140 research outputs found
Viscoelasticity and shear flow of concentrated, non-crystallizing colloidal suspensions: Comparison with Mode-Coupling Theory
We present a comprehensive rheological study of a suspension of
thermosensitive particles dispersed in water. The volume fraction of these
particles can be adjusted by the temperature of the system in a continuous
fashion. Due to the finite polydispersity of the particles (standard deviation:
17%), crystallization is suppressed and no fluid-crystal transition intervenes.
Hence, the moduli and in the linear viscoelastic regime as well as
the flow curves (shear stress as the function of the
shear rate ) could be measured in the fluid region up to the
vicinity of the glass transition. Moreover, flow curves could be obtained over
a range of shear rates of 8 orders of magnitude while and could be
measured spanning over 9 orders of magnitude. Special emphasis has been laid on
precise measurements down to the smallest shear rates/frequencies. It is
demonstrated that mode-coupling theory generalized in the integration through
transients framework provides a full description of the flow curves as well as
the viscoelastic behavior of concentrated suspensions with a single set of
well-defined parameters
Dynamic density functional theory of protein adsorption on polymer-coated nanoparticles
We present a theoretical model for the description of the adsorption kinetics
of globular proteins onto charged core-shell microgel particles based on
Dynamic Density Functional Theory (DDFT). This model builds on a previous
description of protein adsorption thermodynamics [Yigit \textit{et al},
Langmuir 28 (2012)], shown to well interpret the available calorimetric
experimental data of binding isotherms. In practice, a spatially-dependent
free-energy functional including the same physical interactions is built, and
used to study the kinetics via a generalised diffusion equation. To test this
model, we apply it to the case study of Lysozyme adsorption on PNIPAM coated
nanoparticles, and show that the dynamics obtained within DDFT is consistent
with that extrapolated from experiments. We also perform a systematic study of
the effect of various parameters in our model, and investigate the loading
dynamics as a function of proteins' valence and hydrophobic adsorption energy,
as well as their concentration and that of the nanoparticles. Although we
concentrated here on the case of adsorption for a single protein type, the
model's generality allows to study multi-component system, providing a reliable
instrument for future studies of competitive and cooperative adsorption effects
often encountered in protein adsorption experiments.Comment: Submitted to Soft Matte
Overshoots in stress strain curves: Colloid experiments and schematic mode coupling theory
The stress versus strain curves in dense colloidal dispersions under start-up
shear flow are investigated combining experiments on model core-shell
microgels, computer simulations of hard disk mixtures, and mode coupling
theory. In dense fluid and glassy states, the transient stresses exhibit first
a linear increase with the accumulated strain, then a maximum ('stress
overshoot') for strain values around 5%, before finally approaching the
stationary value, which makes up the flow curve. These phenomena arise in
well-equilibrated systems and for homogeneous flows, indicating that they are
generic phenomena of the shear-driven transient structural relaxation.
Microscopic mode coupling theory (generalized to flowing states by integration
through the transients) derives them from the transient stress correlations,
which first exhibit a plateau (corresponding to the solid-like elastic shear
modulus) at intermediate times, and then negative stress correlations during
the final decay. We introduce and validate a schematic model within mode
coupling theory which captures all of these phenomena and handily can be used
to jointly analyse linear and large-amplitude moduli, flow curves, and
stress-strain curves. This is done by introducing a new strain- and
time-dependent vertex into the relation between the the generalized shear
modulus and the transient density correlator.Comment: 21 pages, 13 figure
Theory of Solvation-Controlled Reactions in Stimuli-Responsive Nanoreactors
Metallic nanoparticles embedded in stimuli-responsive polymers can be
regarded as nanoreactors since their catalytic activity can be changed within
wide limits: the physicochemical properties of the polymer network can be tuned
and switched by external parameters, e.g. temperature or pH, and thus allows a
selective control of reactant mobility and concentration close to the reaction
site. Based on a combination of Debye's model of diffusion through an energy
landscape and a two-state model for the polymer, here we develop an analytical
expression for the observed reaction rate constant . Our formula
shows an exponential dependence of this rate on the solvation free enthalpy
change , a quantity which describes the partitioning
