An advanced light scattering apparatus for investigating soft matter onboard the International Space Station.

Colloidal solids (COLIS) is a state-of-the-art light scattering setup developed for experiments onboard the International Space Station (ISS). COLIS allows for probing the structure and dynamics of soft matter systems on a wide range of length scales, from a few nm to tens of microns, and on time scales from 100 ns to tens of hours. In addition to conventional static and dynamic light scattering, COLIS includes depolarized dynamic light scattering, a small-angle camera, photon correlation imaging, and optical manipulation of thermosensitive samples through an auxiliary near-infrared laser beam, thereby providing a unique platform for probing soft matter systems.

Short and long-term effect of polycarbophil vaginal gel on vaginal atrophy of peri- and post-menopausal women. The TRIPLE study.

Objectives: This TRIPLE study was aimed to evaluate the efficacy of polycarbophil vaginal gel (PCV) in treating symptoms of vaginal atrophy (VA) of peri- and post-menopausal women.

Materials And Methods: Sexually active women in peri- (n = 29) and post-menopause (n = 54) suffering from VA, were progressively enrolled and treated for 30 days with PCV. Those wishing to continue (n = 73) were treated for additional 180 days.

Optothermal crystallization of hard spheres in an effective bidimensional geometry.

Using colloids effectively confined in two dimensions by a cell with a thickness comparable to the particle size, we investigate the nucleation and growth of crystallites induced by locally heating the solvent with a near-infrared laser beam. The particles, which are "thermophilic," move towards the laser spot solely because of thermophoresis with no convection effects, forming dense clusters whose structure is monitored using two order parameters that gauge the local density and the orientational ordering. We find that ordering takes place when the cluster reaches an average surface density that is still below the upper equilibrium limit for the fluid phase of hard disks, meaning that we do not detect any sign of a proper "two-stage" nucleation from a glass or a polymorphic crystal structure.

Prevalence and burden of nausea and vomiting in pregnant women: Interim analysis of the PURITY survey.

Objectives: Nausea and vomiting in pregnancy is a common and invalidating condition in early pregnancy. However, no data are available on its prevalence in Italy. This survey aims to evaluate the prevalence and impact of nausea and vomiting during pregnancy on the quality of life of Italian women.

A Light Scattering Investigation of Enzymatic Gelation in Self-Assembling Peptides.

Self-assembling peptides (SAPs) have been increasingly studied as hydrogel-former gelators because they can create biocompatible environments. A common strategy to trigger gelation, is to use a pH variation, but most methods result in a change in pH that is too rapid, leading to gels with hardly reproducible properties. Here, we use the urea-urease reaction to tune gel properties, by a slow and uniform pH increase.

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 α, a feature allowing minute density changes to be detected with high sensitivity in a tiny sample volume, using a simple optical scheme.

Microrheology of a thermosensitive gelling polymer for cell culture.

We investigate the rheo-mechanical properties of Mebiol Gel®, a thermosensitive gel-forming polymer extensively used as a medium for cellular culture, using passive microrheology made either by standard dynamic light scattering or by photon correlation imaging. In the dilute limit, Mebiol displays a Newtonian behavior with an effective viscosity that decreases with temperature, consistent with a peculiar aggregation mechanism characterized by an increase of the molecular weight with a simultaneous reduction of the aggregate size. By increasing concentration and approaching gelation, both the storage and loss moduli show a nonmonotonic dependence with temperature, with a pronounced maximum around T ≃ 28-30 °C, the value above which, in the dilute limit, the individual Mebiol chains are fully compacted.

Unraveling gelation kinetics, arrested dynamics and relaxation phenomena in filamentous colloids by photon correlation imaging.

The fundamental understanding of the gelation kinetics, stress relaxation and temporal evolution in colloidal filamentous gels is central to many aspects of soft and biological matter, yet a complete description of the inherent complex dynamics of these systems is still missing. By means of photon correlation imaging (PCI), we studied the gelation of amyloid fibril solutions, chosen as a model filamentous colloid with immediate significance to biology and nanotechnology, upon passage of ions through a semi-permeable membrane. We observed a linear-in-time evolution of the gelation front and rich rearrangement dynamics of the gels, the magnitude and the spatial propagation of which depend on how effectively electrostatic interactions are screened by different ionic strengths.

Spatially uniform dynamics in equilibrium colloidal gels.

