New fluid separation devices and absorption materials are disclosed. Hollow fibers with an axial capillary slit act as very high efficiency absorption materials, as well as high-surface-area fluid separation devices. The hollow fibers with an axial capillary slit are constructed to preferentially absorb or repel different fluids and arranged to maximize that action over a plurality of fibers to separate different fluids. These separation devices can also function as injection devices and very effective micro-reactors.