Near-critical spreading of droplets.

We study the spreading of droplets in a near-critical phase-separated liquid mixture, using a combination of experiments, lubrication theory and finite-element numerical simulations. The classical Tanner's law describing the spreading of viscous droplets is robustly verified when the critical temperature is neared. Furthermore, the microscopic cut-off length scale emerging in this law is obtained as a single free parameter for each given temperature. In total-wetting conditions, this length is interpreted as the thickness of the thin precursor film present ahead of the apparent contact line. The collapse of the different evolutions onto a single Tanner-like master curve demonstrates the universality of viscous spreading before entering in the fluctuation-dominated regime. Finally, our results reveal a counter-intuitive and sharp thinning of the precursor film when approaching the critical temperature, which is attributed to the vanishing spreading parameter at the critical point.

A versatile interferometric technique for probing the thermophysical properties of complex fluids.

Laser-induced thermocapillary deformation of liquid surfaces has emerged as a promising tool to precisely characterize the thermophysical properties of pure fluids. However, challenges arise for nanofluid (NF) and soft bio-fluid systems where the direct interaction of the laser generates an intriguing interplay between heating, momentum, and scattering forces which can even damage soft biofluids. Here, we report a versatile, pump-probe-based, rapid, and non-contact interferometric technique that resolves interface dynamics of complex fluids with the precision of ~1 nm in thick-film and 150 pm in thin-film regimes below the thermal limit without the use of lock-in or modulated beams. We characterize the thermophysical properties of complex NF in three exclusively different types of configurations. First, when the NF is heated from the bottom through an opaque substrate, we demonstrate that our methodology permits the measurement of thermophysical properties (viscosity, surface tension, and diffusivity) of complex NF and biofluids. Second, in a top illumination configuration, we show a precise characterization of NF by quantitively isolating the competing forces, taking advantage of the different time scales of these forces. Third, we show the measurement of NF confined in a metal cavity, in which the transient thermoelastic deformation of the metal surface provides the properties of the NF as well as thermo-mechanical properties of the metal. Our results reveal how the dissipative nature of the heatwave allows us to investigate thick-film dynamics in the thin-film regime, thereby suggesting a general approach for precision measurements of complex NFs, biofluids, and optofluidic devices.

Numerical simulation of universal morphogenesis of fluid interface deformations driven by radiation pressure.

We report on numerical simulation of fluid interface deformations induced by either acoustic or optical radiation pressure. This is done by solving simultaneously the scalar wave propagation equation and the two-phase flow equations using the boundary element method. Using dimensional analysis, we show that interface deformation morphogenesis is universal, i.e., depends on the same dimensionless parameters in acoustics and electromagnetics. We numerically investigate a few selected phenomena-in particular the shape of large deformations and the slenderness transition and its hysteresis-and compare with existing and novel experimental observations. Qualitative agreement between the numerical simulations and experiments is found when the mutual interaction between wave propagation and wave-induced deformations is taken into account. Our results demonstrate the leading role of the radiation pressure in morphogenesis of fluid interface deformations and the importance of the propagation-deformation interplay.

Hole Scavenging and Electron-Hole Pair Photoproduction Rate: Two Mandatory Key Factors to Control Single-Tip Au-CdSe/CdS Nanoheterodimers.

Metal/semiconductor hetero-nanostructures are now considered as benchmark functional nanomaterials for many light-driven applications. Using laser-driven photodeposition to control growth of gold nanodots (NDs) onto CdSe/CdS dot-in-rods (DRs), we show that the addition of a dedicated hole scavenger (MeOH) is the cornerstone to significantly reduce to less than 3.5% the multiple-site nucleation and 2.5% the rate of gold-free DRs. This means, from a synthetic point of view, that rates up to 90% of single-tip DRs can be reproducibly achieved. Moreover, by systematically varying this hole scavenger concentration and the Au/DRs ratio on the one hand, and the irradiation intensity and the time exposure on the other hand, we explain how gold deposition switches from multisite to single-tipped and how the growth and final size of the single photodeposited ND can be controlled. A model also establishes that the results obtained based on these different varying conditions can be merged onto a single "master behavior" that summarizes and predicts the single-tip gold ND growth onto the CdSe/CdS DRs. We eventually use data from the literature on growth of platinum NDs onto CdS nanorods by laser-deposition to extend our investigation to another metal of major interest and strengthen our modeling of single metallic ND growth onto II-VI semiconducting nanoparticles. This demonstrated strategy can raise a common methodology in the synthesis of single-tip semiconductor-metal hybrid nanoheterodimers (NHDs), leading to advanced nanoparticles architectures for applications in areas as different as photocatalysis, hydrogen production, photovoltaics, and light detection.