Gels of DNA nanostars, besides providing a compatible scaffold for biomedical applications, are ideal model systems for testing the physics of equilibrium colloidal gels. Here, using dynamic light scattering and photon correlation imaging (a recent technique that, by blending light scattering and imaging, provides space-resolved quantification of the dynamics), we follow the process of gel formation over 10 orders of magnitude in time in a model system of tetravalent DNA nanostars in solution, a realization of limited-valence colloids. Such a system, depending on the nanostar concentration, can form either equilibrium or phase separation gels.

2D perfusion angiography as quantitative method to evaluate iloprost effect on foot circulation.

Two-dimensional (2D) perfusion angiography is useful for the evaluation of foot perfusion in patients with critical limb-threatening ischemia (CLTI). Iloprost is a synthetic prostacyclin analogue presenting vasodilating properties. Aim of this study was to demonstrate the utility of 2D perfusion angiography as quantitative method to evaluate iloprost effect on foot circulation.

Relaxation in aging thermoreversible gels: the role of thermal history.

The fast setting of gels originating from an arrested phase separation leads to solid structures that incorporate a substantial amount of frozen-in stresses. Using a colloidal system made of particles whose interactions can accurately be tuned with temperature and exploiting Photon Correlation Imaging (PCI), an optical correlation technique blending the powers of scattering and imaging, we show that the relaxation of these internal stresses, which occurs through a cascade of microscopic restructuring events, is strongly influenced by the thermal history of the sample. By changing with a temperature jump the interparticle interactions in an already set gel, we specifically show that gels formed by a deep quench within the coexistence region store a lot of residual stress.

Dispersions of Zirconia Nanoparticles Close to the Phase Boundary of Surfactant-Free Ternary Mixtures.

The achievement of a homogeneous dispersion of nanoparticles is of paramount importance in supporting their technological application. In wet processing, stable dispersions were largely obtained via surfactant or surface functionalization: although effective, the use of dispersant can alter, or even impair, the functional properties of the resulting nanostructured systems. Herein, we report a novel integrated modeling and experimental approach to obtain stable ZrO nanoparticle (NP) dispersions at native dimensions (about 5 nm) in homogeneous ternary mixtures of solvents (i.

Multiscale relaxation in aging colloidal gels: From localized plastic events to system-spanning quakes.

Relaxation of internal stresses through a cascade of microscopic restructuring events is the hallmark of many materials, ranging from amorphous solids like glasses and gels to geological structures subjected to a persistent external load. By means of photon correlation imaging, a recently developed technique that blends the powers of scattering and imaging, we provide a spatially and temporally resolved survey of the restructuring and aging processes that spontaneously occur in physical gels originating from an arrested phase separation. We show that the temporal dynamics is characterized by an intermittent sequence of spatially localized "microquakes" that eventually lead to global rearrangements occurring at a rate that scales with the gel age.

Thermophoresis in self-associating systems: probing poloxamer micellization by opto-thermal excitation.

Due to its exquisite sensitivity to interfacial properties, thermophoresis, i.e., particle motion driven by thermal gradients, can provide novel, exclusive, and often surprising information on the structural properties of colloidal or macromolecular fluids and on particle/solvent interactions at the nanoscale.

Compressive yield stress of depletion gels from stationary centrifugation profiles.

We have investigated the stationary sedimentation profiles of colloidal gels obtained by an arrested phase-separation process driven by depletion forces, which have been compressed either by natural gravity or by a centrifugal acceleration ranging between 6g and 2300g. Our measurements show that the gel rheological properties display a drastic change when the gel particle volume fraction exceeds a value [Formula: see text], which barely depends on the strength of the interparticle attractive forces that consolidate the network. In particular, the gel compressive yield stress [Formula: see text], which increases as [Formula: see text] for [Formula: see text], displays a diverging behaviour for [Formula: see text], with an asymptotic value that is close to the random close packing value for hard spheres.

Intermittent turbulence in flowing bacterial suspensions.

Dense suspensions of motile bacteria, possibly including the human gut microbiome, exhibit collective dynamics akin to those observed in classic, high Reynolds number turbulence with important implications for chemical and biological transport, yet this analogy has remained primarily qualitative. Here, we present experiments in which a dense suspension of Bacillus subtilis bacteria was flowed through microchannels and the velocity statistics of the flowing suspension were quantified using a recently developed velocimetry technique coupled with vortex identification methods. Observations revealed a robust intermittency phenomenon, whereby the average velocity profile of the suspension fluctuated between a plug-like flow and a parabolic flow profile.

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 ϕ, however, hydrodynamic hindrance eventually takes over. Hence, v(ϕ) actually displays a nonmonotonic trend that may threaten the stability of the settling front to thermal perturbations.

