Subretinal injections (SI) are used to deliver gene therapies for inherited retinal diseases, yet the optimal injection parameters remain undefined. This study used theoretical and experimental models to quantify the relationship between injection pressure, flow dynamics, and residual flow. A theoretical model (TM) was developed based on the Hagen-Poiseuille law and the theory of a jet immersed in the same liquid. An experimental model (EM) was constructed to allow for measuring flow and residual flow across injection pressures ranging from 0 to 20 psi. We assessed the effects of ambient pressure, injection system tubing length, and syringe priming technique. A minimum pressure of 6 psi was required to generate a detectable flow in the EM. Jet speed increased with the square root of injection pressure, aligning with theoretical predictions. Residual flow persisted for 28-47 seconds after injection and increased logarithmically with injection pressure. Elevated ambient pressure (45 mmHg) only reduced flow at lower injection pressures. The "lock-and-load" priming method decreased jet speed and increased residual flow compared to the "load-and-lock" method. Both TM and EM quantified SI flow dynamics, with EM demonstrating residual flow at all tested pressures. To minimize complications, clinicians should use the lowest injection pressure and allow sufficient time for the cannula to be withdrawn from the subretinal space.

We present a combined theoretical and numerical investigation of the inertial exit dynamics of a long horizontal circular cylinder vertically lifted out of a finite-size liquid bath at constant velocity. The various steps of the exit dynamics are studied in detail: from the formation of a bulge on the surface ahead of the cylinder to the coating of the cylinder by a liquid film while crossing the interface. We focus on inertial dynamics, a regime characteristic of large exit velocities, i.e. large Reynolds numbers ( $500 \lt \textit{Re} \lt 10\,000$ ) and negligible interfacial effects. The dynamics is investigated through two-dimensional computations of the Navier-Stokes equations using a finite element method with moving boundaries. We describe in detail the exit dynamics while emphasising the effect of various parameters on surface deformation and resistive force. We identify subtle effects and interplay, such as initial free-surface response after impulsive start-up, the important role of the lateral bounding of the reservoir, and the close relationship between wake size and surge amplitudes as revealed by comparing with free-slip cylinder simulations. All these aspects are shown to be crucial to accurately predict the coated film thickness and the exit force.

Liquid films on moving substrates are used in dip-coating processes to form uniform protective layers. Controlling free-surface waves is essential due to the film's inherent linear instability. Therefore, we develop a linear feedback controller to regulate the film toward a desired flat state by modulating the free-surface shear and pressure, with feedback gains derived analytically from linearised equations. Control performance is assessed for finite-amplitude waves using a Weighted Integral Boundary-Layer (WIBL) model at reduced Reynolds number δ= 8. We identify parameter regimes in which pressure feedback is linearly destabilising while shear is stabilising, and vice versa, with the control mechanisms determined by the balance between the kinematic and dynamic wave velocities. Both stabilising and destabilising combinations of feedback coefficients can drive finite-amplitude waves toward the flat state h̅=1.1 in finite time. In pressure-unstable regimes, the control induces a limit-cycle behaviour, in which long waves decay slowly due to the interplay between thickness and slope terms. The travelling-wave solution, although it decays slowly, moves against gravity, whereas other combinations reduce the wave amplitude in the direction of uncontrolled propagation. These results provide a foundation for higher-Reynolds-number studies and the design of industrially feasible actuator layouts.

An antibubble is an object in which a liquid core is separated from a surrounding liquid by a thin spherical gas shell. Such structures can be generated when a droplet crosses a soap film with sufficient kinetic energy to become wrapped by the film, which then merges upon contact with a liquid bath. When droplets fall through multiple soap films, multilayer antibubbles can be generated. In this paper, the conditions required to successfully generate monolayer and multilayer packed droplets are studied in order to optimize the formation of monolayer and multilayer antibubbles. In particular, the geometrical factors and the composition of the liquid that makes up both the droplets and the soap films are discussed. The stability of antibubbles generated using this method is also investigated as a case study. The robust formation of monolayer and multilayer antibubbles is a key to open opportunities in drug delivery without the use of either oil or solid compounds. Moreover, this method allows for the production of monodisperse, millimetric, and multilayered antibubbles.

