[Paper Review] Capillary Rise and Imbibition of Liquids in Nanoporous Matrices: Rheological Concepts and Experiments

Simon Gruener, Patrick Huber|arXiv (Cornell University)|May 5, 2010

Pickering emulsions and particle stabilization2 references3 citations

TL;DR

This study investigates capillary-driven imbibition in nanoporous silica glass (Vycor) with pore radii of 3.4–4.9 nm, using gravimetric measurements to analyze liquid flow dynamics. It reveals square-root-of-time imbibition kinetics across diverse liquids (water, n-alkanes, silicon oils, liquid crystals), explained by conserved bulk fluidity and capillarity with a sticky (negative slip length) boundary layer, while observing meniscus freezing in n-tetracosane and suppressed viscosity changes in a confined nematic liquid crystal.

ABSTRACT

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.

Motivation & Objective

• To understand the rheological behavior of liquids in extreme spatial confinement, particularly at the nanoscale where continuum hydrodynamics may break down.
• To determine whether bulk fluidity and capillarity are preserved in nanopores, and how interfacial interactions affect flow dynamics.
• To investigate phase transition behavior and flow anomalies in nanopore-confined liquids, including liquid crystals and long-chain alkanes.
• To assess the validity of classical hydrodynamic models (e.g., Darcy’s law) in nanoporous networks with complex pore topologies.
• To explore the role of interfacial layers, such as preadsorbed films and meniscus freezing, in modifying imbibition kinetics.

Proposed method

• Gravimetric measurement of spontaneous imbibition in monolithic nanoporous Vycor glass with controlled pore radii (3.4 nm and 4.9 nm).
• Systematic variation of liquid complexity: water, n-alkanes (C24), silicon oils, and rod-like liquid crystals (8OCB).
• Use of Darcy’s law extended to nanoporous networks, assuming a conserved bulk fluidity and capillarity with a negative slip length boundary condition.
• Analysis of time-dependent mass uptake to extract imbibition dynamics and infer slip length from square-root-of-time scaling.
• Incorporation of preadsorbed liquid layers and vapor-phase transport effects into the hydrodynamic model.
• Complementary birefringence and thermal analysis to characterize equilibrium phase behavior and its influence on dynamic flow.

Experimental results

Research questions

• RQ1 Does capillary-driven imbibition in nanopores follow square-root-of-time kinetics, and can this be explained by bulk fluidity and capillarity with a modified boundary condition?
• RQ2 How do interfacial interactions—particularly a sticky (negative slip length) boundary layer—affect imbibition dynamics in nanopores?
• RQ3 To what extent is the fluidity of liquids preserved in the pore center, and how does this compare to bulk behavior?
• RQ4 What role do preadsorbed liquid layers and vapor-phase transport play in the imbibition process at the nanoscale?
• RQ5 How does spatial confinement alter the phase transition behavior of liquid crystals, and is the viscosity drop upon nematic transition preserved?

Key findings

• All liquids studied exhibit square-root-of-time imbibition dynamics, consistent with a conserved bulk fluidity and capillarity in the pore center.
• A negative slip length of b = (-1.11 ± 0.23) nm for V5 and b = (-1.54 ± 0.31) nm for V10 indicates a sticky boundary layer, implying no slip-flow at the pore walls.
• Meniscus freezing—formation of a surfactant-like, rectified monolayer—was observed for n-tetracosane in Vycor, indicating strong interfacial structuring.
• For the rod-like liquid crystal 8OCB, no viscosity drop upon entering the nematic phase was observed, indicating suppression of the typical bulk rheological transition.
• The nematic phase in 8OCB is stabilized under confinement, with the paranematic-to-nematic transition broadened due to flow-induced orientational alignment.
• The findings suggest a two-phase flow model: a fast, bulk-like flow in the pore center and a slow, interfacial layer with strong liquid-substrate interactions, analogous to film- and capillary-condensed states in sorption isotherms.