Measuring surface energy of solid surfaces using centrifugal adhesion balance.

The standard way to evaluate the solid surface energy using probe liquids relies on contact angle measurements. The measured contact angles rely on visible means and are different from their nanoscopic thermodynamic values. This compromises the surface-energy predictions so much that the surface energy-values can be hundreds of percentages higher than expected based on comparisons with different methods as reported in several studies. We consider the Owen-Wendt approach, which breaks the surface energy to polar and dispersive components, and present a technique for measuring surface energy of solids using probe liquids. Our method avoids the need to measure contact angles; instead, it uses solid-liquid work of adhesion measurements which are performed using a centrifugal adhesion balance. In agreement with the studies mentioned above, we found that indeed, the surface energies of the measured solids are significantly lower than those based on contact angle measurements. More importantly we found that our method results in a reasonable breakdown of the surface energy to polar and dispersive components with a higher polar component for more polar solids. This is in contrast with the surface energy based on contact angle measurements for which the breakdown did not make sense, i.e., the measurements reflected higher polar components of the surface energy for less polar solids.

Droplets Sliding down a Vertical Surface under Increasing Horizontal Forces.

We have investigated the retention forces of liquid drops on rotating, vertical surfaces. We considered two scenarios: in one, a horizontal, centrifugal force pushes the drop toward the surface (?pushed drop? case), and in the other, a horizontal, centrifugal force pulls the drop away from the surface (?pulled drop? case). Both drops slide down as the centrifugal force increases, although one expects that the pushed drop should remain stuck to the surface. Even more surprising, when the centrifugal force is low, the pushed drop moves faster than the pulled drop, but when the centrifugal force is high, the pushed drop moves much slower than the pulled drop. We explain these results in terms of interfacial modulus between the drop and the surface.