8. The Importance of Interfaces

Question 1

What is the difference between bulk and surface atoms?

Answer 1

Bulk atoms are in the middle of a material and are surrounded by more nearest neighbours than surface atoms.

Question 2

Give the equation for the interaction energy of a bulk atom

Answer 2

U = interaction energy
N = number of nearest neighbours
u = interaction energy of one atom

Question 3

Give the equation for the interaction energy of a surface atom

Answer 3

U = interaction energy
N = number of nearest neighbours
u = interaction energy of one atom

Question 4

Give the equation for the energy cost associated with each surface atom

Answer 4

∆U = energy cost
U = interaction energy
N = number of nearest neighbours
u = interaction energy of one atom

Question 5

Define surface energy

Answer 5

The excess energy required per unit area to create a surface in vacuum (or air).

Question 6

Give the equation for the energy required to create two new surfaces from a material, each having an area, S

Answer 6

W = energy required
γ = surface energy
S = Area

Question 7

Give the equation for inter-surface potential energy

Answer 7

U = potential energy
A = Hamaker constant
S = surface area
D = atom diameter

Question 8

Give the equation to calculate the surface energy using the inter-surface potential energy

Answer 8

γ = surface energy
U = potential energy
S = surface area
A = Hamaker constant
D = atom diameter

Question 9

Define interfacial energy

Answer 9

The excess energy per unit area required to create an interface between two different materials, represented by γ₁₂.

Question 10

Define work of adhesion

Answer 10

The energy required per unit area to separate two surfaces of materials 1 and 2 in a third medium (medium 3) and form two new interfaces.

Question 11

Give the equation for the work of adhesion

Answer 11

W = work of adhesion
γ = surface energy

Question 12

Give the equation for the energy stored by a bar under stress

Answer 12

U = energy stored
σ = stress
A = cross-sectional area
L = length
E = Young’s modulus

Question 13

Define surface tension

Answer 13

The force required per unit length to extend a surface, measured in N/m.

Question 14

Define interfacial tension

Answer 14

The force required per unit length to extend an interface, measured in N/m.

Question 15

Are surface/interfacial energy and surface/interfacial tension equivalent?

Answer 15

Yes: they both act to prevent a surface from increasing in area.

Question 16

Why are suspended droplets spherical?

Answer 16

Spheres have the smallest surface area for a given volume, so the free energy is minimised.

Question 17

Give the equation for the spreading parameter

Answer 17

S = spreading parameter
γ = surface energy

Question 18

How does the spreading parameter impact a wetting interaction?

Answer 18

If S < 0 then the surface will only be partially wet because the liquid will form a droplet.
If S > 0 then the whole surface will be wet because the liquid will spread out.

Question 19

What is the contact angle of a droplet?

Answer 19

The angle made by a liquid droplet on a surface.

Question 20

What is the Young-Laplace equation for droplet contact angle?

Answer 20

θ = angle
γ = surface energy

Question 21

What does the contact angle a droplet on a surface depend on?

Answer 21

The balance of the cohesive and adhesive forces.

Question 22

Why do different materials have different contact angles?

Answer 22

Materials have different cohesive and adhesive forces associated with them.

Question 23

Give one way to change the contact angle between a droplet and a surface

Answer 23

The surface can be covered in small structures to change the balance of surface energies.

Question 24

A liquid-air interface _______ contact angle.
A liquid-solid interface ________ contact angle.

Answer 24

Increases
Decreases