Ref HAL: hal-01063623_v1
Exporter : BibTex | endNote
Résumé:

Agricultural spraying involves atomizing a liquid stream through a hydraulic nozzle forming a liquid sheet, which is subsequently destabilized into droplets. Standard solution adjuvants as dilute oil-in-water emulsions are known to influence the spray drop size distribution. To elucidate the mechanisms causing the changes on the drop size distribution, we investigate the influence of dilute emulsions on the destabilization mechanisms of liquid sheets. Model laboratory experiments based on the collision of a liquid drop on a small solid target are used to produce and visualize liquid sheets. With dilute oil-in-water emulsions, the liquid sheet is destabilized by the nucleation of holes in the sheet that perforate it during its expansion. The emulsion concentration is varied to rationalize its influence on the sheet destabilization mechanisms. The results obtained with the drop impact experiments are compared to the measurement of the spray drop size distribution. The very good correlation between the number of nucleation events and the volume fraction of small drops in the spray suggests (i) that the model experiment on liquid sheet is appropriate to investigate and gain an understanding of the physical mechanisms governing the spray drop size distribution and (ii) that the perforation destabilization mechanism of liquid sheets occurring for dilute emulsions is at the origin of the increase of size of the spray drops.

Ref HAL: hal-01063053_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. 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-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.

Ref HAL: hal-00984966_v1
Exporter : BibTex | endNote
Résumé:

Je présenterai une revue sur les phénomènes de fracture dans les réseaux transitoires, qui représentent une classe importante de fluides viscoélastiques. Après avoir défini ce qu'est une fracture dans un fluide complexe je rappellerai les principales propriétés structurales et rhéologiques des réseaux transitoires à base polymères associatifs. Je présenterai ensuite les observations expérimentales de fracture dans ces réseaux dans différentes configurations: fractures induites par cisaillement, fractures en cellule Hele-Shaw et fractures en géométrie extensionnelle. Je tenterai aussi de montrer comment on peut étendre les concepts de fragilité ou ductilité issus de la mécanique des solides aux fluides complexes. Enfin, les différentes approches théoriques développées pour La nucléation de fractures dans les réseaux transitoires seront présentées et confrontées aux expériences

Ref HAL: hal-00840494_v1
Ref Arxiv: 1306.5101
DOI: 10.1039/c3sm51320c
WoS: 000322230300014
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
13 Citations
Résumé:

Fracture processes are ubiquitous in soft materials, even in complex fluids, subjected to stresses. To investigate these processes in a simple geometry, we use a model self-assembled transient gel and study the instability patterns obtained in a radial Hele-Shaw cell when a low viscosity oil pushes the more viscous transient gel. Thanks to an analysis of the morphology of the patterns, we find a discontinuous transition between the standard Saffman-Taylor fingering instability and a fracturing instability as the oil injection rate increases. Our data suggest that the flow properties of the gel ahead of the finger tip controls the transition towards fracturing. By analyzing the displacement field of the gel in the vicinity of the fingers and cracks, we show that in the fingering regime, the oil gently pushes the gel, whereas in the fracturing regime, the crack tears apart the gel, resulting in a strong drop of the gel velocity ahead of the crack tip as compared to the tip velocity. We find a unique behavior for the whole displacement field of a gel around a crack, which is drastically different from that around a finger, and reveals the solid-like behavior of the gel at short time. Our experiments and analysis provide quantitative yet simple tools to unambiguously discriminate a finger from a crack in a visco-elastic material.