Manuel Rubio

The Liebau effect refers to the generation of unidirectional net fluid flow induced by periodic compression and release actions on a compliant tube within systems comprising tubes with varying elastic properties. This study introduces a semi-empirical model that elucidates Liebau pumping in scenarios involving impedance pumping, asymmetric pumping, or a combination thereof. The authors have established a Liebau pump configuration, previously employed in their experimental and numerical investigations. Experiments were conducted to validate the model in a facility designed to replicate the model configuration. A parametric analysis was performed using this model, emphasizing the most critical parameters influencing the pump operation and their interrelations. Furthermore, a dimensionless formulation is proposed, which not only aids in parametric analysis but also provides practical guidelines for optimizing the pump performance. This work extends previous models by incorporating the impedance pumping mechanism and examining the impact of the mechanical properties of the compliant tube.

The Liebau effect generates a net flow without the need for valves. For the Liebau effect pumping phenomenon to occur, the pump must have specific characteristics. It needs tubes with different elastic properties and an actuator to provide energy to the fluid. The actuator periodically compresses the more flexible element. Furthermore, asymmetry is a crucial factor that differentiates between two pumping mechanisms: impedance pumping and asymmetric pumping. In this work, a model based on the fluid dynamics of an asymmetric valveless pump under resonant conditions is proposed to determine which parameters influence the pumped flow rate. Experimental work is used to validate the model, after which each of the parameters involved in the pump performance is dimensionlessly analysed. This highlights the most significant parameters influencing the pump performance such as the actuator period, length tube ratio and tube diameters. The results point out ways to increase a valveless asymmetric pump’s net-propelled flow rate, which has exciting applications in fields such as biomedicine. The model also allows for predicting the resonance period, a fundamental operating parameter for asymmetric pumping.

Extracorporeal Membrane Oxygenation (ECMO) is a modality of extracorporeal life support which allows temporary support in cases of cardiopulmonary failure and cardiogenic shock. This study presents a valveless pump that works by the Liebau effect as a possible pumping system in ECMO circuits, replacing the current roller and centrifugal pumps. For this purpose, a mock circulatory loop emulating the haemodynamic of the right part of the heart has been constructed. A veno-venous ECMO circuit with the integrated Liebau pump has been incorporated to analyse its performance. The Liebau pump in the ECMO circuit showed a flow assistance in the range of paediatric ECMO and low blood flow range for adults. In addition, experimental tests conducted demonstrated the advantage of the Liebau pump over currently used pumps as the ability to generate a pulsatile flow, which has many advantages in biomedical applications.

We numerically study the influence of a soluble surfactant on the microjetting mode of the liquid–liquid flow focusing configuration. The surfactant adsorbs on the interface next to the feeding capillary and accumulates in front of the emitted jet, significantly lowering the surface tension there. The resulting Marangoni stress substantially alters the balance of the tangential stresses at the interface but does not modify the interface velocity. The global stability analysis at the minimum flow rate stability limit shows that the Marangoni stress collaborates with soluto-capillarity to stabilize the microjetting mode. Our analysis unveils the noticeable effect of the Marangoni stress associated with the surface tension perturbation. Surfactant diffusion and desorption hardly affect the stability limit. Transient numerical simulations show how subcritical and supercritical base flows respond to a spatially localized initial perturbation. Our parametric study indicates that the minimum flow rate ratio depends on the adsorption constant and the surfactant concentration through the product of these two variables. The surfactant stabilizing effect increases with the outer stream flow rate. We show that surfactants not only stabilize the microemulsion resulting from the jet breakup in hydrodynamic focusing, but also allow for the reduction of droplet size. Our findings advance the fundamental understanding of the complex role of surfactants in tip streaming via hydrodynamic focusing. In particular, our results contradict the common assumption that adding surfactant favours tip streaming simply because it reduces the meniscus tip surface tension.

We study numerically the microjetting mode obtained when a fluid is injected through a tube submerged in a uniaxial extensional flow. The steady solution to the full nonlinear Navier–Stokes equations is calculated. We obtain the linear global modes determining the linear stability of the steady solution. For sufficiently large outer viscosity, the flow remains stable for infinitely small values of the injected flow rate. This implies that jets with vanishing diameters can be produced regardless of the jet viscosity and outer flow strength. For a sufficiently small inner-to-outer viscosity ratio, the microjetting instability is associated only with the flow near the entrance of the jet. The tapering meniscus stretches and adopts a slender quasiconical shape. Consequently, the cone tip is exposed to an intense outer flow, which stabilizes the flow in the cone–jet transition region. This work presents the first evidence that fluid jets with arbitrarily small diameters can be stably produced via tip streaming. The results are related to those of a droplet in a uniaxial extensional flow with its equator pinned to an infinitely thin ring. The pinning of the equator drastically affects the droplet stability and breakup.

Impedance pumping and asymmetric pumping are the main mechanisms of the Liebau effect, a type of valveless pumping. The primary objective of this study is to enhance the current understanding of the mechanics and functionality of a Liebau pump. To achieve this, a prototype pump is constructed, incorporating several novel features: a pinch frequency control system to optimize pump performance, a planar actuator developed using soft robotic technology, and a compliant chamber designed to mitigate flow fluctuations and prevent reverse flow, a common feature of this pumping method. Experimental tests conducted with this prototype demonstrate that the Liebau pump holds potential as a viable technology for fluid pumping in biomedical applications, especially when pulsatile flow is essential or when hemolysis needs to be minimized.

We study experimentally and numerically the onset of tip streaming in an electrified droplet. The experiments show that, for a sufficiently small dimensionless conductivity, the droplet apex oscillates before ejecting a liquid jet. This effect is caused by the limited charge transfer from the bulk to the interface. This reduces the electrostatic pressure at the droplet's stretching tip, preventing liquid ejection. This reduction of the electrostatic pressure is compensated for by the electric shear stress arising during apex oscillations, which eventually leads to the jet formation. The stability limit calculated from the global stability analysis perfectly agrees with experimental results. However, this analysis predicts non-oscillatory, non-localized instability in all the cases, suggesting that both the oscillatory behavior and the small local scale characterizing tip streaming arise during the nonlinear droplet deformation.

We study experimentally and numerically the thinning of Newtonian leaky-dielectric filaments subjected to an axial electric field. We consider moderately viscous liquids with high electrical permittivity. We analyze the influence of the electric field on the formation of satellite droplets from the breakup of the filaments in the experiments. The electric force delays the free surface pinching. Two electrified filaments with the same minimum radius are thin at the same speed regardless of when the voltage is applied. The numerical simulations show that the polarization stress is responsible for the pinching delay observed in the experiments. Asymptotically close to the pinching point, the filament pinching is dominated by the diverging hydrodynamic forces. The polarization stress becomes subdominant even if this stress also diverges at this finite-time singularity.