TABLE OF CONTENTS For a flow of gas formed inside or around microscaled devices, the properties of the gas are expected to vary in the flow field. As in the case of gas flows in macroscaled devices, these changes can be caused by the distributed type of external forcing that are, for example, electrical, magnetic, or gravitational in nature. The momentum, heat, and chemical interactions between the gas and the surfaces of the microfluidic devices that are in contact with the gas flows can also have significant influences on the properties of the gas flow. Due to the small molecular weight of gases, the gravitational effects can be small in micro gas flows of single species, similar to what has generally been observed in macro gas flows. The effects of heat transfer through either contact surfaces or dissipation, on the other hand, are expected to be of equal if not greater importance in microflows. However, the difference in the length scale of the devices that the gas flows are associated with brings in additional concerns.
An intrinsic length scale in dilute gases is the mean free path, which measures the average distance the gas molecules travel between collisions. The ratio of the molecular mean free path of gas ? to a flow characteristic length scale L is defined as the Knudsen number Kn.
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For gas flows found in conventional macroscale devices at atmospheric conditions, the flow characteristic length scale can be orders of magnitude larger than...