Design of Metal@Titanium Oxide Nano-heterodimers by Laser-Driven Photodeposition: Growth Mechanism and Modeling.

In order to circumvent the usual nucleation of randomly distributed tiny metallic dots photodeposited on TiO2 nanoparticles (NPs) induced by conventional UV lamps, we propose to synthesize well-controlled nanoheterodimers (NHDs) using lasers focused inside microfluidic reactors to strongly photoactivate redox reactions of active ions flowing along with nanoparticles in water solution. Since the flux of photons issued from a focused laser may be orders of magnitude higher than that reachable with classical lamps, the production of electron-hole pairs is tremendously increased, ensuring a large availability of carriers for the deposition and favoring the growth of a single metallic dot as compared to secondary nucleation events. We show that the growth of single silver or gold nanodots can be controlled by varying the beam intensity, the concentration of the metallic salt, and the flow velocity inside the microreactor. The confrontation to a build-in model of the metallic nanodot light-induced growth onto the surface of TiO2 NPs shows the emergence of a predictable "master behavior" on which individual growths obtained from various tested conditions do collapse. We also characterized the associated quantum yield. Eventually, we successfully confronted our model to growth data from the literature in the case of silver on TiO2 and gold on II-VI semiconducting NPs triggered by UV lamps. It shows that for the photosynthesis of NHDs the efficiency of the electron-hole pair production rate matters much more than the number of pairs produced and that the use of laser light can provide a photodeposition-based synthesis at the nanoscale.

Contactless thin-film rheology unveiled by laser-induced nanoscale interface dynamics.

One of the classical limitations for the investigation of the local rheology of small scale soft objects and/or confined fluids is related to the difficulty to control mechanical contact and its consequences. In order to overcome these issues, we implement a new local, active, fast and contactless optical strategy, called optorheology, which is based on both the optical radiation pressure of a laser wave to dynamically deform a fluid interface and interferometry to probe this deformation with nanometric resolution. This optical approach is first validated by measuring the surface tension and the viscosity of transparent Newtonian liquids. We also show how non-equilibrium situations, such as continuous evaporation, can be used to deduce the thickness dependence of the rheology of thin films and the concentration dependence of the viscosity of binary liquid mixtures and suspensions. We further extend the investigation to elasticity and viscoelasticity measurements of polymer solutions. Finally, since liquids may absorb light, we discuss the influence of a weak laser heating and the triggering of interface deformations by thermocapillary tangential stresses that could represent a complementary approach to probe the rheology at small scale.

Hydrothermal Transformation of Titanate Scrolled Nanosheets to Anatase over a Wide pH Range and Contribution of Triethanolamine and Oleic Acid to Control the Morphology.

