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.

Water-in-Oil Emulsions Stabilized by Water-Dispersible Poly(N-isopropylacrylamide) Microgels: Understanding Anti-Finkle Behavior

Emulsions were prepared using poly(N-isopropylacrylamide) microgels as thermoresponsive stabilizers. The latter are well-known for their sensitivity to temperature: they are swollen by water below the socalled volume phase transition temperature (VPTT = 33°C) and shrink when heated above it. Most of the studies reported in the literature reveal that the corresponding emulsions are of the oil-in-water type (O/W) and undergo fast destabilization upon warming above the VPTT. In the present study, whereas O/W emulsions were obtained with a wide panel of oils of variable polarity and were all thermoresponsive, water-in-oil (W/O) emulsions were found only in the presence of fatty alcohols and did not exhibit any thermal sensitivity. To understand the peculiar behavior of emulsions based on fatty alcohols, we investigated the organization of microgels at the oil-water interface and we studied the interactions of pNIPAM microgels with octanol. By combining several microscopy methods and by exploiting the limited coalescence process, we provided evidence that W/O emulsions are stabilized by multilayers of nondeformed microgels located inside the aqueous drops. Such behavior is in contradiction with the empirical Finkle rule stating that the continuous phase of the preferred emulsion is the one in which the stabilizer is preferentially dispersed. The study of microgels in nonemulsified binary water/octanol systems revealed that octanol diffused through the aqueous phase and was incorporated in the microgels. Thus, W/O emulsions were stabilized by microgels whose properties were substantially different from the native ones. In particular, after octanol uptake, they were no longer thermoresponsive, which explained the loss of responsiveness of the corresponding W/O emulsions. Finally, we showed that the incorporation of octanol modified the interfacial properties of the microgels: the higher the octanol uptake before emulsification, the lower the amount of particles in direct contact with the interface. The multilayer arrangement was thus necessary to ensure efficient stabilization against coalescence, as it increased interface cohesiveness. We discussed the origin of this counterexample of the Finkle's rule

Sugar-responsive Pickering emulsions mediated by switching hydrophobicity in microgels

International audienceHypothesis: Pickering emulsions stabilized by soft and responsive microgels can demulsify on demandupon microgel collapse. The concept has been explored with simple model microgels such as poly(Nisopropylacrylamide)(pNIPAM) and their derivatives, but the role of functionalization is largely unexplored.Experiments: Saccharide-responsive phenylboronic-modified microgels are used as Pickering emulsionstabilizers. Emulsion stability and microgel organization at drop surface are studied as a function of saccharideconcentration. Better insight into their behavior at interfaces is gained through adsorption kineticsand Langmuir film studies at air-water interface.Findings: The functionalization of water-swollen microgels by phenylboronic functions imparts somehydrophobicity to the structure, at the origin of additional internal cross-links analogous which rigidifythe structure compared to non-functionalized microgels, as proved by their slow adsorption kinetics andpoor interfacial compressibility. Upon boronate ester formation with diol groups of the saccharide, thehydrophobic character of the phenylboronic acid decreases, increasing the adsorption kinetics and theirinterfacial compressibility. Emulsions are stable in the presence of saccharide, given the high deformabilityof the yet-hydrophilic microgels, and mechanically unstable with less deformable particles in lo

Soft microgels as Pickering emulsion stabilisers: role of particle deformability

We synthesised soft uncharged microgels made of poly(N-isopropylacrylamide) of variable cross-linking degrees and we probed their efficiency as stabilisers to fabricate oil-in-water emulsions. Interestingly, such particles undergo a swollen-to-collapsed transition above a critical temperature. By combining several microscopy methods and by exploiting the limited coalescence process, we were able to determine both the particle concentration and structure at the interface, as a function of the crosslinking density. Being deformable, the initially spherical microgels adopt a ''fried egg-like'' structure when adsorbed at the oil/water interface. As expected, the interfacial deformation is increasingly pronounced as the cross-linking degree decreases. The most deformable microgels tend to form 2D connected networks characterised by significant overlapping of the peripheral parts. When the deformability is lost, by increasing the cross-linking density or the temperature, the stabilisation efficiency is considerably reduced. Our results strongly suggest that emulsion stability is mainly determined by the microgels' deformability and we discuss the origin of that empirical link in terms of lateral overlapping and interfacial elasticity

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

Journal of Colloid and Interface Science

A simple route to deliver on demand hydrosoluble molecules such as peptides, packaged in biocompatible and biodegradable microgels, is presented. Hyaluronic acid hydrogel particles with a controlled structure are prepared using a microfluidic approach. Their porosity and their rigidity can be tuned by changing the crosslinking density. These negatively-charged polyelectrolytes interact strongly with positively-charged linear peptides such as poly-l-lysine (PLL). Their interactions induce microgel deswelling and inhibit microgel enzymatic degradability by hyaluronidase. While small PLL penetrate the whole volume of the microgel, PLL larger than the mesh size of the network remain confined at its periphery. They make a complexed layer with reduced pore size, which insulates the microgel inner core from the outer medium. Consequently, enzymatic degradation of the matrix is fully inhibited and non-affinity hydrophilic species can be trapped in the core. Indeed, negatively-charged or small neutral peptides, without interactions with the network, usually diffuse freely across the network. By simple addition of large PLL, they are packaged in the core and can be released on demand, upon introduction of an enzyme that degrades selectively the capping agent. Single polyelectrolyte layer appears as a simple generic method to coat hydrogel-based materials of various scales for encapsulation and controlled delivery of hydrosoluble molecules