Wall slip across the jamming transition of soft thermoresponsive particles

Flows of suspensions are often affected by wall slip, that is the fluid velocity $v_{f}$ in the vicinity of a boundary differs from the wall velocity $v_{w}$ due to the presence of a lubrication layer. While the slip velocity $v_s=\vert v_{f}-v_{w}\vert$ robustly scales linearly with the stress $\sigma$ at the wall in dilute suspensions, there is no consensus regarding denser suspensions that are sheared in the bulk, for which slip velocities have been reported to scale as a $v_s\propto\sigma^p$ with exponents $p$ inconsistently ranging between 0 and 2. Here we focus on a suspension of soft thermoresponsive particles and show that $v_s$ actually scales as a power law of the viscous stress $\sigma-\sigma_c$, where $\sigma_c$ denotes the yield stress of the bulk material. By tuning the temperature across the jamming transition, we further demonstrate that this scaling holds true over a large range of packing fractions $\phi$ on both sides of the jamming point and that the exponent $p$ increases continuously with $\phi$, from $p=1$ in the case of dilute suspensions to $p=2$ for jammed assemblies. These results allow us to successfully revisit inconsistent data from the literature and paves the way for a continuous description of wall slip above and below jamming.Comment: 6 pages, 4 figures - accepted for publication as a Rapid Communication in Phys. Rev.

Surface compaction versus stretching in Pickering emulsions stabilised by microgels

Colloidal gel particles called microgels have shown their ability to adsorb at an oil-water interface and stabilise emulsion named Pickering emulsions. Such particles are soft, deformable, and porous, and they can swell or contract under the action of an external stimulus. These specificities make thememulsifiers of special interest as they offer a large versatility to emulsions and materials elaborated thereof. This modularity is in counterpart at the origin of an abundant and often contradictory literature. The aim of this paper is to review recent advances in the emulsion stabilisation mechanism, particularly focusing on the microgel conformation at the interface in relation with themechanical interface behaviour and the emulsionmacroscopic stability. A sum up of the unambiguous knowledge is also proposed as well as few central questions that remain to be answered to in the domain

Chapter 4 -Experimental multiscale approach and instrumental techniques for the characterization of Pickering emulsions

International audienceThe aim of this chapter is to propose a step-by-step approach for the study of emulsions stabilized by particles independently of the system chemical nature. This includes tools as simple as macroscopic observations of the obtained emulsions up to more sophisticated techniques, like electron microscopy or interfacial rheology. The progressive characterization allows collecting information at different length scales; at the level of the emulsion properties, at the level of drops and down to the particle packing at the oil-water interface, highlighting interaction between either drops and adsorbed particles. The link between the various length scales is also enhanced. For example, bridging of particles adsorbed at the interface of neighboring drops is responsible for the adhesion between drops leading to a plug flow that can be assessed by naked eyes. Therefore, the chapter pretends to be a general guideline for studying a new unknown system, starting from scratch

Single-Crystalline Gold Nanoplates from a Commercial Gold Plating Solution

A novel route was proposed to synthesize gold nanoplates using a commercial gold plating solution as the reactant. Single-crystalline gold nanoplates can be successfully synthesized by reacting gold plating solution with HCl. The as-prepared nanoplates are from several micrometers to tens of micrometers in size. The effects of reactant concentration and temperature on the morphology of the gold products were investigated. The size of the gold nanoplate increases with the decrease of the amount of gold plating solution, while irregular gold nanoparticles are formed as the HCl concentration becomes low. When the reaction temperature is as low as room temperature, nanoplates with a concavity form. Specifically, it is found that the Cl− plays an important role for the formation of these gold nanoplates. The formation mechanism of the gold nanoplates is studied in detail

