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Wheat flours are essential ingredients of daily food products like bread, cookies or pastries. Their quality depends on the milling process and mechanical strength of wheat grains. Although it is well known that the strength and rupture of grains are strongly controlled by the endosperm microstructure, the respective roles of the starch and polymer volume fractions and their adhesion are not yet fully understood. This typical biological microstructure can be modeled as a cemented granular material, where the two size populations of starch granules (large:A-type, small:B-type) are the particles, and the protein matrix, which partially fills the space between granules, plays the role of a cement. This structural model of wheat endosperm is used, together with mechanical characteristics of starch and proteins obtained by means of Atomic Force Microscopy (AFM) measurements, to simulate the mechanical behavior and breakage of wheat endosperm in milling process. We find that the porosity outweighs the effect of other parameters for the elastic modulus, which declines as a nearly linear function of porosity. We also show that the tensile strength is an increasing function of the amount and connectivity of starch granules with increasing concentration of stresses along chains of granules. This effect is more significant at low porosity where stress distribution is mainly controlled by the contact network between starch granules. This effect explains why the protein content is not fully correlated to vitreousness, and samples of similar protein content can be different in vitreosity. Finally, we find that the starch-granule adhesion strongly affects the tensile strength whereas the effect of starch volume fraction appears mainly at high interface adhesion, which is the case of hard type wheat grains.

The structure of polymer nanocomposites has important consequences on final properties, like e.g.mechanical reinforcement. The structure of the hard filler phase is usually characterized by electronmicroscopy and small-angle X-ray scattering (SAXS), and we will review recent examples of fillerstructure in model and industrial nanocomposites. The chain conformation can only bemeasured by small-angle neutron scattering (SANS) with isotopic substitution. Over the past 15 years,considerable efforts have been dedicated to such measurements. Results appear to converge towardGaussian statistics of chains, unperturbed by the presence of filler NPs, and exceptions seem to be dueto system-dependent effects. In nearly all studies, however, unexplained pollution of the scatteringdata in the low-q range has been observed, in spite of careful contrast-matching.In this study, the conformation of polymer chains in silica-latex nanocomposites has been studiedunder zero-average contrast (ZAC) conditions using SANS. Samples have been prepared by dryingcolloidal suspensions of silica and polymer nanoparticles (NPs) for two different silica NPs (radius of5 and 15 nm) and two chain masses (17 and 100 kg/mol). By mixing appropriate amounts ofhydrogenated and deuterated polymer, chain scattering contrast is introduced, and in principle silicascattering is suppressed. However, the SANS scattering intensities displayed distinct deviations at lowangles. Comparison of the SANS and SAXS intensities proves that the measurement of chain structureby SANS is perturbed by unwanted silica contributions, i.e., the silica is ‘mysteriously’ visible. Aquantitative model rationalizing these contributions for the first time is proposed. It is based on astatistical H/D-mismatch in the local environment of the silica NPs. Such a mismatch leads to theexistence of a nanometer-thick polymer shell surrounding silica NPs, which carries H/D contrast andthereby produce an undesired scattering contribution. These shells are presumably maintained by thereduced mobility of polymer close to interfaces or attractive polymer-silica interactions. The effect ofunwanted filler scattering is NP size-dependent − it is pronounced only for smaller beads. This isprobably why ZAC turned out to be so difficult with polymer nanocomposites.

Janus colloidal particles show remarkable properties in terms of surface activity, self-assembly and wetting. Moreover they can perform autonomous motion if they can chemically react with the liquid in which they are immersed. In order to understand the self-propelled motion of catalytic Janus colloids at the air-water interface, wetting and the orientation of the catalytic surface are important properties to be investigated. Wetting plays a central role in active motion since it determines the contact between fuel and catalytic surface as well as the efficiency of transduction of chemical reaction into motion. Active motion is not expected to occur either when the catalytic face is completely out of the aqueous phase or when the Janus boundaries are parallel to the interfacial plane. The design of a Janus colloid possessing two hydrophilic faces is required to allow the catalytic face to react with the fuel (e.g. H2O2 for Platinum) in water and to permit some rotational freedom of the Janus colloid in order to generate propulsion parallel to the interfacial plane.Here, we discuss some theoretical aspects that should be accounted when studying Janus colloids at the surface of water. The free energy of ideal Janus colloidal particles at the interface is modeled as a function of the immersion depth and the particle orientation. Analytical expressions of the energy profiles are established. Energetic aspects are then discussed in relation to the particle ability to rotate at the interface. By introducing contact angle hysteresis we describe how the effects of contact line pinning modifies the scenario described in the ideal case. Experimental observations of the contact angle hysteresis of Janus colloids at the interface reveal the effect of pinning; and orientations of silica particles half covered with a platinum layer at the interface do not comply with the ideal scenarios. Experimental observations suggest that Janus colloids at the fluid interface behave as kinetically driven system, where the contact line motion over defects decorating the Janus faces rules the orientation and rotational diffusion of the particle.

