Lecture 7a: 5 October 2005. 2003
Blakely, J.M., Introduction to the Properties of Crystal Surfaces, 261 pp., Pergamon
Press, Oxford, 1973.
Guéguen, Y., and V. Palciauskas, Introduction to the Physics of Rocks, 294 pp., Princeton
University Press, Princeton, NJ, 1994.
Surface Energy, Surface Tension and Fracture Surface Energy
Surface Energy
Surface tension is the work to create unit area of new surface:
γ dA→0.0
lim
dW
dA
Conc. of Solute
Grain boundary or surface segregation:
Phase 1
Phase 2
c1
c2
dividing surface
Position
Notice there is an excess amount of
material
ns = ntotal − (n1 + n2 ) where ni is the
number of moles of phase i if the
material were homogeneous.
The specific molar excess per unit area is
d (n1 + n2 )
Γ=
dA
Then at constant T and V, work done is
the Helmholtz Free Energy:
dFtotal = d ( F1 + F2 ) + dFs = γ dA
= μ ⋅ d (n1 + n2 ) + dFs
then from the definition of Γ, and expressing the parameters as specific,
i.e. per unit area,
γ = fs − μ
Conclusions:
Suface tension, γ, (a measurable quantity) is not necessarily
= specific surface free energy, especially if segregation
occurs
It seems that
dγ
< 0 from experimental measurements,
dT
although not a lot of data exist. Values of γ≈1-10 J/m2(0.4- 4.0ev or 1-10 x103 ergs/cm2 ) Intersection of Three Surfaces
Blakely, J. M., Introduction to the Properties of Crystal Surfaces, Chapt. 1&2, Pergamon
Press, Oxford, U. K., 1973. Suppose three B
interfaces meet. Consider a small C
δα2
displacement from the equilibrium
δα
P
configuration. For 3
example, if boundary 1 2 α
grows a bit. Then 3
dF = γ dA + Ad γ
α
2
3
^
t3
^
t2
1 ^
t1
0 = γ 1t̂1 + γ 2 t̂ 2 + γ 3 t̂ 3 +
dF surface = γ 1 (OP) + γ 2 ( BP − BO) + γ 3 (CP −
^
t3’
dt3
δα3
^
t3
+
Assume that the
ensemble is
approaching
equilibrium and that
dFsurface =0.
∂γ 2 t̂ 3 × t̂ 2 × t̂1 ∂γ 3 t̂1 × t̂ 2 × t̂ 3
+
∂α 2 t̂ 3 × t̂ 2 × t̂1 ∂α 3 t̂1 × t̂ 2 × t̂ 3
Conclusions: Two forces arise:
First, capillarity forces that are tangent to the surface at
each point and are proportional to the magnitude of the
surface tension.
Second, torque forces that are perpendicular to the surface
and tend to rotate the surface normal into a new direction.
These forces may be thought of as causing faceting. In the
case of fracture surfaces, the torque terms would arise
whenever the material had a good cleavage.
∂γ
∂γ 2
δα 2 ( BP) +
∂α 2
∂α
Capillarity Forces and Laplace’s Equation
Suppose that the volume occupied by phase 1 increases slightly.
Vtotal = V1 + V2 and dV2 = −dV1
r+dr
1
The total work done is the change in the PdV work plus a surface
(interface) energy term; if the process is reversible then the sum is 0:
− P1 dV1 − P2 dV2 + γ dA = 0
4
Originally, V1 = π r 3 so dV1 = 4π r 2 dr and dA = 8π rdr
3
So P1 − P2 =
2γ
r
In general, for a surface or interface that is not spherical
Should insert
1 1⎛ 1 1 ⎞
= ⎜ + ⎟ where r1,2 are the principal radii of curvature.
rm 2 ⎝ r1 r2 ⎠