Bioinspired shape shifting of liquid-infused ribbed sheets.

The recent emergence of stimuli-responsive, shape-shifting materials offers promising applications in fields as different as soft robotics, aeronautics, or biomedical engineering. Targeted shapes or movements are achieved from the advantageous coupling between some stimulus and various materials such as liquid crystalline elastomers, magnetically responsive soft materials, swelling hydrogels, etc. However, despite the large variety of strategies, they are strongly material dependent and do not offer the possibility to choose between reversible and irreversible transformations. Here, we introduce a strategy applicable to a wide range of materials yielding systematically reversible or irreversible shape transformations of soft ribbed sheets with precise control over the local curvature. Our approach-inspired by the spore-releasing mechanism of the fern sporangium-relies on the capillary deformation of an architected elastic sheet impregnated by an evaporating liquid. We develop an analytical model combining sheet geometry, material stiffness, and capillary forces to rationalize the onset of such deformations and develop a geometric procedure to inverse program target shapes requiring fine control over the curvature gradient. We finally demonstrate the potential irreversibility of the transformation by UV-curing a photosensitive evaporating solution and show that the obtained shells exhibit enhanced mechanical stiffness.

Thermal Antibubbles: When Thermalization of Encapsulated Leidenfrost Drops Matters.

Antibubbles are ephemeral objects composed of a liquid drop encapsulated by a thin gas shell immersed in a liquid medium. When the drop is made of a volatile liquid and the medium is superheated, the gas shell inflates at a rate governed by the evaporation flux from the drop. This thermal process represents an alternate strategy for delaying the antibubble collapse. We model the dynamics of such "thermal" antibubbles by incorporating to the film drainage equation the heat-transfer-limited evaporation of the drop, which nourishes the gas shell with vapor, as for Leidenfrost drops. We demonstrate that the inflation of the gas shell is drastically inhibited by the thermalization of the initially colder drop. Because of this thermalization effect, smaller drops evaporate much faster than larger ones.

Lifetime of Surface Bubbles in Surfactant Solutions.

Despite the prevalence of surface bubbles in many natural phenomena and engineering applications, the effect of surfactants on their surface residence time is not clear. Numerous experimental studies and theoretical models exist but a clear understanding of the film drainage phenomena is still lacking. In particular, theoretical work predicting the drainage rate of the thin film between a bubble and the free surface in the presence and absence of surfactants usually makes use of the lubrication theory. On the other hand, in numerous natural situations and experimental works, the bubble approaches the free surface from a certain distance and forms a thin film at a later stage. This article attempts to bridge these two approaches. In particular, in this article, we review these works and compare them to our direct numerical simulations where we study the coupled influence of bubble deformation and surfactants on the rising and drainage process of a bubble beneath a free surface. In the present study, the level-set method is used to capture the air-liquid interfaces, and the transport equation of surfactants is solved in an Eulerian framework. The axisymmetric simulations capture the bubble acceleration, deformation, and rest (or drainage) phases from nondeformable to deformable bubbles, as measured by the Bond number (Bo), and from surfactant-free to surfactant-coated bubbles, as measured by the Langmuir number (La). The results show that the distance h between the bubble and the free surface decays exponentially for surfactant-free interfaces (La = 0), and this decay is faster for nondeformable bubbles (Bo ≪ 1) than for deformable ones (Bo ≫ 1). The presence of surfactants (La > 0) slows the decay of h, exponentially for large bubbles (Bo ≫ 1) and algebraically for small ones (Bo ≪ 1). For Bo ≈ 1, the lifetime is the longest and is associated with the (Marangoni) elasticity of the interfaces.

Surfactant-induced rigidity of interfaces: a unified approach to free and dip-coated films.

