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London-van der Waals Force
The London-van der Waals force, which is generally attractive in nature, is a short range force and decays rapidly to zero away from a surface. The origin of the London- van der Waals force lies in the instantaneous dipole generated by the fluctuation of electron cloud surrounding the nucleus of electrically neutral atoms. For a spherical particle of diameter d near a flat surface, the interaction energy is given by:
]
1 x
x 2 ln 1 x
x
[
A
φ =− + , (1)
where x=z/d and z is the distance of the sphere from the surface and A is the Hamaker constant. As the particle approaches the surface,
12 z
Ad φ ≈ − as z → 0. (2)
Thus, the energy becomes infinite for z=0. Hence, the surface acts as a perfect sink for aerosol diffusion. The range of operation of the van der Waals force may be estimated by comparing the thermal energy with φ. Values of Hamaker constant A are in the range of
10 −^20 to 10 −^19 J. Thus,
~ 0. 2 d 12 kT
Ad z < for A ~ 10 −^20. (3)
In Table 1 values for van der Waals force for a number of materials are listed and the values of van der Waals force is compared with the Stokes drag force acting on a particle that is moving with a velocity of 1 m/s in aid and in water. It is seen that the van der Waals force in air is comparatively larger than that in water. Furthermore, van der Waals force is much larger that the drag force. The ratio of the van der Waals force to drag force in water is generally less than that in air.
Table 1. van der Waals force for a 1 μm particles. For comparison U = 1 m/sand a
separation of
o z (^) o = 4 Ais assumed.
Particle Surface (^) Fν × 108 N
(In air)^3 dU
F
πμ
ν F^10 N
×^8
ν (In water)^3 dU
F
πμ
ν
Polystyrene Polystynene 1.2-1.8 70-100 0.2 12 Si Si 13.6-14.4 800-850 7 410 Cu Cu 17 1000 9.8 580 Ag Ag 18 1060 15.5 910
Table 2. Variation of Forces (N) versus particle diameter, d (μm), f or a flow velocity of (^) U 0 = 10 m / s Van der Waals
Surface Tension
Added Mass
Drag/Lift Basset
Diameter 2 o
123 12 z
d Fν~A Fst ~ 2 πγ d dt
dV Fam ~ρd 3
f 2 2 FD ~ρd V t
d V F ~
2 B ν
μ
Air d (μm) (^) Fv Fst Fam FD FB
0.2 (^3) × 10 −^89 × 10 −^510 −^18 10 −^12 4 × 10 −^15
(^2 3) × 10 −^79 × 10 −^410 −^15 10 −^10 4 × 10 −^13
(^20 3) × 10 −^69 × 10 −^310 −^12 10 −^8 4 × 10 −^11
Water d (μm) (^) Fv Fst Fam FD FB
0.2 (^2) × 10 −^9 ___ (^8) × 10 −^168 × 10 −^1010 −^12
(^2 2) × 10 −^8 ___ (^8) × 10 −^138 × 10 −^810 −^10
(^20 2) × 10 −^7 ___ (^8) × 10 −^108 × 10 −^610 −^8
Values of van der Waals, surface tension, added mass, drag and Basset forces acting on a sphere of different sizes moving with a velocity of 10 m/s are shown in Table
- For a particle in air, it is seen that the surface tension force is order of magnitudes larger than the other forces, which is followed by the van der Waals force. Among the hydrodynamic forces, drag is the dominating force and the virtual mass and the Basset forces are negligibly small. In water, the surface tension force is absent and the drag force acting on large particles beomes comparable with the van der Waals adhesion force.
z (^) o
Figure 3. Schematics of a two planar surface at a separation distance of zo.
Hamaker Constants for Dissimilar Materials
For two dissimilar materials, the Hamaker constant may be estimated in term of Haymaker constant of each material. That is
A 12 ≈ A 11 A 22 (7)
or alternatively
11 22
11 22 (^12) A A
2 A A
A
For contact of two dissimilar materials in the presence of a third media,
A 132 = A 12 +A 33 −A 13 −A 23 (9)
From Equation (8) it follows that
2 11 33 11 33
2 11 33 (^131 113313) A A ~( A A )
(A A )
A A A 2 A −
or
A 132 ~( A 11 − A 33 )( A 22 − A 33 ). (11)
Lifshitz developed the “macroscopic theory” relating the Hamaker constant to dielectric constants of the materials. Accordingly,
132 h 132 4
A ω π
Values of hω 132 are given in Tables 3 and 4 for a number of materials. (Note that
1 ev= 1. 602 × 10 −^19 J.)
Table 3. Values of Lifshitz -van der Waals Constant hω 131 for some materials (Visser, 1976).
Homogenous Combinations
h ω 131 (eV ) Combinations Vacuum Water Au-Au 14.3 9. Ag-Ag 11.7 7. Cu-Cu 8.03 4. Diamond-Diamond 8.6 3. Si-Si 6.76 3. Ge-Ge 8.36 4. MgO-MgO 3.03 0. KCl-KCl 1.75 0. KBr-KBr 1.87 0. KI-KI 1.76 0. Al 2 O 3 -Al 2 O 3 4.68 1. CdS-CdS 4.38 1. H 2 O-H 2 O 1.43 - Polystyrene-Polystyrene 1.91 0.
Table 4. Values of Lifshitz -van der Waals Constant hω 131 for some materials (Visser, 1976).
Heterogeneous Combinations
h ω 131 (eV ) Combinations Water Polystyrene Au-Ag - 8. Au-Cu 6.41 5. Au-Diamond 6.11 5. Au-Si 5.32 4. Au-Ge 6.50 5. Au-MgO 1.99 1. Au-KBr 0.73 0. Au-Al 2 O 3 - 2. Au-CdS - 2. Au-Polystyrene 0.72 -
London-van der Waals Surface Energy Between Particles
The London-van der Waals surface energy and force between two spherical particles of diameters d 1 and d 2 as show in Figure 4 was evaluated by Hamaker (1937).
The corresponding surface energy is given as
]
d d R (
d d R ( ln ) 2
d d R (
dd / 2
) 2
d d R (
dd / 2 [ 6
A
2 1 2 2
2 1 2 2
2 1 2 2
1 2 2 1 2 2
1 2 − −
where
s 2
d d R 1 2 +
is the distance between particles center and s is the separation distance between surfaces.
Figure 4. Schematics of contact of two dissimilar spheres.
For equal size particles, d 1 = d 2 =d, r = d+s, and
)]
r
d ln( 1 2 (r d )
d 2 r
d [ 6
A
2
2 2 2
2 2
2
As noted before, A is typically of the order of 10 -19^ to 10 -21^ J and depends on the
properties of particles (of composition 1) and suspending medium (composition 2).
Accordingly, the effective Hamaker constant is given by
A 121 = A 11 +A 22 −A 12 ≈ (A 11 −A 22 ) (16)
d 1
d 2
R
s