We investigate the transport and rapid dissolution of carbon dioxide (CO2) bubbles in methanol within a horizontal circular microchannel operated under a pressure of 3 bar. This experimental configuration, featuring coaxial gas injection and high gas volume fractions, enables the observation of nearly the entire dissolution of individual spherical bubbles within the field of view. The vertical off-centering and progressive size reduction of the bubbles allow a broad exploration of key parameters such as bubble size, eccentricity, and inter-bubble distance. Unlike classical microfluidic studies focusing on isolated bubbles in fully developed flows, our system captures complex transient phenomena including initial bubble acceleration, lateral migration, and strong hydrodynamic interactions between closely spaced bubbles. We show that axial and vertical velocities significantly deviate from quasi-steady-state predictions, especially in the presence of confinement and collective effects. In contrast, the Sherwood number Sh remains well described by the theoretical scaling root law Sh proportional to Pe(d*)3, with Pe the P & eacute;clet number and d* the reduced bubble diameter, for bubbles near the microchannel centerline, highlighting the robustness of boundary-layer-based models. Importantly, our experiments reach an unprecedented range of high P & eacute;clet numbers for microfluidics (Pe is an element of [8700-52, 000]), providing the first direct experimental access to high-dissolution regimes, and reveal that transient and collective events can enhance the overall mass transfer by about 8% and up to 60% compared to isolated bubbles. These findings demonstrate the importance of accounting for such dynamics when designing efficient gas-liquid absorption processes, and provide a detailed experimental framework for future model validation under realistic microfluidic conditions, while opening new perspectives for optimizing mass transfer strategies.

Film flow on vertical fibers has been studied for the past 30 years. However, it is only recently that the influence of the nozzle on the film and bead dynamics has been demonstrated. Consequently, similar to the regimes observed with a faucet, both a dripping and a jetting regime have been reported for film flow on fibers. This study investigates the dynamics of thin liquid films on vertical fibers near the nozzle outlet using both experimental and numerical approaches, drawing insights from dripping faucet studies. This allows for a clear definition of the recently observed dripping and jetting regimes, with a particular focus on gaining a deeper understanding of the flow and formation of beads in the dripping regime, as well as the transition to jetting.

In this paper, we investigate the breakup dynamics of an inviscid liquid bridge. The volume of the liquid bridge is reduced through perturbative drainage on one side, which leads to spontaneous breakup and forms a satellite droplet. A two-dimensional model is applied to describe the liquid dynamics, and the whole breakup process up to the second pinchoff is considered. Following the previous experimental work conducted by D. Li et al. [Phys. Fluids 34, 084105 (2022)], we focus on the asymmetric behavior and the momentum of satellite droplets. The process can be sequentially divided into a quasistatic stage, an instability stage, and a pinchoff stage. The key parameter is the length-to-radius ratio of the liquid bridge, denoted L. For a short liquid bridge with L 4.1, the profile remains symmetric throughout the process. The pinchoff occurs simultaneously on both sides, leading to a satellite droplet without axial momentum. For a long liquid bridge with L > 4.1, it becomes asymmetric at the onset of capillary instability, leading to nonsimultaneous pinchoff and nonzero satellite momentum. The transition from symmetric to asymmetric breakup occurs when the dominant perturbation modes of the critical state shift from even-modes to odd-modes. In contrast to the conclusion by Li et al., we argue that the momentum of the satellite droplet stems primarily from the capillary impulses after the flattening moment before the first pinchoff. This work can help eliminate or utilize satellite droplets in practical applications.

Metallic coatings are used to enhance the durability of metal surfaces by protecting them from corrosion. These protective layers are typically deposited in a fluid state via a liquid film. Controlling instabilities in the liquid film is crucial to achieving uniform, high-quality coatings. This study explores the possibility of controlling liquid films on a moving substrate using a combination of gas jets and electromagnetic actuators. To model the three-dimensional liquid film, we extend existing integral models to incorporate the effects of electromagnetic actuators. The control strategy was developed within a reinforcement learning framework, in which the proximal policy optimization (PPO) algorithm interacts with the liquid film via pneumatic and electromagnetic actuators to optimize a reward function that accounts for instability-wave amplitude through a trial-and-error process. The PPO identified an optimal control law that reduced interface instabilities via a novel mechanism: gas jets push crests, and electromagnets raise troughs via the Lorentz force.

