Titanium foams are characterized structurally by their pore topology (relative percentage of open vs. closed pores), porosity (the multiplicative inverse of relative density), pore size and shape, and anisotropy. Microstructures are most often examined by optical microscopy, scanning electron microscopy and X-ray tomography.
Categorizing titanium foams in terms of pore structure (as either open- or close-celled) is the most basic form of differentiation. In close-celled foams, pores are composed of bubbles entrapped in the metallic solid. These foams consist of a continuous network of sealed pores wherein interconnections between the pores are virtually non-existent. Alternatively, in open-celled foams, the pores are interconnected and solid struts allow fluid to pass through.
Most manufactured foams contain both types of pores, although in many cases the subtype is minimal.
According to the IUPAC, pore sizes are classified into three categories: micro (less than 2 nm), meso (between 2 and 50 nm) and macro (larger than 50 nm) pores.