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Journal Articles Physical Review Fluids Year : 2020

Dynamics and flow characterization of liquid fountains produced by light scattering

Hugo Chesneau
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Julien Petit
Hamza Chraibi
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Abstract

5 We present a detailed study of the dynamics of soft liquid interfaces undergoing viscous stresses due 6 to bulk flows generated by momentum transfer from light to a turbid liquid. Using a continuous laser 7 wave, light-induced flow is observed and analyzed through the deformability of very soft interfaces 8 up to instability and jet formation. These dynamics are investigated experimentally and numerically 9 below and above the interface instability threshold. Below instability, we show that the dynamics 10 of the interface deformation at short time scale does not vary with the parameters of the laser 11 excitation. We confirm that the mechanism responsible for the interface deformation is a non-local 12 effect associated to the viscous stress induced by the bulk flow. Then, we characterize the jetting 13 instability regarding the field velocity within the jet, the jet radius and the fluid flow rate. A 14 satisfying agreement is obtained when comparing quantitatively experimental results and numerical 15 predictions. Our investigation illustrates how light can induce a bulk flow in a turbid liquid, such 16 as a suspension, and how this flow can be used to deform an interface and produce well-controlled 17 liquid jets. 18 Light is able to set fluids in movement transiently or permanently by transfer of energy or momentum. The most 19 famous example is thermal Marangoni flow (called optocapillarity when induced by light) due to heat transfer to 20 the interface when a liquid layer absorbs light at the used optical wavelength. It was widely studied experimentally, 21 theoretically and numerically as it is easily induced by a local laser-heating [1-4]. With the development of microfluidic 22 toolboxes on the one hand, and the enhancement of the surface to volume coupling contributions with miniaturization, 23 on the other hand, many efforts were concentrated towards the developments of optically induced surface-tension-24 driven flows at the microscale. Among these examples, one can cite the control of the spreading of films [5], the 25 production of droplets [6], the manipulation of jets [7], or the actuation of particles floating at a free surface [8]. The 26 investigation of flows induced by laser heating was further extended in bulk using absorbing suspensions [9-11] or at 27 interfaces considering light-sensitive surfactants [12]. 28 However, as shown in the present study, light may also transfer momentum isothermally to produce flows. We 29 concentrate here on this totally different mechanism where thermal effects are irrelevant. For this purpose the fluid 30 system at work, a phase-separated liquid mixtures close to criticality, has an extremely weak optical absorption at 31 the used optical wavelength making thermal effects negligible. Pure transfer of light momentum to a fluid can be set 32 in a non-absorbing liquid suspension and the mechanism is the following : when the liquid is constituted by randomly 33 distributed sub-wavelength particles in suspension or density fluctuations as in critical fluids, these refractive index 34 heterogeneities scatter the incident light beam which eventually loses forward momentum during its propagation 35 in the medium ; this phenomenon corresponds to the well-known critical opalescence in critical phenomena. As 36 a consequence, momentum conservation produces a density force, called scattering force, that sets the liquid in 37 permanent motion. As optical absorption is discarded, this density force should be only proportional to the wave 38 momentum (nI/c)z and to the beam attenuation = (−1/I)dI/dz due to the liquid turbidity, with n the refractive 39 index of the medium, I the intensity of the laser beam and c the light celerity ; may also be called extinction 40 coefficient when dealing with sub-wavelength particle suspensions. The expected scattering force density f scatt (r) 41 is then proportional to (nI/c)z for L 1 where L is the thickness of the sample. As illustrated in a recent 42 review [13], this scattering force at the level of the suspended particles is at the origin of many theoretical and 43 experimental works dedicated to particle manipulations in fluids. A similar physical approach was also described 44 to control the advection of small particles at free surfaces efficiently [14]. In the opposite case Savchenko et al. 45 [10], theoretically showed for the first time that it should be possible to induce a convective flow, by transfer of 46 momentum from light to a liquid crystal. Skipetrov et al. [15, 16] demonstrated that multiple scattering events also 47 drive bulk flow and measured the scatterer velocity by determining the temporal autocorrelation function of the 48 scattered electromagnetic field. Later Casner and Delville [17] strongly deformed very soft fluid/fluid interfaces in 49 near critical thermodynamic conditions using the optical radiation pressure. They evidenced a jetting instability at 50 the tip of the interface deformation with droplet production, both unexplained by the surface radiation pressure which 51 cannot actuate steady flows. In the absence of optical absorption, this effect was attributed to light scattering [18] 52 * These authors contributed equally to this work.
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Dates and versions

hal-02915822 , version 1 (16-08-2020)

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Hugo Chesneau, Julien Petit, Hamza Chraibi, Jean-Pierre Delville. Dynamics and flow characterization of liquid fountains produced by light scattering. Physical Review Fluids, 2020, 5 (2), ⟨10.1103/PhysRevFluids.5.024002⟩. ⟨hal-02915822⟩

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