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.

Isoniazid (INH) is one of the primary drugs used in tuberculosis treatment and its encapsulation in liposomal vesicles can both improve its therapeutic index and minimize toxicity. Here we consider mixtures of hydrogenated soy phosphatidylcholine-phosphatidylglycerol (HSPC-DPPG) to get novel biocompatible liposomes for INH delivery. We determined INH encapsulation efficiency by coupling for the first time UV and Laser Transmission Spectroscopy and we showed that HSPC-DPPG liposomes can load more INH than expected from simple geometrical arguments, thus suggesting the presence of drug-lipid association. To focus on this aspect, which has never been explored in liposomal formulations, we employed several complementary techniques, such as dynamic and static light scattering, calorimetry and surface pressure measurements on lipid monolayers. We find that INH-lipid interaction increases the entrapment capability of liposomes due to INH adsorption. Moreover, the preferential INH-HSPC dipole-dipole interaction promotes the modification of lipid ordering, favoring the formation of HSPC-richer domains in excess of DPPG. Our findings highlight how investigating the fundamental aspects of drug-lipid interactions is of paramount importance for the optimal design of liposomal nanocarriers.

PNIPAm microgels synthesized via free radical polymerization (FRP) are often considered as neutral colloids in aqueous media, although it is well known, since the pioneering works of Pelton and coworkers, that the vanishing electrophoretic mobility characterizing swollen microgels largely increases above the lower critical solution temperature (LCST) of PNIPAm, at which microgels partially collapse. The presence of an electric charge has been attributed to the ionic initiators that are employed when FRP is performed in water and that stay anchored to microgel particles. Combining dynamic light scattering (DLS), electrophoresis, transmission electron microscopy (TEM) and atomic force microscopy (AFM) experiments, we show that collapsed ionic PNIPAm microgels undergo large mobility reversal and reentrant condensation when they are co-suspended with oppositely charged polyelectrolytes (PE) or nanoparticles (NP), while their stability remains unaffected by PE or NP addition at lower temperatures, where microgels are swollen and their charge density is low. Our results highlight a somehow double-faced electrostatic behavior of PNIPAm microgels due to their tunable charge density: they behave as quasi-neutral colloids at temperature below LCST, while they strongly interact with oppositely charged species when they are in their collapsed state. The very similar phenomenology encountered when microgels are surrounded by polylysine chains and silica nanoparticles points to the general character of this twofold behavior of PNIPAm-based colloids in water.

Large deformations of soft elastic beads spinning at high angular velocity in a denser background fluid are investigated theoretically, numerically, and experimentally using millimeter-size polyacry-lamide hydrogel particles introduced in a spinning drop tensiometer. We determine the equilibrium shapes of the beads from the competition between the centrifugal force and the restoring elastic and surface forces. Considering the beads as neo-Hookean up to large deformations, we show that their elastic modulus and surface energy constant can be simultaneously deduced from their equilibrium shape. Also, our results provide further support to the scenario in which surface energy and surface tension coincide for amorphous polymer gels.

We exploit the deformation of drops spinning in a denser background fluid to investigate the effective interfacial tension (EIT) between miscible molecular fluids. We find that, for sufficiently low interfacial tension, spinning drops develop dumbbell shapes, with two large heads connected by a thinner central body. We show that this shape depends not only on the density and viscosity contrast between the drop and background fluids, but also on the fluid molecular structure, and hence on the stresses developing at their interface due to a different molecular interaction. We systematically investigate the dynamics of dumbbell-shaped drops of water-glycerol mixtures spinning in a pure glycerol reservoir. By developing a model for the deformation based on the balance of the shear stress opposing the deformation, the imposed normal stress on the drop, and an effective interfacial tension, we exploit the time evolution of the drop shape to measure the EIT. Our results showthat the EIT in water-glycerol systems is orders of magnitude lower than that reported in previous experimental measurements, and in excellent agreement with values calculated via the phase field model proposed by Truzzolillo et al. [Phys. Rev. X 6, 041057 (2016)].