  We have developed a hierarchical nano-porous layer (HNL) on a silicate glass
by simple alkaline etching method. The HNL has a three-dimensionally-continuous
sponge-like structure with a pore size of a few tens of nm at its apparent
surface. The pore size gradually decreases from the apparent surface to the
interface of the porous layer and the bulk substrate. This HNL gives two
significant properties to glass: (1) higher optical transparency than untreated
glass by 7% for all over the visible light. (2) long persistent
superhydrophilicity with a water contact angle of 5 degree for more than 140
days. The superhydrophilicity also realizes anti-fogging and anti-fouling
functionalities.
