  Liquid flow propelled by capillary forces is one of the most important
transport mechanisms in porous environments. It is governed by a fascinating
interplay of interfacial, viscous drag as well as gravitational forces which
liquids encounter upon invasion into geometries with often complex topologies,
such as capillary networks of trees or interconnected fractures in soils and
ice. Here, we present fundamentals, concepts and an experimental, gravimetric
study on the capillarity-driven invasion dynamics of liquids in networks of
pores a few nanometers across in monolithic, nanoporous silica glass (porous
Vycor). A variation of the complexity of the building blocks of the liquids
investigated along with a variation of the humidity and the temperature upon
spontaneous imbibition allows us to gain information regarding the fluidity and
capillarity of liquids in such nanoporous environments. We observe square-root
of time imbibition dynamics for all liquids applied, which we can
quantitatively describe by both a conserved bulk fluidity in the pore center
and bulk capillarity at the advancing menisci, if we assume a sticky boundary
layer (negative velocity slip length). Moreover, pecularities of
nanopore-confined liquids, such as transport via the vapor phase leading to
preadsorbed liquid layers, have to be properly accounted for. Upon increasing
the chain-length in the case of the n-alkanes, we found hints towards a
transition from stick- to slip-flow at the pore walls with increasing
chain-length and thus polymeric behavior. Meniscus freezing is reported for
n-tetracosane confined in porous Vycor. For the rheology of a rod-like liquid
nematogen (8OCB) we found no hints of the viscosity drop upon entering into the
nematic phase, typical of the bulk rheology of this liquid crystal.
