NOVELTY - A mass transport apparatus comprises a porous membrane (12) proximate to and covering a substrate (14a); and a substrate port (38) covered by the porous membrane. The porous membrane directs a source of fluid to flow between the substrate and the porous membrane and through the substrate port. The fluid flow modifies the state of fluid and maintains the porous membrane proximate the substrate.    USE - For modifying the state of a fluid proximate a substrate or for increasing the mass transport at a liquid-solid diffusion boundary layer of a solid work piece exposed to a liquid environment, wherein the substrate is an electrode, chemical reagent, chiller and/or catalyst (claimed). E.g. for fuel cell.    ADVANTAGE - The invention controls fluid flow at the solid and fluid interface and enhances the mass or energy transport rate at the solid interface without circulating a large volume of fluid.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (a) a method for modifying the state of a fluid in contact with a substrate by flowing a source of fluid through a port in the apparatus; and modifying the state of the fluid by transfer of mass or energy between the fluid and the substrate; and    (b) a method for increasing mass transport at a liquid-solid diffusion boundary layer of a work piece layer having surface(s) in contact with a liquid by providing a surface liquid flow at the surface of the workpiece; controlling the liquid-solid diffusion boundary layer at the work piece surface by segregating the surface liquid flow from a source of liquid and limiting the height of the segregated surface liquid flow along the work piece surface; and continuously removing liquid from the surface of the work piece in a manner that induces the liquid flow at the surface of the work piece.    DESCRIPTION OF DRAWING(S) - The figure shows a mass transport apparatus of the invention.    Porous membrane (12)    Substrate (14a)    Source of fluid (16)    Base port (20)    Boundary layer fluid flow (22)    Membrane fluid flow (28)    Standoffs (30)    Average separation height (32)    Initial thickness (34a)    Substrate port (38)