The dynamics of solid bodies near the interface between two co-flowing immiscible fluids in a microchannel is examined. The transverse position and velocity of the particle as well as the associated pressure drop in the channel are assessed. The dependence of the results on the internal forces due to viscosity, inertia and capillarity is examined. Besides, we analyse the influence of the different governing parameters, namely, density, viscosity and thickness ratios, Reynolds and Capillary numbers, particle size and uniform body force. The resulting transverse body force, which can be balanced by an external field such as magnetic or gravitational fields, indicates the stable or unstable character of the position of the particle. The numerical simulations are carried out using the Finite Element Method to solve the full steady Navier-Stokes equations and its asymptotic expansions for small Reynolds and Capillary numbers. To model the deformation of the liquid-liquid deformable interface, the Arbitrary Lagrangian-Eulerian (ALE) method is implemented.

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 eventually 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.

Capillary-driven microflows are usually studied in environments where substrates are developed by methods that yield surfaces with smooth topographies. However, these methods often find limitations when it comes to manufacturing devices larger than a few centimeters or with complicated geometries. As an alternative, additive manufacturing can overcome these limitations. Moreover, it is also particularly suited to develop lightweight structures, to perform topology optimization and to achieve AIT (Assembly, Integration and Testing) effort reduction. Unfortunately, it yields surface finishings that are not smooth, but offer a surface topography that is hard to control. This presents advantages such as the enhancement of capillary transport along the surface, but also makes modelling harder. For instance, it is not easy to define constant advancing-receding contact angles. In this work, a capillarity-driven transport model is developed to quantify the transport and contact-angle dynamics in 3D-printed surface topographies which use lightweight materials typical of the aerospace industry. For that purpose, four simple geometries scaled at five different sizes are manufactured in aluminium using 3D printing technology. Capillary transport was studied with both water and acetone. We compare the experimental and theoretical imbibited height, the latter computed according to Washburn’ s Law. On the one hand, experiments with water were recorded with a high-speed camera. To facilitate the quantification of the imbibition speed, water has been mixed with fluorescein and illuminated with UV light. The results obtained vary qualitatively depending on the microchannel geometry. Moreover, we have observed that a substantial part of the transport takes place, not along the channels, but along the surface of the sample between them. Besides water, acetone was also used. The imbibition of this liquid in the sample was characterized by simultaneous measurements with a thermographic camera and a fast-response digital weighting scale. These experiments are hard to interpret due to the high evaporation ratio of acetone at ambient temperature. In the case of acetone, the flow transport observed in the experiments remarkably overcome theoretical predictions done by Washburn's law, even if the contact angle was assumed to be zero. This suggests that, for acetone, the surface transport mechanism is much more effective than the transport along the microchannels. Based on the results, acetone showed a general high rate of capillary ascension for all cross-sections while water ascended only for certain channel geometries such as triangular and circular. In addition, these preliminary results seem to confirm that both liquids do not follow Washburn imbibition law since transient and stationary behaviour is affected by surface and microchannel roughness.

Porous media are an integral part of energy conversion and storage electrochemical devices. Among them, we have gas diffusion layers (GDLs) and catalyst layers used in polymer electrolyte membrane fuel cells (PEMFCs), as well as fibrous electrodes used in redox flow batteries (RFBs). These porous media must fulfill several critical functions, such as providing a transport pathway for reactants/products through its pore volume and ensuring charge and heat conduction through its solid structure. Catalyst layers and active electrodes have the added functionality of providing a reactive surface area. Conduction in proton-exchange membranes (PEM) through water-filled ionic channels can also be modeled using percolation theory. In this talk, a multiscale continuum-pore network formulation is presented to model two-phase transport in GDLs. The composite model incorporates a control volume mesh at the layer scale, which embeds a structured cubic pore network. Capillary transport is simulated using the discrete pore network, considering the Purcell toroid model to determine the local entry capillary pressures of the fibrous material. The pore-network model is also used to determine analytically local anisotropic effective transport properties (local effective diffusivity and permeability), which are mapped into the CV mesh to simulate gas transport within the porous layer using a continuum formulation. A comparison with experimental data is presented in terms of capillary pressure curves, water distribution and effective diffusivity in carbon-paper GDLs.

