Phase Behavior and Structure of a New Colloidal Model System of Bowl-Shaped Particles
Entry by Pichet Adstamongkonkul, AP 225, Fall 2011
Work in progress
Title: Phase Behavior and Structure of a New Colloidal Model System of Bowl-Shaped Particles
Authors: Matthieu Marechal, Rob J. Kortschot, Ahmet Faik Demirors, Arnout Imhof, and Marjolein Dijkstra
Journal: Nano Letters, 2010, Vol. 10, No. 5
This paper investigated the structural organization of the bowl-shaped particles, which may have many possible applications such as nanocontainers, coatings due to the aggregation behavior of the particles, and perhaps in the field of sensors and shape-memory materials. The bowl-shaped particles appear to form stacks in the wormlike phase or columnar and other ordered structures, depending on many factors, including the particle dimensions, densities, sedimentation, free energy of crystal structures, and packing fraction. In addition, the authors also tried to capture the important features of the phase behaviors of the particles by implementing a computer simulation model, which was shown to support the experimental observations that these particles have a strong tendency to form bent, non-aligned stacks. The calculations also suggested that the wormlike phase is not the equilibrium state of the system and the columnar phase is more thermodynamically stable for deep nanobowls and at high densities. Lastly, this study also proposed a phase diagram, in which the phase behavior changes depending on the dimensions and the packing fractions of these particles, which may have some benefits in designing the particles of this kind for other applications.
The first step in the synthesis of these bowl-shaped particles is the preparation of uniform oil-in-water emulsion droplets of silicone via the hydrolysis and polymerization of the precursor, dimethyldiethoxysilane. The droplets are then coated with a solid shell of tunable thickness of tetraethoxysilane. The silicone inside is then dissolved by ethanol. During the drying step in air, the shells collapse inside, resulting in hemispherical double-walled bowls.
The model used in the computer simulation viewed a particle as the solid of revolution of a crescent. The shape parameter was defined by a reduced thickness, <math>D/\sigma</math>, when D is the thickness of the bowl measured from the bottom of the bowl to the top, and <math>\sigma</math> is the diameter of the bowl. This can be easily visualized, as this quantity equals to 0 in the infinitely thin hemispherical surfaces, and it equals to 0.5 in solid hemispheres. This model then captured the continuous range of shapes of the particles.
Next, colloidal particles were suspended in an index-matched mixture of dimethylsulfoxide and ethanol and the suspension were allowed to sediment