22 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
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
A Theoretical and Simulation Study of the Self-assembly of a Binary Blend of Diblock Copolymers
Pure diblock copolymer melts exhibit a narrow range of conditions at which bicontinuous and cocontinuous phases are stable; such conditions and the morphology of such phases can be tuned by the use of additives. In this work, we have studied a bidisperse system of diblock copolymers using theory and simulation. In particular, we elucidated how a short, lamellar-forming diblock copolymer modifies the phase behavior of a longer, cylinder-forming diblock copolymer. In a narrow range of intermediate compositions, self-consistent field theory predicts the formation of a gyroid phase although particle-based simulations show that three phases compete: the gyroid phase, a disordered cocontinuous phase, and the cylinder phase, all having free energies within error bars of each other. Former experimental studies of a similar system have yielded an unidentified, partially irregular bicontinuous phase, and our simulations suggest that at such conditions the formation of a partially transformed network phase is indeed plausible. Close examination of the spatial distribution of chains reveals that packing frustration (manifested by chain stretching and low density spots) occurs in the majority-block domains of the three competing phases simulated. In all cases, a double interface around the minority-block domains is also detected with the outer one formed by the short chains, and the inner one formed by the longer chains.Publisher Version is available at: http://jcp.aip.org/resource/1/jcpsa6/v136/i23/p234905_s1?view=fulltextThis work was supported by Grant CBET 0756248 from the National Science Foundation. This publication is also
based on work supported in part by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). F.J.M.V. was supported by the ERC
(Advanced Grant Agreement No. 227758). We are thankful to Professor David Morse from University of Minnesota for the code for implementing SCFT
Nanoparticle Organization in Sandwiched Polymer Brushes
The organization of nanoparticles
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 nanoparticles
in between. We show that the pattern formation can be efficiently
controlled by the applied pressure, while the characteristic length-scale,
that is, 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 nanosandwiches