Uniform and ultrathin coatings are needed on the surface of micromachined silicon filters in order to manipulate microfluid flow and minimize non specific protein adsorption. This work reports vapor phase deposition of nanometer thick alkylsilanes at atmospheric pressure using nitrogen as a carrier gas. The coatings were characterized with ellipsometry, SEM, AFM, contact angle goniometry, and zeta potential meter. The method is particularly advantageous when it is necessary to coat small channels in microdevices. Micromachined silicon filters were coated with different alkylsilanes and tested with nitrogen gas and a variety of liquids. Interesting flow rate reversal of water versus ethanol was observed when the minimum channel dimension shrank from 74 microns to 1.8 microns.