We have recently completed a comprehensive analysis of the morphological and kinematical properties of ~ 50 PNe which are known to possess low-ionization (micro)structures (LISs; Gonçalves et al. 2001). This study has shown that density is a crucial parameter for distinguishing among several physical processes responsible for their formation and evolution.
We are at present analysing long-slit, medium-resolution optical spectra of 18 PNe in order to determine the physical parameters of the LISs and their environments. These data were obtained with the 1.5 m Danish (ESO) and the 2.5 m Isaac Newton (ORM) telescopes.
Here we show the first results of this survey, presenting the density variation of NGC 7009 and comparing it with the derived densities of K 4-47 and Wray 17-1.