Bruno Issenmann

Viscosity of deeply supercooled water and its coupling to molecular diffusion.

Significance Water is the most ubiquitous liquid but also the most anomalous. In usual fluids far from their glass transition, viscosity and molecular diffusion are coupled through the Stokes–Einstein relations. For water, viscosity already starts decoupling from translational diffusion below 20°C. Simulations have suggested a connection with the putative separation of supercooled water into two distinct liquid phases. Whereas experimental diffusion data extend far in the supercooled region, accurate viscosity data were lacking due to the readiness of supercooled water to crystallize under the slightest perturbation. Using Brownian motion of spheres suspended in water, we have measured its viscosity down to −34°C without freezing. We find that whereas viscosity decouples increasingly from molecular translation upon cooling, it remains coupled to rotation. The viscosity of a liquid measures its resistance to flow, with consequences for hydraulic machinery, locomotion of microorganisms, and flow of blood in vessels and sap in trees. Viscosity increases dramatically upon cooling, until dynamical arrest when a glassy state is reached. Water is a notoriously poor glassformer, and the supercooled liquid crystallizes easily, making the measurement of its viscosity a challenging task. Here we report viscosity of water supercooled close to the limit of homogeneous crystallization. Our values contradict earlier data. A single power law reproduces the 50-fold variation of viscosity up to the boiling point. Our results allow us to test the Stokes–Einstein and Stokes–Einstein–Debye relations that link viscosity, a macroscopic property, to the molecular translational and rotational diffusion, respectively. In molecular glassformers or liquid metals, the violation of the Stokes–Einstein relation signals the onset of spatially heterogeneous dynamics and collective motions. Although the viscosity of water strongly decouples from translational motion, a scaling with rotational motion remains, similar to canonical glassformers.

Deformation of acoustically transparent fluid interfaces by the acoustic radiation pressure

We experimentally study the deformations of liquid-liquid interfaces induced by a high-intensity focused ultrasonic beam. We quantitatively verify that small-amplitude deformations of a transparent chloroform-water interface are well described by the theory of Langevin acoustic radiation pressure, in both static and dynamic regimes. The large-amplitude deformations depend on the direction of propagation of the beam and are qualitatively similar to those induced by electromagnetic radiation pressure.

Fluid flows driven by light scattering

We report on the direct experimental observation of laser-induced flows in isotropic liquids that scatter light. We use a droplet microemulsion in the two-phase regime, which behaves like a binary mixture. Close to its critical consolute line, the microemulsion undergoes large refractive index fluctuations that scatter light. The radiation pressure of a laser beam is focused onto the soft interface between the two phases of the microemulsion and induces a cylindrical liquid jet that continuously emits droplets. We demonstrate that this dripping phenomenon takes place as a consequence of a steady flow induced by the transfer of linear momentum from the optical field to the liquid due to light scattering. We first show that the cylindrical jet guides light as a step-index liquid optical fiber whose core diameter is self-adapted to the light itself. Then, by modelling the light-induced flow as a low-Reynoldsnumber, parallel flow, we predict the dependence of the dripping flow rate on the thermophysical properties of the microemulsion and the laser beam power. Satisfying agreement is found between the model and experiments.

Pressure dependence of viscosity in supercooled water and a unified approach for thermodynamic and dynamic anomalies of water

Significance Commuting during rush hours teaches us that the denser the crowd, the slower the motion. Liquids follow this law, except water below ambient temperature: Increasing density decreases viscosity! This anomaly witnesses the progressive breaking of the hydrogen bond network in water. Because this network develops upon cooling, dramatic effects of pressure on viscosity are expected in water supercooled below its freezing point. However, no data were hitherto available under these extreme conditions. Using a time-of-flight viscometer, we provide viscosity data down to −29 °C and up to 3,000 atmospheres, revealing that pressure can reduce viscosity by nearly one half. We propose a model that treats water as a mixture of two species, and explains its thermodynamic and dynamic properties. The anomalous decrease of the viscosity of water with applied pressure has been known for over a century. It occurs concurrently with major structural changes: The second coordination shell around a molecule collapses onto the first shell. Viscosity is thus a macroscopic witness of the progressive breaking of the tetrahedral hydrogen bond network that makes water so peculiar. At low temperature, water at ambient pressure becomes more tetrahedral and the effect of pressure becomes stronger. However, surprisingly, no data are available for the viscosity of supercooled water under pressure, in which dramatic anomalies are expected based on interpolation between ambient pressure data for supercooled water and high pressure data for stable water. Here we report measurements with a time-of-flight viscometer down to 244K and up to 300MPa, revealing a reduction of viscosity by pressure by as much as 42%. Inspired by a previous attempt [Tanaka H (2000) J Chem Phys 112:799–809], we show that a remarkably simple extension of a two-state model [Holten V, Sengers JV, Anisimov MA (2014) J Phys Chem Ref Data 43:043101], initially developed to reproduce thermodynamic properties, is able to accurately describe dynamic properties (viscosity, self-diffusion coefficient, and rotational correlation time) as well. Our results support the idea that water is a mixture of a high density, “fragile” liquid, and a low density, “strong” liquid, the varying proportion of which explains the anomalies and fragile-to-strong crossover in water.

