30 research outputs found
Controlling colloidal phase transitions with critical Casimir forces
The critical Casimir effect provides a thermodynamic analogue of the
well-known quantum mechanical Casimir effect. It acts between two surfaces
immersed in a critical binary liquid mixture, and results from the confinement
of concentration fluctuations of the solvent. Unlike the quantum mechanical
effect, the magnitude and range of this attraction can be adjusted with
temperature via the solvent correlation length, thus offering new opportunities
for the assembly of nano and micron-scale structures. Here, we demonstrate the
active assembly control of equilibrium phases using critical Casimir forces. We
guide colloidal particles into analogues of molecular liquid and solid phases
via exquisite control over their interactions. By measuring the critical
Casimir particle pair potential directly from density fluctuations in the
colloidal gas, we obtain insight into liquefaction at small scales: We apply
the Van der Waals model of molecular liquefaction and show that the colloidal
gas-liquid condensation is accurately described by the Van der Waals theory,
even on the scale of a few particles. These results open up new possibilities
in the active assembly control of micro and nanostructures
Long-range strain correlations in sheared colloidal glasses
Glasses behave as solids on experimental time scales due to their slow
relaxation. Growing dynamic length scales due to cooperative motion of
particles are believed to be central to this slow response. For quiescent
glasses, however, the size of the cooperatively rearranging regions has never
been observed to exceed a few particle diameters, and the observation of
long-range correlations that are signatures of an elastic solid has remained
elusive. Here, we provide direct experimental evidence of long-range
correlations during the deformation of a dense colloidal glass. By imposing an
external stress, we force structural rearrangements that make the glass flow,
and we identify long-range correlations in the fluctuations of microscopic
strain, and elucidate their scaling and spatial symmetry. The applied shear
induces a transition from homogeneous to inhomogeneous flow at a critical shear
rate, and we investigate the role of strain correlations in this transition.Comment: 11 pages, 3 figures. Accepted for publication in PR
Experimental observation of the intricate free-energy landscape for a soft glassy system
In the free energy landscape picture of glassy systems, the slow dynamics
characteristic of these systems is believed to be due to the existence of a
complicated free-energy landscape with many local minima. We show here that for
a colloidal glassy material multiple paths can be taken through the free energy
landscape, that can even lead to different 'final' non-ergodic states at the
late stages of aging. We provide clear experimental evidence for the
distinction of gel and glassy states in the system and show that for a range of
colloid concentrations, the transition to non-ergodicity can occur in either
direction (gel or glass), and may be accompanied by 'hesitations' between the
two directions. This shows that colloidal gels and glasses are merely global
free-energy minima in the same free energy landscape, and that the paths
leading to these minima can indeed be complicated.Comment: 5 pages, 5 figure
Dynamics of colloidal aggregation in microgravity by critical Casimir forces
Using the critical Casimir force, we study the attractive-strength dependence
of diffusion-limited colloidal aggregation in microgravity. By means of near
field scattering we measure both the static and dynamic structure factor of the
aggregates as the aggregation process evolves. The simultaneous measurement of
both the static and dynamic structure factor under ideal microgravity
conditions allows us to uniquely determine the ratio of the hydrodynamic and
gyration radius as a function of the fractal dimension of the aggregate,
enabling us to elucidate the internal structure of the aggregates as a function
of the interaction potential. We find that the mass is evenly distributed in
all objects with fractal dimension ranging from 2.55 for a shallow to 1.75 for
the deepest potential.Comment: 5 pages, 4 figure
Fluctuation-dissipation theorem in an aging colloidal glass
We provide a direct experimental test of the Stokes-Einstein relation as a
special case of the fluctuation-dissipation theorem (FDT) in an aging colloidal
glass. The use of combined active and passive microrheology allows us to
independently measure both the correlation and response functions in this
non-equilibrium situation. Contrary to previous reports, we find no deviations
from the FDT over several decades in frequency (1 Hz-10 kHz) and for all aging
times. In addition, we find two distinct viscoelastic contributions in the
aging glass, including a nearly elastic response at low frequencies that grows
during aging. This is the clearest change in material properties of the system
with aging.Comment: 5 pages,4 figure
Revealing Driving Forces in Quantum Dot Supercrystal Assembly
The assembly of semiconductor nanoparticles, quantum dots (QDs), into dense crystalline nanostructures holds great promise for future optoelectronic devices. However, knowledge of the sub-nanometer scale driving forces underlying the kinetic processes of nucleation, growth, and final densification during QD assembly remains poor. Emulsion-templated assembly has recently been shown to provide good control over the bulk condensation of QDs into highly ordered 3D supercrystals. Here, emulsion-templated assembly is combined with in situ small-angle X-ray scattering to obtain direct insight into the nanoscale interactions underlying the nucleation, growth, and densification of QD supercrystals. At the point of supercrystal nucleation, nanoparticles undergo a hard-sphere-like crystallization into a hexagonal-close-packed lattice, slowly transforming into a face-centered-cubic lattice. The ligands play a crucial role in balancing steric repulsion against attractive van der Waals forces to mediate the initial equilibrium assembly, but cause the QDs to be progressively destabilized upon densification. The rich detail of this kinetic study elucidates the assembly and thermodynamic properties that define QD supercrystal fabrication approaching single-crystal quality, paving the way toward their use in optoelectronic devices.</p
Colloidal aggregation in microgravity by critical Casimir forces
By using the critical Casimir force, we study the attractive strength
dependent aggregation of colloids with and without gravity by means of Near
Field scattering. Significant differences were seen between microgravity and
ground experiments, both in the structure of the formed fractal aggregates as
well as the kinetics of growth. Ground measurements are severely affected by
sedimentation resulting in reaction limited behavior. In microgravity, a purely
diffusive behavior is seen reflected both in the measured fractal dimensions
for the aggregates as well as the power law behavior in the rate of growth.
Formed aggregates become more open as the attractive strength increases.Comment: 4 pages, 3 figure