Clustering, wetting and demixing in colloid-microemulsion composites

The behaviour of colloids dispersed in a phase-separating microemulsion (micellar solution) are explored using microscopy techniques. It is well known that colloids will cluster approaching a demixing transition of the host solvent; the mechanism is still unresolved. Here a system is used which is intrinsically slow, making experimental studies easier by direct imaging.

On raising the temperature the pure microemulsion separates into micellar liquid and micellar gas phases. The gas phase contains a smaller proportion of the micelles and is less dense. The dynamics are slow, partly because of the size of the micelles. Well before demixing, clustering of the colloids is observed: initially the clusters are irregular in shape and continuously aggregate. When the system gets closer to the phase boundary a wetting layer of the gas phase becomes visible around the colloid aggregates.

Following complete phase separation the colloids prefer the micellar gas phase. The way the separation mechanism is influenced, depends on the microemulsion composition. The colloidal clusters originating prior to demixing cream to the top of the container.