The adsorption of a sunflower protein extract at two air− water and oil−water interfaces is investigated using tensiometry, dilational viscoelasticity, and ellipsometry. For both interfaces, a three step mechanism was evidenced thanks to master curve representations of the data taken at different aging times and protein concentrations. At short times, a diffusion limited adsorption of proteins at interfaces is demonstrated. First, a two-dimensional protein film is formed with a partition of the polypeptide chains in the two phases that depends strongly on the nature of the hydrophobic phase: most of the film is in the aqueous phase at the air−water interface, while it is mostly in the organic phase at the oil−water interface. Then a three-dimensional saturated monolayer of proteins is formed. At short times, adsorption mechanisms are analogous to those found with typical globular proteins, while strong divergences are observed at longer adsorption times. Following the saturation step, a thick layer expands in the aqueous phase and appears associated with the release of large objects in the bulk. The kinetic evolution of this second layer is compatible with a diffusion limited adsorption of the minor population of polymeric complexes with hydrodynamic radius R H ∼ 80 nm, evidenced in equilibrium with hexameric globulins (R H ∼ 6 nm) in solution. These complexes could result from the presence of residual polyphenols in the extract and raise the question of the role of these compounds in the interfacial properties of plant protein extracts.

We report on the response of semi-dilute shear-banding wormlike micelles to time-dependent flow protocols such as step stress and shear startup. We use global rheology coupled to direct optical visualisations in Taylor-Couette flow to provide a detailed 2D description of the spatio-temporal dynamics of the shear banding flow under imposed stress. We compare these dynamics to their extensively studied counterpart under imposed shear rate. We show that, for a given asymptotic state, the onset of banding is delayed and the transients are longer when the stress is used as control parameter. The way the banding structure is built up strongly depends on the control parameter. In the two cases, secondary flows develop on top of the shear-banding instability: after shear startup, the interface between bands is formed in the bulk of the system and migrates inwards to its stationary position in the gap of the Taylor-Couette device, before being destabilized due to elastic instability; under creep flow, the interface between bands forms at the inner wall and becomes wavy while it is still migrating outwards to its steady position, leading to permanent rearrangements of the flow structure until steady state is reached, potentially explaining longer transients. We also outline the possible mechanical signature of the onset of shear banding in terms of shape of the bulk mechanical signals, which is the subject of the second part of this work [Briole et al., submitted].

Complex systems often exhibit shear banding-the coexistence of two different states characterized by their internal structuring and local shear rates. For some of them, the heterogeneous flow corresponds to the final steady state response while for others, shear banding can only be transient, the banding structure healing back to homogeneous flow in the ultimate steady state after long-lived periods. In order to explain the diversity of observations, Moorcroft and Fielding have established general criteria for the onset of banding in time-dependent flows of complex systems, ranging from polymeric fluids to soft glassy materials [Moorcroft et al., Phys. Rev. Lett., 2013, 110, 086001]. The proposed criteria are based on the time evolution of the bulk rheological response function of the system to a given time-dependent flow protocol and are associated with a specific signature in the mechanical response. In this contribution, we test the validity of these criteria in the case of two common time-dependent flow protocols: a step stress and a shear startup. Two types of fluids are examined. On the one hand, a wormlike micelles system exhibiting steady shear banding is studied experimentally, using rheometry coupled with direct visualisations and particle image velocimetry. On the other hand, we analyse previous literature on yield stress fluids exhibiting transient shear banding. Under creep flow, for both types of fluids the onset of banding arises in a time window compatible with the Moorcroft-Fielding criterion. However, the mechanical signature, i.e. the shape of the bulk mechanical signal as a function of time is not the one expected within some of the specific models with which the general Moorcroft-Fielding criteria were tested numerically. Under shear startup, both types of fluids behave differently. The criterion holds for yield stress fluids, the onset of banding arising just after the stress overshoot, as expected. On the contrary, for wormlike micelles the window of instability is delayed, even if the overshoot clearly plays a crucial role in the nucleation of the shear-induced structures. Regardless of the flow protocol or the system, wall slip seems to go hand in hand with banding indicating that it is a key ingredient to take into account.