The interface between two fluids is never flat at the nanoscale, and this is important for transport across interfaces. In absence of any external field, the surface roughness is due to thermally excited capillary waves possessing subnanometric amplitudes in the case of simple liquids. Here, we investigate the effect of ultrasound on the surface roughness of liquid-gas and liquid-liquid interfaces. MHz frequency ultrasound was applied normal to the interface at relatively low ultrasonic pressures (< 0.6 MPa), and the amplitudes of surface fluctuations have been measured by light reflectivity and ellipsometry. We found a dramatic enhancement of surface roughness, roughly linear with intensity, with vertical displacements of the interface as high as 50-100 nm. As a consequence, the effective contact area between two fluids can be increased by ultrasound. This result has a clear impact for enhancing interface based processes such as mass or heat transfer.

Dynamics of anisotropic particles at a fluid interface governs trapping and particles subsequentassembly. Measuring and controlling this viscous dynamics open the way to new materials [1], andapplications [2]. Despite these domains of applications, measurements of the viscous coefficient ofanisotropic particles were mainly restricted to the bulk [3-4] or in quasi-2D confinement between twoglass plates [5]. Here we measure the complete set of viscous coefficients of single micrometricprolate ellipsoids near and trapped at an air-water interface. These coefficients are obtained from theBrownian motion of the particle coupling interferometry and particle tracking. For particles trapped atthe interface we measured anomalously large drags even larger than in the bulk water (ratio of surface/bulk rotational drags rR > 1 in Figure) [6]. Such an intriguing behaviour is discussed in terms ofadditional random forces due to thermally activated fluctuations of the interface and their coupling tothe particle drag through the fluctuation–dissipation theorem.Figure: Rotational viscous-drag ratio rR (surface/bulk) of polystyrene ellipsoidal particles at anair/water interface versus particle’s aspect ratio φ. Inset: mean square angular displacements (MSADs)versus the time lag τ.[1] M. Cavallaro, L. Botto, E. Lewandowski, M. Wang, and K. Stebe, PNAS 108, 20923 (2011).[2] Z. Huang, D. Legendre, and P. Guiraud, Chem. Eng. Sci. 66, 982 (2011).[3] D. Mukhija and M. Solomon, J. Colloid Interface Sci. 314, 98 (2007).[4] T. H. Besseling, M. Hermes, A. Kuijk et al., Journal of Physics: Condensed Matter 27, 194109 (2015).[5] Y. Han, A. Alsayed, M. Nobili, J. Zhang, T. Lubensky, and A. Yodh, Science 314, 626 (2006).[6] G. Boniello, C. Blanc, D. Fedorenko, M. Medfai, N. Ben Mbarek, M. In, M. Gross, A. Stocco, M. Nobili,Nature Materials 14, 908 (2015).

We study the destabilization mechanism of thin liquid sheets expanding in air and show that dilute oilin-water emulsion-based sheets disintegrate through the nucleation and growth of holes that perforatethe sheet [1]. The velocity and thickness fields of the sheet outside the holes are not perturbed by holesand hole opening follows a Taylor-Culick law. We find that a pre-hole, which widens and thins out thesheet with time, systematically precedes the hole nucleation. The growth dynamics of the pre-holefollows the law theoretically predicted for a liquid spreading on another liquid of higher surfacetension due to Marangoni stresses. Classical Marangoni spreading experiments quantitativelycorroborate our findings.