Ref HAL: hal-01187419_v1
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Résumé:

We study the destabilization mechanism of thin liquid sheets expanding in air and show that dilute oil-in-water emulsion-based sheets disintegrate through the nucleation and growth of holes that perforate the sheet. The velocity and thickness fields of the sheet are not perturbed by holes and hole opening follows a Taylor-Culick law. We find that a pre-hole, which widens and thins out the sheet with time, systematically precedes the hole nucleation. The growth dynamics of the pre-hole follows the law theoretically predicted for a liquid spreading on another liquid of higher surface tension due to Marangoni stresses. Classical Marangoni spreading experiments quantitatively corroborate those findings.

Ref HAL: hal-01187396_v1
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Agricultural spraying involves atomizing a liquid stream through a hydraulic nozzle, thus forming a liquid sheet that is subsequently destabilized into drops. Standard adjuvants as dilute oil-in-water emulsions are known to influence the spray drop size distribution: the fraction of small drops responsible of spray drift is reduced. Although being documented, the physical mechanisms at the origin of the size increase of the drops remain unclear. 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 tear 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 and the size of the oil droplet of the emulsion are varied to rationalize their influence on the sheet destabilization mechanisms. The results obtained with the model laboratory experiments are compared to the measurement of the drop size distribution resulting from the destabilization of a conventional agricultural spray. The very good correlation between the number of perforation 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, which dominate for dilute emulsions, is at the origin of the increase of size of the spray. Finally the analysis of the of the kinetics of formation and growth of the holes allows to identify the physical origin of the perforation.

Ref HAL: hal-01148904_v1
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Liquid sheets are formed 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. 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 now holes perforate the sheet before the retraction, as already observed for some surfactant solutions [1]. The holes do not perturb significantly the velocity field of the liquid sheet; they growth with a constant velocity given by the Culicks’s law until they merge together and form a web of ligaments, which are then destabilized into droplets. We use a simple experimental optical method we have developed recently to get time and space resolved measurements of the thickness field of the liquid sheet [2]. We show that each perforation’s event (hole) is preceded by a hole’s precursor (thinning zone of the liquid sheet) whose thickness profile and growth’s velocity have been measured . Interestingly each rupture event (transition from holes’s precursor to true hole) of the sheet is clearly evidenced by a discontinuity of the growth’s velocity of the instability. These experiments are appropriate to gain an understanding on the physical mechanisms governing the perforation of thick films of emulsion.

Résumé:

Les réseaux transitoires auto-assemblés constituent une classe importante de fluides viscoélastiques aux propriétés rhéologiques linéaires particulièrement simples . Après les avoir décrits je tenterai de définir ce qu'est une fracture dans un fluide complexe et j'illustrerai différentes situations de fractures que nous avons explorées en utilisant ses systèmes expérimentaux modèles formulés au laboratoire: fractures induites par cisaillement, fractures en géométrie élongationnelle ( experiences de gouttes pendantes incluant l'étude de la propagation de fractures)), fractures en cellules Hele-Shaw (transition digitation/fracture), fractures dans des nappes liquides de fluides faiblement viso-élastiques . Je montrerai aussi comment nous avons tenté de généraliser la notion de transition fragile/ductile au cas de fluides visco-élastiques.

Ref HAL: hal-01104008_v1
Ref Arxiv: 1412.3930
DOI: 10.1017/jfm.2014.714
WoS: 000348128700022
Ref. & Cit.: NASA ADS
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32 Citations
Résumé:

The collision of a liquid drop against a small target results in the formation of a thin liquid sheet that extends radially until it reaches a maximum diameter. The subsequent retraction is due to the air-liquid surface tension. We have used a time- and space-resolved technique to measure the thickness field of this class of liquid sheet, based on the grey level measurement of the image of a dyed liquid sheet recorded using a fast camera. This method enables a precise measurement of the thickness in the range $(10-450) \, \mathrm{\mu m}$, with a temporal resolution equal to that of the camera. We have measured the evolution with time since impact, $t$, and radial position, $r$, of the thickness, $h(r,t)$, for various drop volumes and impact velocities. Two asymptotic regimes for the expansion of the sheet are evidenced. The scalings of the thickness with $t$ and $r$ measured in the two regimes are those that were predicted in \citet{Rozhkov2004} fort the short-time regime and \citet{Villermaux2011} for the long time regime, but never experimentally measured before. Interestingly, our experimental data also evidence the existence of a maximum of the film thickness $h_{\rm{max}}(r)$ at a radial position $r_{\rm{h_{max}}}(t)$ corresponding to the crossover of these two asymptotic regimes. The maximum moves with a constant velocity of the order of the drop impact velocity, as expected theoretically. Thanks to our visualization technique, we also evidence an azimuthal thickness modulation of the liquid sheets.

Commentaires: accepted for publication in Journal of Fluid Mechanics. Réf Journal: J. Fluid Mech. (2015), vol. 764, pp. 428-444

Ref HAL: hal-01101051_v1
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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 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 emulsion, the liquid sheet is destabilized by the nucleation of holes in the sheet that perforate it during its expansion. The physico-chemical parameters of the emulsion, such as the emulsion concentration and the emulsion drop size distribution, are varied to rationalize their 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 that experiments on liquid sheet are appropriate model experiments to gain an understanding of the physical mechanisms governing the spray drop size distribution.

Résumé:

The collision of a liquid drop against a small target results in the formation of a thin liquid sheet that extends radially until it reaches a maximum diameter. The subsequent retraction is due to the air-liquid surface tension. We have used a time- and space-resolved technique to measure the thickness field of this class of liquid sheet, based on the grey level measurement of the image of a dyed liquid sheet recorded using a fast camera. This method enables a precise measurement of the thickness in the range (10 − 450) μm, with a temporal resolution equals to that of the camera. We have measured the evolution with time since impact, t, and radial position, r, of the thickness, h(r, t), for various drop volumes and impact velocities. Two asymptotic regimes for the expansion of the sheet are evidenced. The scalings of the thickness with t and r measured in the two regimes are those that were predicted in Rozhkov et al. (2004) fort the short-time regime and Villermaux & Bossa (2011) for the long time regime, but never experimentally measured before. Interestingly, our experimental data also evidence the existence of a maximum of the film thickness hmax(r) at a radial position rhmax (t) corresponding to the crossover of these two asymptotic regimes. The maximum moves with a constant velocity of the order of the drop impact velocity, as expected theoretically. Thanks to our visualization technique, we also evidence an azimuthal thickness modulation of the liquid sheets.