Colloidal spheres confined by liquid droplets: Geometry, physics, and physical chemistry

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Original Entry: Aaron Goldfain, AP 225, Fall 2012


General Information

Author: Vinothan N. Manoharan

Publication: Vinothan Manoharan. Colloidal spheres confined by liquid droplets: Geometry, physics, and physical chemistry. Solid State Communications (2006) vol. 139 (11-12) pp. 557-561

Keywords: diffusion, colloids, emulsion, interfaces, self-assembly

Summary

Figure 1, taken from [1].
Figure 2, taken from [1].
Figure 3, taken from [1].

This paper describes the self-assembly of colloidal aggregates within water and oil emulsions. In general, for the experiments discussed, micrometer sized colloids are dispersed in emulsions composed of spherical droplets of controlled sizes, ranging from a few micrometers to a millimeter in diameter. Different preparation methods and droplets sizes were shown to produce three types of structures; colloidosomes (Pickering Emulsions), colloidal clusters, and supraparticles.


The interactions between the colloids and the oil-water interface, interactions between colloids, and the thermal energy of the system determine what structures are formed. The energy of colloid-interface interaction is described by a 3 phase contact line between the colloid, oil, and water. Using different types of colloids, colloid functionaliziations, and surfactants in the liquids enables the experimenters to tune this interaction energy to range from <math>10^4kT</math> to <math>\sim\,\!</math> <math>kT</math>. The interactions between colloids depend on the colloids themselves. Such interactions can be short ranged and steric if the colloids are coated in polymer chains, or long ranged if the colloids are coated in charged groups. The strength of these interactions can be readily tuned by changing how the colloids are functionalized and what salts and surfactants are in the water and oil. However, the functional form of the energy of colloid groups is not well established.


Colloidalsomes are formed when the colloids bind to the spherical interfaces in the emulsion. Accordingly, the colloids form structures where each colloid is on the surface of a sphere. These structures are formed when the binding energy to the interface is much larger than <math> kT </math>. At high colloid concentrations, the colloids close-pack around the interface in configurations independent of the form of the inter-colloid interaction potential. The colloids form a hexagonal lattice, but with defects due to the curvature of the droplets. At low concentrations, when the area of colloids in the sphere is small compared to the sphere's surface area, the equilibrium configuration depends on the functional form of the inter-colloid interaction energy. Since this potential is not well characterized, precise configurations cannot be predicted. The 12 particle, icosahedron configuration shown in Figure 1, however can be described by an <math>r^{-p}</math> potential where <math>r</math> is the distance between two colloids and <math>p</math> is either <math>1</math> or <math>\infty</math>.


Colloidal clusters are formed by removing the liquid from the inside of a low density colloidalsome (Figure 2). The liquid can either be removed by evaporating the inner liquid or having it diffuse out to a lower pressure container. As the colloidalsome shrinks, the colloids remain adsorbed to the interface until they are in contact and become bound by Van der Waals forces. Geometrical arguments dictate that clusters with fewer than 19 colloids are unique, and clusters of greater numbers can be simulated using a linear interaction force. Clusters with fewer than 15 colloids have been experimentally examined and agree with the geometrical arguments, but larger colloidal clusters have not been tested.


Supraparticles are formed when the colloids do not adsorb to the water-oil interface (Figure 3). This is achieved by increasing the three-phase contact angle so the binding energy to interface is approximately <math>kT</math>. The contact angle can be increased either by an appropriate choice of fluids and particles, or by adding surfactant to the water and/or oil. Supraparticles are formed from relatively dense colloidal suspensions (volume fraction ~ 10-30%) and thus the colloids pack tightly within the droplets. Their structure depends mostly on the ratio of droplet size to colloid size. When this ratio is around 1000, the colloids form a face-centered cubic lattice capable of diffracting light. When the ratio is around 10-100 the interiors of the supraparticles are mostly disordered while their surface forms a lattice similar to a colloidalsome's.

Discussion

This article provides a good summary of structures that can be created by colloids in emulsions. It brings together very clean experimental data from multiple research groups and attempts to describe the data with physical and mathematical models. The models are far from complete, and the author acknowledges this, but can still describe many of the observed qualitative features. Mainly, the models lack a detailed, quantitative description of the inter-colloid interactions. Having such a description could provide a much more rigorous backing to the models described. Additionally, to me, it would be interesting to read about possible applications of the described colloidal structures. For example, could the large supraparticles be used as a model system for studying crystals?

References

[1] Vinothan Manoharan. Colloidal spheres confined by liquid droplets: Geometry, physics, and physical chemistry. Solid State Communications (2006) vol. 139 (11-12) pp. 557-561