Text

theses

Janus particles are a class of colloids characterized by two regions of distinct surface properties. Those with hydrophilic–hydrophobic regions tend to strongly adsorb to liquid–fluid interfaces and may exhibit unique equilibrium and dynamic behavior not observed in homogeneous colloids. When in bulk phase (i.e. suspension), Janus particles are shown to self–assemble into strings and lattices. Interfacial behavior of such particles however is less explored, especially those related to transport and dynamics under the influence of external fields. Such knowledge is crucial not only to predict the response of systems with particles at interfaces (e.g. particle–stabilized emulsions and foams) to external fields, but also to design and enable novel materials and applications. In this thesis, we first provide a quasi–static analysis on the equilibrium orientation of single and capillary–induced interactions between particle pairs. For Janus spheres, we show the existence of dipolar capillary forces, and quantify them in terms of particle size and amphiphilicity. Moreover, breaking the symmetry in distribution of the two Janus regions can enhance particle surface activity. In Janus ellipsoids, shape anisotropy results in capillary hexapoles, which govern their preferred side–by–side alignment at an interface. In the second part, we investigate hydrodynamics of Janus particles at fluid interfaces by first exploring their interfacial thermal diffusion. We demonstrate that the diffusivity is not only a function of particle size, but also depends on amphiphilicity: thermal diffusion reduces as amphiphilicity increases. We then explore dynamic response of Janus particles to a symmetric shear at the interface. For isolated particles, depending on shape, amphiphilicity, and the shear rate, two unique rotational dynamics are observed: tilting and tumbling. For a cluster of randomly distributed Janus particles, we show that the interfacial shear is capable of ordering them into chains normal to shear direction. The order parameter and separation between the chains depends on the surface coverage and strength of capillary dipoles. We obtain an optimum range of surface coverage in which ordered structures are obtained. An interesting feature of this method is that the resulting ordered structure is preserved after the field is removed.

ETD_6660

Ph.D.

Includes bibliographical references

by Hossein Rezvantalab

doi:10.7282/T3ST7RT1

ETD doctoral

The author owns the copyright to this work.