Everything about Surface Energy totally explained
Surface energy quantifies the disruption of intermolecular bonds that occurs when a surface is created. In the
physics of
solids, surfaces must be intrinsically less energetically favourable than the bulk of a material; otherwise there would be a driving force for surfaces to be created, and surface is all there would be (see
sublimation (physics)). Cutting a solid body into pieces disrupts its bonds, and therefore consumes energy.
If the cutting is done reversibly (see
reversible), then
conservation of energy means that the energy consumed by the cutting process will be equal to the energy inherent in the two new surfaces created. The unit surface energy of a material would therefore be half of its energy of
cohesion, all other things being equal; in practice, this is true only for a surface freshly prepared in vacuum. Surfaces often change their form away from the simple "cleaved bond" model just implied above. They are found to be highly dynamic regions, which readily rearrange or react, so that energy is often reduced by such processes as
passivation or
adsorption.
As first described by
Thomas Young in 1805 in the
Philosophical Transactions of the Royal Society of London, it's the interaction between the forces of cohesion and the forces of
adhesion which determines whether or not
wetting, the spreading of a liquid over a surface, occurs. If complete wetting doesn't occur, then a bead of liquid will form, with a
contact angle which is a function of the surface energies of the system.
Surface energy is most commonly quantified using a contact angle
goniometer and a number of different methods.
Thomas Young described surface energy as the interaction between the forces of cohesion and the forces of adhesion which, in turn, dictate if wetting occurs. If wetting occurs, the drop will spread out flat. In most cases, however, the drop will bead to some extent and by measuring the contact angle formed where the drop makes contact with the solid the surface energies of the system can be measured.
Young established the well-regarded Young's Equation which defines the balances of forces caused by a wet drop on a dry surface. If the surface is hydrophobic then the contact angle of a drop of water will be larger. Hydrophilicity is indicated by smaller contact angles and higher surface energy. (Water has rather high surface energy by nature; it's polar and forms hydrogen bonds).
The Young equation gives the following relation,
»
In the case of "dry wetting", one can use the
Young-Dupré equation which is expressed by the work of adhesion. This method accounts for the surface pressure of the liquid vapor which can be significant.
Pierre-Gilles De Gennes, a Nobel Prize Laureate in Physics, describes wet and dry wetting and how the difference between the two relate to whether or not the vapor is saturated .
Further Information
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