An experimental investigation was conducted to evaluate the possible use of highly-charged spherical nanoparticles to stabilize an aqueous dispersion of weakly-charged microspheres. At low pH values, the surface of silica is weakly charged, which leads to flocculation of colloidal suspensions of silica microspheres. Binary solutions of weakly charged silica microspheres and highly charged polystyrene latex nanoparticles result in adsorption of the nanoparticles onto the surface of the silica microspheres. This effectively "recharges" the silica spheres, with effective zeta potentials increased to the range that is unfavorable for flocculation of microspheres in a silica-only solution. However, this does not guarantee stability, and comparisons between positively charged amidine latex nanoparticles and negatively charged sulfate latex nanoparticles indicate that the degree of coverage plays an important role in the restabilization. The sulfate latex nanoparticles do not cover the surface sufficiently, and though they seemingly provide sufficient charge, the weakly charged patches of the exposed silica substrate can lead to flocculation. The amidine latex nanoparticles, on the other hand, cover the surface more completely, and effectively prevent flocculation of the silica microspheres. The mechanisms responsible for this different adsorption and stabilizing behavior are not entirely understood, as both the amidine and sulfate latex nanoparticles are of similar size and the magnitude of the zeta potentials of the different particle types are comparable. / Master of Science

This thesis presents work on the DNA directed assembly of colloids at liquid and solid interfaces under specifically sculpted attractive interactions via depletion forces and/or magnetic fields. The highly specific and thermally reversible nature of binding between two complementary single strands of DNA allows us to encode binding rules among various (solid or liquid) components of the system. The thesis begins by presenting a new approach for introducing mobile DNA linkers on oil droplets, enabling a reversible adsorption of colloids at the oil/water interface. In comparison to previous cumbersome approaches involving expensive biotinylated lipids, this simple method provides a relatively higher grafting density of DNA anchors at the interface. Further, it is possible to kinetically control the surface coverage of oil droplets with colloidal particles while preserving fully ergodic colloidal dynamics on the droplets. The equilibrium nature of the absorbed colloids is illustrated by exploring the quasi-two-dimensional (2d) phase behaviour under the influence of depletion interactions. Colloids bound to the oil water interface are found to be significantly less diffusive compared to their bulk counterparts. Simulation studies from collaboration reaffirm the experimentally observed phase behaviour and the nature of compositional arrest. Further, some preliminary results on the phase behaviour of binary colloidal mixtures at the oil/water interface are also presented. The last section of this thesis demonstrates an approach for creating novel superstructures of DNA coated colloids (DNAcc) directed via an externally applied magnetic field. Raspberry-like and long coaxial skeletons of smaller colloids around larger superparamagnetic colloidal cores in a two component system are shown. The rigidity of these mesoscopic superstructures is enhanced by adding a suitably functionalized third component. Finally, the thesis concludes by presenting various dimensions that have emerged out of this work and are being currently pursued.