13 research outputs found
Discontinuous shear modulus determines the glass transition temperature
A solid - amorphous or crystalline - is defined by a finite shear modulus
while a fluid lacks such. We thus experimentally investigate the elastic
properties of a colloidal glass former near the glass transition: spectroscopy
of vibrational excitations yields the dispersion relations of longitudinal and
transverse phonons in the glassy state. From the long wavelength limit of the
dispersion relation we extract the bulk and the shear modulus. As expected, the
latter disappear in a fluid and we measure a clearly resolved discontinuous
behaviour of the elastic moduli at the glass transition. This not only
determines the transition temperature T_G of the system but also directly
addresses recent discussions about elasticity during vitrification. We show
that low frequency excitations in our system are plane waves such that
continuum elasticity theory can be used to describe the macroscopic behaviour.Comment: 8 pages, 6 figure
Structural and dynamical features of multiple metastable glassy states in a colloidal system with competing interactions
Systems in which a short-ranged attraction and long-ranged repulsion compete
are intrinsically frustrated, leading their structure and dynamics to be
dominated either by mesoscopic order or by metastable disorder. Here we report
the latter case in a colloidal system with long-ranged electrostatic repulsions
and short-ranged depletion attractions. We find a variety of states exhibiting
slow non-diffusive dynamics: a gel, a glassy state of clusters, and a state
reminiscent of a Wigner glass. Varying the interactions, we find a continuous
crossover between the Wigner and cluster glassy states, and a sharp
discontinuous transition between the Wigner glassy state and gel. This
difference reflects the fact that dynamic arrest is driven by repulsion for the
two glassy states and attraction in the case of the gel
Mermin-Wagner fluctuations in 2D amorphous solids
In a recent comment, M. Kosterlitz described how the discrepancy about the
lack of broken translational symmetry in two dimensions - doubting the
existence of 2D crystals - and the first computer simulations foretelling 2D
crystals at least in tiny systems, motivated him and D. Thouless to investigate
melting and suprafluidity in two dimensions [Jour. of Phys. Cond. Matt.
\textbf{28}, 481001 (2016)]. The lack of broken symmetries proposed by D.
Mermin and H. Wagner is caused by long wavelength density fluctuations. Those
fluctuations do not only have structural impact but additionally a dynamical
one: They cause the Lindemann criterion to fail in 2D and the mean squared
displacement not to be limited. Comparing experimental data from 3D and 2D
amorphous solids with 2D crystals we disentangle Mermin-Wagner fluctuations
from glassy structural relaxations. Furthermore we can demonstrate with
computer simulations the logarithmic increase of displacements predicted by
Mermin and Wagner: periodicity is not a requirement for Mermin-Wagner
fluctuations which conserve the homogeneity of space on long scales.Comment: 7 pages, 4 figure
Glass Elasticity from Particle Trajectories
Using positional data from video microscopy of a two-dimensional colloidal system and from simulations of hard disks, we determine the wave-vector-dependent elastic dispersion relations in glass. The emergence of rigidity based on the existence of a well defined displacement field in amorphous solids is demonstrated. Continuum elastic theory is recovered in the limit of long wavelengths which provides the glass elastic shear and bulk modulus as a function of temperature. The onset of a finite static shear modulus upon cooling marks the fluid-glass transition in an intuitive and unique way
Novel kinetic trapping in charged colloidal clusters due to self-induced surface charge organization
Colloidal clusters are an unusual state of matter where tunable interactions enable a sufficient reduction in their degrees of freedom that their energy landscapes can become tractable — they form a playground for statistical mechanics and promise unprecedented control of structure on the submicron lengthscale. We study colloidal clusters in a system where a short-ranged polymer-induced attraction drives clustering, while a weak, long-ranged electrostatic repulsion prevents extensive aggregation. We compare experimental yields of cluster structures with theory which assumes simple addition of competing isotropic interactions between the colloids. Here we show that for clusters of size 4 ≤ m ≤ 7, the yield of minimum energy clusters is much less than expected. We attribute this to an anisotropic self-organized surface charge distribution which leads to unexpected kinetic trapping. We introduce a model for the coupling between counterions and binding sites on the colloid surface with which we interpret our findings