The behavior of thin liquid films is known to be strongly affected by the presence of surfactants at the interfaces. The detailed mechanism by which the latter enhance film stability is still a matter of debate, in particular concerning the influence of surface elastic effects on the hydrodynamic boundary condition at the liquid/air interfaces. In the present work, "twin" hydrodynamic models neglecting surfactant transport to the interfaces are proposed to describe the coating of films onto a solid plate (Landau-Levich-Derjaguin configuration) as well as soap film pulling (Frankel configuration). Experimental data on the entrained film thickness in both configurations can be fitted very well using a single value of the surface elasticity, which is in good agreement with independent measurements by mean of surface expansion experiments in a Langmuir through. The analysis thus shows that soap films or dip coating experiments may be used to measure the surface elasticity of surfactant solutions in the insoluble limit, namely as long as the film generation dynamics is fast compared to the surfactant adsorption timescale.

Shear-enhanced sorting of ovoid and filamentous bacterial cells using pinch flow fractionation.

In this paper, we experimentally investigate the influence of the flow rate on the trajectory of ovoid and filamentous bacterial cells of E. coli in a low aspect ratio pinch flow fractionation device. To that aim, we vary the Reynolds number over two orders of magnitude, while monitoring the dynamics of the cells across our device. At low flow rates, filamentous cells adopt several rotational motions in the pinched segment, which are induced both by the shear rate and by their close interactions with the nearest wall. As a result, the geometrical centre of the filamentous cells deviates towards the centre of the channel, which increases their effective sorting diameter depending on the length of their major axis as well as on the rotational mode they adopt in the pinch. As the flow rate increases, particles are forced to align vertically in the pinch, in the direction of the main shear gradient, which reduces the amplitude of the lateral deviation generated by their rotation. The trajectory of the particles in the expansion is directly determined by their position at the pinch outlet. As a consequence, the position of the filamentous cells at the outlet of the device strongly depends on the flow rate as well as on the length of their major axis. Based on these observations we optimized the flow conditions to successfully extract an ultra high purity sample of filamentous cells from a solution containing mainly ovoid cells.

Coalescence of surface bubbles: The crucial role of motion-induced dynamic adsorption layer.

The formation of motion-induced dynamic adsorption layers of surfactants at the surface of rising bubbles is a widely accepted phenomenon. Although their existence and formation kinetics have been theoretically postulated and confirmed in many experimental reports, the investigations primarily remain qualitative in nature. In this paper we present results that, to the best of our knowledge, provide a first quantitative proof of the influence of the dynamic adsorption layer on drainage dynamics of a single foam film formed under dynamic conditions. This is achieved by measuring the drainage dynamics of single foam films, formed by air bubbles of millimetric size colliding against the interface between n-octanol solutions and air. This was repeated for a total of five different surfactant concentrations and two different liquid column heights. All three steps preceding foam film rupture, namely the rising, bouncing and drainage steps, were sequentially examined. In particular, the morphology of the single film formed during the drainage step was analyzed considering the rising and bouncing history of the bubble. It was found that, depending on the motion-induced state of adsorption layer at the bubble surface during the rising and the bouncing steps, single foam film drainage dynamics can be spectacularly different. Using Direct Numerical Simulations (DNS), it was revealed that surfactant redistribution can occur at the bubble surface as a result of the bouncing dynamics (approach-bounce cycles), strongly affecting the interfacial mobility, and leading to slower rates of foam film drainage. Since the bouncing amplitude directly depends on the rising velocity, which correlates in turn with the adsorption layer of surfactants at the bubble surface during the rising step, it is demonstrated that the lifetime of surface bubbles should intimately be related to the history of their formation.

Antibubble dynamics: the drainage of an air film with viscous interfaces.

An antibubble is a spherical air film that is immersed in a surfactant mixture and drains under the action of hydrostatic pressure. A dynamical model of this film is proposed that accounts for the surface shear viscosity effects in the case of purely viscous interfaces, which applies for surfactants whose adsorption rate is much larger than advection rate and at a concentration much above the critical micelle concentration. Our model shows that the lifetime of the antibubbles in this case increases with surface shear viscosity, denoted ε, whose value is measured independently, all in agreement with experimental measurements. We also found that the critical thickness, h(c), at film rupture due to van der Waals interactions slightly depends on the surface shear viscosity, namely h(c) ∝ ε(1/6).

