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Résumé:

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.