A gel consists of a three dimensional continuous network, which encloses a
liquid phase, In a colloidal gel, the network is built from agglomeration of
colloidal particles. In a polymer gel the particles have a polymeric sub-structure
made by aggregates of sub-colloidal particles. Generally, the sol particles may
interact by van der Waals forces or hydrogen bonds. A gel may also be formed
from linking polymer chains. In most gel systems used for materials synthesis,
the interactions are of a covalent nature and the gel process is irreversible. The
gelation process may be reversible if other interactions are involved.