Mild hydrothermal conditions used for the treatment of titanate scrolled nanosheets (SNSs) suspensions (140 °C, 72 h) resulted in a large variety of anatase TiO2 anisotropic nano-objects depending on the studied parameters: influence of the medium pH and the presence or not of structuring agents (SAs). The present work shows that such a hydrothermal treatment of the SNSs, whatever the pH, resulted in the formation of single-crystalline anatase nanoneedles (NNs) with a specific crystal-elongation direction and a pH-dependent morphological anisotropy with aspect ratios (ARs) from 1 to 8. The SNSs suspensions were prepared by the conventional ultrabasic treatment of TiO2 with NaOH, followed by washing with HNO3 to different pH values. The crystal size of the anatase TiO2 obtained from this hydrothermal treatment increased with the pH of the suspensions, from 15 nm nanoparticles (NPs; AR = 1) at pH 2.2 to 500 nm NNs (AR = 8) at a pH 10.8 with a long axis systematically along the anatase [001] direction. Triethanol amine and oleic acid were used as SAs. Their respective influence, when acting on their own, had little influence on the control of the size, shape, or polydispersity of the NNs. However, their concomitant use provided a much better control of not only the size and polydispersity, which was strongly reduced, but also on (i) the shape and morphology giving rise to a controlled access to well-defined nanorods as opposed to nanoneedles and (ii) the crystal phase purity eliminating the few percent brookite still visible in the X-ray diffraction patterns of samples prepared in SA-free conditions. This approach offers an on-demand control over the production of anatase morphologies with defined aspect ratios.

Convection flows driven by laser heating of a liquid layer.

When a fluid is heated by the absorption of a continuous laser wave, the fluid density decreases in the heated area. This induces a pressure gradient that generates internal motion of the fluid. Due to mass conservation, convection eddies emerge in the sample. To investigate these laser-driven bulk flows at the microscopic scale, we built a setup to perform temperature measurements with a fluorescent-sensitive dye on the one hand, and measured the flow pattern at different beam powers, using a particle image velocimetry technique on the other hand. Temperature measurements were also used in numerical simulations in order to compare predictions to the experimental velocity profiles. The combination of our numerical and experimental approaches allows a detailed description of the convection flows induced by the absorption of light, which reveals a transition between a thin and a thick liquid layer regime. This supports the basis of optothermal approaches for microfluidic applications.

Fulminant isolated adenovirus hepatitis 5 months after haplo-identical HSCT for AML.

The principal limitation of allogeneic hematopoietic stem cell transplantation except relapse remains the transplant-related mortality (TRM). In addition to graft-versus-host disease (GvHD), infections contribute to TRM in many patients. We describe herein a case of an adult patient presenting 5 months after haplo-identical transplantation an isolated fulminant hepatitis due to adenovirus.

Enhancing optofluidic actuation of micro-objects by tagging with plasmonic nanoparticles.

We report experimentally and theoretically on the significant exaltation of optical forces on microparticles when they are partially coated by metallic nanodots and shined with laser light within the surface plasmon resonance. Optical forces on both pure silica particles and silica-gold raspberries are characterized using an optical chromatography setup to measure the variations of the Stokes drag versus laser beam power. Results are compared to the Mie theory prediction for both pure dielectric particles and core-shell ones with a shell described as a continuous dielectric-metal composite of dielectric constant determined from the Maxwell-Garnett approach. The observed quantitative agreement demonstrates that radiation pressure forces are directly related to the metal concentration on the microparticle surface and that metallic nanodots increase the magnitude of optical forces compared to pure dielectric particles of the same overall size, even at very low metal concentration. Behaving as "micro-sized nanoparticles", the benefit of microparticles coated with metallic nanodots is thus twofold: it significantly enhances optofluidic manipulation and motion at the microscale, and brings nanometric optical, chemical or biological capabilities to the microscale.

Break-up dynamics of fluctuating liquid threads.

The thinning dynamics of a liquid neck before break-up, as may happen when a drop detaches from a faucet or a capillary, follows different rules and dynamic scaling laws depending on the importance of inertia, viscous stresses, or capillary forces. If now the thinning neck reaches dimensions comparable to the thermally excited interfacial fluctuations, as for nanojet break-up or the fragmentation of thermally annealed nanowires, these fluctuations should play a dominant role according to recent theory and observations. Using near-critical interfaces, we here fully characterize the universal dynamics of this thermal fluctuation-dominated regime and demonstrate that the cross-over from the classical two-fluid pinch-off scenario of a liquid thread to the fluctuation-dominated regime occurs at a well-defined neck radius proportional to the thermal length scale. Investigating satellite drop formation, we also show that at the level of the cross-over between these two regimes it is more probable to produce monodisperse droplets because fluctuation-dominated pinch-off may allow the unique situation where satellite drop formation can be inhibited. Nonetheless, the interplay between the evolution of the neck profiles from the classical to the fluctuation-dominated regime and the satellites' production remains to be clarified.

