liquid and gas
phases. This interaction can be characterized by the contact angle between solid and liquid surfaces in the vicinity of the
three phase contact line. For macroscopic systems a number of experimental techniques are available for measurement of
contact angles. Recently intermediate micron size systems have been studied as well. However, in the nano-scale contact angles
are hardly accessible. Here we report the first direct experimental determination of contact angles and contact line curvatures
on a scale of 1 nm. We have considered measurements of heterogeneous nucleation of super saturated water vapour on nearly
spherical and mono dispersed Ag particles with well defined seed particle radius rp down to about 1.5 nm. From the slope of
the activation curves we obtained the number n* of molecules in the critical cluster using the nucleation theorem. On the other
hand the onset saturation ratio, where 50% of the seed particles are activated, allows to determine the radius r* of the critical
cluster using the Kelvin relation. Based on rp, r* and n* the microscopic contact angle as well as radius and curvature of the
contact line can be directly obtained. We find microscopic contact angles around 15 degrees compared to 90 degrees for the
macroscopic equilibrium angle. This difference can be attributed to line tension as originally postulated by Gibbs. Line tension
is becoming increasingly dominant with increasing curvature of the contact line.