  Non-linear effects of the Navier-Stokes equations disappear under the Stokes
regime of Newtonian fluid flows disallowing the fluid flow rectification. Here
we show mathematically and experimentally that passive flow rectification of
Newtonian fluids is obtainable under the Stokes regime of both compressible and
incompressible flows by introducing nonlinearity into the otherwise linear
Stokes equations. Asymmetric flow resistances arise in shallow nozzle/diffuser
microchannels with deformable ceiling, in which the fluid flow is governed by a
non-linear coupled fluid-solid mechanics equation. Fluid flow rectification has
been demonstrated for low-Reynolds-number flows (Re ~ O(0.001)-O(1)) of common
Newtonian fluids such as air, water, and alcohol. This mechanism can pave the
way for regulating the low-Reynolds-number fluid flows with potential
applications in precise low-flow-rate micropumps, drug delivery systems, etc.