Nanogels de polysaccharides pour la délivrance d'insuline

Les nanogels sont de bons candidats pour la délivrance d actifs. Ces réseaux de polymères réticulés et de taille nanométrique, sont gonflés d eau. Ils sont donc capables d encapsuler une protéine à l intérieur de leurs pores et de la libérer en fonction de l état de gonflement du réseau. Cet état peut être modulé par la densité de réticulation du réseau ou par l application d un stimulus externe tel que le pH, la température ou encore une biomolécule telle que le glucose. Ainsi, les nanogels sensibles au glucose se présentent comme des candidats idéaux pour administrer l insuline de façon asservie à la glycémie. Afin de satisfaire aux critères de biocompatibilité et de biorésorption des vecteurs, nous avons choisi de développer des nanogels à base de polysaccharide, en particulier à base d acide hyaluronique (HA). Ceux-ci sont obtenus par réticulation du HA, préalablement modifié par des fonctions réticulables telles que les méthacrylates, dans des nanogouttes d'émulsion eau-dans-huile. Des nanogels de taille et de porosité modulables ont été synthétisés grâce à un bon contrôle 1) de la modification chimique des précurseurs par des fonctions réticulables (taux de méthacrylation), 2) de l émulsion matricielle (taille, stabilité), 3) des conditions de réticulation par photopolymérisation gouvernant le taux de conversion des méthacrylates. Ce savoir-faire a ensuite été appliqué à la synthèse de nanogels modifiés par des dérivés de l acide phénylboronique, ligand du glucose, afin d obtenir des matériaux dont le taux de gonflement varie en fonction de la glycémie. L intérêt applicatif de ces objets a été évalué vis-à-vis des propriétés d encapsulation de l insuline, de dégradabilité enzymatique, et de biocompatibilité.Nanogels are an attractive class of delivery systems. These soft particles, made of highly swollen polymer network, can physically entrap a drug and release it at a rate depending on its diffusion though the network. Therefore, any change in the swelling degree can trigger the release kinetics. This parameter can be tuned by modifying the density of cross-links in the gel matrix or by changing the environmental conditions such as pH, temperature or analyte such as glucose. Thus, glucose-responsive nanogels are good candidates to be used as self-regulated systems for insulin delivery. To fulfill both biocompatibility and biodegradability criteria, our attention has been focused on the design of new nanogels made of polysaccharides, in particular made of hyaluronic acid (HA), as a main constituent. HA was at first covalently modified with polymerizable methacrylate functions and confined in nanoreactors during photopolymerization using water-in-oil miniemulsions as template. Biodegradable nanogels with a well-defined size and various cross-linking degrees were thus achieved, thanks to a good control of 1) the chemical modification of HA with methacrylates (degree of methacrylation) 2) the emulsion template (size, stability), 3) the photopolymerization conditions which governed the conversion rate of the polymerization. Further modification of the polysaccharide with phenylboronic acid as a glucose-sensitive group yielded nanogels whose swelling behavior could vary as a function of glucose concentration. These systems were further studied as insulin delivery systems. Moreover, their biodegradability, stability and biocompatibility were assessed.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Multicomponent macroporous materials with a controlled architecture

We describe the elaboration of organized macroporous electrode surfaces with a complex composition using the hard sphere template procedure. In a first approach, different metals are subsequently generated in the template leading finally to a multicomponent porous material showing interesting electrochemical features combining the redox properties of the different metals. In a complementary second approach, alternating porous and compact metal layers are formed, with the compact layer being introduced on purpose as a defect layer allowing the controlled destruction of the sandwich-type architecture using a chemical or electrochemical trigger

Impact of Electrostatics on the Adsorption of Microgels at the Interface of Pickering Emulsions

The importance of electrostatics on microgel adsorption at a liquid interface is studied, as well as its consequence on emulsion stabilization. In this work, poly(Nisopropylacrylamide) (pNIPAM) microgels bearing different numbers of charges and various distribution profiles are studied, both in solution and at the oil−water interface of emulsion drops. Charged microgels are compared to neutral ones, and electrostatic interactions are screened by adding salt to the aqueous solution. In solution, electrostatics has a significant impact on microgel swelling, as induced by the osmotic pressure exerted by mobile counterions in the gel network. At the interface of drops, microgels pack in a hexagonal array, whose lattice parameter is independent of the number of charges and range of electrostatic interactions. Microgel morphology and packing are ruled only by the adsorption of the pNIPAM chain at the interface. Conversely, decreasing the charge density of microgels by the protonation of the carboxylic groups leads to unstable emulsions, possibly as a result of the impact of hydrogen bonding on microgel deformability

Modulation of Wetting Gradients by Tuning the Interplay between Surface Structuration and Anisotropic Molecular Layers with Bipolar Electrochemistry

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Interfacial rheology of model water-air microgels laden interfaces: effect of cross-linking

International audienceHypothesis The mechanical properties of model air/water interfaces covered by poly(Nisopropylacrylamide) microgels depend on the microgels deformability or in other words on the amount of cross-linker added during synthesis.Experiments The study is carried out by measuring the apparent dilational, the compression and the shear moduli using three complementary methods: 1) the pendant drop method with perturbative areas, 2) the Langmuir trough compression, and 3) shear rheology using a double wall ring cell mounted onto a Langmuir through.Findings In the range of surface coverages studied, the interfaces exhibit a solid-like behavior and elasticity goes through a maximum as a function of the surface pressure. This is observable whatever the investigation method. This maximum elasticity depends on the microgel deformability: the softer the microgels the higher the value of the moduli. The mechanical behavior of model interfaces is discussed, taking into account the core-shell structure of the particles and their packing at the interface