  The addition of small amounts of a secondary fluid to a suspension can,
through the attractive capillary force, lead to particle bridging and network
formation. The capillary bridging phenomenon can be used to stabilize particle
suspensions and precisely tune their rheological properties. This effect can
even occur when the secondary fluid wets the particles less well than the bulk
fluid. These materials, so-called capillary suspensions, have been the subject
of recent research studying the mechanism for network formation, the properties
of these suspensions, and how the material properties can be modified. Recent
work in colloidal clusters is summarized and the relationship to capillary
suspensions is discussed. Capillary suspensions can also be used as a pathway
for new material design and some of these applications are highlighted. Results
obtained to date are summarized and central questions that remain to be
answered are proposed in this review.
