35 research outputs found
Salt-induced reentrant stability of polyion-decorated particles with tunable surface charge density
The electrostatic complexation between DOTAP-DOPC unilamellar liposomes and
an oppositely charged polyelectrolyte (NaPA) has been investigated in a wide
range of the liposome surface charge density. We systematically characterized
the "reentrant condensation" and the charge inversion of
polyelectrolyte-decorated liposomes by means of dynamic light scattering and
electrophoresis. We explored the stability of this model
polyelectrolyte/colloid system by fixing each time the charge of the bare
liposomes and by changing two independent control parameters of the
suspensions: the polyelectrolyte/colloid charge ratio and the ionic strength of
the aqueous suspending medium. The progressive addition of neutral DOPC lipid
within the liposome membrane gave rise to a new intriguing phenomenon: the
stability diagram of the suspensions showed a novel reentrance due to the
crossing of the desorption threshold of the polyelectrolyte. Indeed, at fixed
charge density of the bare DOTAP/DOPC liposomes and for a wide range of polyion
concentrations, we showed that the simple electrolyte addition first (low salt
regime) destabilizes the suspensions because of the enhanced screening of the
residual repulsion between the complexes, and then (high salt regime)
determines the onset of a new stable phase, originated by the absence of
polyelectrolyte adsorption on the particle surfaces. We show that the observed
phenomenology can be rationalized within the Velegol-Thwar model for
heterogeneously charged particles and that the polyelectrolyte desorption fits
well the predictions of the adsorption theory of Winkler and Cherstvy. Our
findings unambiguously support the picture of the reentrant condensation as
driven by the correlated adsorption of the polyelectrolyte chains on the
particle surface, providing interesting insights into possible mechanisms for
tailoring complex colloids via salt-induced effects.Comment: 34 pages, 7 figure
Bulk and interfacial stresses in suspensions of soft and hard colloids
We explore the influence of particle softness and internal structure on both
the bulk and interfacial rheological properties of colloidal suspensions. We
probe bulk stresses by conventional rheology, by measuring the flow curves,
shear stress vs strain rate, for suspensions of soft, deformable microgel
particles and suspensions of near hard-sphere-like silica particles. A similar
behavior is seen for both kind of particles in suspensions at concentrations up
to the random close packing volume fraction, in agreement with recent
theoretical predictions for sub-micron colloids. Transient interfacial stresses
are measured by analyzing the patterns formed by the interface between the
suspensions and their own solvent, due to a generalized Saffman-Taylor
hydrodynamic instability. At odd with the bulk behavior, we find that microgels
and hard particle suspensions exhibit vastly different interfacial stress
properties. We propose that this surprising behavior results mainly from the
difference in particle internal structure (polymeric network for microgels vs
compact solid for the silica particles), rather than softness alone.Comment: 20 pages, 8 figure
Transition from confined to bulk dynamics in symmetric star-linear polymer mixtures
We report on the linear viscoelastic properties of mixtures comprising
multiarm star (as model soft colloids) and long linear chain homopolymers in a
good solvent. In contrast to earlier works, we investigated symmetric mixtures
(with a size ratio of 1) and showed that the polymeric and colloidal responses
can be decoupled. The adopted experimental protocol involved probing the linear
chain dynamics in different star environments. To this end, we studied mixtures
with different star mass fraction, which was kept constant while linear chains
were added and their entanglement plateau modulus () and terminal
relaxation time () were measured as functions of their concentration.
Two distinct scaling regimes were observed for both and : at low
linear polymer concentrations, a weak concentration dependence was observed,
that became even weaker as the fraction of stars in the mixtures increased into
the star glassy regime. On the other hand, at higher linear polymer
concentrations, the classical entangled polymer scaling was recovered. Simple
scaling arguments show that the threshold crossover concentration between the
two regimes corresponds to the maximum osmotic star compression and signals the
transition from confined to bulk dynamics. These results provide the needed
ingredients to complete the state diagram of soft colloid-polymer mixtures and
investigate their dynamics at large polymer-colloid size ratios. They also
offer an alternative way to explore aspects of the colloidal glass transition
and the polymer dynamics in confinement. Finally, they provide a new avenue to
tailor the rheology of soft composites.Comment: 9 Figure
Overcharging and reentrant condensation of thermoresponsive ionic microgels
We investigated the complexation of thermoresponsive anionic
poly(N-isopropylacrylamide) (PNiPAM) microgels and cationic
-polylysine (-PLL) chains. By combining electrophoresis,
light scattering, transmission electron microscopy (TEM) and dielectric
spectroscopy (DS) we studied the adsorption of -PLL onto the microgel
networks and its effect on the stability of the suspensions. We show that the
volume phase transition (VPT) of the microgels triggers a large polyion
adsorption. Two interesting phenomena with unique features occur: a
temperature-dependent microgel overcharging and a complex reentrant
condensation. The latter may occur at fixed polyion concentration, when
temperature is raised above the VPT of microgels, or by increasing the number
density of polycations at fixed temperature. TEM and DS measurements
unambiguously show that short PLL chains adsorb onto microgels and act as
electrostatic glue above the VPT. By performing thermal cycles, we further show
that polyion-induced clustering is a quasi-reversible process: within the time
of our experiments large clusters form above the VPT and partially re-dissolve
as the mixtures are cooled down. Finally we give a proof that the observed
phenomenology is purely electrostatic in nature: an increase of the ionic
strength gives rise to the polyion desorption from the microgel outer shell.Comment: 15 Figure
Depletion gels from dense soft colloids: Rheology and thermoreversible melting
Truzzolillo, D., Vlassopoulos, D., Munam, A., & Gauthier, M. (2014). Depletion gels from dense soft colloids: Rheology and thermoreversible melting. Journal of Rheology, 58(5), 1441–1462. https://doi.org/10.1122/1.4866592Upon addition of small nonadsorbing linear polymers, colloidal glasses composed of large hard spheres melt and eventually revitrify into the so-called attractive glass regime. We show that, when replacing the hard spheres by star polymers representing model soft particles, a reentrant gel is formed. This is the result of compression and depletion of the stars due to the action of the osmotic pressure from the linear homopolymers. The viscoelastic properties of the soft dense gel were studied with emphasis on the shear-induced yielding process, which involved localized breaking of elements with a size of the order of the correlation length. Based on these results, a phenomenological attempt was made at describing the universal rheological features of colloid/nonadsorbing polymer mixtures of varying softness. The star gel was found to undergo thermoreversible melting, despite the fact that conventional hard-sphere depletion gels are invariant to heating. This phenomenon is attributed to the hybrid internal microstructure of the stars, akin to a dry-to-wet brush transition, and is characterized by slow kinetics, on the time scale of the osmotic gel formation process. These results may be useful in finding generic features in colloidal gelation, as well as in the molecular design of new soft composite materials.Financial support from the EU (ITN-COMPLOIDS FP7-234810, FP7 Infrastructure ESMI, GA 262348 and FP7-SMALL-Nanodirect CP-FP-213948) and the Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged