The idea of using capillarity as a mechanism to passively control the propellant flow in microthrusters is explored. A capillary tube consisting of two different radius sections can be employed to passively 'pump' a fluid within the tube and keep a microthruster continuously supplied with propellant. A simple, one-dimensional fluid model is developed and tested using five different fluids and five different combinations of larger and smaller radius tube sections. Non-dimensional similarity parameters are derived from the theoretical fluid model, and used to transform the dimensional experimental data from a set spanning four orders of magnitude to a set falling within one order of magnitude of the one-dimensional theory. A gravitational correction, based on physical arguments, enables the theoretical model and data set to more closely agree. The resulting experimentally verified scaling laws provide the fundamental tools for designing passive capillary mass feeding systems for micropropulsion.