Prof. dr. H.N.W. (Henk) Lekkerkerker

Algemeen Scheikunde

The study of the liquid crystal phase behavior of suspensions of rodlike and plate-like colloids

We have developed new model systems for hard colloidal rods and colloidal plates. The hard colloidal rods consist of a Boehmite (AlOOH) core and a steric stabilization layer of polyisobutylene. Dispersions of these particles show an isotropic-nematic phase separation. Using polarization microscopy and small angle light scattering we observed that depending on the concentration the pathway of phase separation is either nucleation and growth or spinodal decomposition. The effect of attractive interactions on the phase behavior of colloidal rods has been studied by the addition of nonadsorbing polymer. Now four pathways of phase separation can be distinguished; nucleation and growth, spinodal decomposition aggregation and gelation. The hard colloidal plates we developed consist of a Gibbsite (Al(OH)3) core and again a steric stabilization layer of polyisobutylene. In dispersions of these particles we discovered an Isotropic -Nematic and Nematic-Columnar phase transition. Especially the latter transition is fascinating and unexpected as it persists for a polydispersity up to 25%. Our results imply that liquid crystalline order in synthetic mesoscopic materials may be easier to achieve than previously thought. We also investigated the phase behavior of mixed dispersions of hard colloidal rods and colloidal platelets. Here we were able to show experimentally that phase separation in two coexisting uniaxial nematic phases occurs and not a single biaxial nematic . This issue has been the subject of considerable theoretical debate. 

The study of the phase behavior of mixed colloidal systems

We developed a theory of the phase behavior of colloidal rod-sphere mixtures. From this theory it follows that in such systems a colloidal fluid colloidal crystal phase transition can occur at very low concentrations of added colloidal rods. In subsequent experimental work we showed this indeed to be the case. At the same time we were able to follow the morphology of the aggregation process that eventually leads to crystallization.

The study of interfaces in phase separated colloid polymer suspensions 

Using the spinning drop method we were the first to determine the ultra low interfacial tensions of the interface in a demixed colloidal polymer system. As predicted by simple scaling relations these interfacial tensions are indeed as low as a few µN/m. We were able to confirm this result by analyzing the rate of break up of long filamentous droplets. Using ellipsometry we studied for the first time the interfacial profile. The thickness of the interfacial profile is of the order of the diameter of the colloidal particles.