Solution blow spinning (SBS), a technique for nanofibers production, is being in use since 2009. for the preparation of polymeric nonwoven fibrous membranes. During the spinning, a polymer solution is pumped through the inner nozzle while the high-velocity gas flows through the outer nozzle, which causes rapid solvent evaporation and formation of nanofibrous membrane on a collector positioned at a certain distance from the nozzle. The diameter of produced fibers is usually in the submicronic and nanometer range, whereby nanofibers have specific morphology and exhibit some unique properties, different from their micrometer-sized counterparts. The SBS technique up-to-date is optimized for a wide variety of synthetic polymers. Their current and potential application lies in fluid filtration, medical scaffolds, energy storage units, and many more. At the Department of Materials Science and Engineering and Chemical Engineering at UC3M (Group of Polymer Composite Materials and Interphases), the current research is being conducted to optimize SBS production of materials from biopolymers, such as cellulose, and chitosan. The advantages of biopolymeric materials are many, the most important being that they are abundant in nature, and biodegradable. Their potential use can be in many fields, e.g. food packaging or medical materials, due to their non-toxicity and biocompatibility.

Acknowledgement: Researcher Ana Kramar acknowledges support from the CONEX-Plus program funded by Universidad Carlos III de Madrid and the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 801538.

Electricity demand in the world is continuingly increasing, and this tendence is expected to be even more pronounced as electric vehicles will replace conventional ones. Ideally, all this energy will be produced from renewable sources, so given their intrinsic intermittency, energy storage devices are needed. From them, redox flow batteries (RFBs) are the most promising for this kind of so-called stationary applications.

Nowadays, the most implemented technology for RFBs is the one based in vanadium, due to the large number of soluble species with different oxidation states for this element, which permits to have the same electrolyte in both half-cells. Nevertheless, the main drawback for vanadium are its low abundance and high cost, joint to the fact that very acidic and corrosive solutions are used. Therefore, alternatives such as coordination compounds and water-soluble organic molecules that work in neutral media have been explored. In this case the battery capacity is limited, due to the poor solubility of the species, so solid boosters need to be added. For this system to work, several parameters have to be considered: electrochemical potential of the compound in solution (mediator) and of the solid material has to be the same or as close as possible; kinetic of the reaction need to be fast; shape of the solid should permit to insert it in the flow system without hindering the flow of the liquid phase.

To maximize the kinetics and hydrodynamic of the system, controlling and optimizing the porosity and shape of the solid booster is crucial. In this work, we use powder extrusion moulding to obtain LiFePO4 (LFP) in the shape of films, as previously employed for Li-ion battery cathodes. These films will act as solid boosters for ferrocyanide catholyte in a RFB, showing the importance of the geometry and the porosity of the solid to improve its electrochemical performance.

Mechanical forces influence the development and behavior of biological tissues. In many situations these forces are exerted or resisted by elastic compliant structures such as the own-tissue cellular matrix or other surrounding tissues. This kind of tissue-elastic body interactions are also at the core of many state-of-the-art in situ force measurement techniques employed in biophysics. This creates the need to model tissue interaction with the surrounding elastic bodies that exert these forces, raising the question: which are the minimum ingredients needed to describe such interactions? We conduct experiments where migrating cell monolayers push on carbon fibers as a model problem. Although the migrating tissue is able to bend the fiber for some time, it eventually recoils before coming to a stop. This stop occurs when cells have performed a fixed mechanical work on the fiber, regardless of its stiffness. Based on these observations we develop a minimal active-fluid model that reproduces the experiments and predicts quantitatively relevant features of the system. This minimal model points out the essential ingredients needed to describe tissue-elastic solid interactions: an effective inertia and viscous stresses.

During the last decade, organ-on-a-chip technologies have become good candidates for disease modelling and drug testing showing more physiologically relevant cell and system behaviors. This turns to be specially interesting for skin models due to the European prohibition of cosmetic testing on animals. Although a growing field of research, several limitations are present in these technologies and tissue models are still simple and mostly based on cell monolayers. We have developed a system based on a parallel flow to generate a three-dimensional construct inside a microfluidic chip resembling the structure of the skin. Using the parallel flow, a fibrin gel containing fibroblasts was introduced in the device as the dermal compartment of the skin; once gelled, a monolayer of keratinocytes were seeded on top to form the epidermal layer. Later work will focus on characterizing this dermo-epidermal equivalent and to induce epidermal differentiation to generate a completely mature skin.

