Studies of physical systems in solid porous hosts date back several decades, for example, to studies of the helium superfluid transition in jewelers rouge. Research of the superfluid properties in porous media kept center stage until the nineties where they had to share their supremacy with confined liquid crystal studies (those of us working in liquid crystals would like to believe that).
Point in case, from an applied perspective, a liquid crystal display is arguably one the better known confined physical systems. Their operation strongly depends on the interaction between the liquid crystal molecules and the host solid surfaces. From a fundamental point of view, liquid crystals imbedded in solid porous materials or with dispersed nanoparticles are incredibly rich systems that allow the study of a variety of physical phenomena including probing effects on different order phase transitions. These phenomena include dramatic changes at phase transitions and in particular, size-dependent critical exponents; surface-induced liquid crystal alignment; bilayer-by-bilayer smectic growth; complete, quasi-complete and partial wetting, and molecular configurational transitions, among others. It should be mentioned that much of the above experimental work heavily relied on the theoretical contributions of S. Zumer and his co-workers.
Specifically, some of these effects were born out by NMR and ac calorimetry studies with cyanobiphenyl liquid crystals embedded in the well-defined cylindrical cavities of aluminum oxide Anopore or polycarbonate Nucleopore membranes; in the randomly interconnected networks of pores like in Vycor and Aerogel glasses; in the Millipore filter cellulose voids or by dispersing spherical silica nanometer particles, Aerosil, in the liquid crystal; and of course, the original liquid crystals in polymer networks.
After reviewing some of the aforementioned studies, we will touch upon where future studies might be. These may range from confining networks formed from metallic nanoparticles or nanowires to photovoltaics to drug delivery to understanding trans-to-cis photo-isomerization transition in porous media.
*These studies would not have been possible without the many contributions of B. Zalar and S. Zumer at the University of Ljubljana, Slovenia, and G. Iannacchione, T. Jin, S. Qian, H. Zeng, and G. Crawford at Kent State University.