Structure of adhesive emulsions

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J. Bibette,tJ T. G. Mason,$ Hu Gang,$ D. A. Weitz,*J and P. Poulint

"Structure of adhesive emulsions"

Entry by Fei Pu, AP 225, Fall 2012


Keywords: contact angle, surface forces, adhesives, droplets


Summary

When oil is dropped in water emulsions, the interactions between the droplets are so strong that they adhere together and retain their integral shapes. The structure of the strongly adhesive emulsions reflects a complex interplay among the strength of the adhesion, the droplet volume fraction, contact angle phi, and the time evolution of the adhesion. Initially, strong adhesion of the droplets leads to the formation of an emulsion gel. Moreover, the gel possesses a well-defined characteristic length scale, d,, as evidenced by an intense ring of small angle light scattering. The characteristic length scale decreases as the droplet volume fraction increases. At low phi, the gelation mechanism is controlled by diffusion-limited cluster aggregation. However, at higher phi, the short range structure is more compact, rather than fractal, and a different mechanism must be responsible for the gelation. If the strength of the adhesion is increased still further, the droplets become more deformed, resulting in massive restructuring of the emulsion gel. The structure fractures into independent, more compact flocs, eliminating the overall rigidity of the emulsion gel. These results help rationalize some of the diverse structures that are observed upon flocculation of the more usually studied polydisperse emulsions.

Materials and Methods

Colloidal particles consisting of one smooth and one rough sphere were synthesized following a modified synthesis by Kim et al. (27). Roughness on the seed particles was obtained through adsorption of polystyrene particles nucleated during polymerization. The synthesized colloids were washed and redispersed in 0.3% w∕w aqueous polyvinyl alcohol (Mw ¼ 30–50 kg∕mol).

Monte Carlo simulations were used in the canonical ensemble (NVT) to calculate the probability distribution of the cluster. Also, The free energy of clusters of different sizes was calculated using grand-canonical Monte Carlo (GCMC) simulations on single clusters (41).

Results & Discussion

When two colloids overlap each other, the depletion entropy increases, and such phenomenon makes the colloids attract more closely. The effect of rough particles interacting with smooth particles and other rough particles was recorded and analyzed. At higher concentrations and thus stronger attractions, the roughness anisotropic colloidal particles spontaneously organized into clusters, in which the attractive parts constitute the core of the aggregate and the non-attractive rough sides are located at the outside. These structures look like micelles, as shown below in Figure 1.

Figure 1. The rough and smooth colloidal particles interact and spontaneously bond into clusters, which could be called colloidal micelles


Monte Carlo simulation was further done on the smooth and rough colloids, shown in Figure 2. As time went on, it seems like smooth particles attracted into clusters and formed the core of the micelles and that the rough particles stayed outside and surrounded the core.

Figure 2. Smooth particles clustered on the inside of the micelles while the rough particles surrounded the outside.

Finally,cluster size distributions changed as interactions increased and geometry overlapped more. From Figure 3, it's clear that as density, p, increased, clusters were more prone and easily formed. When density was low at the beginning, the particles flowed freely and even repelled. Only when density reached a threshold that the surface energy and forces favored clustering.

Figure 3. As density of particles increased, they overlapped more and formed clusters.

Soft Matter Applications

Due to the available variety of colloids and their straightforward assembly even between different patch sizes, it is expected that these soft colloidal particles with smooth and rough surfaces could self-assemble in a controlled manner into superstructures with desired topology and properties. This has significant applications. For example, the virus macromolecules, protein subunits, and building cell blocks in our body are often complex and challenging to identify key elements for self-assembly processes. By mimicking such self-assembly processes on a colloidal scale, insights into the paramount elements that control the assembly can be obtained in situ and applied to build up superstructures with new and desirable properties. The findings in this article have fundamental and practical importance in the field of colloidal and macromolecular self assembly.