structural analysis with a SAXS laboratory beamline on a microfluidic chip.

Recent advances have been made in coupling microfluidic chips with X-ray equipment, enabling structural analysis of samples directly in microfluidic devices. This important step mainly took place at powerful synchrotron facilities because of the need for a beam reduced in size to fit the microfluidic channel dimensions but still intense. In this work, we discuss how improvements of an X-ray laboratory beamline and an optimal design of a microfluidic device allow reliable structural information to be obtained without the need for a synchrotron.

On the in situ 3D electrostatic directed assembly of CdSe/CdZnS colloidal quantum nanoplatelets towards display applications.

Hypothesis: Due to their unique quantum yield and photostability performances, quantum nanoplatelets are very promising building blocks for future generations of displays. The directed assembly of such colloidal nano-objects in the shape of micro-pixels is thus the next mandatory step to reach this goal. Selectively trapping them on electrostatically charged patterns by nanoxerography could be a versatile and appealing strategy but requires a full understanding of the assembly mechanisms in order to make the most of their integration.

Microfluidic osmotic compression of a charge-stabilized colloidal dispersion: Equation of state and collective diffusion coefficient.

We show, using a model coupling mass transport and liquid theory calculations for a charge-stabilized colloidal dispersion, that diffusion significantly limits measurement times of its equation of state (EOS), osmotic pressure vs composition, using the osmotic compression technique. Following this result, we present a microfluidic chip allowing one to measure the entire EOS of a charged dispersion at the nanoliter scale in a few hours. We also show that time-resolved analyses of relaxation to equilibrium in this microfluidic experiment lead to direct estimates of the collective diffusion coefficient of the dispersion in Donnan equilibrium with a salt reservoir.

Shear-induced glass-to-crystal transition in anisotropic clay-like suspensions.

A new numerical framework based on Stokesian dynamics is used to study a shear-induced glass-to-crystal transition in suspensions of clay-like anisotropically charged platelets. The structures obtained in quiescent conditions are in agreement with previous Monte Carlo results: a liquid phase at very short interaction range (high salt concentration), phase separation and a gel without large scale density fluctuations at intermediate interaction ranges, and glassy states at very large interaction ranges. When initially glassy suspensions are sheared, hydrodynamic torques first rotate platelets so they can reach a transient quasi-nematic disordered state.

Surfactant mediated particle aggregation in nonpolar solvents.

The aggregation behavior of particles in nonpolar media is studied with time-resolved light scattering. At low surfactant concentrations particles are weakly charged and suspensions are not stable. The suspensions become progressively more stable with increasing surfactant concentration as particles become more highly charged.

Drying colloidal systems: Laboratory models for a wide range of applications.

The drying of complex fluids provides a powerful insight into phenomena that take place on time and length scales not normally accessible. An important feature of complex fluids, colloidal dispersions and polymer solutions is their high sensitivity to weak external actions. Thus, the drying of complex fluids involves a large number of physical and chemical processes.

Surface and extrapolated point charge renormalizations for charge-stabilized colloidal spheres.

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory is widely used to model interactions between weakly charged spheres in dilute suspensions. For particles bearing a higher charge, the linearized electrostatics underlying the DLVO theory is no longer valid but it is possible to map the real colloidal system to an auxiliary one that still obeys linear electrostatics but which involves a different, effective pair potential. This procedure, termed renormalization, can be performed in various ways, the most widely used being surface charge renormalization (SCR) based on the cell model.

Fast, Robust Evaluation of the Equation of State of Suspensions of Charge-Stabilized Colloidal Spheres.

Increasing demand is appearing for the fast, robust prediction of the equation of state of colloidal suspensions, notably with a view to using it as input data to calculate transport coefficients in complex flow solvers. This is also of interest in rheological studies, industrial screening tests of new formulations, and the real-time interpretation of osmotic compression experiments, for example. For charge-stabilized spherical particles, the osmotic pressure can be computed with standard liquid theories.

Modeling the Electrostatics of Hollow Shell Suspensions: Ion Distribution, Pair Interactions, and Many-Body Effects.

Electrostatic interactions play a key role in hollow shell suspensions as they determine their structure, stability, thermodynamics, and rheology and also the loading capacity of small charged species for nanoreservoir applications. In this work, fast, reliable modeling strategies aimed at predicting the electrostatics of hollow shells for one, two, and many colloids are proposed and validated. The electrostatic potential inside and outside a hollow shell with a finite thickness and a specific permittivity is determined analytically in the Debye-Hückel (DH) limit.

A Three-Step Scenario Involved in Particle Capture on a Pore Edge.

A scenario is proposed to describe the capture of a spherical particle around a cylindrical pore. This geometry, "ideal" as far as the problem of particle capture on a filtration membrane is concerned, is clearly relevant in view of the pore-scale geometry of nucleopore or microsieve filtration membranes, and also of some microfluidic systems used to perform fluid-particle separation. The present scenario consists of three successive steps: particle deposition on the membrane away from the pore, subsequent reentrainment of some of the deposited particles by rolling on the membrane surface, and final arrest by a stabilizing van der Waals torque when the particle rolls over the pore edge.

Quantitative assessment of the accuracy of the Poisson-Boltzmann cell model for salty suspensions.

The cell model is a ubiquitous, fast, and relatively easily implemented model used to estimate the osmotic pressure of a colloidal dispersion. It has been shown to yield accurate approximations of the pressure in dispersions with a low salt content. It is generally accepted that it performs well when long-ranged interactions are involved and the structure of the dispersion is solidlike.