95 research outputs found
Complex depletion forces
Most experimental studies of the effects of depletion forces in a colloidal suspensions have so far been performed on systems where the depletion agent can be regarded as ideal. Here, we review our recent results on systems where depletants present self-interactions. In the first case we focus on a system where strong electrostatic coupling is present in the suspension. At fixed colloid volume fraction, colloidal aggregation takes place when the surfactant concentration reaches a critical value which raises for increasing ionic strength. Screening repulsive electrostatic interactions inhibits the depletion mechanism and weakens the effective colloid-colloid attraction. In the second case, investigating the depletion effects
brought in by surfactants that show a liquid-liquid phase separation with water, we shall conversely deal with a situation where long-range spatial correlations are of
primary importance in setting the phase behavior of the colloidal fluid. Our experimental and theoretical results show that, in the proximity of the critical demixing
point, depletion effects merge continuously into critical Casimir effects, displaying distinctive scaling properties
Colloidal swarms can settle faster than isolated particles
Colloid sedimentation has played a seminal role in the development of statistical physics thanks to the celebrated experiments by Perrin, which gave a concrete demonstration of molecular reality. Recently, the investigation of sedimentation equilibrium has provided valuable information on a wide class of systems, ranging from simple colloids to active particles and biological fluids [1]. Yet, many aspects of the sedimentation kinetics deserve to be further investigated. Here we present some rather surprising results concerning the effect of interactions on particle settling [2]. Usually, the settling velocity of a colloidal suspension decreases with concentration: this well-known effect is called “hindered’’ settling. By experimenting on model colloids in which depletion forces can carefully be tuned, we conversely show that attractive interactions consistently “promote particle settling, so much that, close to a phase-separation line, the sedimentation velocity of a moderately concentrated dispersion can even exceed its single-particle value. At larger particle volume fraction , however, hydrodynamic hindrance eventually takes over. Hence, v() actually displays a non-monotonic trend that may threaten the stability of the settling front to thermal perturbations. By discussing a representative case, we show that these results are relevant to the investigation of protein weak association effects by ultracentrifugation.
References.
[1] R. Piazza, Reports of Progress in Physics, 2014, 77, 056602.
[2] E. Lattuada, S. Buzzaccaro, R. Piazza, Phys. Rev. Lett. 2016, 116, 03830
Highly nonlinear dynamics in a slowly sedimenting colloidal gel
We use a combination of original light scattering techniques and particles
with unique optical properties to investigate the behavior of suspensions of
attractive colloids under gravitational stress, following over time the
concentration profile, the velocity profile, and the microscopic dynamics.
During the compression regime, the sedimentation velocity grows nearly linearly
with height, implying that the gel settling may be fully described by a
(time-dependent) strain rate. We find that the microscopic dynamics exhibit
remarkable scaling properties when time is normalized by strain rate, showing
that the gel microscopic restructuring is dominated by its macroscopic
deformation.Comment: Physical Review Letters (2011) xxx
Thermal Lens Measurements of Thermal Expansivity in Thermosensitive Polymer Solutions
The weak absorption of a laser beam generates in a fluid an inhomogeneous refractive
index profile acting as a negative lens. This self-effect on beam propagation, known as Thermal
Lensing (TL), is extensively exploited in sensitive spectroscopic techniques, and in several all-optical
methods for the assessment of thermo-optical properties of simple and complex fluids. Using the
Lorentz–Lorenz equation, we show that the TL signal is directly proportional to the sample thermal
expansivity a, a feature allowing minute density changes to be detected with high sensitivity in a tiny
sample volume, using a simple optical scheme. We took advantage of this key result to investigate the
compaction of PniPAM microgels occurring around their volume phase transition temperature, and
the temperature-driven formation of poloxamer micelles. For both these different kinds of structural
transitions, we observed a significant peak in the solute contribution to a, indicating a decrease in the
overall solution density—rather counterintuitive evidence that can nevertheless be attributed to the
dehydration of the polymer chains. Finally, we compare the novel method we propose with other
techniques currently used to obtain specific volume changes
