NOVELTY - An apparatus (100) comprises a rigid fluid-permeable body (102) having a first fluid-permeable body surface (104) and a second fluid-permeable body surface (108); fluid-permeable cells (122) in the body, each having a cell wall defining a fluid pathway (126) communicating with openings (128, 130) in both the body surfaces; and several raised micro-scale features (106) on the first fluid-permeable body surface. Each micro-scale feature includes an end (110). The ends are spaced apart at a distance from the body surface. The ends form a superhydrophobic surface (114).    USE - As an apparatus for treating a liquid with a fluid (claimed); useful in blood oxygenation; blood dialysis; commercial, chemical and biotechnological manufacturing; conveying a fluid within an apparatus at an increased flow rate, including microfluidics; in fuel cell; and for removal of a gas or vapor from a liquid.    ADVANTAGE - The apparatus has rigid fluid-permeable body having superhydrophobic surfaces. The apparatus maintains a fluid flow of the liquid within the first fluid containment structure at an elevated pressure without compromising the superhydrophobic surface, by counteracting the elevated pressure of the liquid by a selected pressure of the gas.    DETAILED DESCRIPTION - An apparatus (100) comprises a rigid fluid-permeable body (102) having a first fluid-permeable body surface (104), and a second fluid-permeable body surface (108); several fluid-permeable cells (122) in the fluid-permeable body, each of the fluid-permeable cells having a cell wall defining a fluid pathway (126) communicating with openings (128, 130) in both the first and second fluid-permeable body surfaces; and several raised micro-scale features (106) on the first fluid-permeable body surface. Each micro-scale feature includes an end (110). The ends are spaced apart at a distance from the first fluid-permeable body surface, and form a superhydrophobic surface (114). The fluid-permeable body includes a first fluid containment structure. The apparatus further comprises a fluid containment body forming, together with the second fluid-permeable body surface, a second fluid containment structure. INDEPENDENT CLAIMS are included for the following:    (1) treating a liquid with a fluid involving providing the apparatus; and introducing a first liquid into the first fluid containment structure, and a fluid into the second fluid containment structure, and allowing interaction to occur between the first liquid and the fluid through the fluid-permeable body; and    (2) method of maintaining a superhydrophobic surface involving providing an apparatus; and introducing a liquid into the first fluid containment structure, introducing a gaseous fluid into the second fluid containment structure, and maintaining a pressure in the first fluid containment structure within a selected range relative to a pressure in the second fluid containment structure.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of an apparatus.    Apparatus (100)    Fluid-permeable body (102)    First fluid-permeable body surface (104)    Micro-scale features (106)    Second fluid-permeable body surface (108)    Superhydrophobic surface (114)    Ends (110)    Fluid-permeable cells (122)    Fluid pathway (126)    Openings. (128, 130)