Shear-Induced Formation of Ordered Monodisperse Emulsions Stabilized by an Associating Amphiphilic Polyelectrolyte

• Published in: Langmuir 2001-05, Vol.17 (9), p.2656-2663
• Main Authors: Perrin, Patrick, Devaux, Nicolas, Sergot, Philippe, Lequeux, François
• Format: Article
• Language: English
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la001492w

An associating amphiphilic polyelectrolyte was used as an emulsifier of the n-dodecane−water system. First, the dynamic rheological behavior of polydisperse concentrated direct emulsions with dispersed phase volume fractions up to 0.91 was investigated. Both the Princen and Kiss (J. Colloid Interface Sci. 1983, 91, 160; 1986, 112, 427) and Mason et al. (Phys. Rev. Lett. 1995, 75, 2051) approaches on emulsion elasticity give an adequate fitting of the rheological behavior leading to the conclusion that the emulsion elasticity mainly arises from droplet compression. Second, the shear-induced formation of ordered monodisperse emulsions was studied. More specifically, the conditions under which ordered structures are obtained by shearing polydisperse premixed emulsions within the two-parallel circular glass plates of a homemade shear apparatus were reported. The light scattering patterns, first-order ring, first-order diffuse ring with 6 bright spots, and second-order diffuse ring with 12 bright spots successively observed while shearing the emulsion samples reveal the progressive formation of ordered layers of hexagonal close-packed planes of oil droplets as the shear rate increases. The observation of the diffraction patterns also shows that the level of droplet organization decreases after cessation of flow thus suggesting that the droplets occupy well-defined spatial positions within a sample under shear as compared to a sample after cessation of flow. The radius of the droplets forming the colloidal crystal was systematically measured under shear as a function of the radial distance from the center of the glass slides, the gap spacing, the polymer concentration, and the oil content to follow the development of long-range ordering within the emulsions. The formation of the droplet crystal was found to depend only on the shear rate. Smaller droplets are obtained with increasing both the shear rate and the polymer concentration, and only a slight change in the droplet size could be detected upon varying the volume fraction of the dispersed phase. The results are in qualitative agreement with a coalescence−fractionation mechanism of the droplets under shear.