Thermocapillary migration in small-scale temperature gradients: application to optofluidic drop dispensing.

We experimentally investigate the thermocapillary migration induced by local laser heating of the advancing front of a growing droplet confined in a microfluidic channel. When heating implies an effective increase in interfacial tension, the laser behaves as a "soft door" whose stiffness can be tuned via the optical parameters (beam power and waist). The light-driven thermocapillary velocity of a growing droplet, which opposes the basic flow, is characterized for different types of fluid injection, either pressure or flow rate driven, and various channel aspect ratios. Measurements are interpreted using a simplified model for the temperature gradient at the interface, based on a purely diffusive, three-layer system. Considering the mean temperature gradient, we demonstrate that the classical large-scale temperature gradient behavior is retrieved in the opposite case when the thermal gradient length scale is smaller than the droplet size. We also demonstrate that the thermocapillary velocity is proportional to the smallest droplet curvature imposed by the channel confinement. This suggests that the thermocapillary velocity is in fact proportional to the mean temperature gradient and to the largest interface curvature radius, which both coincide with the imposed one and the spherical droplet radius in large-scale and unconfined situations. Furthermore, as used surfactant concentrations are largely above the critical micelle concentration, we propose a possible explanation, relying on state-of-the-art considerations on high-concentration surfactant-covered interfaces for the observed effective increase in interfacial tension with temperature. We also propose a mechanism for explaining the blocking effect at the scaling-law level. This mechanism involves the temporal evolution of hydrodynamic and thermocapillary forces, based on experimental observations. We finally show that this optocapillary interaction with a microfluidic droplet generator allows for controlling either the flow rate (valve) or the droplet size (sampler), depending on the imposed fluid injection conditions.

Liquid-column sustainment driven by acoustic wave guiding.

We report on the formation and sustainment of liquid columns with aspect ratios much larger than the value at the onset of the Rayleigh-Plateau instability. This is achieved by using the passive feedback of the radiation pressure applied on the column surface by an acoustic beam injected at the upper end of the column and guided along it. We develop an analytical model that describes the coupling between the acoustic wave guiding and the balance between acoustic and capillary surface forces exerted on the column surface and find a satisfactory agreement with the experiment.

Liquid optical fibers with a multistable core actuated by light radiation pressure.

We report on spatiotemporal behavior of self-adapted dielectric liquid columns generated and sustained by light radiation pressure. We show that single- or multivalued liquid column diameter depends on the excitation light beam. When the beam diameter is sufficiently small, we observe a well-defined stationary column diameter. In contrast, at a larger beam diameter, the liquid column experiences complex spatiotemporal dynamics whose statistical analysis evidences an underlying multistable structure. Experimental observations are all supported by a full electromagnetic model that accounts for the wave guiding properties of the liquid column viewed as a step-index liquid-core liquid-cladding optical fiber having an optically tunable core diameter.

Stretching and squeezing of sessile dielectric drops by the optical radiation pressure.

We study numerically the deformation of sessile dielectric drops immersed in a second fluid when submitted to the optical radiation pressure of a continuous Gaussian laser wave. Both drop stretching and drop squeezing are investigated at steady state where capillary effects balance the optical radiation pressure. A boundary integral method is implemented to solve the axisymmetric Stokes flow in the two fluids. In the stretching case, we find that the drop shape goes from prolate to near-conical for increasing optical radiation pressure whatever the drop to beam radius ratio and the refractive index contrast between the two fluids. The semiangle of the cone at equilibrium decreases with the drop to beam radius ratio and is weakly influenced by the index contrast. Above a threshold value of the radiation pressure, these "optical cones" become unstable and a disruption is observed. Conversely, when optically squeezed, the drop shifts from an oblate to a concave shape leading to the formation of a stable "optical torus." These findings extend the electrohydrodynamics approach of drop deformation to the much less investigated "optical domain" and reveal the openings offered by laser waves to actively manipulate droplets at the micrometer scale.

