Active Role of Vapor Clouds around Evaporating Sessile Droplets in Microgravity: Marangoni Jets and Electroconvection.

A benchmark microgravity experiment (dubbed "ARLES") is analyzed. It concerns evaporation of several-µL sessile droplets with a pinned millimetric circular contact line on a flat substrate into a vast calm (here nitrogen) atmosphere at nearly normal conditions. Hydrofluoroether (HFE-7100) is used as a working liquid whose appreciable volatility and heavy vapor accentuate the contrast between the micro- and normal gravity. A possibility of switching on a DC electric field (EF) of several kV/mm orthogonally to the substrate is envisaged. We here focus on the findings intimately associated with the visualization of the vapor cloud by means of interferometry and rationalized by means of extensive simulations. In particular, with different degrees of unexpectedness, we discover and explore a Marangoni jet (without EF) and electroconvection (with EF) in the gas, which would otherwise be masked by buoyancy convection. Using the same tools, we examine some malfunctions of the space experiment.

Controlling the wetting and evaporation dynamics of non-ideal volatile binary solutions.

HYPOTHESIS: Volatile binary liquid samples on wetting substrates are known to undergo either spreading or contraction tendencies, as a result of solutal Marangoni stresses due to differential volatility. Enhanced spreading is commonly thought to occur when the lower surface tension component is more 'volatile', while contraction is expected otherwise. We seek to test the limits of this scenario for various configurations such as sessile drops with free or pinned contact lines, without or with microparticles, and tears-of-wine menisci. EXPERIMENTS: We consider isopropanol- and ethanol-water mixtures, important in numerous applications. We conduct interferometric experiments with sessile droplets for multiple combinations of the initial concentration and controlled ambient humidity (water vapour only), essentially covering the entire range of these parameters. Experiments are also carried out for other configurations mentioned above. FINDINGS: Contraction regimes are found in certain situations where spreading is expected, despite the alcohols being more volatile than water. Furthermore, regime reversals occur between cases with different initial liquid concentrations even at zero humidity, and are not necessarily associated with the existence of an azeotropic composition. Such surprising observations are rationalized by a simple model highlighting the often overlooked role of the diffusion coefficient ratio of the two vapours in conjunction with the non-ideality of the mixture. Our picture of the phenomenon is demonstrated to be universal for all configurations studied.

Miniscope3D: optimized single-shot miniature 3D fluorescence microscopy.

Miniature fluorescence microscopes are a standard tool in systems biology. However, widefield miniature microscopes capture only 2D information, and modifications that enable 3D capabilities increase the size and weight and have poor resolution outside a narrow depth range. Here, we achieve the 3D capability by replacing the tube lens of a conventional 2D Miniscope with an optimized multifocal phase mask at the objective's aperture stop. Placing the phase mask at the aperture stop significantly reduces the size of the device, and varying the focal lengths enables a uniform resolution across a wide depth range. The phase mask encodes the 3D fluorescence intensity into a single 2D measurement, and the 3D volume is recovered by solving a sparsity-constrained inverse problem. We provide methods for designing and fabricating the phase mask and an efficient forward model that accounts for the field-varying aberrations in miniature objectives. We demonstrate a prototype that is 17 mm tall and weighs 2.5 grams, achieving 2.76 µm lateral, and 15 µm axial resolution across most of the 900 × 700 × 390 µm3 volume at 40 volumes per second. The performance is validated experimentally on resolution targets, dynamic biological samples, and mouse brain tissue. Compared with existing miniature single-shot volume-capture implementations, our system is smaller and lighter and achieves a more than 2× better lateral and axial resolution throughout a 10× larger usable depth range. Our microscope design provides single-shot 3D imaging for applications where a compact platform matters, such as volumetric neural imaging in freely moving animals and 3D motion studies of dynamic samples in incubators and lab-on-a-chip devices.

Collective diffusion coefficient of a charged colloidal dispersion: interferometric measurements in a drying drop.

In the present work, we use Mach-Zehnder interferometry to thoroughly investigate the drying dynamics of a 2D confined drop of a charged colloidal dispersion. This technique makes it possible to measure the colloid concentration field during the drying of the drop at a high accuracy (about 0.5%) and with a high temporal and spatial resolution (about 1 frame per s and 5 µm per pixel). These features allow us to probe mass transport of the charged dispersion in this out-of-equilibrium situation. In particular, our experiments provide the evidence that mass transport within the drop can be described by a purely diffusive process for some range of parameters for which the buoyancy-driven convection is negligible. We are then able to extract from these experiments the collective diffusion coefficient of the dispersion D(φ) over a wide concentration range φ = 0.24-0.5, i.e. from the liquid dispersed state to the solid glass regime, with a high accuracy. The measured values of D(φ) ≃ 5-12D0 are significantly larger than the simple estimate D0 given by the Stokes-Einstein relation, thus highlighting the important role played by the colloidal interactions in such dispersions.

Vapor-Mediated versus Substrate-Mediated Interactions between Volatile Droplets.