Electromagnetic wiping systems allow to pre-meter the coating thickness of the liquid metal on a moving substrate. These systems have the potential to provide more uniform coating and significantly higher production rates compared to pneumatic wiping, but they require substantially larger amounts of energy. This work presents a multi-objective optimization accounting for (1) maximal wiping efficiency (2) maximal smoothness of the wiping meniscus, and (3) minimal Joule heating. We present the Pareto front, identifying the best wiping conditions given a set of weights for the three competing objectives. The optimization was based on a 1D steady-state integral model, whose prediction scales according to the Hartmann number (Ha). The optimization uses a multi-gradient approach, with gradients computed with a combination of finite differences and variational methods. The results show that the wiping efficiency depends solely on Ha and not on the magnetic field distribution. Moreover, we show that the liquid thickness becomes insensitive to the intensity of the magnetic field above a certain threshold and that the current distribution (hence the Joule heating) is mildly affected by the magnetic field’s intensity and shape.

Passive and effective fluid capture and transport at small scale is crucial for industrial and medical applications, especially for the realisation of point-of-care tests. Performing these tests involves several steps, including capturing biological fluid, aliquoting, reacting with reagents, and reading the results. Ideally, these tests must be fast and offer a large surface-to-volume ratio to achieve rapid and precise diagnostics with a reduced amount of fluid. Such constraints are often contradictory as a high surface-to-volume ratio implies a high hydraulic resistance and hence a decrease in the flow rate. Inspired by the feeding mechanism of hummingbirds, we propose a frugal fluid capture device that takes advantage of elastocapillary deformations to enable concomitant fast liquid transport, aliquoting, and high confinement in the deformed state. The hierarchical design of the device - that consists in vertical grooves stacked on an elastic sheet - enables a two-step sequential fluid capture. Each unit groove mimics the hummingbird's tongue and closes due to capillary forces when a wetting liquid penetrates, yielding the closure of the whole device in a tubular shape, in the core of which additional liquid is captured. Combining elasticity, capillarity, and viscous flow, we rationalise the fluid-structure interaction at play both when liquid is scarce and abundant. By functionalising the surface of the grooves, such a passive device can concomitantly achieve all the steps of point-of-care tests, opening the way for the design of optimal devices for fluid capture and transport in microfluidics.

When transported by a pressure driven flow in a cylindrical pipe, bubbles may exhibit very fast velocities. In this paper, we show that, when the bubbles are largely deformable, that is, at large capillary numbers Ca, the velocity of the bubble can be larger than the maximal velocity of the flow that transports them. We call this regime "super-fast". However, the situation changes when inertia comes at play for increasing Reynolds numbers Re, and the relative velocity of the bubble drops for sufficiently large Laplace number, defined as La = Re/Ca. In this article, we uncover the conditions for which the super-fast regime exists : the deformability of the drop is crucial, and hence the capillary number needs to be larger than a critical value, yet smaller than a threshold above which the bubble breaks up. The two limiting capillary numbers are presented in a phase diagram as a function of the bubble size and the Laplace number.

This paper presents an empirical analysis of a film flow dropping-off from an inclined fiber substrate, where the fiber diameter is on the order of the capillary length. The investigation aims to elucidate the dynamics of beads traveling down the inclined fibers. We explore the parameters influencing the bead’s final shape and their stability on the substrate (in terms of detaching or not detaching) in a flow driven by gravity and affected by surface tension and inertia. Three different growth modes and their connection to the two main existing drop-off mechanisms of beads on inclined fibers, namely drop-off due to unsaturated growth and drop-off due to coalescence events, are established and demonstrated. A new parameter, Φ , is introduced, which includes information about the shape of the beads and its connection to the flow conditions. If Φ >1 , the bead exhibits a rather sinusoidal shape, indicating that its formation is predominantly influenced by surface tension. If Φ < 1 , the bead is subjected to front-steepening due to an inertia-dominated flow. If Φ =1 , the bead’s tail aligns with the direction of gravity, experiencing unsaturated growth until it drops off, which indicates a flow condition driven primarily by gravitational forces. Furthermore, this study offers a quantitative experimental basis for flow parameters on inclined fibers, including Nusselt film thickness, healing length within the unperturbed film and rate of growth of beads at the end of healing length, beads crest height, beads velocity, and parameter Φ for future comparison to theoretical and numerical modelling.

Metallic coatings are used to enhance the durability of metal surfaces by protecting them from corrosion. These protective layers are typically deposited in a fluid state via a liquid film. Controlling instabilities in the liquid film is crucial to achieving uniform, high-quality coatings. This study explores the possibility of controlling liquid films on a moving substrate using a combination of gas jets and electromagnetic actuators. To model the 3D liquid film, we extend existing integral models to incorporate the effects of electromagnetic actuators. The control strategy was developed within a reinforcement learning framework, in which the Proximal Policy Optimisation (PPO) algorithm interacts with the liquid film via pneumatic and electromagnetic actuators to optimise a reward function that accounts for instability-wave amplitude through a trial-and-error process. The PPO identified an optimal control law that reduced interface instabilities via a novel mechanism: gas jets push crests, and electromagnets raise troughs via the Lorentz force.