Plate coating: influence of concentrated surfactants on the film thickness.

We present a large range of experimental data concerning the influence of surfactants on the well-known Landau-Levich-Derjaguin experiment where a liquid film is generated by pulling a plate out of a bath. The thickness h of the film was measured as a function of the pulling velocity V for different kinds of surfactants (C(12)E(6), which is a nonionic surfactant, and DeTAB and DTAB, which are ionic) and at various concentrations near and above the critical micellar concentration (cmc). We report the thickening factor α = h/h(LLD), where h(LLD) is the film thickness obtained without a surfactant effect, i.e., as for a pure fluid but with the same viscosity and surface tension as the surfactant solution, over a wide range of capillary numbers (Ca = ηV/γ, with η being the surfactant solution viscosity and γ its surface tension) and identify three regimes: (i) at small Ca α is large due to confinement and surface elasticity (or Marangoni) effects, (ii) for increasing Ca there is an intermediate regime where α decreases as Ca increases, and (iii) at larger (but still small) Ca α is slightly higher than unity due to surface viscosity effects. In the case of nonionic surfactants, the second regime begins at a fixed Ca, independent of the surfactant concentration, while for ionic surfactants the transition depends on the concentration, which we suggest is probably due to the existence of an electrostatic barrier to surface adsorption. Control of the physical chemistry at the interface allowed us to elucidate the nature of the three regimes in terms of surface rheological properties.

Microfluidic droplet generation based on non-embedded co-flow-focusing using 3D printed nozzle.

Most commercial microfluidic droplet generators rely on the planar flow-focusing configuration implemented in polymer or glass chips. The planar geometry, however, suffers from many limitations and drawbacks, such as the need of specific coatings or the use of dedicated surfactants, depending on the fluids in play. On the contrary, and thanks to their axisymmetric geometry, glass capillary-based droplet generators are a priori not fluid-dependent. Nevertheless, they have never reached the market because their assembly requires fastidious and not scalable fabrication techniques. Here we present a new device, called Raydrop, based on the alignment of two capillaries immersed in a pressurized chamber containing the continuous phase. The dispersed phase exits one of the capillaries through a 3D-printed nozzle placed in front of the extraction capillary for collecting the droplets. This non-embedded implementation of an axisymmetric flow-focusing is referred to non-embedded co-flow-focusing configuration. Experimental results demonstrate the universality of the device in terms of the variety of fluids that can be emulsified, as well as the range of droplet radii that can be obtained, without neither the need of surfactant nor coating. Additionally, numerical computations of the Navier-Stokes equations based on the quasi-steadiness assumption allow to provide an explanation to the underlying mechanism behind the drop formation and the mechanism of the dripping to jetting transition. Excellent predictions were also obtained for the droplet radius, as well as for the dripping-jetting transition, when varying the geometrical and fluid parameters, showing the ability of this configuration to enventually enhance the dripping regime. The monodispersity ensured by the dripping regime, the robustness of the fabrication technique, the optimization capabilities from the numerical modelling and the universality of the configuration confer to the Raydrop technology a very high potential in the race towards high-throughput droplet generation processes.

Controlling the lifetime of antibubbles.