Understanding the behavior of sessile drops can be rewarding in many applications while also fostering progress in the rapidly evolving field of capillarity and wetting. Our experiments reveal that two evaporating sessile drops on a solid substrate do attract even if, unlike the binary-liquid drops recently studied in the literature [Cira et al., Nature, 2015], they are made of the same pure liquid. Several perfectly wetting liquids of different volatilities are tested to unveil and quantify the mechanisms enabling droplets to communicate. While all recent works focusing on the topic consider vapor-mediated interactions only, we here identify not less than three substrate-mediated forces, important for not too heat-conducting substrates (e.g. glass) and driven by the thermal Marangoni effect (favoring droplet motion toward colder regions) and by evaporation-induced variations of the apparent contact angles (acting similarly to a wettability gradient). In addition to an attractive mechanism and a (generally weaker) repelling one, the third effect acts on each droplet individually due to the self-centering cold spot it induces in the substrate. Interestingly, in the force balance used to rationalize our results, this "cold-trap resistance" enters as an effective drag force opposing any motion, like the viscous drag does. The interaction mechanisms described here could hopefully open new directions of research about thermal effects as a mean of self-organizing evaporating/condensing liquid entities on substrates of various shapes and thermal properties.

Hybrid Two-Scale Fabrication of Sub-Millimetric Capillary Grippers.

Capillary gripping is a pick-and-place technique that is particularly well-suited for handling sub-millimetric components. Nevertheless, integrating a fluid supply and release mechanism becomes increasingly difficult to manufacture for these scales. In the present contribution, two hybrid manufacturing procedures are introduced in which the creation of the smallest features is decoupled from the macro-scale components. In the first procedure, small scale features are printed directly (by two-photon polymerisation) on top of a 3D-printed device (through stereolithography). In the second approach, directional ultraviolet (UV)-illumination and an adapted design allowed for successful (polydimethylsiloxane, PDMS) moulding of the microscopic gripper head on top of a metal substrate. Importantly, a fully functional microchannel is present in both cases through which liquid to grip the components can be supplied and retracted. This capability of removing the liquid combined with an asymmetric pillar design allows for a passive release mechanism with a placement precision on the order of 3% of the component size.

Magnetomigration of Rare-Earth Ions Triggered by Concentration Gradients.

Mach-Zehnder interferometry was applied to explore the effects of inhomogeneous magnetic fields on the mobility of rare-earth ions in aqueous solutions. No migration of ions was observed in a thermodynamically closed system when a homogeneous solution was subjected to a magnetic field gradient alone. However, magnetomigration could be triggered by a concentration gradient of the rare-earth ions in the solution. When a concentration gradient was introduced in the sample by solvent evaporation, consistent migration of paramagnetic Dy3+ ions from the bulk solution to regions with stronger magnetic fields was observed. By contrast, no movement was detected for diamagnetic Y3+ ions in the presence of a concentration gradient.

Soft Encapsulation of Flexible Electrical Stimulation Implant: Challenges and Innovations.

In this document we discuss the main challenges encountered when producing flexible electrical stimulation implants, and present our approach to solving them for prototype production. We include a study of the optimization of the flexible PCB design, the selection of additive manufacturing materials for the mold, and the chemical compatibility of the different materials. Our approach was tested on a flexible gastro-stimulator as part of the ENDOGES research program.

Effect of Marangoni Flows on the Shape of Thin Sessile Droplets Evaporating into Air.

Freely receding evaporating sessile droplets of perfectly wetting liquids, for which the observed finite contact angles are attributed to evaporation, are studied with a Mach-Zehnder interferometer. The experimentally obtained droplet shapes are found to depart, under some conditions, from the classical macroscopic static profile of a sessile droplet. The observed deviations (or the absence thereof) are explained in terms of a Marangoni flow due to evaporation-induced thermal gradients along the liquid-air interface. When such a Marangoni effect is strong, the experimental profiles exhibit a maximum of the slope at a certain distance from the contact line. In this case, the axisymmetric flow is directed from the contact line to the apex (along the liquid-air interface), hence delivering more liquid to the center of the droplet and making it appear inflated. These findings are quantitatively confirmed by predictions of a lubrication model accounting for the impact of the Marangoni effect on the droplet shape.

Nonequilibrium Gibbs' criterion for completely wetting volatile liquids.

During the spreading of a liquid over a solid substrate, the contact line can stay pinned at sharp edges until the contact angle exceeds a critical value. At (or sufficiently near) equilibrium, this is known as Gibbs' criterion. Here, we show both experimentally and theoretically that, for completely wetting volatile liquids, there also exists a dynamically-produced contribution to the critical angle for depinning, which increases with the evaporation rate. This suggests that one may introduce a simple modification of the Gibbs' criterion for (de)pinning that accounts for the nonequilibrium effect of evaporation.

Vapor-based interferometric measurement of local evaporation rate and interfacial temperature of evaporating droplets.

The local evaporation rate and interfacial temperature are two quintessential characteristics for the study of evaporating droplets. Here, it is shown how one can extract these quantities by measuring the vapor concentration field around the droplet with digital holographic interferometry. As a concrete example, an evaporating freely receding pending droplet of 3M Novec HFE-7000 is analyzed at ambient conditions. The measured vapor cloud is shown to deviate significantly from a pure-diffusion regime calculation, but it compares favorably to a new boundary-layer theory accounting for a buoyancy-induced convection in the gas and the influence upon it of a thermal Marangoni flow. By integration of the measured local evaporation rate over the interface, the global evaporation rate is obtained and validated by a side-view measurement of the droplet shape. Advective effects are found to boost the global evaporation rate by a factor of 4 as compared to the diffusion-limited theory.