What buoyancy really is. A generalized Archimedes' principle for sedimentation and ultracentrifugation
Particle settling is a pervasive process in nature, and centrifugation is a versatile separation technique. Yet, the results of settling and ultracentrifugation experiments often appear to contradict the very law on which they are based: Archimedes' principle - arguably, the oldest physical law. The purpose of this paper is delving into the very roots of the concept of buoyancy by means of a combined experimental-theoretical study on sedimentation profiles in colloidal mixtures. Our analysis shows that the standard Archimedes' principle is only a limiting approximation, valid for mesoscopic particles settling in a molecular fluid, and we provide a general expression for the actual buoyancy force. This "Generalized Archimedes' Principle" accounts for unexpected effects, such as denser particles floating on top of a lighter fluid, which in fact we observe in our experiments
Spatially: Resolved heterogeneous dynamics in a strong colloidal gel
We re-examine the classical problem of irreversible colloid aggregation, showing that the application of Digital Fourier Imaging (DFI), a class of optical correlation methods that combine the power of light scattering and imaging, allows one to pick out novel useful evidence concerning the restructuring processes taking place in a strong colloidal gel. In particular, the spatially-resolved displacement fields provided by DFI strongly suggest that the temporally-intermittent local rearrangements taking place in the course of gel ageing are characterized by very long-ranged spatial correlations
Biopolymer gels with "physical" cross-links: gelation kinetics, aging, heterogeneous dynamics, and macroscopic mechanical properties
Alginate is a natural biopolymer that forms, in the presence of divalent cations, ionic-bound gels typifying a large class of biological gels stabilized by non-covalent cross-links, and displaying a consistent restructuring kinetics. We investigate the kinetics of formation and aging of alginate gels by slow permeation of a curing CaCl2 agent by means of photon correlation imaging, a novel optical technique that allows obtaining the microscopic dynamics of the sample, while retaining at the same time the spatial resolution of imaging techniques. Specifically, the gelling kinetics displays a peculiar non-diffusive behavior, and the subsequent restructuring of the gel structure shares several features in common with the aging of colloidal gels, in particular for what concerns the occurrence of heterogeneous dynamics effects. A comparative analysis of the gel macroscopic mechanical properties at different aging stages further highlights distinctive effects arising from the non-permanent nature of the bonds
Colloidal Swarms Can Settle Faster than Isolated Particles: Enhanced Sedimentation near Phase Separation
By experimenting on model colloids where depletion forces can be carefully tuned and quantified, we
show that attractive interactions consistently “promote” particle settling, so much that the sedimentation
velocity of a moderately concentrated dispersion can even exceed its single-particle value. At larger particle
volume fraction Ď•, however, hydrodynamic hindrance eventually takes over. Hence, v(Ď•) actually displays
a nonmonotonic trend that may threaten the stability of the settling front to thermal perturbations. Finally,
by discussing a representative case, we show that these results are relevant to the investigation of protein
association effects by ultracentrifugation
Optical correlation techniques for the investigation of colloidal systems
This review aims to provide a simple introduction to the application of
optical correlation methods in colloidal science. In particular, I plan to show
that full appraisal of the intimate relation between light scattering and
microscopy allows designing novel powerful investigation techniques that
combine their powers. An extended version of this paper will appear in
"ColloidalFoundations of Nanoscience", edited by D. Berti and G. Palazzo,
Elsevier (ISBN 978-0-444-59541-6). I am very grateful to the publisher for
having granted me the permission to post this preprint on arXiv.Comment: 19 pages, 5 figure
Critical Casimir forces and colloidal aggregation: A numerical study
We present a numerical study of the effective potential
between two hard-sphere colloids dispersed in a solvent of interacting
particles, for several values of temperature and solvent density, approaching
the solvent gas-liquid critical point. We investigate the stability of a system
of particles interacting via to evaluate the locus of
colloidal aggregation in the solvent phase-diagram, and its dependence on the
colloid size. We assess how the excluded volume depletion forces are modified
by solvent attraction and discuss under which conditions solvent critical
fluctuations, in the form of critical Casimir forces, can be used to
effectively manipulate colloidal aggregation
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