2,033 research outputs found
Hydrodynamics of Random-Organizing Hyperuniform Fluids
Disordered hyperuniform structures are locally random while uniform like
crystals at large length scales. Recently, an exotic hyperuniform fluid state
was found in several non-equilibrium systems, while the underlying physics
remains unknown. In this work, we propose a non-equilibrium
(driven-dissipative) hard-sphere model and formulate a hydrodynamic theory
based on Navier-Stokes equations to uncover the general mechanism of the
fluidic hyperuniformity (HU). At a fixed density, this model system undergoes a
smooth transition from an absorbing state to an active hyperuniform fluid, then
to the equilibrium fluid by changing the dissipation strength. We study the
criticality of the absorbing phase transition. We find that the origin of
fluidic HU can be understood as the damping of a stochastic harmonic oscillator
in space, which indicates that the suppressed long-wavelength density
fluctuation in the hyperuniform fluid can exhibit as either acoustic
(resonance) mode or diffusive (overdamped) mode. Importantly, our theory
reveals that the damping dissipation and active reciprocal interaction
(driving) are two ingredients for fluidic HU. Based on this principle, we
further demonstrate how to realize the fluidic HU in an experimentally
accessible active spinner system and discuss the possible realization in other
systems.Comment: Supplementary information can be found at
https://www.dropbox.com/s/ksic8v9chw7a7ir/SIpnas.pdf?dl=
Crystal nucleation of colloidal hard dumbbells
Using computer simulations we investigate the homogeneous crystal nucleation
in suspensions of colloidal hard dumbbells. The free energy barriers are
determined by Monte Carlo simulations using the umbrella sampling technique. We
calculate the nucleation rates for the plastic crystal and the aperiodic
crystal phase using the kinetic prefactor as determined from event driven
molecular dynamics simulations. We find good agreement with the nucleation
rates determined from spontaneous nucleation events observed in event driven
molecular dynamics simulations within error bars of one order of magnitude. We
study the effect of aspect ratio of the dumbbells on the nucleation of plastic
and aperiodic crystal phases and we also determine the structure of the
critical nuclei. Moreover, we find that the nucleation of the aligned CP1
crystal phase is strongly suppressed by a high free energy barrier at low
supersaturations and slow dynamics at high supersaturations.Comment: Accepted by J. Chem. Phy
Self-assembled multi-layer simple cubic photonic crystals of oppositely charged colloids in confinement
Designing and fabricating self-assembled open colloidal crystals have become
one major direction in soft matter community because of many promising
applications associated with open colloidal crystals. However, most of the
self-assembled crystals found in experiments are not open but close-packed.
Here by using computer simulation, we systematically investigate the
self-assembly of oppositely charged colloidal hard spheres confined between two
parallel hard walls, and we find that the confinement can stabilize multi-layer
NaCl-like (simple cubic) open crystals. The maximal layers of stable NaCl-like
crystal increases with decreasing the inverse screening length. More
interestingly, at finite low temperature, the large vibrational entropy can
stabilize some multi-layer NaCl-like crystals against the most energetically
favoured close-packed crystals. In the parameter range studied, we find upto
4-layer NaCl-like crystal to be stable in confinement. Our photonic calculation
shows that the inverse 4-layer NaCl-like crystal can already reproduce the
large photonic band gaps of the bulk simple cubic crystal, which open at low
frequency range with low dielectric contrast. This suggests new possibilities
of using confined colloidal systems to fabricate open crystalline materials
with novel photonic properties
Entropy stabilizes floppy crystals of mobile DNA-coated colloids
Grafting linkers with open ends of complementary single-stranded DNA makes a
flexible tool to tune interactions between colloids,which facilitates the
design of complex self-assembly structures. Recently, it has been proposed to
coat colloids with mobile DNA linkers, which alleviates kinetic barriers
without high-density grafting, and also allows the design of valency without
patches.However, the self-assembly mechanism of this novel system is poorly
understood.Using a combination of theory and simulation, we obtain phase
diagrams forthe system in both two and three dimensional spaces, and find
stable floppy squareand CsCl crystals when the binding strength is strong, even
in the infinite bindingstrength limit. We demonstrate that these floppy phases
are stabilized by vibrational entropy, and "floppy" modes play an important
role in stabilizing the floppy phases for the infinite binding strength limit.
This special entropic effect in the self-assembly of mobile DNA-coated colloids
is very different from conventional molecular self-assembly, and it offers new
axis to help design novel functional materials using mobile DNA-coated
colloids.Comment: Accepted in Physical Review Letter
Driving dynamic colloidal assembly using eccentric self-propelled colloids
Designing protocols to dynamically direct the self-assembly of colloidal
particles has become an important direction in soft matter physics because of
the promising applications in fabrication of dynamic responsive functional
materials. Here using computer simulations, we found that in the mixture of
passive colloids and eccentric self-propelled active particles, when the
eccentricity and self-propulsion of active particles are high enough, the
eccentric active particles can push passive colloids to form a large dense
dynamic cluster, and the system undergoes a novel dynamic demixing transition.
