Imbibition in mesoporous silica: rheological concepts and experiments on water and a liquid crystal

TL;DR

This study investigates the capillarity-driven imbibition of water and liquid crystal in mesoporous silica, confirming theoretical models and revealing temperature-induced phase behavior within nanopores.

Contribution

It provides experimental validation of imbibition dynamics in nanoporous silica and uncovers temperature-dependent phase phenomena of liquid crystals confined in such environments.

Findings

Imbibition follows square root of time dynamics.

A sticky velocity boundary condition is observed.

Temperature induces a paranematic phase in confined liquid crystal.

Abstract

We present, along with some fundamental concepts regarding imbibition of liquids in porous hosts, an experimental, gravimetric study on the capillarity-driven invasion dynamics of water and of the rod-like liquid crystal octyloxycyanobiphenyl (8OCB) in networks of pores a few nanometers across in monolithic silica glass (Vycor). We observe, in agreement with theoretical predictions, square root of time invasion dynamics and a sticky velocity boundary condition for both liquids investigated. Temperature-dependent spontaneous imbibition experiments on 8OCB reveal the existence of a paranematic phase due to the molecular alignment induced by the pore walls even at temperatures well beyond the clearing point. The ever present velocity gradient in the pores is likely to further enhance this ordering phenomenon and prevent any layering in molecular stacks, eventually resulting in a suppression of the smectic phase in favor of the nematic phase.