117 research outputs found
Nanoparticle ordering in sandwiched polymer brushes
The organization of nano-particles inside grafted polymer layers is governed
by the interplay of polymer-induced entropic interactions and the action of
externally applied fields. Earlier work had shown that strong external forces
can drive the formation of colloidal structures in polymer brushes. Here we
show that external fields are not essential to obtain such colloidal patterns:
we report Monte Carlo and Molecular dynamics simulations that demonstrate that
ordered structures can be achieved by compressing a `sandwich' of two grafted
polymer layers, or by squeezing a coated nanotube, with nano-particles in
between. We show that the pattern formation can be efficiently controlled by
the applied pressure, while the characteristic length--scale, i.e. the typical
width of the patterns, is sensitive to the length of the polymers. Based on the
results of the simulations, we derive an approximate equation of state for
nano-sandwiches.Comment: 18 pages, 4 figure
Procedure to construct a multi-scale coarse-grained model of DNA-coated colloids from experimental data
We present a quantitative, multi-scale coarse-grained model of DNA coated
colloids. The parameters of this model are transferable and are solely based on
experimental data. As a test case, we focus on nano-sized colloids carrying
single-stranded DNA strands of length comparable to the colloids' size. We show
that in this regime, the common theoretical approach of assuming pairwise
additivity of the colloidal pair interactions leads to quantitatively and
sometimes even qualitatively wrong predictions of the phase behaviour of
DNA-grafted colloids. Comparing to experimental data, we find that our
coarse-grained model correctly predicts the equilibrium structure and melting
temperature of the formed solids. Due to limited experimental information on
the persistence length of single-stranded DNA, some quantitative discrepancies
are found in the prediction of spatial quantities. With the availability of
better experimental data, the present approach provides a path for the rational
design of DNA-functionalised building blocks that can self-assemble in complex,
three-dimensional structures.Comment: 17 pages, 10 figures; to be published in Soft Matte
Layering, freezing and re-entrant melting of hard spheres in soft confinement
Confinement can have a dramatic effect on the behavior of all sorts of
particulate systems and it therefore is an important phenomenon in many
different areas of physics and technology. Here, we investigate the role played
by the softness of the confining potential. Using grand canonical Monte Carlo
simulations, we determine the phase diagram of three-dimensional hard spheres
that in one dimension are constrained to a plane by a harmonic potential. The
phase behavior depends strongly on the density and on the stiffness of the
harmonic confinement. Whilst we find the familiar sequence of confined
hexagonal and square-symmetric packings, we do not observe any of the usual
intervening ordered phases. Instead, the system phase separates under strong
confinement, or forms a layered re-entrant liquid phase under weaker
confinement. It is plausible that this behavior is due to the larger positional
freedom in a soft confining potential and to the contribution that the
confinement energy makes to the total free energy. The fact that specific
structures can be induced or suppressed by simply changing the confinement
conditions (e.g. in a dielectrophoretic trap) is important for applications
that involve self-assembled structures of colloidal particles.Comment: 5 pages, 5 figure
Quantitative prediction of the phase diagram of DNA-functionalized nano-colloids
We present a coarse-grained model of DNA-functionalized colloids that is
computationally tractable. Importantly, the model parameters are solely based
on experimental data. Using this highly simplified model, we can predict the
phase behavior of DNA-functionalized nano-colloids without assuming pairwise
additivity of the inter-colloidal interactions. Our simulations show that for
nano-colloids, the assumption of pairwise additivity leads to substantial
errors in the estimate of the free energy of the crystal phase. We compare our
results with available experimental data and find that the simulations predict
the correct structure of the solid phase and yield a very good estimate of the
melting temperature. Current experimental estimates for the contour length and
persistence length of single-stranded DNA sequences are subject to relatively
large uncertainties. Using the best available estimates, we obtain predictions
for the crystal lattice constants that are off by a few percent: this indicates
that more accurate experimental data on ssDNA are needed to exploit the full
power of our coarse-grained approach.Comment: 4 pages, 2 figures; accepted for publication in Phys. Rev. Let
Controlling the temperature sensitivity of DNA-mediated colloidal interactions through competing linkages
We propose a new strategy to improve the self-assembly properties of
DNA-functionalised colloids. The problem that we address is that
DNA-functionalised colloids typically crystallize in a narrow temperature
window, if at all. The underlying reason is the extreme sensitivity of
DNA-mediated interactions to temperature or other physical control parameters.
We propose to widen the window for colloidal crystallization by exploiting the
competition between DNA linkages with different nucleotide sequences, which
results in a temperature-dependent switching of the dominant bond type.
Following such a strategy, we can decrease the temperature dependence of
DNA-mediated self assembly to make systems that can crystallize in a wider
temperature window than is possible with existing systems of DNA functionalised
colloids. We report Monte Carlo simulations that show that the proposed
strategy can indeed work in practice for real systems and specific, designable
DNA sequences. Depending on the length ratio of the different DNA constructs,
we find that the bond switching is either energetically driven (equal length or
`symmetric' DNA) or controlled by a combinatorial entropy gain (`asymmetric'
DNA), which results from the large number of possible binding partners for each
DNA strand. We provide specific suggestions for the DNA sequences with which
these effects can be achieved experimentally
Designing stimulus-sensitive colloidal walkers.
Colloidal particles with DNA "legs" that can bind reversibly to receptors on a surface can be made to 'walk' if there is a gradient in receptor concentration. We use a combination of theory and Monte Carlo simulations to explore how controllable parameters, e.g. coating density and binding strength, affect the dynamics of such colloids. We find that competition between thermodynamic and kinetic trends imply that there is an optimal value for both the binding strength and the number of "legs" for which transport is the fastest. Using available thermodynamic data on DNA binding, we indicate how directionally reversible, temperature-controlled transport of colloidal walkers can be achieved. In particular, the present results should make it possible to design a chromatographic technique that can be used to separate colloids with different DNA functionalizations
Complex crystal structures formed by the self assembly of di-tethered nanospheres
We report the results from a computational study of the self-assembly of
amphiphilic di-tethered nanospheres using molecular simulation. As a function
of the interaction strength and directionality of the tether-tether
interactions, we predict the formation of four highly ordered phases not
previously reported for nanoparticle systems. We find a double diamond
structure comprised of a zincblende (binary diamond) arrangement of spherical
micelles with a complementary diamond network of nanoparticles (ZnS/D); a phase
of alternating spherical micelles in a NaCl structure with a complementary
simple cubic network of nanoparticles to form an overall crystal structure
identical to that of AlCu_2Mn (NaCl/SC); an alternating tetragonal ordered
cylinder phase with a tetragonal mesh of nanoparticles described by the [8,8,4]
Archimedean tiling (TC/T); and an alternating diamond phase in which both
diamond networks are formed by the tethers (AD) within a nanoparticle matrix.
We compare these structures with those observed in linear and star triblock
copolymer systems
Effects of compositional asymmetry in phase behavior of ABA triblock copolymer melts from Monte Carlo simulation
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