Le fractionnement du grain de blé, qui permet la séparation de l’albumen amylacé (au cœur) de ses parties périphériques pour produire de la farine, dépend de ses propriétés mécaniques. On distingue, chez le blé tendre, des blés dits « soft », dont l’albumen est friable, et « hard » plus résistant. Ils différent par la présence de gènes codant deux protéines spécifiques, les Puroindolines (PINs) A et B, qui sont de forme sauvage (cas des soft) ou mutés (cas des hard). Ces protéines sont impliquées dans les interactions entre les constituants majeurs du grain (amidon et protéines de réserve qui entourent les granules, capables de former un réseau aux propriétés uniques appelé gluten). Nous avons pu disposer de lignées quasi-isogéniques pour la dureté qui ne différent que par la présence de ces gènes sauvages ou mutées, alors que le fond génétique est identique, et montré que ces seuls gènes affectent les caractéristiques physiques et le comportement au broyage des grains [1,2]. Nous avons pu caractériser in situ les propriétés mécaniques respectives de l’amidon et du gluten et montré leurs différences, contrairement à ce qui est établi dans la littérature, grâce au développement d’une méthode originale où la pointe d’un Microscope à Force Atomique (AFM) est utilisée pour abraser la surface de l’échantillon à analyser [3]. Nous avons également mis en évidence des différences d’interfaces entre les polymères en fonction de la dureté du grain [4]. Enfin, nous avons utilisé la méthode de caractérisation mécanique par AFM par résonance de contact (CR-AFM) pour mesurer le module d’indentation (élastique) au sein de granules d’amidon (échelle de la centaine de nanomètres), moyennant une correction originale prenant en compte la topographie locale [5]. Cette méthode a été utilisée sur des sections de grains (céréales) et graines (légumineuses) de différentes origines botaniques pour comparer les propriétés de leurs amidons.

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.

HypothesisAlthough the dynamics of colloids in the vicinity of a solid interface has been widely characterized in the past, experimental studies of Brownian diffusion close to an air–water interface are rare and limited to particle-interface gap distances larger than the particle size. At the still unexplored lower distances, the dynamics is expected to be extremely sensitive to boundary conditions at the air–water interface. There, ad hoc experiments would provide a quantitative validation of predictions.ExperimentsUsing a specially designed dual wave interferometric setup, the 3D dynamics of 9 μm diameter particles at a few hundreds of nanometers from an air–water interface is here measured in thermal equilibrium.FindingsIntriguingly, while the measured dynamics parallel to the interface approaches expected predictions for slip boundary conditions, the Brownian motion normal to the interface is very close to the predictions for no-slip boundary conditions. These puzzling results are rationalized considering current models of incompressible interfacial flow and deepened developing an ad hoc model which considers the contribution of tiny concentrations of surface active particles at the interface. We argue that such condition governs the particle dynamics in a large spectrum of systems ranging from biofilm formation to flotation process.

In this article, we present the mobilities of prolate ellipsoidal micrometric particles close to an air–water interface measured by dual wave reflection interference microscopy. Particle's position and orientation with respect to the interface are simultaneously measured as a function of time. From the measured mean square displacement, five particle mobilities (3 translational and 2 rotational) and two translational–rotational cross-correlations are extracted. The fluid dynamics governing equations are solved by the finite element method to numerically evaluate the same mobilities, imposing either slip and no-slip boundary conditions to the flow at the air–water interface. The comparison between experiments and simulations reveals an agreement with no-slip boundary conditions prediction for the translation normal to the interface and the out-of-plane rotation, and with slip ones for parallel translations and in-plane rotation. We rationalize these evidences in the framework of surface incompressibility at the interface.