Thermoresponsive microgels are one of the most investigated types of soft colloids, thanks to their ability to undergo a VolumePhase Transition (VPT) close to ambient temperature. However, this fundamental phenomenon still lacks a detailed microscopicunderstanding, particularly regarding the presence and the role of charges in the deswelling process. This is particularly important for the widely used poly(N-isopropylacrylamide)–based microgels, where the constituent monomers are neutral but charged groups arise due to the initiator molecules used in the synthesis. Here, we address this point combining experiments with state-of-the-art simulations to show that the microgel collapse does not happen in a homogeneous fashion, but through a two-step mechanism, entirely attributable to electrostatic effects.The signature of this phenomenon is the emergence of a minimum in the ratio between gyration and hydrodynamic radii at the VPT. Thanks to simulations of microgels with different cross-linker concentrations, charge contents, and charge distributions, we provide evidence that peripheral charges arising from the synthesis are responsible for this behavior and we further build a universal master curve able to predict the twostep deswelling. Our results have direct relevance on fundamental soft condensed matter science and on applications where microgels are involved, ranging from materials to biomedical technologies.

The development of smart nanomaterials has attracted great attention in several fields like nanoscience and nanotechnology due to their unique response to external stimuli. Many of them are based on polymers that can exhibit a shape-changes when submitted to environmental modifications. Poly(N-isopropylacrylamide), PNIPAM, is a thermo-responsive polymer. Linear chains are water soluble at room temperature but undergo a reversible coil-to-globule transition at a lower critical solution temperature (LCST) close to 32°C due to the dehydration and subsequent collapse of its chains into compact globules. [1] This phenomenon results in a volume phase transition (VPT) in PNIPAM based crosslinked microgels and can be used to promote original thermal effects.Hybrid nanocomposite microgels associating PNIPAM and gold nanoparticles (GNP) have thus been designed in order to take advantage of the outstanding plasmonic and photo-thermal properties of GNP to promote the VPT of the microgels through an efficient photo-thermal conversion. [2] With their strong diameter-dependent optical absorption in the near infrared (NIR) and their large surface area favoring photo-thermal transfer, semiconducting single-walled carbon nanotubes (s-SWNT) are also good candidates for photo-thermal conversion in the NIR (Figure 1a). However, to the best of our knowledge, no thorough studies of nanomaterials based on both SWNT and PNIPAM have been reported so far.Here we describe the preparation of SWNT/PNIPAM hybrid microgels through a non-covalent functionalization technique. These nanoparticles are stable in water and show a VPT, which can be promoted either by direct heating or by excitation of the resonant absorption of s-SWNT in the NIR (Figure 1b-c). The photoluminescence (PL) signal can be used to monitor the VPT by a redshift observed when crossing the LCST, while the Raman signatures remain essentially the same.

Avec la plupart des couches d’alignement classiques des cellules commerciales, il est difficile d’obtenir une texture uniforme des phases nématiques twist bend (NTB) en géométrie planaire, y compris dans des cellules très fines (~1 μm). Même lorsqu’un composé présente une phase nématique N bien alignée, on observe en effet la formation spontanée de bandes lorsque l’on passe en phase NTB [1,2,3] (voir Fig.1-a). Cette texture en rayures est caractérisée par un pas égal à deux fois l’épaisseur de la cellule [1]. Son origine est attribuée à l’amincissement rapide des pseudo-couches NTB en dessous de la température de transition N-NTB qui conduit à une instabilité d’ondulation.Nous avons étudié en détail le rôle de la force d’ancrage des couches d’alignement sur les textures NTB d’un dimère cyanophyphényle, le CB7CB (1,7-bis-4-(4-cyanobiphenyl) heptane). Pour cela nous avons utilisé un polymère dont la force d’ancrage sur les nématiques cyanobiphényles (monomères) était connue pour varier fortement selon les conditions de préparation. Une étude systématique de l’ancrage de ce polymère sur le CB7CB a été faite en phase nématique et nous avons étudié l’influence des énergies d’ancrages zénithale et azimutale sur les textures obtenues en phase NTB. Dans des cellules symétriques, nous avons ainsi montré que la périodicité des instabilités dépendait fortement de l’ancrage et notamment de sa composante azimutale.Par ailleurs, ce travail a permis de montrer qu’avec des conditions d’ancrage adéquates, il était possible d’obtenir un alignement spontané uniforme de la phase NTB (voir Fig.1-b-c). Cet alignement est conservé même en s’éloignant de la température de transition, ouvrant la possibilité future d’utiliser les phases NTB dans des dispositifs électro-optiques.