Spatially-resolved heterogeneous dynamics in a strong colloidal gel [corrected].

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.

Optical extinction, refractive index, and multiple scattering for suspensions of interacting colloidal particles.

We provide a general microscopic theory of the scattering cross-section and of the refractive index for a system of interacting colloidal particles, exact at second order in the molecular polarizabilities. In particular: (a) we show that the structural features of the suspension are encoded into the forward scattered field by multiple scattering effects, whose contribution is essential for the so-called "optical theorem" to hold in the presence of interactions; (b) we investigate the role of radiation reaction on light extinction; (c) we discuss our results in the framework of effective medium theories, presenting a general result for the effective refractive index valid, whatever the structural properties of the suspension, in the limit of particles much larger than the wavelength; (d) by discussing strongly-interacting suspensions, we unravel subtle anomalous dispersion effects for the suspension refractive index.

Time-evolution scenarios for short-range depletion gels subjected to the gravitational stress.

By exploiting photon correlation imaging and ghost particle velocimetry, two novel optical correlation techniques particularly suited to the investigation of the microscopic dynamics of spatially heterogeneous samples, we investigate the settling and restructuring dynamics of colloidal gels generated by short-ranged depletion interactions. Three distinct regions can be clearly set apart within the liquid-liquid coexistence region of the phase diagram where gel formation is observed. When depletion forces are barely sufficient to drive the system within the metastable region, an initial disordered gel hosts the rapid nucleation of crystallites, which stress the gel structure until it fully collapses, leading to the formation of a macroscopic colloidal crystal.

Settled and unsettled issues in particle settling.

Colloid sedimentation has played a seminal role in the development of statistical physics thanks to the celebrated experiments by Perrin, which provided a concrete demonstration of molecular reality and gave strong support to Einstein's theory of Brownian motion. This review, which mostly focuses on settling at low Peclét number, where Brownian fluctuations are dominant, aims to show that a lot more can be learnt both from the sedimentation equilibrium and from the particle settling dynamics of a wide class of systems, ranging from simple colloids to mesogenic suspensions, from soft solids to active particles and living organisms. At the same time, the occurrence of unexpected and surprising effects brings about challenging questions in statistical and fluid mechanics that make sedimentation an exciting field of research.

Ghost particle velocimetry: accurate 3D flow visualization using standard lab equipment.

We describe and test a new approach to particle velocimetry, based on imaging and cross correlating the scattering speckle pattern generated on a near-field plane by flowing tracers with a size far below the diffraction limit, which allows reconstructing the velocity pattern in microfluidic channels without perturbing the flow. As a matter of fact, adding tracers is not even strictly required, provided that the sample displays sufficiently refractive-index fluctuations. For instance, phase separation in liquid mixtures in the presence of shear is suitable to be directly investigated by this "ghost particle velocimetry" technique, which just requires a microscope with standard lamp illumination equipped with a low-cost digital camera.

On the general concept of buoyancy in sedimentation and ultracentrifugation.

Gravity or ultracentrifuge settling of colloidal particles and macromolecules usually involves several disperse species, either because natural and industrial colloids display a large size polydispersity, or because additives are put in on purpose to allow for density-based fractionation of the suspension. Such 'macromolecular crowding', however, may have surprising effects on sedimentation, for it strongly affects the buoyant force felt by a settling particle. Here we show that, as a matter of fact, the standard Archimedes' principle is just a limiting law, valid only for mesoscopic particles settling in a molecular fluid, and we obtain a fully general expression for the actual buoyancy force providing a microscopic basis to the general thermodynamic analysis of sedimentation in multi-component mixtures.

Sedimentation equilibrium and the generalized Archimedes' principle.

The buoyancy concept is critically re-examined for applications to dispersions of nano-particles, such as colloids, proteins, or macromolecules. It is shown that when the size of the buoyant particle is not too different (say, at most a factor of ten) from the size of the dispersed particles, new intriguing phenomena emerge, leading to the violation of the Archimedes' principle. The resulting buoyancy force depends not only on the volume of the particle and on the mass density of the dispersion, but also on the relative size of the particles, on their geometry, and on the interactions between the buoyant particle and the fluid.

The unbearable heaviness of colloids: facts, surprises, and puzzles in sedimentation.

Sedimentation has played a key role in the development of colloid science. In fact, it is because of the celebrated experiments by Perrin, yielding a concrete demonstration of molecular reality and giving strong support to Einstein's theory of Brownian motion, that colloids enter the realm of basic physics. Subsequent investigations have shown that a lot more can be learnt both from sedimentation equilibrium and from particle settling dynamics.