The cerebrospinal fluid (CSF) in the spinal canal undergoes an oscillatory motion, driven mainly by intracranial pressure fluctuations that occur with each heartbeat as a result of the cyclic variation of the cerebrovascular blood volume. This pressure fluctuation drives CSF periodically into and out of the compliant spinal canal, where it is accommodated by the compression of the venous and fatty tissue in the epidural space that surrounds the dural sac. The resulting motion has peak velocities of the order of a few cm/s in the cervical region, progressively diminishing along the canal to reach much smaller values in the lumbar region.

In addition to the cardiac cycle, respiration also produces an oscillation of the CSF in the spinal canal, at a much lower frequency (12-18 cycles per minute in adults). Contrary to the cardiac-driven motion that is mainly driven from pressure variations at the cerebral end of the canal, the respiratory-driven motion has been hypothesized to be induced by pressure variations in the venous plexus surrounding the entire length of the canal, which can become especially important during abdominal breathing.

Here we present novel cardiac- and respiratory-gated measurements of CSF motion using phase-contrast magnetic resonance imaging, together with simple models to elucidate the physical mechanisms behind both modes of motion.

Excitable systems occur frequently in both living and engineering systems. Forest fires, the propagation of axon potentials or the cAMP waves of the amoebae Dictyostelium, are familiar yet still fascinating systems that exhibit excitability. Previous works have shown that topologically complex networks interconnecting explicitly oscillatory or excitable elements that are subject to a refractory time after each excitation, can display rich emerging dynamics. But what if such excitable elements are not (presumably) available? In this talk, we propose a realization of a fluidic resistor with non-monotonic differential resistance, and discuss how a connected series of such fluidic elements could result in excitatory-like behavior, without an explicit refractory time. In the absence of any time dependence in the pressure input and output the system exhibits emerging dynamics in the form of self-sustained waves, which travel through the tubes. Using finite element hydrodynamic simulations we explore the behavior of the non-linear fluidic element, show internal accumulation and depletion of volume in the tube, akin to a fluidic capacitance, and a long range volume pressure coupling, all necessary components for the excitable behavior of the fluidic system.

The objective of this work is the manufacturing of a monopole antenna prototype using a novel metal called eGaIn as a conductor. This alloy is liquid at room temperature, which makes it ideal for different applications. Furthermore, unlike its traditional competitors such as mercury or cesium, it is not toxic nor flammable. Hence, this project belongs to two main fields, antenna theory and material engineering.

Nonetheless, working with a liquid metal implies that it is necessary to take into account not only its electrical performance but also its fluidic behavior. As a consequence, several challenges would emerge, conditioning its applications and use cases.

Since the project is carried out from zero, the metal is observed and studied to design, manufacture, refine and measure a model device which can prove whether its behavior corresponds to what is found in literature. The manufacturing process would also allow to grasp how liquid metal should be manipulated.

The outcome is a successful prototype that could be manufactured and measured indeed, ensuring that it works as a monopole should do. Nevertheless, the result does not behave exactly as it was predicted or established, as described during the presentation.

In mildly supersaturated solutions, bubbles generally grow by diffusion. In this environment, when a gas bubble is exposed to ultrasonic waves, it will experience a sudden massive growth rate enhancement. This event takes place when the working frequency of the ultrasound matches the natural frequency of the bubble and is of the order of hundreds of milliseconds. We show that when a bubble approaches resonance, it undergoes non-linear oscillations which generate a strong microstreaming flow. This results in a bubble growth rate which exceeds the diffusive mass transfer capabilities by two orders of magnitude. By the application of PIV techniques, we are able to track and model the streaming patterns around the bubble as it oscillates, differentiating two oscillatory modes: fountain (weak) and antifountain (strong) modes. The gross increase in the bubble radius specifically occurs during the latter. In addition, we achieve a quasi-continuous mass transfer enhancement by programming chirps of decreasing frequency. This configuration is potentially relevant to novel medical treatments involving targeted drug delivery and industrial applications where bubble accumulation becomes detrimental.