Water electrolysis is a cornerstone of sustainable hydrogen production; however, gas bubble formation on electrode surfaces reduces efficiency by increasing resistance and hindering mass transport. In this study, we examine bubbling behaviors using a novel protocol to determine the supersaturation required for hydrogen nucleation in a membraneless micro-electrolyzer. Three distinct bubbling regimes are identified: (i) region 0 (bubble-free electrolysis), (ii) region 1 (hydrogen-only bubbles), and (iii) region 2 (hydrogen-oxygen bubbles). A comprehensive modeling framework, which combines numerical simulations and an analytical boundary layer approximation, is used to describe the evolution of dissolved gases along the microchannel. Experiments reveal that hydrogen nucleation occurs consistently when the local supersaturation ratio exceeds 3 to 5, while oxygen requires higher current thresholds. The good agreement between the experimental results and the model validates the accuracy of the approach. Furthermore, we establish a correlation between the nucleation position of the hydrogen bubbles along the electrode and both the reaction rate and the flow rate. This correlation is of practical interest for the prediction of bubble formation in electrolyzers. These insights pave the way for selectively promoting or suppressing specific bubbling regimes, enabling more cost-effective electrolysis systems with improved hydrogen purity and minimized gas cross-over.

When transported by a pressure-driven flow in a cylindrical capillary, bubbles may exhibit very fast velocities. In this paper, we show that when the bubbles are largely deformable, that is, at large capillary numbers Ca, the velocity of the bubble can be larger than the maximal velocity of the flow that transports them. We call this regime “super-fast”. However, the situation changes when inertial effects become significant at higher Reynolds numbers ( Re ), leading to a decrease in the bubble’s relative velocity for sufficiently large values of the Laplace number, defined as La= Re/ Ca . In this article, we uncover the conditions for which the super-fast regime exists: the deformability of the bubble is crucial, and hence the capillary number needs to be larger than a critical value, yet smaller than a threshold above which the bubble breaks up. The two limiting capillary numbers are presented in a phase diagram as a function of the bubble size and the Laplace number.

The fabrication of microgels, particularly those ranging from tens to hundreds of micrometers in size, represents a thriving area of research, particularly for biologists seeking controlled and isotropic media for cell encapsulation. In this article, we present a novel and robust method for producing structurally homogeneous alginate beads with a reduced environmental footprint, employing a co-flow focusing microfluidic device. These beads can be easily recovered in an oil-free aqueous medium, making the fabrication method highly suitable for diverse applications. We demonstrate precise control over the production of perfectly spherical beads across a wide range of diameters, from about 30 to 300 mu m. We then measure Young's moduli of the beads, revealing a wide accessible range from 90 Pa to 11 kPa, contingent upon controlling the type (e.g. chain length) and concentration of alginate. A novel microfluidic method produces perfectly spherical and structurally homogeneous alginate beads directly in water, reducing environmental impact. By adjusting the alginate composition, this method allows access to a wide range of Young's moduli.

Experimental investigations of the exit dynamics of a horizontal cylindrical object were performed in water and silicone oil (50 cSt). The fully immersed cylinder was initially at rest in a still fluid tank before being pushed (or pulled according to the measurement procedure) upwards at a constant velocity. Firstly, we demonstrate that these conditions are better satisfied for a large aspect ratio cylinder equipped with vertical side plates. Secondly, the influence of the initial depth on the liquid entrained and the wake generated by the cylinder is discussed. The deformation of the bath is found to be independent of the starting depth when the starting depth is larger than 6 times the cylinder diameter. In the present case, this criterion reflects also the finite acceleration of the cylinder to reach the determined constant exit velocity. Measurements in a range of exit speeds between 0.1 and 1 m/s indicate that the thickness of the liquid above the cylinder, when the cylinder starts crossing the interface, increases with the speed according to a logarithmic law of the Froude number. During the subsequent drainage, the evolution of the coated liquid thickness is found to first decrease exponentially with time just after the crossing of the interface. At later times, a change of regime occurs and the drainage follows the inverse of the square root of time irrespective of the crossing speed. Finally, the force necessary to maintain a constant exit speed during the motion of the cylinder inside and outside the bath is analyzed. This global measurement of the entrained liquid confirms the square root scaling of the thinning with time during the drainage process.