8. The Importance of Interfaces Flashcards
What is the difference between bulk and surface atoms?
Bulk atoms are in the middle of a material and are surrounded by more nearest neighbours than surface atoms.
Give the equation for the interaction energy of a bulk atom
U = interaction energy
N = number of nearest neighbours
u = interaction energy of one atom
Give the equation for the interaction energy of a surface atom
U = interaction energy
N = number of nearest neighbours
u = interaction energy of one atom
Give the equation for the energy cost associated with each surface atom
∆U = energy cost
U = interaction energy
N = number of nearest neighbours
u = interaction energy of one atom
Define surface energy
The excess energy required per unit area to create a surface in vacuum (or air).
Give the equation for the energy required to create two new surfaces from a material, each having an area, S
W = energy required
γ = surface energy
S = Area
Give the equation for inter-surface potential energy
U = potential energy
A = Hamaker constant
S = surface area
D = atom diameter
Give the equation to calculate the surface energy using the inter-surface potential energy
γ = surface energy
U = potential energy
S = surface area
A = Hamaker constant
D = atom diameter
Define interfacial energy
The excess energy per unit area required to create an interface between two different materials, represented by γ₁₂.
Define work of adhesion
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.
Give the equation for the work of adhesion
W = work of adhesion
γ = surface energy
Give the equation for the energy stored by a bar under stress
U = energy stored
σ = stress
A = cross-sectional area
L = length
E = Young’s modulus
Define surface tension
The force required per unit length to extend a surface, measured in N/m.
Define interfacial tension
The force required per unit length to extend an interface, measured in N/m.
Are surface/interfacial energy and surface/interfacial tension equivalent?
Yes: they both act to prevent a surface from increasing in area.
Why are suspended droplets spherical?
Spheres have the smallest surface area for a given volume, so the free energy is minimised.
Give the equation for the spreading parameter
S = spreading parameter
γ = surface energy
How does the spreading parameter impact a wetting interaction?
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.
What is the contact angle of a droplet?
The angle made by a liquid droplet on a surface.
What is the Young-Laplace equation for droplet contact angle?
θ = angle
γ = surface energy
What does the contact angle a droplet on a surface depend on?
The balance of the cohesive and adhesive forces.
Why do different materials have different contact angles?
Materials have different cohesive and adhesive forces associated with them.
Give one way to change the contact angle between a droplet and a surface
The surface can be covered in small structures to change the balance of surface energies.
A liquid-air interface _______ contact angle.
A liquid-solid interface ________ contact angle.
Increases
Decreases
