10 research outputs found
Crystal nuclei and structural correlations in two-dimensional colloidal mixtures: experiment versus simulation
We examine binary mixtures of superparamagnetic colloidal particles confined
to a two-dimensional water-air interface both by real-space experiments and
Monte-Carlo computer simulations at high coupling strength. In the simulations,
the interaction is modelled as a pairwise dipole-dipole repulsion. While the
ratio of magnetic dipole moments is fixed, the interaction strength governed by
the external magnetic field and the relative composition is varied. Excellent
agreement between simulation and experiment is found for the partial pair
distribution functions including the fine structure of the neighbour shells at
high coupling. Furthermore local crystal nuclei in the melt are identified by
bond-orientational order parameters and their contribution to the pair
structure is discussed
The experimental realization of a two-dimensional colloidal model system
We present the technical details of an experimental method to realize a model
system for 2D phase transitions and the glass transition. The system consists
of several hundred thousand colloidal super-paramagnetic particles confined by
gravity at a flat water-air interface of a pending water droplet where they are
subjected to Brownian motion. The dipolar pair potential and therefore the
system temperature is not only known precisely but also directly and
instantaneously controllable via an external magnetic field B. In case of a one
component system of monodisperse particles the system can crystallize upon
application of B whereas in a two component system it undergoes a glass
transition. Up to 10000 particles are observed by video microscopy and image
processing provides their trajectories on all relative length and time scales.
The position of the interface is actively regulated thereby reducing surface
fluctuations to less than one micron and the setup inclination is controlled to
an accuracy of 1 microrad. The sample quality being necessary to enable the
experimental investigation of the 2D melting scenario, 2D crystallization, and
the 2D glass transition, is discussed.Comment: 13 pages, 11 figure
Dynamics of particles and cages in an experimental 2D glass former
We investigate the dynamics of a glass forming 2D colloidal mixture and show
the existence of collective motions of the particles. We introduce a mean
square displacement MSD with respect to the nearest neighbors which shows
remarkable deviations from the usual MSD quantifying the individual motion of
our particles. Combined with the analysis of the self part of the Van Hove
function this indicates a coupled motion of particles with their cage as well
as intra cage hopping processes.Comment: Submitted to EP
Partial clustering prevents global crystallization in a binary 2D colloidal glass former
A mixture of two types of super-paramagnetic colloidal particles with long
range dipolar interaction is confined by gravity to a flat interface of a
hanging water droplet. The particles are observed by video microscopy and the
dipolar interaction strength is controlled via an external magnetic field. The
system is a model system to study the glass transition in 2D, and it exhibits
partial clustering of the small particles. This clustering is strongly
dependent on the relative concentration of big and small particles.
However, changing the interaction strength reveals that the clustering
does not depend on the interaction strength. The partial clustering scenario is
quantified using Minkowski functionals and partial structure factors. Evidence
that partial clustering prevents global crystallization is discussed
Local crystalline order in a 2D colloidal glass former
A mixture of two types of super-paramagnetic colloidal particles with long-range dipolar interaction is confined by gravity to a flat interface of a hanging water droplet. The particles are observed by video microscopy and the dipolar interaction strength is controlled by an external magnetic field. The local structure as obtained by pair correlation functions and bond order statistics is investigated as a function of system temperature and relative concentration. Although the system has no long-range order and exhibits glassy dynamics, different types of stable crystallites coexist. The local order of the globally disordered structure is explained by a small set of specific crystal structures. The statistics of crystal unit cells show a continuous increase of local order with decreasing system temperature as well as a dependence on sample history and local composition
Correlation between dynamical heterogeneities, structure and potential-energy distribution in a 2D amorphous solid
We investigate the collective properties of particles in a 2D experimental system which consists of a bi-disperse mixture of colloidal particles confined at an air/water interface. We find a direct correlation between structure and dynamical heterogeneities in this system: particles belonging to locally ordered structures have lower potential energy and are slower than other particles. In a more general way we show that particles with high potential energy are dominating the dynamics especially in the α-relaxation regime