what is a membrane?
the "membrane" is a general term for a thin layered substance for isolation, of which the thickness can range from a few microns to a few millimeters. the "membrane" here refers specifically to materials with selective separation function, also known as separation membrane, through which one or several substances in the fluid can permeate while the other one or several substances cannot, thereby playing the role of separation, purification and concentration. the membrane is also defined as "a three-dimensional structure" internationally. "the size of one dimension in three dimensions is much smaller than the other two, and mass transfer can be realized due to various driving forces.”