In the context of a more sustainable economy, bio-surfactants become increasingly important, due to their independence of petrol-based chemistry, their usually mild synthesis conditions, and in certain cases their pharmacological activity. We have recently discussed self-assembly studies in binary systems of bio-surfactants of microbial origin, or saponins extracted from plants (Hellweg et al., Frontiers in Soft Matter, 2023, 2). In the present review, we focus on the formation of microemulsions based on these molecules. We review the formation and structure of microemulsion systems formed by oil, water, and biosurfactants, with a particular focus on Quillaja saponins and rhamnolipids.

In this paper, we numerically and experimentally show that the director field orientation degeneracy within the Translationally Invariant Configuration (TIC) of a cholesteric liquid crystal under an electric field can be lifted by imposing a magnetic field B parallel to the electrodes. The configuration can be either parallel or perpendicular to the magnetic field depending on the values of the sample thickness, pitch, and applied voltage, with two equiprobable orientations in each case. The transition between the parallel and perpendicular orientations has hysteresis, suggesting that it is first order. When B is slightly tilted with respect to the electrode plane, the indeterminacy on the TIC orientation is removed when the TIC is directed along B.

3D halide perovskites used in light emitters and PV systems exhibit an unusualanharmonicity, which is not accounted for in nowadays descriptions of phonons andelectron-phonon coupling. Therefore, the low frequency lattice vibrations andrelaxations were investigated in single crystals of the four 3D hybrid organoleadperovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone centerusing Raman and Brillouin scattering and at the zone boundary using inelasticneutron scattering. The temperature dependence of the PbX6 lattice modes in thefour compounds can be renormalized into universal curves and no soft vibration isobserved excluding a displacive-like transitional dynamics related to structural phasetransitions. The reorientational (pseudospin) motions of the molecular cations exhibita seemingly order-disorder character recalling that of plastic crystals, but attributedto a secondary order-parameter. At ultra-low frequency, a quasi-elastic componentevidenced by Brillouin scattering and associated to the unresolved central peakobserved in neutron scattering, is attributed to center of mass anharmonic motionsand rattling of the molecular cations in the perovskite cavities. 3D halide perovskitesare therefore in a very unusual situation where anharmonicity prevails. It is leading topartial, extensive and ultraslow critical and stochatic dynamics connected or not tostructural instabilities. The extensive disorder of the lattice at normal operationtemperatures for devices is similar to premelting where only long-range acousticnormal modes survive.[1] A.Létoublon, S. Paofai, B. Ruffle, P. Bourges, B. Hehlen, T. Michel, C.Ecolivet, O.Durand, S. Cordier, C. Katan, J. Even. Elastic Constants, Optical Phonons andMolecular Relaxations in the High Temperature Plastic Phase of the CH3NH3PbBr3Hybrid Perovskite. J. Phys. Chem.Lett. 7, 3776 (2016)[2]C. Katan, A. D. Mohite, J. Even, Entropy in halide perovskites, Nature Materials,17, 377-379 (2018)[3]A. Ferreira, A. Létoublon, S. Paofai, S. Raymond, C. Ecolivet, B. Ruffle, S.Cordier, C. Katan, M. I. Saidaminov, A. A. Zhumekenov, O. M. Bakr, J. Even, P.Bourges, Elastic softness of hybrid lead halide perovskites, Phys. Rev. Lett., 121,085502 (2018)[4] A. Ferreira, S. Paofai, A. Létoublon, J. Ollivier, S. Raymond, B. Hehlen, B. Rufflé,S. Cordier, C. Katan, J. Even & P. Bourges, Direct evidence of weakly dispersed andstrongly anharmonic optical phonons in hybrid perovskites, Comm. Phys. 3, 48(2020).[5] B. Hehlen, P. Bourges, B. Rufflé, S. Clément, R. Vialla, A. C. Ferreira, C.Ecolivet, S. Paofai, S. Cordier, C. Katan, A. Létoublon, J. Even, Pseudospin-phononpretransitional dynamics in lead halide hybrid perovskites, Phys. Rev. B, 105,024306 (2022)

The evaporation of drops of colloidal suspensions plays an important role in numerous contexts, such as the production of powdered dairies, the synthesis of functional supraparticles, and virus and bacteria survival in aerosols or drops on surfaces. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. Previousworks propose that, for drops evaporating very fast, the instability occurs when the particles form a rigid porous solid, constituted of permanently aggregated particles at random close packing. To date, however, no measurements could directly test this scenario and assess whether it also applies todrops drying at lower evaporation rates, severely limiting our understanding of this phenomenon and the possibility of harnessing it in applications. Here, we combine macroscopic imaging and spaceand time-resolved measurements of the microscopic dynamics of colloidal nanoparticles in drying drops sitting on a hydrophobic surface, measuring the evolution of the thickness of the shell and the spatial distribution and mobility of the nanoparticles. We find that, above a threshold evaporationrate, the drop undergoes successively two distinct shape instabilities, invagination and cracking. While permanent aggregation of nanoparticles accompanies the second instability, as hypothesized in previous works on fast-evaporating drops, we show that the first one results from a reversible glass transition of the shell, unreported so far. We rationalize our findings and discuss their implicationsin the framework of a unified state diagram for the drying of colloidal drops sitting on a hydrophobic surface.

We explore the impact of three water-soluble polyelectrolytes (PEs) on the flow of concentrated suspensions of poly(N-isopropylacrylamide) (PNIPAm) microgels with thermoresponsive anionic charge density. By progressively adding the PEs to a jammed suspension of swollen microgels, we show that the rheology of the mixtures is remarkably influenced by the sign of the PE charge, PE concentration and hydrophobicity only when the temperature is raised above the microgel volume phase transition temperature Tc, namely when microgels collapse, they are partially hydrophobic and form a volume-spanning colloidal gel. We find that the original gel is strengthened close to the isoelectric point, attained when microgels are mixed with cationic PEs, while PE hydrophobicity rules the gel strengthening at very high PE concentrations. Surprisingly, we find that polyelectrolyte adsorption or partial embedding of PE chains inside the microgel periphery occurs also when anionic polymers of polystyrene sulfonate with high degree of sulfonation are added. This gives rise to colloidal stabilization and to the melting of the original gel network above Tc. Contrastingly, the presence of polyelectrolytes in suspensions of swollen, jammed microgels results in a weak softening of the original repulsive glass, even when an apparent isoelectric condition is met. Our study puts forward the crucial role of electrostatics in thermosensitive microgels, unveiling an exciting new way to tailor the flow of these soft colloids and highlighting a largely unexplored path to engineer soft colloidal mixtures.