Our simulations show that the dynamic demixing occurs when the eccentric active
particles move much faster than the passive particles such that the dynamic
trajectories of different active particles can overlap with each other while
passive particles are depleted from the dynamic trajectories of active
particles. Our results suggest that this is in analogy to the entropy driven
demixing in colloid-polymer mixtures, in which polymer random coils can overlap
with each other while deplete the colloids. More interestingly, we find that by
fixing the passive colloid composition at certain value, with increasing the
density, the system undergoes an intriguing re-entrant mixing, and the demixing
only occurs within certain intermediate density range. This suggests a new way
of designing active matter to drive the self-assembly of passive colloids and
fabricate dynamic responsive materials.Comment: Accepted in Soft Matter. Supplementary information can found at
https://www.dropbox.com/sh/xb3u5iaoucc2ild/AABFUyqjXips7ewaie2rFbj_a?dl=
Entropy-Driven Phase Transitions in Colloidal Systems
This thesis can be divided into two independent parts. In the first part of
this thesis, we focus on studying the kinetic pathways of nucleation in
colloidal systems. In Chapter 2, we briefly introduce the relevant theory of
nucleation, i.e., classic nucleation theory. Then in Chapter 3, we investigate
the crystal nucleation in the "simplest" model system for colloids, i.e., the
monodisperse hard-sphere system, by using three different simulation methods,
i.e., molecular dynamics, forward flux sampling and umbrella sampling
simulations. Subsequently, we apply our simulation methods to a more realistic
system of colloidal hard spheres in Chapter 4. Furthermore, we study the
nucleation in a variety of systems consisting of hard particles, i.e., hard
dumbbells (Chapter 5), hard rods (Chapter 6), hard colloidal polymers (Chapter
7) and binary hard-sphere mixtures (Chapter 8). In the second part of this
thesis, we study the phase behavior of several colloidal systems. In Chapter 9,
we study the equilibrium phase diagram of colloidal hard superballs whose shape
interpolates from cubes to octahedra via spheres. We investigate the
micellization of asymmetric patchy dumbbells induced by the depletion
attraction in Chapter 10.Comment: PhD thesis, Utrecht University, July 2012. 188 page
Non-Equilibrium Strongly Hyperuniform Fluids of Circle Active Particles with Large Local Density Fluctuations
Disordered hyperuniform structures are an exotic state of matter having
vanishing long-wavelength density fluctuations similar to perfect crystals but
without long-range order. Although its importance in materials science has been
brought to the fore in past decades, the rational design of experimentally
realizable disordered strongly hyperuniform microstructures remains
challenging. Here we find a new type of non-equilibrium fluid with strong
hyperuniformity in two-dimensional systems of chiral active particles, where
particles perform independent circular motions of the radius R with the same
handedness. This new hyperuniform fluid features a special length scale, i.e.,
the diameter of the circular trajectory of particles, below which large density
fluctuations are observed. By developing a dynamic mean-field theory, we show
that the large local density fluctuations can be explained as a
motility-induced microphase separation, while the Fickian diffusion at large
length scales and local center-of-mass-conserved noises are responsible for the
global hyperuniformity
Pushing the glass transition towards random close packing using self-propelled hard spheres
Although the concept of random close packing with an almost universal packing
fraction of ~ 0.64 for hard spheres was introduced more than half a century
ago, there are still ongoing debates. The main difficulty in searching the
densest packing is that states with packing fractions beyond the glass
transition at ~ 0.58 are inherently non-equilibrium systems, where the dynamics
slows down with a structural relaxation time diverging with density; hence, the
random close packing is inaccessible. Here we perform simulations of
self-propelled hard spheres, and we find that with increasing activity the
relaxation dynamics can be sped up by orders of magnitude. The glass transition
shifts to higher packing fractions upon increasing the activity, allowing the
study of sphere packings with fluid-like dynamics at packing fractions close to
random close packing. Our study opens new possibilities of investigating dense
packings and the glass transition in systems of hard particles
Self-Assembled Chiral Photonic Crystals From Colloidal Helices Racemate
Chiral crystals consisting of micro-helices have many optical properties
while presently available fabrication processes limit their large-scale
applications in photonic devices. Here, by using a simplified simulation
method, we investigate a bottom-up self-assembly route to build up helical
crystals from the smectic monolayer of colloidal helices racemate. With
increasing the density, the system undergoes an entropy-driven
co-crystallization by forming crystals of various symmetries with different
helical shapes. In particular, we identify two crystals of helices arranged in
the binary honeycomb and square lattices, which are essentially composed by two
sets of opposite-handed chiral crystal. Photonic calculations show that these
chiral structures can have large complete photonic bandgaps. In addition, in
the self-assembled chiral square crystal, we also find dual polarization
bandgaps that selectively forbid the propagation of circularly polarized lights
of a specific handedness along the helical axis direction. The self-assembly
process in our proposed system is robust, suggesting possibilities of using
chiral colloids to assemble photonic metamaterials.Comment: Accepted in ACS Nan
Linker-mediated self-assembly of mobile DNA-coated colloids
Developing construction methods of materials tailored for given applications
with absolute control over building block placement poses an immense challenge.
DNA-coated colloids offer the possibility of realising programmable
self-assembly, which, in principle, can assemble almost any structure in
equilibrium, but remains challenging experimentally. Here, we propose an
innovative system of linker-mediated mobile DNA-coated colloids (mDNACCs), in
which mDNACCs are bridged by the free DNA linkers in solution, whose two
single-stranded DNA tails can bind with specific single-stranded DNA receptors
of complementary sequence coated on colloids. We formulate a mean-field theory
efficiently calculating the effective interaction between mDNACCs, where the
entropy of DNA linkers plays a nontrivial role. Particularly, when the binding
between free DNA linkers in solution and the corresponding receptors on mDNACCs
is strong, the linker-mediated colloidal interaction is determined by the
linker entropy depending on the linker concentration
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