Surface energy

  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.

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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 is 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 does not 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 developed 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 is polar and forms hydrogen bonds). The Young equation gives the following relation: Y_SL + Ycos(a_E) = Y_SV

Where Y_SL , Y, and Y_SV are the interfacial tensions between the liquid and the solid, the liquid and the vapor and the solid and the vapor respectively, and a_E is the equilibrium contact angle the drop makes with the surface. To derive the Young equation, normally the interfacial tensions are described as forces per unit length and from the one-dimensional force balance along the x axis Young equation is obtained. The Young equation assumes perfectly flat surface, and in many cases surface roughness and impurities deviated the equilibrium contact angle from the Young contact angle. Even in a perfectly smooth surface a drop will assume a wide spectrum of contact angles ranging from the so called "advancing contact angle", a_A to the so called receding contact angle a_R. The equilibrium contact angle can be calculated from a_A and a_R as was shown by Tadmor ^[1] as:

a_E = arccos{[R_A cos(a_A)+R_R cos(a_R)]/[R_A+R_R]}

Where

R_A = {sin³(a_A)/[2-3cos(a_A)+cos³(a_A)]}^1/3

and R_R = {sin³(a_R)/[2-3cos(a_R)+cos³(a_R)]}^1/3

In the case of "dry wetting", one can use the Young-Dupree 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 relates to whether or not the vapor is saturated ^[2].

See also

• Contact angle
• Surface tension

References