Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces

Abstract

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.

Adsorption trajectories and free-energy separatrices for colloidal particles in contact with a liquid-liquid interface

“…There is also an impetus to theoretically describe the colloidal adsorption process more accurately, in order to gain a deeper insight into the mechanisms at work in experimental systems. Especially 2D fluid phenomena and phase transitions in 2D fluids of anisotropic particles 4,5 are of interest. The tunability and variety of colloidal particles currently available, coupled with our still limited knowledge on colloidal adsorption, leave the study of their interfacial phenomena an open field.…”

“…To better understand the complex systems [4][5][6][7][8][9] which arise when colloidal particles are brought in contact with an interface, we developed a method to determine the free energy associated with the adsorption of a single particle. 10 This free-energy calculation is based on similar surface tension arguments as proposed by Pieranski, 1 in his ground breaking study of colloidal adsorption phenomena.…”

Effects of Particle Shape on Growth Dynamics at Edges of Evaporating Drops of Colloidal Suspensions

“…The experiments are reproducible across many droplets (4-10 for each aspect ratio), enabling accumulation of sufficient statistics to test continuum equation predictions of surface roughness scaling, and more. Strong shape-based capillary attractions between particles on the air-water interface [26][27][28][29][30][31][32] permit us to establish relationships between particle interaction and interfacial growth processes.…”

“…Superficially, our experiments appear to have different conditions than those needed for KPZQ. However, the highly anisotropic ellipsoids induce strong capillary attraction on the air-water interface [26][27][28][29][30][31][32], which causes regions with many particles to strongly attract additional particles. Even particles that adsorb on the air-water interface in regions nearly void of particles are strongly attracted to particle-rich regions, and eventually are deposited in particle-rich regions.…”

Improving the microscopic sweep efficiency of water flooding using silica nanoparticles

“…Onyekonwu and Ogolo (2010) studied capability of three different nanoparticles as an operator to change wettability and oil recovery purposes. Studies also involved particle size and shape (Hunter et al 2008;Madivala and Fransaer 2009), particle concentration Zhou et al 2013;Chevalier and Bolzinger 2013). Moreover, the effects of salinity (Yoon et al 2012;Lee et al 2014) on emulsion stability have also been studied (Lee et al 2014).…”