Monodisperse elastic shells: a 3D study of jammed granular matter and microencapsulation

Abstract

This thesis focusses on two, admittedly rather different, applications of spherical elastic shells. First in the field of microencapsulation as a smart container to encapsulate and release functional materials. Second, in jammed matter research as a model system to probe the microstructure and contact force networks in jammed packings in a disordered state. What unites these two subjects is that they both depend upon the reversible elastic response of the shells: in microencapsulation this response allows loading and release to take place, while in granular matter the response allows the shells to act as sensors of the local stress. We describe the synthesis of stable spherical shells of size larger than 3 μm. They are prepared on a bulk scale with high monodispersity and fluorescently labeled for real space studies using a confocal microscope. The synthesis is based on an emulsion templating technique in which a solid shell of tetraethylorthosilicate (TEOS) cross-linked with dimethyldiethoxysilane (DMDES) is coated around liquid droplets, in our case polydimethylsiloxane (PDMS) oil droplets. Various robust methods to make monodisperse PDMS droplets are also discussed. We explored the possibilities of capsules filled with PDMS oil in microencapsulation applications like drug delivery. We propose a new method to efficiently unload and reload preformed capsules with apolar liquids that can contain fluorescent tracer molecules. The morphological changes that the capsules underwent during these processes were monitored and analyzed both qualitatively and quantitatively for a range of capsules with different ratio of shell thickness to radius. We quantitatively studied the amorphous structure and contact force network in jammed packings of elastic shells. Instead of capsules filled with PDMS oil we have chosen shells with an empty core for these studies. By empty core we mean that the liquid inside and outside the shell remain the same; in our system it was an index matched solvent. The 3D coordinates and radii of the deformed shells in jammed packings were identified with sub pixel resolution using Image J algorithms. In jammed packings the shells were deformed in such a way that one of the shells formed a dimple and the other remained spherical upon contact. On the whole the volume of the shells was not conserved in a jammed state. We also identified the shell that contains a dimple and hence the number of dimples per particle, which is a unique property of these packings. Having the knowledge of positions and radii of the shells we studied the geometry of the packings and the contact forces between pairs of shells, and hence the force network under static compression and shear, respectively.