58 research outputs found
Influence of Micro-mixing on the Size of Liposomes Self-Assembled from Miscible Liquid Phases
Ethanol injection and variations of it are a class of methods where two
miscible phases---one of which contains dissolved lipids---are mixed together
leading to the self-assembly of lipid molecules to form liposomes. This method
has been suggested, among other applications, for in-situ synthesis of
liposomes as drug delivery capsules. However, the mechanism that leads to a
specific size selection of the liposomes in solution based self-assembly in
general, and in flow-focussing microfluidic devices in particular, has so far
not been established. Here we report two aspects of this problem. A simple and
easily fabricated device for synthesis of monodisperse unilamellar liposomes in
a co-axial flow-focussing microfluidic geometry is presented. We also show that
the size of liposomes is dependent on the extent of micro-convective mixing of
the two miscible phases. Here, a viscosity stratification induced hydrodynamic
instability leads to a gentle micro-mixing which results in larger liposome
size than when the streams are mixed turbulently. The results are in sharp
contrast to a purely diffusive mixing in macroscopic laminar flow that was
believed to occur under these conditions. Further precise quantification of the
mixing characteristics should provide the insights to develop a general theory
for size selection for the class of ethanol injection methods. This will also
lay grounds for obtaining empirical evidence that will enable better control of
liposome sizes and for designing drug encapsulation and delivery devices.Comment: 11 pages, 14 Figure
Characterization of the stationary states of a dilute vibrofluidized granular bed
This paper reports two phenomena in an event driven simulation of a dilute vibrofluidized granular material in two dimensions. Both phenomena show inhomogeneity in the horizontal direction. They are convection rolls similar to the Rayleigh-Benard thermal convection in fluids, and a clustering instability, where the bed spontaneously phase separates into coexisting dense and dilute regions. Detailed investigations show that these are different from the known instabilities in a vibrated granular medium. Characterization of these instabilities is carried out with a phase diagram using suitable parameters from the kinetic theory of vibrofluidized beds
Optimisation of a Brownian dynamics algorithm for semidilute polymer solutions
Simulating the static and dynamic properties of semidilute polymer solutions
with Brownian dynamics (BD) requires the computation of a large system of
polymer chains coupled to one another through excluded-volume and hydrodynamic
interactions. In the presence of periodic boundary conditions, long-ranged
hydrodynamic interactions are frequently summed with the Ewald summation
technique. By performing detailed simulations that shed light on the influence
of several tuning parameters involved both in the Ewald summation method, and
in the efficient treatment of Brownian forces, we develop a BD algorithm in
which the computational cost scales as O(N^{1.8}), where N is the number of
monomers in the simulation box. We show that Beenakker's original
implementation of the Ewald sum, which is only valid for systems without bead
overlap, can be modified so that \theta-solutions can be simulated by switching
off excluded-volume interactions. A comparison of the predictions of the radius
of gyration, the end-to-end vector, and the self-diffusion coefficient by BD,
at a range of concentrations, with the hybrid Lattice Boltzmann/Molecular
Dynamics (LB/MD) method shows excellent agreement between the two methods. In
contrast to the situation for dilute solutions, the LB/MD method is shown to be
significantly more computationally efficient than the current implementation of
BD for simulating semidilute solutions. We argue however that further
optimisations should be possible.Comment: 17 pages, 8 figures, revised version to appear in Physical Review E
(2012
Gaussian approximation for finitely extensible bead-spring chains with hydrodynamic interaction
The Gaussian Approximation, proposed originally by Ottinger [J. Chem. Phys.,
90 (1) : 463-473, 1989] to account for the influence of fluctuations in
hydrodynamic interactions in Rouse chains, is adapted here to derive a new
mean-field approximation for the FENE spring force. This "FENE-PG" force law
approximately accounts for spring-force fluctuations, which are neglected in
the widely used FENE-P approximation. The Gaussian Approximation for
hydrodynamic interactions is combined with the FENE-P and FENE-PG spring force
approximations to obtain approximate models for finitely-extensible bead-spring
chains with hydrodynamic interactions. The closed set of ODE's governing the
evolution of the second-moments of the configurational probability distribution
in the approximate models are used to generate predictions of rheological
properties in steady and unsteady shear and uniaxial extensional flows, which
are found to be in good agreement with the exact results obtained with Brownian
dynamics simulations. In particular, predictions of coil-stretch hysteresis are
in quantitative agreement with simulations' results. Additional simplifying
diagonalization-of-normal-modes assumptions are found to lead to considerable
savings in computation time, without significant loss in accuracy.Comment: 26 pages, 17 figures, 2 tables, 75 numbered equations, 1 appendix
with 10 numbered equations Submitted to J. Chem. Phys. on 6 February 200
The viscosity radius in dilute polymer solutions: Universal behaviour from DNA rheology and Brownian dynamics simulations
The swelling of the viscosity radius, , and the universal
viscosity ratio, , have been determined experimentally for linear
DNA molecules in dilute solutions with excess salt, and numerically by Brownian
dynamics simulations, as a function of the solvent quality. In the latter
instance, asymptotic parameter free predictions have been obtained by
extrapolating simulation data for finite chains to the long chain limit.
Experiments and simulations show a universal crossover for and
from to good solvents in line with earlier observations
on synthetic polymer-solvent systems. The significant difference between the
swelling of the dynamic viscosity radius from the observed swelling of the
static radius of gyration, is shown to arise from the presence of hydrodynamic
interactions in the non-draining limit. Simulated values of and
are in good agreement with experimental measurements in synthetic
polymer solutions reported previously, and with the measurements in linear DNA
solutions reported here.Comment: 19 pages, 14 figures, two column, Supporting Information added, to
appear in Macromolecule
Shear thinning in dilute and semidilute solutions of polystyrene and DNA
The viscosity of dilute and semidilute unentangled DNA solutions, in steady
simple shear flow, has been measured across a range of temperatures and
concentrations. For polystyrene solutions, measurements of viscosity have been
carried out in the semidilute unentangled regime, while results of prior
experimental measurements in the dilute regime have been used for the purpose
of data analysis, and for comparison with the behaviour of DNA solutions.
Interpretation of the shear rate dependence of viscosity in terms of suitably
defined non-dimensional variables, is shown to lead to master plots,
independent of temperature and concentration, in each of the two concentration
regimes. In the case of semidilute unentangled solutions, defining the
Weissenberg number in terms of a concentration dependent large scale relaxation
time is found not to lead to data collapse across different concentrations. On
the other hand, the use of an alternative relaxation time, with the
concentration dependence of a single correlation blob, suggests the existence
of universal shear thinning behaviour at large shear rates.Comment: 24 pages, 13 figures, supplementary material (see ancillary
directory), to appear in Journal of Rheolog
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