Consider two surfaces pressed against each other. Now the air at the interface is evacuated. Will the thermal contact resistance at the interface increase or decrease as a result?

Heat transfer is a fundamental concept in physics that describes the movement of heat energy from one place to another. This can occur in various ways, such as conduction, convection, and radiation. In conduction, heat is transferred through direct contact between materials, a process that is heavily influenced by their thermal conductivity.

With convection, heat is carried away by flowing fluids, which could be liquids or gases. Lastly, radiation heat transfer occurs through electromagnetic waves and does not require a medium. Understanding the mechanisms of heat transfer is essential in many practical applications, including designing heat sinks, insulating homes, and analyzing thermal systems.

Thermal conductivity is a property of materials that measures their ability to conduct heat. It's defined as the quantity of heat, typically measured in watts, that passes through a material with a given area and thickness over a time and temperature difference. Materials with high thermal conductivity, such as metals, are efficient at transferring heat, while those with low conductivity, like rubber or air, are good insulators.

In the context of the textbook problem, understanding thermal conductivity is crucial as it helps explain why eliminating air from the interface between two surfaces can improve the overall heat transfer—since air's low thermal conductivity is a bottleneck in the process.

Conduction in solids is a mode of heat transfer occurring within a solid material or between solid surfaces in contact. The thermal motion of particles within a solid increases with temperature, causing neighboring particles to vibrate more vigorously. This motion and subsequent interaction between particles is how energy is passed through the solid.

This process is also described by Fourier's law of heat conduction, which states that the rate of heat transfer through a material is proportional to the negative gradient of temperatures and the area to which the heat is being transferred, with a constant of proportionality known as thermal conductivity. In the instance of two surfaces in contact, increasing the contact area by evacuating air enhances this conductive transfer significantly.

Radiation heat transfer differs from conduction and convection as it involves the transfer of heat through electromagnetic waves, such as infrared radiation. This type of transfer does not require a medium; it can occur through a vacuum. Every object emits, absorbs, and possibly transmits or reflects thermal radiation according to its temperature.

In the process of heat transfer between two surfaces, when there's an air gap, radiation can contribute to the transfer of energy across that gap. However, direct solid contact tends to be more efficient, leading to the conclusion that removing the air gap can significantly reduce the resistance to heat flow via increased conduction, overshadowing the contribution by radiation.

Evacuating air from the interface of two contacting surfaces has a profound effect on thermal contact resistance. When the air is present, its low thermal conductivity compared to that of solids limits the efficiency of heat transfer between those surfaces. Upon evacuation of air, the solid surfaces come into closer contact, rapidly increasing the area of direct solid-to-solid conduction.

This increased contact area reduces the paths' resistance, where heat can flow more freely due to higher thermal conductivity of solids—effectively decreasing thermal contact resistance. This concept is essential when designing interfaces for heat transfer in mechanical and electronic systems to ensure efficient thermal management.