High-precision experimental results are reported showing the tangential mom
entum accommodation coefficient (TMAC) for several gases in contact with si
ngle-crystal silicon to be less than unity. A precise and robust experiment
al platform is demonstrated for measurement of mass flows through silicon m
icromachined channels due to an imposed pressure gradient. Analytic express
ions for isothermal Maxwellian slip flows through long channels are used to
determine the TMAC at a variety of Knudsen numbers. Results from experimen
ts using nitrogen, argon and carbon dioxide are presented. For all three ga
ses the TMAC is found to be lower than one, ranging from 0.75 to 0.85.