Consider two slabs made up of different materials which are brought in contact with each other as shown. Thermal energy is supplied from the left, and it flows through the two slabs towards right.
The surfaces are rough and there are asperities. As a result of this, there will be some points of contact between the two slabs at the interface and the voids will be filled by, say, air.
The sources that I'm referring to say that at the steady state, because of surface roughness there will be a temperature drop at the interface and we can associate a resistance to this temperature drop called the contact resistance.
What is the reason that there is a temperature drop at the interface because of the presence of asperities?
Also, why there is not temperature drop when there is an ideal contact between the surfaces (perfect contact with no roughness)?