Using colloids effectively confined in two dimensions by a cell with a thickness comparable to the particle size, we investigate the nucleation and growth of crystallites induced by locally heating the solvent with a near-infrared laser beam. The particles, which are “thermophilic,” move towards the laser spot solely because of thermophoresis with no convection effects, forming dense clusters whose structure is monitored using two order parameters that gauge the local density and the orientational ordering. We find that ordering takes place when the cluster reaches an average surface density that is still below the upper equilibrium limit for the fluid phase of hard disks, meaning that we do not detect any sign of a proper “two-stage” nucleation from a glass or a polymorphic crystal structure. The crystal obtained at late growth stage displays a remarkable uniformity with a negligible amount of defects, arguably because the incoming particles diffuse, bounce, and displace other particles before settling at the crystal interface. This “fluidization” of the outer crystal edge may resemble the surface enhanced mobility giving rise to ultra-stable glasses by physical vapor deposition.

We investigate the rheo-mechanical properties of Mebiol Gel^®, a thermosensitive Gel-Forming Polymer extensively used as a medium for cellular culture, using passive microrheology made either by standard Dynamic Light Scattering(DLS) or by Photon Correlation Imaging (PCI). In the dilute limit, Mebiol displays a Newtonian behaviour with an effective viscosity that decreases with temperature, consistent with a peculiar aggregation mechanism characterized by anincrease of the molecular weight with a simultaneous reduction of the aggregate size. By increasing concentration and approaching gelation, both the storage and loss moduli shows a nonmonotonic dependence with temperature, with a pronounced maximum around Tm ≃28 −30{degree sign}C, the value above which, in the dilute limit, the individual Mebiol chains are fully compacted. Such a distinctive trend of the elastic and viscous properties persists within the gel, which therefore becomes "softer" above Tm. Although when temperature changes are performed adiabatically the transition from the fluid to the gel phase takes place without any apparent discontinuity, a rapid T -jump leads to the formation of a hard gel at a concentration where a low heating rate conversely yields a fluid phase. This is a visible manifestation of the nonequilibrium nature of these physical gels.

The intrinsic heterogeneity of bacterial niches should be retained in in vitro cultures to represent the complex microbial ecology. As a case study, mucin‐containing hydrogels ‐CF‐Mu³Gel ‐ are generated by diffusion‐induced gelation, bioinspired on cystic fibrosis (CF) mucus, and a microbial niche challenging current therapeutic strategies. At breathing frequency, CF‐Mu³Gel exhibits a G′ and G″ equal to 24 and 3.2 Pa, respectively. Notably, CF‐Mu³Gel exhibits structural gradients with a gradual reduction of oxygen tension across its thickness (280–194 µmol L⁻¹). Over the culture period, a steep decline in oxygen concentration occurs just a few millimeters below the air–mucus interface in CF‐Mu³Gel, similar to those of CF airway mucus. Importantly, the distinctive features of CF‐Mu³Gel significantly influence bacterial organization and antimicrobial tolerance in mono‐ and co‐cultures of Staphylococcus aureus and Pseudomonas aeruginosa that standard cultures are unable to emulate. The antimicrobial susceptibility determined in CF‐Mu³Gel corroborates the mismatch on the efficacy of antimicrobial treatment between planktonically cultured bacteria and those in patients. With this example‐based research, new light is shed on the understanding of how the substrate influences microbial behavior, paving the way for improved fundamental microbiology studies and more effective drug testing and development.

The weak absorption of a laser beam generates in a fluid an inhomogeneous refractive index profile acting as a negative lens. This self-effect on beam propagation, known as Thermal Lensing (TL), is extensively exploited in sensitive spectroscopic techniques, and in several all-optical methods for the assessment of thermo-optical properties of simple and complex fluids. Using the Lorentz–Lorenz equation, we show that the TL signal is directly proportional to the sample thermal expansivity $\alpha$, a feature allowing minute density changes to be detected with high sensitivity in a tiny sample volume, using a simple optical scheme. We took advantage of this key result to investigate the compaction of PniPAM microgels occurring around their volume phase transition temperature, and the temperature-driven formation of poloxamer micelles. For both these different kinds of structural transitions, we observed a significant peak in the solute contribution to $\alpha$, indicating a decrease in the overall solution density—rather counterintuitive evidence that can nevertheless be attributed to the dehydration of the polymer chains. Finally, we compare the novel method we propose with other techniques currently used to obtain specific volume changes.

Self-assembling peptides (SAPs) have been increasingly studied as hydrogel–former gelators because they can create biocompatible environments. A common strategy to trigger gelation, is to use a pH variation, but most methods result in a change in pH that is too rapid, leading to gels with hardly reproducible properties. Here, we use the urea–urease reaction to tune gel properties, by a slow and uniform pH increase. We were able to produce very homogeneous and transparent gels at several SAP concentrations, ranging from $c=1\mathrm{g}/\mathrm{L}$ to $c=10\mathrm{g}/\mathrm{L}$. In addition, by exploiting such a pH control strategy, and combining photon correlation imaging with dynamic light scattering measurements, we managed to unravel the mechanism by which gelation occurs in solutions of ${\left(\mathrm{LDLK}\right)}_3$-based SAPs. We found that, in diluted and concentrated solutions, gelation follows different pathways. This leads to gels with different microscopic dynamics and capability of trapping nanoparticles. At high concentrations, a strong gel is formed, made of relatively thick and rigid branches that firmly entrap nanoparticles. By contrast, the gel formed in dilute conditions is weaker, characterized by entanglements and crosslinks of very thin and flexible filaments. The gel is still able to entrap nanoparticles, but their motion is not completely arrested. These different gel morphologies can potentially be exploited for controlled multiple drug release.