This set of Phase Transformation Multiple Choice Questions & Answers (MCQs) focuses on “Interfacial Free Energy”.
1. The free energy of a system containing an interface of area 5mm2 and free energy 6 KJ/mol*mm2 per unit area is given by_______(G’= 10 KJ/mol, is the free energy of the system assuming that all material in the system has the properties of the bulk).
a) 40 KJ/mol
b) 50 KJ/mol
c) 20 KJ/mol
d) 10 KJ/mol
Explanation: The free energy of a system containing an interface of area A and free energy ᵞ per unit area is given by, G = Go + Aᵞ where Go is the free energy of the system assuming that all material in the system has the properties of the bulk- ᵞ is therefore the excess free energy arising from the fact that some material lies in or close to the interface.
2. Consider for simplicity a wire frame suspending a liquid film. If one bar of the frame is movable it is found that a force 5N per unit length must be applied to maintain the bar in position. If this force moves a small distance so that the total area of the film is increased by 0.5m2. Interfacial free energy per unit area is given as 4 J/m2. Calculate the change in interfacial energy? (Take the area of interface as 2m2)
a) 0.25 J
b) 0.50 J
d) 0.90 J
Explanation: If one bar of the frame is movable it is found that a force F per unit length must be applied to maintain the bar in position. If this force moves a small distance so that the total area of the film is increased by dA the work done by the force is FdA. Then the force is given as F = ᵞ + dᵞ/dA (where ᵞ is the interfacial free energy). That is 4*2=8J.
3. In the case of a liquid film which of the following is true? (Where ᵞ is the interfacial free energy and A is the area of the interface).
a) (dᵞ/dA) > 0
b) (dᵞ/dA) < 0
c) (dᵞ/dA) = 0
d) Cannot be predicted
Explanation: In the case of a liquid film the surface energy is independent of the area of the interface and hence the value of (dᵞ/dA)=0. This leads to the well-known result F = ᵞ i.e. a surface with a free energy ᵞ J m-2 exerts a surface tension of ᵞ Nm-1.
4. Why is the value of (dᵞ/dA) =0 in case of a liquid film?
a) Liquid is unable to support shear stresses
b) Liquid develops viscous forces
c) Liquid can support shear stresses
d) Intermolecular forces in liquid is quite high
Explanation: For a liquid the shear stress is something which is unbearable and it won’t be able to support that, so the atoms within the liquid can rearrange and redistribute at the time when they stretch and thereby maintain a constant surface structure. Solids are much more viscous and here the transfer of atoms occurs from the bulk to the surface.
5. What is the order of interfacial free energy found in Grain Boundary?
Explanation: Interfacial free energies are found in the order of 10-1 in the grain boundaries, 10-2 in Twin boundaries and 10-0 in free surface (of metals).
6. Interfacial free energy is anisotropic in crystalline solids?
Explanation: In a crystalline solid the surface formed with different planes will have different energies because the number of broken bonds per unit areas are different on different planes. Hence, the interfacial is also anisotropic in crystalline solids.
7. Solids are much more viscous than liquid films and there is transfer of atoms from the bulk to the surface.
Explanation: Solids are much more viscous and transfer of atoms from the bulk to the surface, which is necessary to maintain an unchanged surface structure and energy, will take much longer. If this time is long in comparison to the time of the experiment then (dᵞ/dA) is not equal to 0 and the surface free energy and surface tension will not be identical.
8. The anisotropy in crystalline solid is prominent at ___
a) Lower temperature
b) Higher temperature
c) Zero degree Celsius
d) Cannot be predicted
Explanation: This anisotropy is more prominent at lower temperatures. As temperatures rise, the entropy contribution becomes dominant and makes the interfacial energy less anisotropic and if this happens the surface formed with different planes will have same energies in crystalline solids.
9. Which among the following has the typical energy (J/m2) in the order 10-1?
a) Coincident site lattice boundary
b) Anti-phase boundary
c) Solidification front
d) Twin boundary
Explanation: The typical energy in Solidification front is in the order of 10-1, whereas in the Anti-phase boundary, Twin boundary and CSL boundary it is in the order of 10-2.
10. In the equation dG = A*dᵞ+ ᵞ*dA, if dᵞ = 0, what does that mean? (ᵞ is the interfacial free energy, A is the area of interface).
a) The interfacial excess free energy is less than surface tension
b) The interfacial excess free energy is same as surface tension
c) The interfacial excess free energy is greater than surface tension
d) Cannot be predicted
Explanation: If dᵞ = 0, the change in free energy is equal to the work done in increasing the interface area; and, hence, the interfacial free energy is the same as surface tension. On the other hand, in general, in solids, the surface tension is not the same as the interfacial free energy.
11. In a typical fluid-vapor interface, the interfacial excess free energy is same as ________
a) Internal energy
b) Surface tension
c) Kinetic energy
Explanation: Let’s take the case of a typical fluid-vapor interface, here actually the interfacial excess free energy equals the surface tension but this is the case only for only for liquids and gases and this force tries to reduce the surface area.
12. In case of coherent interface elastic energy associated with the interface along with the interfacial energy determines its_______
d) Surface tension
Explanation: The coherent and semi coherent interfaces has been associated with some strain energies. The elastic energy associated with the interface and the interfacial energy is the factor that determines the equilibrium shape in case of a coherent interface.
13. Which of the following can be used to calculate surface energy of a solid?
a) Bond making model
b) Bond breaking model
c) Solid bond theory
d) Interface creation model
Explanation: Bond breaking model can be used to calculate the surface energy of a solid. We make a lot of assumption before creating this model which include that the solid is in contact with its own vapor, we also assume that the temperatures are low enough that the primary contribution to the surface energy comes from the broken bonds.
Sanfoundry Global Education & Learning Series – Phase Transformation.
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