Summer may be almost over, but there’s still time for a splash in the pool or at the beach. Afterwards, jump into the shower to rinse off the salt, sand or pool chlorine. For the shower to do its job, however, it needs to wet your skin. This brings to mind what happens when we clean an industrial product with a liquid. For cleaning and rinsing to be effective, the liquid must actually contact the surface being cleaned. The physical and chemical properties of both the surface and the liquid determine how effective that contact, or wetting, is.
Attractive forces between molecules are either cohesive or adhesive. Cohesive forces occur between like molecules: adhesive forces are between unlike molecules. Cohesive forces are what hold liquids and solids together and differentiate them from gasses. Within the bulk of a liquid or solid, the cohesive forces are essentially balanced, exerted in all directions, because there are other molecules of the material in all directions.
It’s different at the surface
However, the balance of forces at a surface is different, because the cohesive forces can only be exerted from molecules on or below the surface (Figure 1).
Figure 1 Surface tension or surface energy forces
(Figure from Boundless.com—Cohesion and Adhesion)
In a liquid, this imbalanced force at the surface results in what is termed the surface tension. Surface tension forces act like a skin on a balloon to contain the bulk beneath the surface. In both liquids and solids, this imbalance of forces results in a greater energy for the molecules at the surface. This excess energy in a solid is referred to as surface energy. Surface tension and surface energy have the same units, for example Joules per square meter.
For cleaning or rinsing to happen, there must be a phenomenon called wetting. Wetting occurs when the surface energy of the substrate (surface to become wet) is greater than the surface energy or surface tension of the liquid. In other words, the adhesive forces between the substrate and the liquid are greater than the cohesive forces holding the liquid together. In this case, the liquid can flow over and make good contact with the substrate. If the adhesive forces of the liquid are lower than the cohesive forces, the liquid does not flow but will remain as drops and bead the surface.
Polar, hydrogen bonding and non-polar forces
Awhile back, I discussed polar, hydrogen bonding, and non-polar forces in the context of “like dissolves like” [1, 2]. Understanding surface energy helps understand how wetting occurs; and how wetting relates to accessibility to soils. Polar and hydrogen bonding forces are stronger than non-polar or dispersive forces. This is why polar liquids, like water, have stronger surface tension than non-polar liquids, like hexane. Therefore, a substrate must have a higher surface energy to be wetted by water than by hexane. Clean metal surfaces act like polar substances, since, due to the conductivity of the metal, charges can flow and leave the surface with a net charge distribution like that of a polar molecule. Organic contaminants, such as a film of oil, lower the surface energy of a substrate, because the actual surface is not the metal but the top of the oil film, and oils have little or no polar forces.
Clean enough?
The cleanliness of a metal surface is often measured by the degree of wetting by liquids. Whether by water break, contact angle or dyne pen, the measurement is based on the degree of flowing or wetting by the liquid on the substrate.
Dyne pens contain fluids with specified surface tensions. A high number dyne pen has a higher surface tension fluid than a low number pen. Because a clean surface has a higher surface energy, determining the level of cleanliness using dyne pens consists of determining the highest number pen that flows rather than beads. Pure water flows smoothly over a high surface energy substrate, creating the sheeting that characterizes a water break or producing drops with low contact angle.
In subsequent articles, we will continue this discussion of wetting of surfaces and explain how wetting relates to cleaning and coating.
References:
1. E. Kanegsberg, “The Physics of Cleaning, Part 2: May the Forces be With You,” Clean Source, Jan. 2007.
2. E. Kanegsberg, “The Physics of Cleaning, Part 3: Gecko Feet”, Clean Source, July 2007.
