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DOI: 10.1122/1.4997587
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5 Citations
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We study the fracture of reversible double transient networks, constituted of water suspensions of entangled surfactant wormlike micelles reversibly linked by various amounts of telechelic polymers. We provide a state diagram that delineates the regime of fracture without necking of the filament from the regime where no fracture or break-up has been observed. We show that filaments fracture when stretched at a rate larger than the inverse of the slowest relaxation time of the networks. We quantitatively demonstrate that dissipation processes are not relevant in our experimental conditions and that, depending on the density of nodes in the networks, fracture occurs in the linear viscoelastic regime or in a non-linear regime. In addition, analysis of the crack opening profiles indicates deviations from a parabolic shape close to the crack tip for weakly connected networks. We demonstrate a direct correlation between the amplitude of the deviation from the parabolic shape and the amount of non linear viscoelasticity.

Commentaires: . Réf Journal: Journal of Rheology 61(6), 1267-1275, 2017

Résumé:

In a first part I will focus on the destabilization of dilute oil-in-water emulsion-based liquid sheets expanding in air. A sheet results from the collision of a single tear on a small solid target ; it disintegrates through the nucleation and growth of holes that perforate the sheet. We have developed an optical technique that allows the determination of the time and space-resolved thickness of the sheet to gain a, understanding of the physical mechanisms f the perforation events This bursting based-liquid sheet destabilization is at the origin of emulsion-based anti-drift formulations are developed for agricultural spray. In a second part, I will investigate freely expanding sheets formed by ultra soft spherical gel beads of elastic but also liquid droplets with various surface tensions, and simple viscoelastic fluids (Maxwell fluids), produced by impacting them on a silicon wafer covered with a thin layer of liquid nitrogen that suppresses viscous dissipation by an inverse Leidenfrost effect. The experiments reveal a universal behaviour of the impact dynamics with impact velocity, for both solids and liquids, and even viscoelastic fluids, that we have rationalized .

Ref HAL: hal-01513239_v1
DOI: 10.1021/acs.langmuir.7b00170
WoS: 000399263600013
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6 Citations
Résumé:

A single-drop experiment based on the collision of one drop of liquid on a small solid target is used to produce liquid sheets that are visualized with a fast camera. Upon impact, the drop flattens into a sheet that is bounded by a thicker rim and radially expanding in air. Emulsion-based liquid sheets are destabilized through the nucleation of holes that perforate the sheet during its expansion. The holes grow until they merge together and form a web of ligaments, which are then destabilized into drops. We propose the perforation mechanism as a sequence of two necessary steps. The emulsion oil droplets first enter the air/water interface, and then spread at the interface. We show that the formulation of the emulsion is a critical parameter to control the perforation as the addition of salt or amphiphilic copolymers can trigger or completely inhibit the perforation mechanism. We demonstrate that the entering of the droplets at the air/water interface is the limiting step of the mechanism. Thin-film forces such as electrostatic or steric repulsion forces stabilize the thin film formed between the interface and the approaching oil droplets, thus preventing the entering of droplets at the interface and in turn inhibiting the perforation process. We theoretically rationalize the successive steps in the approach and entering of an oil droplet at the film interface and the role of salt and amphiphilic polymer in the different steps.