904 research outputs found
Sphere-to-cylinder transition in hierarchical electrostatic complexes
We report the formation of colloidal complexes resulting from the
electrostatic co-assembly between anionic surfactants and cationic
polyelectrolytes or block copolymers. Combining light and x-ray scattering
experiments with cryogenic transmission and optical microscopy, we emphasize a
feature rarely addressed in the formation of the electrostatic complexes,
namely the role of the mixing concentration on the microstructure. At low
mixing concentration, electrostatic complexes made from cationic-neutral
copolymers and alkyl sulfate surfactants exhibit spherical core-shell
microstructures. With increasing concentration, the complexes undergo a
sphere-to-cylinder transition, yielding elongated aggregates with diameter 50
nm and length up to several hundreds of nanometers. From the comparison between
homo- and diblock polymer phase behaviors, it is suggested that the
unidimensional growth is driven by the ability of the surfactant to
self-assemble into cylindrical micelles, and in particular when these
surfactants are complexed with oppositely charged polymers.Comment: 7 pages, 9 figures, 2 tables paper accepted at Colloid and Polymer
Science, 31-Mar-0
The role of the coating and aggregation state in the interactions between iron oxide nanoparticles and 3T3 fibroblasts
Recent nanotoxicity studies revealed that the physico-chemical
characteristics of engineered nanomaterials play an important role in the
interactions with living cells. Here, we report on the toxicity and uptake of
the iron oxide sub-10 nm nanoparticles by NIH/3T3 mouse fibroblasts. Coating
strategies include low-molecular weight ligands (citric acid) and polymers
(poly(acrylic acid), MW = 2000 g mol-1). We find that most particles were
biocompatible, as exposed cells remained 100% viable relative to controls. The
strong uptake shown by the citrate-coated particles is related to the
destabilization of the dispersions in the cell culture medium and their
sedimentation down to the cell membranes.Comment: 5 pages 3 figure
Magnetic wire as stress controlled micro-rheometer for cytoplasm viscosity measurements
We review here different methods to measure the bulk viscosity of complex
fluids using micron-sized magnetic wires. The wires are characterized by length
of a few microns and diameter of a few hundreds of nanometers. We first draw
analogies between cone-and-plate rheometry and wire-based microrheology. In
particular we highlight that magnetic wires can be operated as
stress-controlled rheometers for two types of testing, the creep-recovery and
steady shear experiments. In the context of biophysical applications, the
cytoplasm of different cell lines including fibroblasts, epithelial and tumor
cells is studied. It reveals that the interior of living cells can be described
as a viscoelastic liquid with a static viscosity comprised between 10 and 100
Pas. We extend the previous approaches and show that the proposed technique can
also provide time resolved viscosity data, which for cells display strong
temporal fluctuations. The present work demonstrates the high potential of the
magnetic wires for quantitative rheometry in confined espaces.Comment: 11 pages, 6 figures, 40 reference
The role of surface charge in the interaction of nanoparticles with model pulmonary surfactants
Inhaled nanoparticles traveling through the airways are able to reach the
respiratory zone of the lungs. In such event, the incoming particles first
enter in contact with the liquid lining the alveolar epithelium, the pulmonary
surfactant. The pulmonary surfactant is composed of lipids and proteins that
are assembled into large vesicular structures. The question of the nature of
the biophysicochemical interaction with the pulmonary surfactant is central to
understand how the nanoparticles can cross the air-blood barrier. Here we
explore the phase behavior of sub-100 nm particles and surfactant substitutes
in controlled conditions. Three types of surfactant mimetics, including the
exogenous substitute Curosurf, a drug administred to infants with respiratory
distress syndrome are tested together with aluminum oxide (Al2O3), silicon
dioxide (SiO2) and polymer (latex) nanoparticles. The main result here is the
observation of the spontaneous nanoparticle-vesicle aggregation induced by
Coulombic attraction. The role of the surface charges is clearly established.
We also evaluate the supported lipid bilayer formation recently predicted and
find that in the cases studied these structures do not occur. Pertaining to the
aggregate internal structure, fluorescence microscopy ascertains that the
vesicles and particles are intermixed at the nano- to microscale. With
particles acting as stickers between vesicles, it is anticipated that the
presence of inhaled nanomaterials in the alveolar spaces could significantly
modify the interfacial and bulk properties of the pulmonary surfactant and
interfere with the lung physiology.Comment: 20 pages, 6 figure
Shear-induced transitions and instabilities in surfactant wormlike micelles
In this review, we report recent developments on the shear-induced
transitions and instabilities found in surfactant wormlike micelles. The survey
focuses on the non-linear shear rheology and covers a broad range of surfactant
concentrations, from the dilute to the liquid-crystalline states and including
the semi-dilute and concentrated regimes. Based on a systematic analysis of
many surfactant systems, the present approach aims to identify the essential
features of the transitions. It is suggested that these features define classes
of behaviors. The review describes three types of transitions and/or
instabilities : the shear-thickening found in the dilute regime, the
shear-banding which is linked in some systems to the isotropic-to-nematic
transition, and the flow-aligning and tumbling instabilities characteristic of
nematic structures. In these three classes of behaviors, the shear-induced
transitions are the result of a coupling between the internal structure of the
fluid and the flow, resulting in a new mesoscopic organization under shear.
This survey finally highlights the potential use of wormlike micelles as model
systems for complex fluids and for applications.Comment: 64 pages, 31 figures, 2 table
- …