Fluctuation-induced order in two-dimensional colloidal clusters

Abstract

PACS. 82.70.Dd -Colloids. PACS. 64.60.Cn -Order-disorder transformations; statistical mechanics of model systems. PACS. 83.10.Tv -Structural and phase changes.Abstract. -The behavior of a finite number (N = 29) of paramagnetic colloidal particles in 2D circular hard-wall cavities is investigated. By applying a magnetic field B, the interaction between the particles is varied. We demonstrate that the angular diffusion of the particles which are arranged in shells is highly anisotropic and shows a non-monotonic behavior as a function of B. When reducing radial particle fluctuations in one shell by a ring-shaped optical tweezer, the relative angular diffusion between adjacent shells increases again. This clearly demonstrates that radial particle fluctuations are responsible for an enhanced registration between adjacent shells.Thermal fluctuations are well known to have a strong influence on the phase behavior of two-dimensional (2D) systems compared to their three-dimensional counterparts [1]. The most striking evidence for such a difference is probably the melting transition which is, in 2D, predicted to occur via two sequential defect-driven continuous phase transitions as described by the KTHNY theory (see, e.g., [2]). Amongst conveniently accessible experimental systems for basic investigations of 2D melting colloidal particles have been established as model systems because they allow direct optical observation of topological defects and comparison to theoretical predictions [3][4][5]. However, while the KTHNY theory applies only for extended 2D systems, much less (both experimentally and theoretically) is known about the properties of 2D systems which are only comprised of a few particles (typically less than N = 100). Due to the finitness of such systems a different melting scenario compared to infinite systems is expected which may be important for the understanding of melting and freezing of, e.g., small 2D clusters or ions in radiofrequency traps [6]. It has been experimentally demonstrated that when super paramagnetic colloidal particles are confined to a circular hard-wall cavity, the particles at low effective temperatures T eff do not crystallize in a triangular lattice, but are rather arranged in a shell-like structure with both positional and orientational order. Upon increasing the temperature first orientational order between adjacent shells is lost which is typically refered to as intershell rotation. As the temperature is increased further, a "re-entrant" ordered phase is observed which is followed by complete melting of the cluster [7]. While intershell rotation is also observed in numerical studies of finite 2D systems of ions or electrons in circular cavities, the "re-entrant" ordered phase as found in the experiments, is still c EDP Sciences

Structural analysis of a dipole system in two-dimensional channels

“…Wall-induced layering is observed in this system analogous to the layering observed in a dusty-plasma study [16] and the shell structure in circular confinements [10][11][12][13][14]. However, in the case of repulsive magnetic dipoles there is a higher density and localization at the walls in the 2D channel system than in the dustyplasma system.…”

“…The structural properties of these confined systems have a strong influence on the nature of the phase transition-where it begins and how it proceeds. In the case of 2D channels we have observed structural properties of the crystal which, in the context of previous studies [10][11][12][13][14], will likely prove important in studying the phase transition in the 2D channel system. These other interesting properties of the 2D channel system will be discussed in future work.…”

“…The point-dipole approximation for MR colloids in 2D is very common in the literature [6,22] and has been shown to be a good approximation for the magnetic behavior of MR colloids [23]. It has even been used to generate meaningful results in the case where hard walls are present in the system [11,13]. Additionally the effects of mutual induction between particles in this system are negligible due to the large separation distances between the particles.…”

“…Others have studied the 2D field-responsive colloid system under various confinements. Most of this research has focused on circular confinements [10][11][12][13][14] and confinement due to a periodic 1D potential [15]. The overriding theme of these studies is that the confinement induces a change in the structural and dynamical behavior of the colloidal crystal and the trends depend upon the nature of the confinement.…”

Mie scattering by soft core-shell particles and its applications to ellipsometric light scattering

“…As it is the softness of core-shell systems that makes them attractive for a number of applications, the ability of the mantle of such a composite particle to deform or fluctuate defines many of these useful properties [5][6][7]. It is desirable to be able to ascribe quantitative measures to the softness of the shell of a particle, as many of the interaction properties are related to the ability of the shell to deform [8].…”