Thermal conductivity measures the speed of heat transfer across a single material, but how do you model heat transfer from one material to another? If you have hot water (0.58 W/(m K)) running across a cold copper plate (401), is the thermal conductivity across the contact surface a mean of the two materials (200)? Or is it limited to the thermal conductivity of the water (0.58)?
In essence, I'm wondering if the material used to construct a heatsink or water cooling block really matters if the block material options (copper, aluminum, titanium) all have a thermal conductivity two orders of magnitude greater than the coolant (water, air) flowing over them. My gut instinct is "yes", but I would like to have a more complete and rational understanding.