Wave turbulence in a two-layer fluid: coupling between free surface and interface waves

We experimentally study gravity-capillary wave turbulence on the interface between two immiscible fluids of close density with free upper surface. We locally measure the wave height at the interface between both fluids by means of a highly sensitive laser Doppler vibrometer. We show that the inertial range of the capillary wave turbulence regime is significantly extended when the upper fluid depth is increased: The crossover frequency between the gravity and capillary wave turbulence regimes is found to decrease whereas the dissipative cut-off frequency of the spectrum is found to increase. We explain these observations by the progressive decoupling between waves propagating at the interface and the ones at the free surface, using the full dispersion relation of gravity-capillary waves in a two-layer fluid of finite depths. The cut-off evolution is due to the disappearance of parasitic capillaries responsible for the main wave dissipation for a single fluid.

Gravity wave turbulence revealed by horizontal vibrations of the container.

We experimentally study the role of forcing on gravity-capillary wave turbulence. Previous laboratory experiments using spatially localized forcing (vibrating blades) have shown that the frequency power-law exponent of the gravity wave spectrum depends on the forcing parameters. By horizontally vibrating the whole container, we observe a spectrum exponent that does not depend on the forcing parameters for both gravity and capillary regimes. This spatially extended forcing leads to a gravity spectrum exponent in better agreement with the theory than by using a spatially localized forcing. The role of the vessel shape has been also studied. Finally, the wave spectrum is found to scale linearly with the injected power for both regimes whatever the forcing type used.

Acoustical spring effect in a compliant cavity

We report on the first dynamic study of acoustical spring effect in a compliant cavity formed between a spherical ultrasonic transducer immersed in water and the free liquid surface located at its focus. As its optical analog, this effect is due to the mutual feedback between the cavity length L and the large acoustical power stored inside the cavity, here through acoustic radiation pressure. We use surface waves to investigate the acoustical spring effect. The amplitude of surface waves above the cavity is observed to vary with the slope of variation of the L -dependent acoustic radiation pressure exerted on the liquid surface, i.e. with the acoustic spring stiffness. The observed simultaneous back-scattering of these surface waves demonstrates that the surface response to the cavity length variations results mainly in an added stiffness, i.e., in an increase of the real part of the surface impedance above the cavity. Finally, when the liquid surface is located out of the focal plane, spontaneous surface oscillations are reproducibly observed, which may be due to a parametric instability.Graphical abstract

Simple and versatile non‐contact technique for measuring the interfacial tension of a liquid‐liquid interface using pulsed acoustic radiation pressure

We present a versatile non‐contact technique for measuring the interfacial tension of a liquid‐liquid interface using the acoustic radiation pressure. This technique is based on the analysis of the shape of the time‐dependent deformation of the liquid interface induced by the radiation pressure of a focused, pulsed ultrasonic beam. It combines a simple optical detection step and a novel analytical model of interface deformation dynamics that accurately takes into account interfacial, gravitational, as well as viscous effects in both weakly and highly viscous limits. The accuracy of this technique is experimentally demonstrated on liquid‐air and liquid‐liquid interfaces.

Opto-hydrodynamic instability of fluid interfaces

The bending of fluid interfaces by the optical radiation pressure is now recognized as an appealing contactless tool to probe microscopic surface properties of soft materials. However, as the radiation pressure is intrinsically weak (typically of the order of a few Pascal), investigations are often limited to the regime of weak deformations. Non-linear behaviors can nevertheless be investigated using very soft fluid interfaces. Either a large stable tether is formed, or else a break-up of the interface occurs above a well-defined beam power threshold, depending on the direction of the beam propagation. This asymmetry originates from the occurrence of total reflection condition of light at deformed interface. Interface instability results in the formation of a stationary beam-centered liquid micro-jet that emits droplets. Radiation-induced jetting can also lead to giant tunable liquid columns with aspect ratio up to 100, i.e. well beyond the fundamental Rayleigh-Plateau limitation. Consequently, the applications range of the opto-hydrodynamic interface instability is wide, going from adaptative micro-optics (lensing and light guiding by the induced columns) to micro-fluidics and microspraying, as fluid transfer is optically monitored and directed in three dimensions.