An antibubble is a liquid droplet wrapped by a thin layer of gas, inside a bulk liquid usually of the same composition. The lifetime of an antibubble is governed by the drainage of the gas between the two liquid-gas interfaces populated by surfactants. Depending on the relative magnitude of surface viscosity and elastic moduli, which directly depend on or are determined by the nature of surfactants, the lifetime of an antibubble may vary a lot, from few seconds to few minutes. While such a difference can be predicted with models that include the role of interfacial properties, they were not observed experimentally in previous studies, due to important sources of dispersion. In this review, the main sources of dispersion are identified, such as (i) the initial amount of gas embedded in the antibubble, (ii) the level of saturation of gas in the bulk liquid, (iii) the presence of dust particles (<0.5 µm) in the gas, and (iv) three-dimensional flow effects. By accounting for these various effects, we obtain a coherent view on the lifetime of an antibubble, as a function of its radius and the surface rheology, with excellent consistency between experiments and modeling. Results thus demonstrate that controlling the size and lifetime of antibubbles is achievable.

Interaction of three-dimensional hydrodynamic and thermocapillary instabilities in film flows.

We study three-dimensional wave patterns on the surface of a film flowing down a uniformly heated wall. Our starting point is a model of four evolution equations for the film thickness h , the interfacial temperature theta , and the streamwise and spanwise flow rates, q and p , respectively, obtained by combining a gradient expansion with a weighted residual projection. This model is shown to be robust and accurate in describing the competition between hydrodynamic waves and thermocapillary Marangoni effects for a wide range of parameters. For small Reynolds numbers, i.e., in the "drag-gravity regime," we observe regularly spaced rivulets aligned with the flow and preventing the development of hydrodynamic waves. The wavelength of the developed rivulet structures is found to closely match the one of the most amplified mode predicted by linear theory. For larger Reynolds numbers, i.e., in the "drag-inertia regime," the situation is similar to the isothermal case and no rivulets are observed. Between these two regimes we observe a complex behavior for the hydrodynamic and thermocapillary modes with the presence of rivulets channeling quasi-two-dimensional waves of larger amplitude and phase speed than those observed in isothermal conditions, leading possibly to solitarylike waves. Two subregions are identified depending on the topology of the rivulet structures that can be either "ridgelike" or "groovelike." A regime map is further proposed that highlights the influence of the Reynolds and the Marangoni numbers on the rivulet structures. Interestingly, this map is found to be related to the variations of amplitude and speed of the two-dimensional solitary-wave solutions of the model. Finally, the heat transfer enhancement due to the increase of interfacial area in the presence of rivulet structures is shown to be significant.

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.

A major secretory defect of tumour-infiltrating T lymphocytes due to galectin impairing LFA-1-mediated synapse completion.

Surface galectin has been shown to contribute to dysfunctions of human tumour-infiltrating lymphocytes (TILs). We show here that galectin-covered CD8 TILs produce normal amounts of intracellular cytokines, but fail to secrete them because of defective actin rearrangements at the synapse. The non-secreting TILs also display reduced adhesion to their targets, together with defective LFA-1 recruitment and activation at the synapse. These defects are relieved by releasing surface galectin. As mild LFA-1 blockade on normal blood T cells emulate the defects of galectin-covered TILs, we conclude that galectin prevents the formation of a functional secretory synapse by preventing optimal LFA-1 triggering. Our results highlight a major secretory defect of TILs that is not revealed by widely used intracellular cytokine immunomonitoring assays. They also provide additional insights into the T-cell response, by showing that different thresholds of LFA-1 triggering are required to promote the intracellular production of cytokines and their secretion.

Hydrogen peroxide concentration by pervaporation of a ternary liquid solution in microfluidics.

Pervaporation in a microfluidic device is performed on liquid ternary solutions of hydrogen peroxide-water-methanol in order to concentrate hydrogen peroxide (H2O2) by removing methanol. The quantitative analysis of the pervaporation of solutions with different initial compositions is performed, varying the operating temperature of the microfluidic device. Experimental results together with a mathematical model of the separation process are used to understand the effect of the operating conditions on the microfluidic device efficiency. The parameters influencing significantly the performance of pervaporation in the microfluidic device are determined and the limitations of the process are discussed. For the analysed system, the operating temperature of the chip has to be below the temperature at which H2O2 decomposes. Therefore, the choice of an adequate reduced operating pressure is required, depending on the expected separation efficiency.