Wide-angle static and dynamic light scattering under shear.

We develop and characterize a wide angle static and dynamic light scattering under shear setup. The apparatus is suitable for the study of the structure and the dynamics of soft materials systems with a sub-micron characteristic length scale. The shear device consists in two parallel plates, and the optical setup allows us to perform light scattering measurements in any plane that contains the gradient of the velocity field direction. We demonstrate several capabilities of our apparatus: a measurement of the evolution with shear of the first peak of the structure factor of a concentrated suspension of spherical particles, both in the compression and extension quadrants of the shear flow, and the measurement of the velocity profile in dynamic light scattering. We present a theoretical treatment of light scattering under flow that takes into account the Gaussian character of the illumination and detection optical paths, in the case where the scattering volume extension is smaller than the gap of the flow cell, and compare with experimental measurements.

Velocity of lateral drying fronts in film formation by drying of colloidal dispersions. A 2D simulation.

Drying of colloids is always heterogeneous and proceeds by progression of drying fronts in various directions at various velocities. The fundamental mechanisms at the origin of appearance and motion of drying fronts are still not totally understood. This article addresses these questions in the case of lateral drying fronts by using the new simulation tool based on cellular automata we recently developed (Langmuir 2015 and 2017). For the first time, a 2D simulation is proposed. Silica dispersions were used as model colloids to test the simulation. Film profiles were measured during drying by optical profilometry as well as front velocities by image processing. In the cases of non-circular deposits (squares and rectangles), drying fronts in the plane of the film (x,y plane, x being the longest side in the case of a rectangle) do not move at the same speed along sides and diagonals, the velocity order being diagonal >x (longest side) >y (shortest side). The velocity contrast (difference between x and y sides) increases with the aspect ratio of the rectangle. This behavior is explained and accounted for by the 2D simulation presented in this article. Experimental results reasonably well validate the simulation.

Horizontal drying fronts in films of colloidal dispersions: influence of hydrostatic pressure and collective diffusion.

The origin and time evolution of heterogeneities in drying colloidal films is still a matter of debate. In this work, we studied the behaviour of horizontal drying fronts in a 1D configuration. The effects of hydrostatic pressure and collective diffusion of charged particles, neglected so far, were introduced. We made use of the new simulation tool based on cellular automata we recently presented (Langmuir 2015 & 2017). To check the simulation results, measurements of film profiles in the wet state and drying front velocities were performed with silica colloids. It was shown that taking hydrostatic pressure into account much improves agreement between theory and experiment. On the other hand, the simulation showed that collective diffusion slows down the drying fronts, even more when the Debye length is increased. This latter effect remains to be checked experimentally. This work opens the way to further improvements of theory and simulation, notably 2D and 3D simulations.

Simulation of Vertical Surfactant Distributions in Drying Latex Films.

Following our previous contribution ( Gromer, A. et al. Langmuir 2015 , 31 , 10983 - 10994 ) presenting a new simulation tool devoted to particle distributions in drying latex films, this Article describes the prediction of surfactant concentration profiles in the vertical direction during the complete film formation process. The simulation is inspired by cellular automata and equations by Routh and co-workers. It includes effects that were not considered before: surfactant convection by water and surfactant desorption upon particle deformation. It is based on five parameters describing the nature of the polymer/surfactant system and on film formation conditions. In particular, the viscoelastic properties of the polymer were taken into account through the λ̅ parameter introduced by Routh and Russel. Results show the importance of convection by water and the influence of the particular deformation mechanism on the final surfactant distribution. Excesses or depletions can be predicted either on the surface or on the substrate sides, in qualitative agreement with the numerous existing experimental studies. The complex interplay between parameters governing surfactant distributions makes the results unpredictable without the help of such a simulation tool. Therefore, it should be of interest to both industrial and academic scientists.

Simulation of Latex Film Formation Using a Cell Model in Real Space: Vertical Drying.

This paper presents a simulation tool applied to latex film formation by drying, a hybrid between a classical numerical resolution method using finite differences and cellular automata, and making use of object-oriented programming. It consists of dividing real space into cells and applying local physical laws to simulate the exchange of matter between neighboring cells. In a first step, the simulation was applied to the simple case of vertical drying of a latex containing only one population of monodisperse particles and water. Our results show how the distribution of latex particles evolves through the different drying stages due to a combination of diffusion, convection, and particle deformation. While repulsive interactions between the particles tend to favor homogeneous distributions in the first drying stage, concentration gradients that develop in opposite ways can be observed depending on the drying regime. The distributions, calculated in various cases, reproduce and extend several theoretical results and are in qualitative agreement with some experimental findings.

