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Liquid flow propelled by capillary forces is one of the most importanttransport mechanisms in porous environments. It is governed by a fascinatinginterplay of interfacial, viscous drag as well as gravitational forces whichliquids encounter upon invasion into geometries with often complex topologies,such as capillary networks of trees or interconnected fractures in soils andice. Here, we present fundamentals, concepts and an experimental, gravimetricstudy on the capillarity-driven invasion dynamics of liquids in networks ofpores a few nanometers across in monolithic, nanoporous silica glass (porousVycor). A variation of the complexity of the building blocks of the liquidsinvestigated along with a variation of the humidity and the temperature uponspontaneous imbibition allows us to gain information regarding the fluidity andcapillarity of liquids in such nanoporous environments. We observe square-rootof time imbibition dynamics for all liquids applied, which we canquantitatively describe by both a conserved bulk fluidity in the pore centerand bulk capillarity at the advancing menisci, if we assume a sticky boundarylayer (negative velocity slip length). Moreover, pecularities ofnanopore-confined liquids, such as transport via the vapor phase leading topreadsorbed liquid layers, have to be properly accounted for. Upon increasingthe chain-length in the case of the n-alkanes, we found hints towards atransition from stick- to slip-flow at the pore walls with increasingchain-length and thus polymeric behavior. Meniscus freezing is reported forn-tetracosane confined in porous Vycor. For the rheology of a rod-like liquidnematogen (8OCB) we found no hints of the viscosity drop upon entering into thenematic phase, typical of the bulk rheology of this liquid crystal.

Capillary Rise and Imbibition of Liquids in Nanoporous Matrices: Rheological Concepts and Experiments