Phase behaviours of colloidal systems with critical Casimir forces

Abstract

The study of phase behaviors elucidates the thermodynamics of condensed matters and thus is vital to the understanding of material properties. Colloids are ideal model systems for the study of phase behaviors because the particles can assemble into extremely rich structures and phases that are governed by the general statistical principles that also work in atomic or molecular systems. And their thermal motions can be directly visualized and measured by video microscopy. Progress in chemistry and particle fabrication enables the implementation of tunable and controllable interaction between particles, opening the door to actively triggering phase transformations in colloidal systems. In this thesis, I report the studies of phase behaviors of colloidal particles in a recently developed critical Casimir system. The attractive critical Casimir forces (CCFs) between the colloidal particles can be precisely tuned by temperature. With CCFs, the spherical and ellipsoidal particles can be assembled into different phases, i.e., glasses, crystals, liquid crystals, and plastic crystals. Bulk phases with a free solid-vapor interface are achieved and studied. Tuning the temperature, the melting transition of these phases is triggered and the kinetics of the transformation are recorded by video microscopy. The interaction between surface and bulk regions as well as the dynamics in different degrees of freedom are investigated systematically. As the introduction, Chapter 1 offers a general background about CCFs and phase behaviors of glasses, liquid crystals and plastic crystals. Research motivations are presented from the perspective of both condensed matter physics and material sciences. Chapter 2 presents the quantification of the CCFs between the spherical colloidal particles. Measurements of the pair-potential between the particles using both total internal reflection microscopy (TIRM) and the insertion method with video microscopy data are presented. Both measurements exhibit consistent results, which are in good agreement with the theoretical prediction of CCFs. In chapter 3, anisotropic curvature-dependent CCFs are observed and quantified between the ellipsoidal particles. Both experiments and results from the insertion method confirmed that the strength of CCFs depends on the local curvature. Elucidation of the forces between particles in Chapter 2 and Chapter 3 facilitates the understanding of phase behaviors of glasses (Chapter 4), liquid crystals, crystals with also orientational order and plastic crystals (Chapter 5). Chapter 4 reports the surface melting of two-dimensional glasses composed of binary colloidal particles. Adjacent to the bulk glass, we observe an unexpected surface glassy layer that has the same density but much faster particle dynamics. This is due to cooperative clusters of highly mobile particles which percolate from the surface and penetrate deep into the material. In addition to resolving the properties of a glassy surface during surface melting, our results are also relevant for the understanding of the largely enhanced surface mobility of glassy materials. Chapter 5 reports the phase behaviors of ellipsoidal particles with different aspect ratios. The curvature-dependent CCFs between the ellipsoidal particles enable the assembly of novel 2D phases that are realized in the colloidal ellipsoidal system for the first time. These phases are nematic liquid crystals (NLCs), crystals with both translational and orientational order (CTOOs) and rotator crystals (RCs). In all three systems, thermodynamic bulk phase transitions and typical surface premelting are observed. Comparing across the systems, the existence of the disorder in a certain degree of freedom has relation to the decoupling between motions in different degrees of freedom or between dynamics and structure. In systems where the disorder exists, i.e., NLCs and RCs, the decoupling between dynamics in translational and rotational degrees is observed in the surface region with the non-monotonic temperature dependence. In contrast, such decoupling does not emerge in the completely ordered system like the CTOOs and crystals composed of spherical particles. The connection between the disorder and dynamical decoupling-induced non-monotonic behaviors seems to be general, although tests in more experiments and simulations may be necessary. Finally, the outlook and summary of the current studies are offered in Chapter 6. Some possibly interesting research directions of phase behavior studies are proposed based on the results in the previous chapters, both within the framework of thermodynamics and connection with other realms.