Imbibition kinetics of spherical colloidal aggregates.

The imbibition kinetics of a millimeter-sized aggregate of 300 nm diameter colloidal particles by a wetting pure solvent is studied. Three successive regimes are observed. First, the imbibition proceeds by compressing the air inside the aggregate. Next, the solvent stops when the pressure of the compressed air is equal to the excess of capillary pressure at the meniscus of the wetting solvent in the porous aggregate. The interface is pinned and the aggregate slowly degases up to the point where the pressure of the entrapped air stops decreasing and is controlled by the capillary pressure. Finally, the imbibition starts again at a constant excess of pressure, smaller than the capillary pressure but larger than the one of the atmosphere. This last stage leads to the complete infiltration of the aggregate.

Subcellular localization and transport kinetics of ruthenium organometallic anticancer compounds in living cells: a dose-dependent role for amino acid and iron transporters.

Ruthenium-based compounds are developed for anticancer treatment, but their mode of action including their import mechanism and subcellular localization remains elusive. Here, we used the intrinsic luminescent properties of cytotoxic organoruthenium (Ru(II)) compounds obtained with an anionic cyclometalated 2-phenylpyridine chelate and neutral aromatic chelating ligands (e.g., phenanthrolines) to follow their behavior in living cells. We established that the difference in sensitivity between cancer cells and noncancerous cells toward one of the compounds correlates with its import kinetics and follows a balance between active and passive transport. The active-transport mechanism involves iron and amino-acid transporters, which are transcriptionally regulated by the drug. We also demonstrated a correlation between the accumulation of these compounds in specific compartments (endoplasmic reticulum, nucleus, mitochondria) and the activation of specific cytotoxic mechanisms such as the mitochondrial stress pathway. Our study pinpoints a novel and complex mechanism of accumulation of ruthenium drugs in cancer cells.

Restricted diffusion of small probe particles in a laponite dispersion.

Evanescent wave microscopy is used to study the dynamics of probe particles inside a laponite suspension, when the size of the latex probes is of the order of the diameter of the laponite disks. A correlation procedure is introduced that allows us to study quantitatively the diffusion of small probes. For all studied sizes, the motion exhibits two modes: a fast relaxation mode and a slow relaxation mode. In the fast relaxation mode, the probes diffuse in a viscous medium, whose viscosity does not depend on the diameter of the probes and is slightly larger than the viscosity of water. Then, the diffusion of the particles is restricted over distances larger than their diameters, which increase when the particle diameter decreases. In this regime, the probe particles experience the elasticity of the solution and the apparent elastic modulus increases when the diameter of the probe particle increases, whereas for large enough particles, the macroscopic behavior is recovered, in which the diffusing particles experience a homogeneous medium, and the macroscopic elastic modulus is recovered.

Stress propagation in a concentrated colloidal suspension under shear.

The stress propagation in a concentrated attractive colloidal suspension under shear is studied using numerical simulations. The spatial correlations of the intercolloidal stress field are studied and an inertia-like tensor is defined in order to characterize the anisotropic nature of the stress field. It is shown that the colloids remain in a liquid order, the intercolloidal stress is strongly anisotropic. A transition under flow is observed: during a transient regime at low deformation, the stress propagates along the compression direction of the shear, whereas at larger deformations, the stress is organized into layers parallel to the (flow, vorticity) plane.

Magnetic force probe for nanoscale biomolecules.

We present a new technique to measure the mechanical properties of small biomolecules. This technique uses long range repulsive colloidal forces together with magnetic attraction as a force probing tool. The biomolecules are grafted between superparamagnetic particles, which are regularly spaced within long chains maintained by an external magnetic field. Varying the magnetic field results in compression or extension of the molecules between the particles. In order to demonstrate this technique we use, as a size controlled model molecule, a short double stranded DNA (151 base pairs) for which the force-extension law is determined and found in agreement with existing predictions.

Effects of the rate of evaporation and film thickness on nonuniform drying of film-forming concentrated colloidal suspensions.

