Gases versus Liquid Flow in Microchannels

There is no distinction between gases and liquids for macroscale flow and heat transfer as long as the governing parameters like the Reynolds number, Prandtl number, Grashof number, and so on are the same for both. However, this is not true for microscale flows. The following observations can be made for microscale gas and liquid flows. 

Mean free path of liquids are much smaller than those of gases. Therefore, the continuum assumption may hold for liquids but fail for gases. 

The Knudsen number provides criterion for the validity of thermodynamic equilibrium and continuum model for gases. However, it does not hold for liquids. 

The failure of thermodynamic equilibrium and continuum is not well defined for liquids. Therefore, the validity of no-slip, no-temperature-jump, linear stress-strain relation and Fourier heat flux-temperature relationship are unknown for liquids. 

Surface forces become important for microchaimel flow. The nature of surface forces in liquids is different from that of gases. Therefore, the boundary conditions for liquids are different from those of gases. 

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