Attenuation and stiffness of artificial, fluid containing cracks are measured from 3 mHz to 10 Hz. The cracks are wedge-shaped; made from glass microscope slides. To explain the frequency dependence of both the attenuation and the stiffness (akin to a modulus), we need to appeal to well known fluid flow mechanisms and to the physicochemical interaction between the fluid and crack surface. By altering the wettability of the crack surfaces, surfactants change the mobility of water and thereby change the frequency dependence of the fluid flow effects by several orders of magnitude. ¿ American Geophysical Union 1996