In this paper, we report on nonuniform distribution of film-forming waterborne colloidal suspensions above the critical concentration phi(c) of the colloidal glass transition during drying. We found that colloidal suspension films dry nonuniformly when the initial rate of evaporation E and/or the initial thickness l(0) are high. We found that a Peclet number Pe, defined as Pe = El(0)/D, where D is the diffusion coefficient of the colloids in the diluted suspensions, does not predict uniformity of drying of the concentrated suspensions, contrary to the reported work on drying of diluted suspensions. Since the colloidal particles are crowded and their diffusive motion is restricted in concentrated suspensions, we assumed that above phi(c) water is transported to the drying surface by hydrodynamic flow along the osmotic pressure gradient. The permeability of water through channels between deforming particles is estimated by adapting the theory of foam drainage. We defined a new Peclet number Pe' by substituting the transport coefficient of flow (defined as the permeability divided by the viscosity, multiplied by the osmotic pressure gradient) for the diffusion coefficient. This extended Peclet number predicted the nonuniform drying with a criterion of Pe' > 1. These results indicate that the mechanism of water transport to the drying surface in concentrated suspensions is water permeation by osmotic pressure, which is faster than mutual diffusion between water and particles --that has been observed in diluted suspensions and discussed by Routh and Russel. The theory fits well the experimental drying curves for various thicknesses and rates of evaporation. The particle distribution in the drying films is also estimated and it is indicated that the latex distribution is nonuniform when Pe' > 1.

Interface composition of multiple emulsions: rheology as a probe.

We have investigated the dynamic rheological properties of concentrated multiple emulsions to characterize their amphiphile composition at interfaces. Multiple emulsions (W1/O/W2) consist of water droplets (W1) dispersed into oil globules (O), which are redispersed in an external aqueous phase (W2). A small-molecule surfactant and an amphiphilic polymer were used to stabilize the inverse emulsion (W1 in oil globules) and the inverse emulsion (oil globules in W2), respectively. Rheological and interfacial tension measurements show that the polymeric surfactant adsorbed at the globule interface does not migrate to the droplet interfaces through the oil phase. This explains, at least partly, the stability improvement of multiple emulsions as polymeric surfactants are used instead of small-molecule surfactants.

Dynamics of concentrated colloidal suspensions during drying--aging, rejuvenation and overaging.

We report on the slow dynamics of concentrated colloidal suspensions during drying and rewetting under conditions of reversible concentration changes without coalescence or aggregation. We used multispeckle diffusing-wave spectroscopy to monitor the slow dynamics of particles. We found that the alpha relaxation of the suspensions exhibits successively slowing-down, acceleration and a stationary regime during drying at constant rates. Under rewetting conditions, we observed slowing-down and a stationary regime. The characteristic time of the stationary regime is inversely proportional to the rate of concentration change and identical for both drying and rewetting. We explain these regimes as aging (overaging), rejuvenation and plastic flow of the suspensions induced by a deviatoric stress (a combination of compressive and elongational stresses) which is induced by the uniaxial compressive strain generated by evaporation.

Partial rejuvenation of a colloidal glass.

We study the effect of shear on the aging dynamics of a colloidal suspension of synthetic clay particles. We find that a shear of amplitude gamma reduces the relaxation time measured just after the cessation of shear by a factor exp(-gamma/gamma(c)), with gamma(c) approximately 5%, and is independent of the duration and the frequency of the shear. This simple law for the rejuvenation effect shows that the energy involved in colloidal rearrangements is proportional to the shear amplitude gamma rather than gamma(2), leading to an Eyring-like description of the dynamics of our system.

Monitoring of flocculation and creaming of sodium-caseinate-stabilized emulsions using diffusing-wave spectroscopy.

Diffusing-wave spectroscopy (DWS) has been used to study the stability of sodium-caseinate-stabilized emulsions. The emulsions underwent creaming as a result of depletion flocculation when excess sodium caseinate was added. The creaming process was monitored over a 3-h period and each autocorrelation function was collected for 2 min to ensure adequate signal-to-noise ratio. The temporal variation of average particle size times the coefficient of viscosity of the continuous phase was derived from the backscattering measurements, and the variation of the scattering mean free path length with time was found from the backscattering and transmission measurements. It was confirmed that the creaming process was delayed at high oil concentrations, presumably due to the formation of oil droplet networks.

Brownian motion of particles embedded in a solution of giant micelles.

We have measured the mean-square displacement of colloidal particles embedded in a semi-dilute solution of worm-like micelles, using diffusing wave spectroscopy. This allowed us to describe their rheological properties over a very wide time range. At very short times, the particles diffuse freely in the solvent, and then, they experience the characteristic relaxation times of the living chains. We deduced directly, from the mean-square displacement of the particles, the mechanical properties of the micellar solution, not only in the high-frequency regime, but also in the low-frequency range, in which we compared our results with direct mechanical measurements, and found good agreement.