A cotton fiber was kept under slight tension and exposed locally to a stream of aqueous 1 N NaOH microdrops of 50 mu m diameter. The resulting "macrodrop" of about 300 mu m size was at the origin of the formation of Na-cellulose I domains extending about 550 mu m from the center of the macrodrop along the fiber. The phase transformation zone between cellulose I and Na-cellulose I was mapped by scanning synchrotron radiation microdiffraction using a 300 nm x 300 nm beam. A stitching technique was used to limit radiation damage. Subsequent exposure of the NaOH containing macrodrop to a stream of H2O or HCl microdrops converted part of the Na-cellulose I back into cellulose I.