Ref HAL: hal-01063053_v1
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Résumé:

Pesticide spraying in agriculture involves atomizing a liquid stream through a hydraulic nozzle. The spray droplets results from the destabilization of a liquid sheet formed by the nozzle. Standard pesticide solution adjuvants as dilute solution of long polymer chains or dilute emulsions are known to influence the spray drop size distribution. Although being documented, these effects are not understood yet. In order to elucidate the physical mechanisms at the origin of the change on the drop size distribution, we investigate the influence of different complex fluids on the destabilization mechanisms of liquid sheets. We here form liquid sheets by the collision of a liquid drop on a small solid target. Upon impact, the drop flattens into a radial sheet expanding in the air bounded by a thicker rim. Different destabilization mechanisms of the sheet are observed depending on the fluid nature. A pure water sheet spreads out radially until it reaches a maximum diameter and then retracts due to the effect of surface tension. The destabilization mechanism is drastically modified when a dilute oil in water emulsion is used. The liquid sheet spreads out radially but holes perforate the sheet before the retraction, as already observed for some surfactant solutions [1]. The holes grow until they merge together and form a web of ligaments, which are then destabilized into droplets. To investigate the different sheet destabilization mechanisms, we use a fast camera imaging coupled to an original technique recently developed to access the time and space-resolved sheet thickness. Physico-chemical parameters of the dilute emulsion are modified to rationalize their influence on the perforation mechanism as for instance the influence of the emulsion concentration. We show in particular that the number of hole nucleation events per liquid sheet is directly governed by the emulsion concentration. In striking concordance, we find that the emulsion concentration directly controls the drop size distributions of a spray, as measured with a diffraction-based size analyzer, suggesting that experiments on liquid sheets are appropriate model experiments to gain an understanding on the physical mechanisms governing the spray drop size distribution. [1] A. Rozhkov, B. Prunet-Foch, M. Vignes-Adler, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 466, 2897-2916 (2010).

Ref HAL: hal-01058850_v1
Ref Arxiv: 1311.1996
DOI: 10.1103/PhysRevLett.113.078301
WoS: 000341115700027
Ref. & Cit.: NASA ADS
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25 Citations
Résumé:

We use confocal microscopy and time-resolved light scattering to investigate plasticity in a colloidal polycrystal, following the evolution of the network of grain boundaries as the sample is submitted to thousands of shear deformation cycles. The grain boundary motion is found to be ballistic, with a velocity distribution function exhibiting non-trivial power law tails. The shear- induced dynamics initially slow down, similarly to the aging of the spontaneous dynamics in glassy materials, but eventually reach a steady state. Surprisingly, the cross-over time between the ini- tial aging regime and the steady state decreases with increasing probed length scale, hinting at a hierarchical organization of the grain boundary dynamics.

Commentaires: main paper + supplementary materials Journal: Phys. Rev. Lett., 113, 078301 (2014)

Résumé:

Wheat storage gluten proteins are among the most complex proteins families comprising at least fifty different proteins, with extremely broad polymorphisms. Wheat gluten proteins are moreover largely insoluble in water, rendering their study difficult. Gluten proteins are responsible for the remarkable viscoelastic properties of dough. However despite extensive studies over more than 200 years, in order to provide structural and mechanistic basis for the improvement of the viscoelastic properties of dough and of the quality of resulting food products, there is still a crucial need to understand supramolecular organization of gluten proteins. We have proposed a novel extraction protocol that allows for the first careful structural and rheological analysis of gluten protein suspensions over a wide range of protein concentrations. Our system appears therefore as a unique model system to investigate the supramolecular organization of gluten proteins and its impact on the viscoelastic properties of gluten gels. Rheological measurements show a spontaneous and very slow and concentration-dependent gelation of the samples which can be rationalized using percolation models. Consistently, scattering data highlight a hierarchical structure strikingly similar to that of polymeric gels, thus providing some factual knowledge to rationalize the viscoelastic properties of wheat gluten and their assemblies.

Résumé:

Wheat storage gluten proteins are among the most complex proteins families comprising at least fifty different proteins, with extremely broad polymorphisms. Wheat gluten proteins are moreover largely insoluble in water, rendering their study difficult. Gluten proteins are responsible for the remarkable viscoelastic properties of dough. However despite extensive studies over more than 200 years, in order to provide structural and mechanistic basis for the improvement of the viscoelastic properties of dough and of the quality of resulting food products, there is still a crucial need to understand supramolecular organization of gluten proteins. We have proposed a novel extraction protocol that allows for the first careful structural and rheological analysis of gluten protein suspensions over a wide range of protein concentrations. Our system appears therefore as a unique model system to investigate the supramolecular organization of gluten proteins and its impact on the viscoelastic properties of gluten gels. Rheological measurements show a spontaneous and very slow and concentration-dependent gelation of the samples which can be rationalized using percolation models. Consistently, scattering data highlight a hierarchical structure strikingly similar to that of polymeric gels, thus providing some factual knowledge to rationalize the viscoelastic properties of wheat gluten and their assemblies.

Ref HAL: hal-01009203_v1
Exporter : BibTex | endNote
Résumé:

Pesticide spraying in agriculture involves atomizing a liquid stream through a hydraulic nozzle. The spray droplets results from the destabilization of a liquid sheet formed by the nozzle. Standard pesticide solution adjuvants as dilute solution of long polymer chains or dilute emulsions are known to influence the spray drop size distribution. Although being documented, these effects are not understood yet. In order to elucidate the physical mechanisms at the origin of the change on the drop size distribution, we investigate the influence of different complex fluids on the destabilization mechanisms of liquid sheets. We here form liquid sheets by the collision of a liquid drop on a small solid target. Upon impact, the drop flattens into a radial sheet expanding in the air bounded by a thicker rim.