Universal behavior of photochemical deposition in liquid solutions driven by a one-photon transition.

Even if photochemical deposition of nearly all types of materials has been used for decades to pattern almost any kind of substrate for various applications (catalysis, chemical sensing, magnetic data storage, optoelectronics, spin-dependent electron transport, and solar cells), a rationalized description is still missing. This paper aims at fulfilling this lack by presenting a unified approach of the photodeposit growth initiated by a one-photon photochemical reaction. We experimentally investigate the robustness of growth scalings predicted for photochemical deposition driven by a continuous laser wave. Three types of one-photon photochemical reactions (photoexcitation of chromates, photodissociation of permanganates, and photocondensation of colloidal selenium) and three parameters (solvent p H variations, concentration in photoactive reagent, and influence of the exciting optical wavelength) were cross analyzed. In all the cases, including data taken from the literature, the same dynamic master behavior emerges from the data rescaling of measured deposit growth laws. The nice agreement observed between system-independent predictions and the whole data set strongly supports a universal description of the photodeposit growth whatever the photosensitive medium and the involved one-photon chemical reaction. Such an approach also points out the quantitative sorting of photochemical reactions in terms of deposition efficiency. This rationalization of the kinetics of photodeposition anticipates new methodologies to predict, design, and control substrate micropatterning for chemical, lithographic, and optoelectronic applications.

An optical toolbox for total control of droplet microfluidics.

The use of microfluidic drops as microreactors hinges on the active control of certain fundamental operations such as droplet formation, transport, division and fusion. Recent work has demonstrated that local heating from a focused laser can apply a thermocapillary force on a liquid interface sufficient to block the advance of a droplet in a microchannel (C. N. Baroud, J.-P. Delville, F. Gallaire and R. Wunenburger, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2007, 75(4), 046302). Here, we demonstrate the generality of this optical approach by implementing the operations mentioned above, without the need for any special microfabrication or moving parts. We concentrate on the applications to droplet manipulation by implementing a wide range of building blocks, such as a droplet valve, sorter, fuser, or divider. We also show how the building blocks may be combined by implementing a valve and fuser using a single laser spot. The underlying fundamentals, namely regarding the fluid mechanical, physico-chemical and thermal aspects, will be discussed in future publications.

Thermocapillary valve for droplet production and sorting.

Droplets are natural candidates for use as microfluidic reactors, if active control of their formation and transport can be achieved. We show here that localized heating from a laser can block the motion of a water-oil interface, acting as a microfluidic valve for two-phase flows. A theoretical model is developed to explain the forces acting on a drop due to thermocapillary flow, predicting a scaling law that favors miniaturization. Finally, we show how the laser forcing can be applied to sorting drops, thus demonstrating how it may be integrated in complex droplet microfluidic systems.

Liquid transport due to light scattering.

Using experiments and theory, we show that light scattering by inhomogeneities in the index of refraction of a fluid can drive a large-scale flow. The experiment uses a near-critical, phase-separated liquid, which experiences large fluctuations in its index of refraction. A laser beam traversing the liquid produces a interface deformation on the scale of the experimental setup and can cause a liquid jet to form. We demonstrate that the deformation is produced by a scattering-induced flow by obtaining good agreements between the measured deformations and those calculated assuming this mechanism.

Bistability of a compliant cavity induced by acoustic radiation pressure.

We report on the first observation of multiple-order bistability due to acoustic radiation pressure in a compliant acoustic cavity formed between a spherical ultrasonic transducer immersed in water and the free liquid surface located at its focus. The hysteretic behavior of the cavity length, observed both with amplitude ramps and frequency sweeps, is accurately described using a one-dimensional model of a compliant Fabry-Pérot resonator assuming the acoustic radiation pressure to be the only coupling between the cavity and the acoustic field.