This study aims to encapsulate polymers, surfactants and nanoparticles from solutions or suspensions in open-ended carbon nanotubes and glass microchannels. The work also demonstrates a novel method of capping water-filled carbon nanotubes using polymer seals of relatively small polymer molecules. The self-sustained diffusion mechanism driving admixtures from solutions into carbon nanotubes, as reported in A. V. Bazilevsky, K. Sun, A. L. Yarin and C. M. Megaridis, Langmuir, 2007, 23, 7451-7455, is shown to be effective for encapsulating a number of compounds in confinements spanning sizes from 50 nm-diameter carbon nanotubes to 300 µm-diameter glass capillaries. For example, surfactants and nanoparticles are encapsulated using this self-sustained diffusion mechanism. Very high filling efficiencies can be achieved with this method. The procedure opens new opportunities for water containment in nanotubes and microchannels. Nanoparticles filling microchannels form colloidal crystals, which, upon illumination, demonstrate opalescence characteristics of long columnar photonic crystals.