Structure of adhesive emulsions
J. Bibette,J T. G. Mason,Hu Gang,$ D. A. Weitz,J and P. Poulint
"Structure of adhesive emulsions"
Entry by Fei Pu, AP 225, Fall 2012
Emulsions are inherently unstable dispersions of one, normally immiscible, fluid in a second. Nevertheless, with appropriate surfactant molecules, or other surface active species, emulsions can be made to be stable nearly indefinitely. 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
The crude emulsion is formed from silicon oil droplets in water, stabilized by sodium dodecyl sulfate (SDS). Highly monodisperse emulsion droplets, about 0.6 pm in diameter, are obtained by repeated crystallization fractionation. The surfactant concentration in the continuous phase is adjusted to about 0.01 M, slightly above the critical micelle concentration, ensuring the stability of the emulsion. To induce adhesion between the droplets, sodium chloride is added to the continuous phase.
The strength of the adhesion is strongly temperature dependent: Above an onset temperature, T, the droplets remain repulsive, whereas adhesion is induced between the droplets when the temperature is reduced below T, and the strength of the adhesion increases as the temperature is further decreased. We believe that this may be associated with the formation of a nonzero contact angle, phi, between droplets, as has been observed for larger, polydisperse emulsion droplets. The onset temperature depends sensitively on the concentration of sodium chloride, increasing with increasing NaCl concentration. All experiments are carried out by rapidly lowering the temperature several degrees below T, to induce the attraction, ensuring strong adhesion between the droplets. The structure of the flocculating emulsions are directly observed using a phase contrast microscope with a variable temperature stage.
While the temperature of the emulsion is maintained above T, the droplets undergo random Brownian motion, and there is a sufficiently large, short-range repulsive barrier between them to ensure stability against flocculation. When the temperature is lowered several degrees below T, a strong adhesion develops between the droplets. However, while the droplets adhere to one another, they retain their integrity and do not coalesce.
The gel also depends on the contact angle phi. At phi = 0.005, the structure of the gel at short length scales consists of a highly random, tenuous network of droplets. The distinct branches of the network are roughly a single droplet in thickness. As phi is increased to 0.01 and then to 0.05, the average thickness of the branches increases slightly, but the structure still exhibits a random, tenuous network at short length scales. As phi is increased still further, the average thickness of the branches increases substantially and the tenuous network is no longer observed at short length scales. Instead, the gel has a dramatically different appearance, with a random, weak and free-flowing network.
A remarkable feature of these emulsion gels is the fact that they form a tenuous network, which must maintain some inherent rigidity, despite the fact that it is composed entirely of liquid droplets. This rigidity has a profound implication about the nature of the adhesion: It suggests that there must be no slip between two adjacent droplets. If slip could occur, the droplets would be able to rotate about their bonds, and the resultant rearrangement would lead to a significantly less tenuous structure. This suggests that a rigid film is formed by the adhesion between neighboring droplets. One possibility is that the surfactant forms a solid layer at the interface, which prevents re-arrangement of the atoms on the surface.
Soft Matter Applications
Under favorable conditions emulsification offers the opportunity to incorporate almost any ratio of one fluid in another. This leads to a wide range of important technological applications, ranging from cosmetics to coatings and from foods to medicines. Emulsions are also fascinating dispersions of deformable spheres, which exhibit novel colloidal properties. For example, the interaction between droplets can be made strongly adhesive, ultimately leading to a nonzero contact angle between droplets. If the droplets retain their integrity, this adhesion causes flocculation of the emulsion droplets. Emulsion flocculation, or creaming, is a commonly encountered phenomenon. If the phenomenon is understood, it could be used in many different colloidal applications.