nanofluidic entropic traps , comprising alternating thin and thick regions , sieve small molecules such as dna or protein polymers and other molecules . the thick region is comparable or substantially larger than the molecule to be separated , while the thin region is substantially smaller than the size of the molecules to be separated . due to the molecular size dependence of the entropic trapping effect , separation of molecules may be achieved . in addition , entropic traps are used to collect , trap and control many molecules in the nanofluidic channel . a fabrication method is disclosed to provide an efficient way to make nanofluidic constrictions in any fluidic devices .