Jérôme Joseph Crassous

Differential dynamic microscopy is used to characterize the dynamics of casein micelle dispersions from the dilute to the concentrated regime and to quantitatively probe the aggregation kinetics of para -casein micelles following a temperature jump.

Core-shell microgels with rigid cores and soft, deformable hydrogel shells can assemble at air-water interfaces, forming freely floating monolayers. The strong adsorption at such interfaces is related to the reduction in interfacial tension, which also causes the microgels to deform laterally. The degree of this deformation is typically controlled through applied surface pressure. Until now, surprisingly little has been known about the impact of interfacial tension imbalances between interfacial areas covered with a microgel monolayer and microgel-free areas in the surroundings. In this work, we systematically study the monolayer evolution at air/water interfaces in dependence of interfacial tension controlled by the addition of sodium dodecyl sulfate or linear poly-N-isopropylacrylamide homopolymer to the free area. We do this by globally monitoring the evolution of the area of freely floating monolayers. Macroscopic changes are also related to the local microstructure studied by atomic force microscopy (AFM). Depending on the interfacial tension imbalance, the monolayer either expands, shrinks, or maintains its conformation. The kinetics of monolayer expansion is compared for core-shell microgels with the same silica core and varying cross-linker densities. Our study reveals the impact of interfacial tension on the behavior of microgel monolayers at liquid interfaces and also provides useful insights into controlling the two-dimensional (2D) microstructure without the need for a Langmuir trough.

Herein, we present a novel liquid crystal (LC)-based sensing platform utilizing microgel-stabilized Pickering LC droplets dispersed in water for simple and label-free detection of proteins in an aqueous environment. This could be achieved by tailoring the surface of 4-cyano-4'-pentylbiphenyl (5CB) LC droplets dispersed in aqueous medium through the interfacial adsorption of poly(N-isopropylacrylamide) (PNIPAM) microgel particles, followed by the introduction of model surfactants, such as anionic sodium dodecyl sulfate and cationic dodecyltrimethylammonium bromide. These surfactant/microgel complex-coated LC droplets underwent a configurational transition from radial-to-bipolar under a polarized optical microscope, upon exposure to model proteins, namely bovine serum albumin and lysozyme. This transition stemmed from the interfacial adsorption of proteins, which was facilitated by their strong interaction with the preadsorbed microgel particles and surfactant molecules. The adsorption of proteins led to the disruption of the interfacial packing density of surfactant molecules, inducing a switch from homeotropic-to-planar surface anchoring of LCs within the droplets. In addition to providing remarkable Pickering stability to the LC droplets, the microgel coating significantly enhanced the sensitivity of the resulting emulsions to proteins. The dose-response behavior and detection limit of these modified LC droplets were strongly influenced by the microgel concentration, surfactant charge, pH of the medium, and the types of proteins. Notably, the droplets exhibited heightened responsiveness under conditions that favor attractive interactions between the proteins and interfacial surfactant molecules. Thus, this study opens avenues for engineering Pickering LC-based biosensors to discern biomolecular interactions, thereby facilitating various interfacial and sensing applications.

The adsorption of ellipsoidal colloidal particles on liquid interfaces induces interfacial deformation, resulting in anisotropic interface-mediated interactions and the formation of superstructures. Soft prolate-shaped microgels at the air-water interface offer an ideal model for studying spontaneous capillary-driven self-assembly due to their tunable aspect ratio, controlled functionality, and softness. These microgels consist of a polystyrene core surrounded by a cross-linked, fluorescently labeled poly( N -isopropylmethylacrylamide) shell. By uniaxially stretching the particles embedded in polyvinyl alcohol films, the aspect ratio ρ can be finely adjusted. ρ was found to vary from 1 to 8.8 as estimated in their swollen conformation at 20 ^° C from confocal laser scanning microscopy. The spontaneous interfacial self-assembly at the air–water interface is investigated through fluorescence microscopy, theoretical calculations, and computer simulations. A structural transition occurs from a seemingly random assembly for small aspect ratios to compact clusters, which transform into a side-to-side assembly forming long chains for high aspect ratios. The influence of the poly( N -isopropylmethacrylamide) shell on the assembly indicates a significant ρ -dependent microgel deformation. This deformation, in turn, determines the average distance between the particles. Consequently, capillary-driven self-assembly of soft anisotropic colloids becomes a powerful mechanism for structuring interfaces and designing microstructured materials.

Depending on their aspect ratio, rod‐shaped particles exhibit a much richer 2D and 3D phase behavior than their spherical counterparts, with additional nematic and smectic phases accompanied by defined orientational ordering. While the phase diagram of colloidal hard rods is extensively explored, little is known about the influence of softness in such systems, partly due to the absence of appropriate model systems. Additionally, investigating higher volume fractions for long rods is usually complicated because non‐equilibrium dynamical arrest is likely to precede the formation of more defined states. This has motivated us to develop micrometric rod‐like microgels with limited sedimentation that can respond to temperature and reversibly reorganize into defined phases via annealing and seeding procedures. A detailed procedure is presented for synthesizing rod‐shaped hollow poly( N ‐isopropylacrylamide) microgels using micrometric silica rods as sacrificial templates. Their morphological characterization is conducted through a combination of microscopy and light scattering techniques, evidencing the unconstrained swelling of rod‐shaped hollow microgels compared to core‐shell microgel rods. Different aspects of their assembly in dispersion and at interfaces are further tested to illustrate the opportunities and challenges offered by such systems that combine softness, anisotropy, and thermoresponsivity.

Well-defined fluorescent microgel capsules responsive to pH, ionic strength, temperature and osmotic stress were synthesized using micrometer-sized silica particles as sacrificial templates and characterized via scattering and microscopy techniques.