of the reactant in the network versus bulk. Thus, changes in , and not in the diffusion coefficient, will be the decisive
factor affecting the reaction rate in most cases. A comparison with recent
experimental data on switchable, thermosensitive nanoreactors demonstrates the
general validity of the concept
Charged dendrimers revisited: Effective charge and surface potential of dendritic polyglycerol sulfate
We investigate key electrostatic features of charged dendrimers at hand of
the biomedically important dendritic polyglycerol sulfate (dPGS) macromolecule
using multi-scale computer simulations and Zetasizer experiments. In our
simulation study, we first develop an effective mesoscale Hamiltonian specific
to dPGS based on input from all-atom, explicit-water simulations of dPGS of low
generation. Employing this in coarse-grained, implicit-solvent/explicit-salt
Langevin dynamics simulations, we then study dPGS structural and electrostatic
properties up to the sixth generation. By systematically mapping then the
calculated electrostatic potential onto the Debye-H\"uckel form -- that serves
as a basic defining equation for the effective charge -- we determine
well-defined effective net charges and corresponding radii, surface charge
densities, and surface potentials of dPGS. The latter are found to be up to one
order of magnitude smaller than the bare values and consistent with previously
derived theories on charge renormalization and weak saturation for high
dendrimer generations (charges). Finally, we find that the surface potential of
the dendrimers estimated from the simulations compare very well with our new
electrophoretic experiments
Nonequilibrium Structure of Colloidal Dumbbells under Oscillatory Shear
We investigate the nonequilibrium behavior of dense, plastic-crystalline
suspensions of mildly anisotropic colloidal hard dumbbells under the action of
an oscillatory shear field by employing Brownian dynamics computer simulations.
In particular, we extend previous investigations, where we uncovered novel
nonequilibrium phase transitions, to other aspect ratios and to a larger
nonequilibrium parameter space, that is, a wider range of strains and shear
frequencies. We compare and discuss selected results in the context of novel
scattering and rheological experiments. Both simulations and experiments
demonstrate that the previously found transitions from the plastic crystal
phase with increasing shear strain also occur at other aspect ratios. We
explore the transition behavior in the strain-frequency phase and summarize it
in a nonequilibrium phase diagram. Additionally, the experimental rheology
results hint at a slowing down of the colloidal dynamics with higher aspect
ratio
Reaction rate of a composite core-shell nanoreactor with multiple, spatially distributed embedded nano-catalysts
We present a detailed theory for the total reaction rate constant of a
composite core-shell nanoreactor, consisting of a central solid core surrounded
by a hydrogel layer of variable thickness, where a given number of small
catalytic nanoparticles are embedded at prescribed positions and are endowed
with a prescribed surface reaction rate constant. Besides the precise geometry
of the assembly, our theory accounts explicitly for the diffusion coefficients
of the reactants in the hydrogel and in the bulk as well as for their transfer
free energy jump upon entering the hydrogel shell. Moreover, we work out an
approximate analytical formula for the overall rate constant, which is valid in
the physically relevant range of geometrical and chemical parameters. We
discuss in depth how the diffusion-controlled part of the rate depends on the
essential variables, including the size of the central core. In particular, we
derive some simple rules for estimating the number of nanocatalysts per
nanoreactor for an efficient catalytic performance in the case of small to
intermediate core sizes. Our theoretical treatment promises to provide a very
useful and flexible tool for the design of superior performing nanoreactor
geometries and with optimized nanoparticle load.Comment: 12 pages, 3 figures, Physical Chemistry Chemical Physics, 201
Adsorption of Mono- and Divalent Ions onto Dendritic Polyglycerol Sulfate (dPGS) as Studied Using Isothermal Titration Calorimetry
The effective charge of highly charged polyelectrolytes is significantly lowered by a condensation of counterions. This effect is more pronounced for divalent ions. Here we present a study of the counterion condensation to dendritic polyglycerol sulfate (dPGS) that consists of a hydrophilic dendritic scaffold onto which sulfate groups are appended. The interactions between the dPGS and divalent ions (Mg2+ and Ca2+) were analyzed using isothermal titration calorimetry (ITC) and showed no ion specificity upon binding, but clear competition between the monovalent and divalent ions. Our findings, in line with the latest theoretical studies, demonstrate that a large fraction of the monovalent ions is sequentially replaced with the divalent ions
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