Microgels display specific features attracting a high research interest in several applications such as bioactives, drug delivery or protein purification. Whey protein microgels (WPM) are obtained by promoting whey protein intramolecular cross-linking through heat treatment. Temperature-induced protein denaturation and aggregation involves several kinds of interactions such as electrostatic repulsions and disulfide bonds. In the present work, the WPM particles topography and nanomechanical properties were investigated at native pH (6.5) and acid pH (5.5 and 3.0) by atomic force microscopy (AFM) in contact mode in a liquid environment. Prior to AFM analysis, WPM particles were captured on a gold substrate via low energy interactions by means of specific monoclonal antibodies. The 2D AFM images clearly showed a swelling of WPM particles induced by pH decrease. At native pH, they displayed an average width and height of 192 ± 27 nm and 44 ± 9 nm, respectively. A decrease in pH to 5.5 led to a significant (p<0.05) increase in WPM particles width (282 ± 9 nm) and height (101 ± 1 nm). At pH 3.0, a further increase in WPM size (a width of 420 ± 25 nm and a height of 78 ± 1 nm). The AFM elasticity (E) data showed a significant (p<0.05) increase in stiffness at pH 5.5 (E: 199 ± 9 kPa) and pH 3.0 (E: 187 ± 12 kPa) compared to native pH (E: 12 ± 1 kPa). These findings indicate that the mechanical profiles of WPM network directly varied with pH decrease. The WPM topographic and nanomechanical changes induced by acidification were most likely due to substantial changes in the shape and the structure of WPM particles. These strengthened internally crosslinked structures, modified by acidification, could display interesting encapsulation properties, providing an additional proof for their use as nanovectors monitoring bioactives release to the desired target after ingestion.

Atomic force microscopy-based nanomechanical methods were directly applied in order to elucidate important questions related to brain metastasis formation and amyotrophic lateral sclerosis (ALS). Force mapping with single cell force spectroscopy was combined in order to gain direct insight into the surface “screening” process of tumor cells during their extravasation into the brain parenchyma. Furthermore, comparative SCFS measurements were performed between three melanoma cell types (WM35, A2058 and A375), showing altered invasive characteristics, and blood vessel lining endothelials in order to examine how the metastatic potential relates to tumor cell’s autonomous and inter-cellular nanomechanical properties. In case of ALS by comparing the adhesion of CD8+ cytotoxic T cells isolated from wild-type as well as SOD1G93A mutant mice against wild-type motor neurons we found that cytotoxic T cells are active contributors of neurodegeneration in the development of ALS. In addition, nanomechanical mapping of wild-type and SOD1G93A mutant mice derived single elongated myoblasts and multinuclear myotubes revealed differences in elasticity during early differentiation stage into myotubes. These findings provide valuable knowledge to the understanding of brain metastasis formation and ALS pathogenesis.

The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photolu-minescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6-13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

Control of film structures made from a polystyrene-polystyrene sodium sulfonate-polystyrene (PS-PNaSS-PS) copolymer micellar solution is investigated in a THF/water mixture. Four different copolymers (varying molecular weights) are synthesised via RAFT (Reversible Addition Fragmentation chain Transfer) polymerisation. Depending on parameters such as copolymer molecular weight, solvent composition and copolymer concentration, the PS-PNaSS-PS triblock self-assembles into different morphologies in solution and dry state. The effect of each parameter is investigated using characterization techniques such as AFM, TEM, Cryo-TEM, SEM and SAXS. The morphologies obtained for PS-PNaSS-PS are found to be extremely sensitive when the water content of the micellar solution is low. Among the structures observed, a highly ordered nano-porous film is obtained using a PS10k-PNaSS6k-PS10k triblock copolymer solution containing 3.0 wt% of water. This micellar solution is used to prepare a porous membrane for filtration applications. Pure water filtration data suggest a pore size in the range of ultrafiltration, making these membranes attractive for applications in the food industry, for bacteria, virus and protein removal.

We use space-resolved dynamic light scattering in the highly multiple scattering regime (Photon Correlation Imaging Diffusing Wave Spectroscopy, PCI-DWS) to investigate temperature-induced phase transitions in polymorphic materials. We study paraffin wax as a simple model system and chocolate, a prototypical example of fat-based products exhibiting complex, history-dependent phase transitions. We find that microscopic dynamics measured using PCI-DWS show remarkable, non-monotonic behavior upon heating: they transiently accelerate when crossing phase transition and slow down above the transition temperature. Sub-micron resolution measurements of the local drift of the sample surface reveal that the speed-up of the dynamics is due to the strain field induced by the change in density at transition temperature. The transition temperatures obtained from PCI-DWS are found to be in excellent agreement with those inferred from complementary differential scanning calorimetry and X-ray scattering experiments, thereby validating PCI-DWS as a new, powerful tool for the characterization of phase transitions in complex soft matter. Finally, we demonstrate the unique possibilities afforded by space-resolved DWS by investigating the spatially heterogeneous response of poorly manufactured or composite chocolate samples.

We explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time τα of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard spheres. Computer simulations confirm that the growth of τα on approaching the glass transition is independent of particle softness. By contrast, softness becomes relevant at very large packing fractions when the system falls out of equilibrium. In this nonequilibrium regime, τα depends surprisingly weakly on packing fraction, and time correlation functions exhibit a compressed exponential decay consistent with stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease in local order with increasing density typical of ultrasoft systems. We propose that these peculiar dynamics results from the combination of the nonequilibrium aging dynamics expected in the glassy state and the tendency of colloids interacting through soft potentials to refluidize at high packing fractions.

Power law rheology is of widespread occurrence in complex materials that are characterized by the presence of a very broad range of microstructural length and time scales. Although phenomenological models able to reproduce the observed rheological features exist, in general a well-established connection with the microscopic origin of this mechanical behavior is still missing. As a model system, this work focuses on a fractal colloidal gel. We thoroughly characterize the linear power law rheology of the sample and its age dependence. We show that at all sample ages and for a variety of rheological tests the gel linear viscoelasticity is very accurately described by a Fractional Maxwell (FM) model, characterized by a power law behavior. Thanks to a unique set-up that couples small-angle static and dynamic light scattering to rheological measurements, we demonstrate that the power law rheology observed in the linear regime originates from reversible non-affine rearrangements and discuss the possible relationship between the FM model and the microscopic structure of the gel.