  Sub-continuum gas conduction is an essentially important phenomenon in
disparate fields of applications ranging from aerospace vehicles to biomedical
sensors, and has been the focus of many computational studies over the past
decades. These studies predicted that the energy exchange mechanisms are driven
by gas-surface interactions, strongly dependent on the gas and surface
characteristics. Despite its fundamental and practical importance, thermal
transport via gas conduction at non-continuum regimes mostly remains
experimentally unverified. Here, we report precision measurements of
sub-continuum gas conduction within parallel micro-cavities and elucidate its
dependence on the gas and surface characteristics. More importantly, we
demonstrate a systematic approach for extracting the energy accommodation
coefficient (EAC), which is necessary to establish gas-surface scattering
kernels or develop diffusive-specular solutions to the Boltzmann transport
equation. EACs are also required for calculating the temperature jump
coefficient in near-continuum conditions to solve classical hydrodynamical
equations. For the first time, we show a correction to the kinetic theory in
the transition to near-continuum regimes (particularly for non-monatomic gases)
by extracting a physical parameter representing the intermolecular collisions
within the Knudsen layer. Our results agree well with the kinetic theory
predictions and are expected to inform the development of technologies such as
thermal switches, gas sensors, and light-driven actuators.
