The destabilization of an initially thick liquid sheet edge

International audienceBy forcing the sudden dewetting of a free soap film attached on one edge to a straight solid wire, we study the recession and subsequent destabilization of its free edge. The newly formed rim bordering the sheet is initially thicker than the film to which it is attached, because of the Plateau border preexisting on the wire. The initial condition is thus that of an immobile massive toroidal rim connected to a thin liquid film of thickness h. The terminal Taylor-Culick receding velocity V = sqrt(2 sigma/rho h), where sigma and rho are the liquid surface tension and density, respectively, is only reached after a transient acceleration period which promotes the rim destabilization. The selected wavelength and associated growth time coincide with those of an inertial instability driven by surface tension

Deformation upon impact of a concentrated suspension drop

We study the impact between a plate and a drop of non-colloidal solid particles suspended in a Newtonian liquid, with a specific attention to the case when the particle volume fraction, $\phi$, is close to - or even exceeds - the critical volume fraction, $\phi_c$, at which the steady effective viscosity of the suspension diverges. We use a specific concentration protocol together with an accurate determination of $\phi$ for each drop and we measure the deformation $\beta$ for different liquid viscosities, impact velocities and particle sizes. At low volume fractions, $\beta$ is found to follow closely an effective Newtonian behavior, which we determine by documenting the low deformation limit for a highly viscous Newtonian drop and characterizing the effective shear viscosity of our suspensions. By contrast, whereas the effective Newtonian approach predicts that $\beta$ vanishes at $\phi_c$, a finite deformation is observed for $\phi>\phi_c$. This finite deformation remains controlled by the suspending liquid viscosity and increases with increasing particle size, which suggests that the dilatancy of the particle phase is a key factor of the dissipation process close to and above $\phi_c$.Comment: Submitted to JF

Pinch-off of a viscous suspension thread

International audienceThe pinch-off of a capillary thread is studied at large Ohnesorge number for non-Brownian, neutrally buoyant, mono-disperse, rigid, spherical particles suspended in a Newtonian liquid with viscosity η 0 and surface tension σ. Reproducible pinch-off dynamics is obtained by letting a drop coalesce with a bath. The bridge shape and time evolution of the neck diameter, h min , are studied for varied particle size d, volume fraction φ and liquid contact angle θ. Two successive regimes are identified: (i) a first effective-viscous-fluid regime which only depends upon φ and (ii) a subsequent discrete regime, depending both on d and φ, in which the thinning localises at the neck and accelerates continuously. In the first regime, the suspension behaves as an effective viscous fluid and the dynamics is solely characterised by the effective viscosity of the suspension, η e ∼ −σ / ˙ h min , which agrees closely with the steady shear viscosity measured in a conventional rheometer and diverges as (φ c − φ) −2 at the same critical particle volume fraction, φ c. For φ 35 %, the thinning rate is found to increase by a factor of order one when the flow becomes purely extensional, suggesting non-Newtonian effects. The discrete regime is observed from a transition neck diameter, h min ≡ h * ∼ d (φ c − φ) −1/3 , down to h min ≈ d, where the thinning rate recovers the value obtained for the pure interstitial fluid, σ /η 0 , and lasts t * ∼ η e h * /σ

Drop deformation by laser-pulse impact

A free-falling absorbing liquid drop hit by a nanosecond laser-pulse experiences a strong recoil-pressure kick. As a consequence, the drop propels forward and deforms into a thin sheet which eventually fragments. We study how the drop deformation depends on the pulse shape and drop properties. We first derive the velocity field inside the drop on the timescale of the pressure pulse, when the drop is still spherical. This yields the kinetic-energy partition inside the drop, which precisely measures the deformation rate with respect to the propulsion rate, before surface tension comes into play. On the timescale where surface tension is important the drop has evolved into a thin sheet. Its expansion dynamics is described with a slender-slope model, which uses the impulsive energy-partition as an initial condition. Completed with boundary integral simulations, this two-stage model explains the entire drop dynamics and its dependance on the pulse shape: for a given propulsion, a tightly focused pulse results in a thin curved sheet which maximizes the lateral expansion, while a uniform illumination yields a smaller expansion but a flat symmetric sheet, in good agreement with experimental observations.Comment: submitted to J. Fluid Mec

Viscoelastic liquid curtains

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Drop Shaping by Laser-Pulse Impact

We show how the deposition of laser energy induces propulsion and strong deformation of an absorbing liquid body. Combining high speed with stroboscopic imaging, we observe that a millimeter-sized dyed water drop hit by a millijoule nanosecond laser pulse propels forward at several meters per second and deforms until it eventually fragments. The drop motion results from the recoil momentum imparted at the drop surface by water vaporization. We measure the propulsion speed and the time-deformation law of the drop, complemented by boundary-integral simulations. The drop propulsion and shaping are explained in terms of the laser-pulse energy, the drop size, and the liquid properties. These findings are, for instance, crucial for the generation of extreme ultraviolet light in nanolithography machines.Comment: Submitted as research article to Physical Review Applied, 6 pages with 6 figure

Drop fragmentation by laser-pulse impact

We study the fragmentation of a liquid drop that is hit by a laser pulse. The drop expands into a thin sheet that breaks by the radial expulsion of ligaments from its rim and the nucleation and growth of holes on the sheet. By combining experimental data from two liquid systems with vastly different time- and length scales we show how the early-time laser-matter interaction affects the late-time fragmentation. We identify two Rayleigh--Taylor instabilities of different origins as the prime cause of the fragmentation and derive scaling laws for the characteristic breakup time and wavenumber. The final web of ligaments results from a subtle interplay between these instabilities and deterministic modulations of the local sheet thickness, which originate from the drop deformation dynamics and spatial variations in the laser-beam profile.Comment: about to be submitted to JF