Scattering studies of complex fluids in confinement

A simulation study of electrostatic effects on mixed ionic micelles confined between two parallel charged plates

Confined colloidal systems have been the subject of extensive theoretical and experimental research, and the recent observation of long-range like-charge attraction in such systems has only highlighted their peculiar behavior. On the other hand, surfactant solutions are often used in small confined space, yet their behavior in confinement has received relatively little attention. A distinct feature of confined self-assembling systems is that the aggregates are capable of adjusting their composition, size, and shape in response to their external environment, which may lead to very different phase characteristics compared to bulk solutions. The primary objective of this study is to explore the effects of varying micelle composition on the structural behavior of a confined mixed ionic micellar solution. Mesoscale canonical Monte Carlo simulations were used to probe the structure of the confined solution, while a molecular-thermodynamic model was used to systematically account for the change in micelle size as we varied its composition. Significant micelle ordering was found under certain conditions, which implies that large deviations from the minimum-energy micelle configuration may not be entropically favorable. Accumulation of micelles along the midplane was observed when the confining walls are weakly charged, suggesting that micelle shape transformation should be considered in more detail. On the other hand, with high wall charge density, apparent attraction was found between like-charged micelles and wall. These findings point to the need for a more quantitative theoretical treatment in describing surfactant self-assembly in confined geometries.

Structure of charged colloids under a wedge confinement

Monte Carlo simulations have been performed to study the influence of a wedge confinement by hard walls on the ordering of charged colloidal particles interacting through screened Coulomb repulsive potential in an aqueous medium. The density distribution of particles for a fixed wedge angle straight theta(0) is studied for different suspension parameters, viz., bulk volume fraction straight phi, salt concentration C(s), and charge Ze on the particles. The density distribution rho(straight theta) along the angular direction and that along the radial direction, rho(r), have been analyzed in different regions of the wedge. Simulations show the formation of layered structure along the angular direction and a large gathering of particles along the wall. The number of layers as well as the density of particles within the layer are found to change as the strength and range of the interaction are varied, whereas the density profiles calculated close to the vertex region showed no significant variation in the density. The radial density profiles rho(r) corresponding to the vertex region show one-dimensional (1D) ordering of particles parallel to the vertex at a distance that is close to a wedge height, h equal to diameter of the particle. This 1D ordering is found to be destroyed upon the addition of salt or lowering the straight phi. The reported experimental observations on the "vacuum phase" are discussed in the light of the present results.