Inhomogeneous Aerogels

A review is presented of techniques that allow one to fabricate patterned aerogels and other inhomogeneous porous materials. Fabrication techniques can be divided into two major categories. In one category, gradients are introduced by layering sols with different properties (e.g., density or hydrophobicity). Fabrication by layering is quite simple and easily upscaled. However, spatial resolution is comparatively poor, and features obtained by layering are on the order of hundreds of micrometers to several millimeters. In a second fabrication category, inhomogeneity is introduced with lithographic techniques. The solvent of a wet gel is exchanged with a solution of precursors that can be photodissociated or photopolymerized. Exposure to a suitable radiation source causes the precursors to react and form nanoparticles and/or polymers within the pores of the matrix with a spatial resolution of a few microns. Patterning with nanoparticles can be used to tune optical absorption, emission, and index of refraction of selected regions of aerogels. Polymers can be attached to the derivatized walls of the matrix pores, allowing one to modulate mechanical strength, hydrophobicity, and optical properties. Prospective research directions and applications of the materials are discussed.