Dynamics of liquid crystalline phase transition in sedimenting platelet-like particles

Self-growing nano-liquid-crystal film from dynamic swollen hydrogel substrates

A hydrogel which spontaneously swells in an aqueous polymer solution was observed to produce a new hydrogel film coated on its swollen surface. Here, inspired by this phenomenon, we theoretically formulate the dynamics of isotropic-to-nematic (I-N) phase transition caused by swelling a hydrogel substrate (HS) in a dilute nanoplatelet suspension, and quantitatively characterize a self-growing nano-liquid-crystal (NLC) film coated on the swollen HS surface. We show that as the HS gets softer, the resulting NLC film can form earlier and achieve greater thickness (up to hundreds of micrometers). Our results and the existing experiments confirm that the growth dynamics of the NLC film or hydrogel film is exclusively regulated by the swelling behaviors of the HS instead of suspension configurations, e.g., I-N phase transition or sol-gel transition, suggesting a universal signature for the solutes ranging from molecules to colloids. However, both the maximum thickness of the NLC film and the corresponding characteristic time rely highly on the inherent elasticity of the HS and nanoplatelet aspect ratio. We demonstrate that the swelling quasiequilibrium state rather than the equilibrium state of the HS is more qualified to formulate a condition which is practically significant in preestimating the moment when the maximum thickness of the NLC film appears. Our theoretical framework serves as a robust paradigm to extensively rationalize (bio)film coatings which self-integrate with diverse nanostructural configurations via swelling-induced phase transition.

Dynamic behaviors of sedimenting colloidal gel materials: hydrodynamic interactions

It is a highly nonlinear poromechanics phenomenon that colloidal gel materials that are exposed to a gravitational stress greater than their yield stress undergo elastic compression.

Stratification in the dynamics of sedimenting colloidal platelet-sphere mixtures

The dynamics of sedimentation in a binary mixture of colloidal platelets-spheres is studied theoretically using the minimal energy model. Revisiting a sedimenting mixture of platelets, we find that nematic phase behaviors are formed, coupled with certain stratification structures in the mixed sediments, which has never been discussed before. Non-equilibrium sedimentation-diffusion equations involving excluded volume interactions between the colloids have been developed to show the occurrence of the stratified structures in the mixed platelet-sphere sediments. The model shows clearly how the nematic configuration occurs corresponding to the stratifications (the peak-like concentration profiles) over time. Both initial sphere concentration and size ratio are found to have great effects on these two coupled structural configurations, which has been illustrated in two state diagrams in detail. It is found that the stratification structure of sphere-on-top corresponds to the nematic bottom phase configuration for a small size ratio; meanwhile, the platelet-on-top structure occurs corresponding to the floating nematic phase configuration for a large size ratio. Interestingly, we can specify a moderate range of size ratio in which the mixture displays only the nematic phase configuration (either the nematic bottom phase or the floating nematic phase depending on the sphere loadings), while the mixed sediments are unstratified as a bulk.

Directional freezing of binary colloidal suspensions: a model for size fractionation of graphene oxide

The performance of graphene oxide (GO)-based materials strongly depends on the lateral size and size distribution of GO nanosheets. Various methods are employed to prepare GO nanosheets with a narrow size distribution. One promising method was proposed recently, directional freezing of a GO aqueous dispersion at a controlled growth rate of the freezing front. We develop a theoretical model of a binary colloidal suspension, incorporating both the moving freezing boundary and the preferential adsorption of colloidal particles to the ice phase. We numerically solve the coupled diffusion equations and present state diagrams of size fractionation. Selective trapping of colloids according to their size can be achieved by a suitable choice of the experimental parameters, such as the adsorption rates and the freezing speed.

Dynamics of the floating nematic phase formation in platelet suspension with thickness polydispersity by sedimentation

An inverted phase coexistence, where an ordered phase appears on top of a disordered phase, has been observed in polydisperse colloidal suspensions. Herein, we studied the dynamics of this phenomenon in a suspension of a mixture of two types of platelets, namely, thick and thin. We show that the thick platelets preferentially sediment first excluding the thin platelets, and create a region enriched with thin platelets at some place above the bottom, which eventually gives the inverted configuration. We show that such interplay between the sedimentation and the isotropic-nematic phase transition can cause a rich and complex sedimentation dynamic. Depending on the initial concentration and the gravity strength, the interface between the isotropic phase and the nematic phase can move up or down during the sedimentation, and a variety of final equilibrium structures can appear.