Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces Basavaraj Madivala, † Jan Fransaer, ‡ and Jan Vermant* ,† Department of Chemical Engineering, K. U. LeuVen, W. de Croylaan 46, B-3001, LeuVen, Belgium, and Department of Metallurgy and Materials Engineering, K. U. LeuVen, Kasteelpark Arenbergpark 44, B-3001, LeuVen, Belgium ReceiVed October 25, 2008. ReVised Manuscript ReceiVed December 16, 2008 Colloidal particles confined at liquid interfaces have important applications, for example in the stabilization of emulsions and foams. Also the self-assembly of particles at interfaces offers potential for novel applications and structured particle films. As the colloidal interactions of colloidal particles at interfaces differ from those in bulk, colloidal microstructures can be achieved at an interface which cannot be produced in bulk. In the present work the particle shape, surface charge, and wetting properties are varied, and the resulting self-assembly of particles at a fluid interface is studied. Model monodisperse micrometer-sized ellipsoidal particles were prepared by a mechanical stretching method. These particles were chosen to be well-suited for investigation by optical microscopy. When deposited at an interface between two fluids, shape-induced capillary interactions compete with the electrostatic repulsion. Changing the surface charge and the position at the interface can be used to manipulate the experimentally observed self-assembly process. The initial microstructure of charged ellipsoids at a decane-water interface consists of individual ellipsoids coexisting with linear chains of ellipsoids, connected at their tips. The aggregation behavior in these monolayers was investigated by optical microscopy combined with quantitative image analysis and a dominant tip-tip aggregation was observed. Microstructural information was quantified by calculating the pair-distribution and orientation-distribution functions, as a function of time. Compared to particles at an oil-water interface, particles of the same surface chemistry and charge at an air-water interface seem to have weaker electrostatic interactions, and they also have a different equilibrium position at the interface. The latter leads to differences in the capillary forces. The subsequent change in the balance between electrostatic and capillary forces gave rise to very dense networks having as a typical building block ellipsoids connected at their tips in triangular or flower-like configuration. These networks were very stable and did not evolve in time. The resulting monolayers responded elastically and buckled under compression. Furthermore, the mechanical properties of these monolayers, as measured by surface shear rheology, showed that the monolayer of ellipsoids exhibit a substantial surface modulus even at low surface coverage and can be used to create more elastic monolayers compared to aggregate networks of spheres of the same size and surface properties. Introduction The possibility of tailoring the interaction forces between colloidal particles at fluid-fluid interfaces, by choosing an appropriate combination of fluids, suitable additives, and other formulation parameters, has been exploited to study a range of physical phenomena, such as dislocation dynamics in crystals, 1 crystallization, 2 and aggregation 3-5 in two-dimensional (2D) model systems. The major advantage of using a 2D suspension is that all the microstructural information is contained in one plane, and high resolution, time-resolved studies are possible using simple bright field microscopy over a wide range of surface coverages. When micrometer-sized particles are pinned at an interface between air-water or oil-water, they stick to the interface irreversibly because of capillary energy on the order of 10 6 times the thermal energy. 6 Charged spherical particles at a fluid-liquid interface have been known to form ordered hexagonal crystals that are stable over several days, because of enhancement of the electrostatic interactions. 6 Aggregation can be induced by tuning the interparticle interaction by the addition of salt and/or surfactant to the subphase. 7 For spherical particles, the screening of the surface charges in the aqueous phase combined with the changes in wetting properties that control the amount of surface area exposed to the water phase control the electrostatic interactions, as has been demonstrated by direct measurements using optical tweezers. 8 In the present work, we will focus on the use of nonspherical particles. The use of shape as a parameter in colloidal systems leads to a number of effects that can be exploited, in 3D as well as in 2D systems. First, one can lower the percolation threshold by using anisotropic particles, which is a critical issue for several applications. 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