arXiv subjects

Roberto Zenit

Publications and source records attributed to Roberto Zenit.

At least 19 recordsLinked to original sources

arXiv2026

Peristaltic pumping under poroelastic confinement

Low Reynolds number flow near a poroelastic interface can be found across scales in biological and engineered systems. We develop a 2D model of peristaltic flow confined under a poroelastic solid. In this geometry, the lower boundary is an infinite train of traveling waves which pump fluid along a channel. The upper boundary of the flow is a poroelastic half space. The flow and deformation are solved analytically by an asymptotic expansion in the peristaltic amplitude and depend nonlinearly on dimensionless poroelastic stiffness, permeability, and interfacial slip. We quantify the effect of material properties on the poroelastic fluid-structure interaction. Peristaltic flow through the channel is inhibited by poroelastic confinement owing to increased viscous dissipation across the interface and energy loss in deforming the elastic solid. Permeability and slip interact with the material stiffness to produce material dependent regimes of forward or backward interstitial flow within the poroelastic domain. The maximum Darcy flow is found to occur at permeability values that optimize the elastic matrix interaction.

physics.flu-dyn↗

arXiv2026

Maximal spreading of impacting viscoelastic droplets

Droplet impact and spreading on solid substrates are well understood for Newtonian fluids, yet how viscoelasticity alone modifies the maximal spreading remains unclear. To identify the mechanisms governing the spreading dynamics, we conducted impact experiments and measured the maximal spreading diameter to quantify how fluid elasticity modifies the maximal spreading of impacting droplets. Experiments were performed using fluids within a narrow range of viscosity and surface tension, but with varying relaxation times. For a wide range of conditions, viscoelastic droplets follow a similar behavior as Newtonian ones; however, their maximal spreading diameter is significantly reduced compared with the Newtonian behavior when the Deborah number is of order unity. These observations are rationalized by incorporating the viscoelastic effects into a classical energy balance model. The scaling argument obtained from this model explains the reported reduction in maximal spreading and identifies the range of fluid properties for which the strongest viscoelastic effects emerge.

physics.flu-dyn↗

arXiv2025

The mechanics of the squash nick shot

Squash is a widely popular racket sport, practiced by millions of people worldwide, played inside a walled court. When played well, players can last for several minutes before the ball bounces twice on the floor. There is, however, an unanswerable shot. When the ball hits the nick between a vertical wall and the floor, under certain conditions, it rolls without any vertical bounce. We study this process experimentally. We determined that the ball must hit the vertical wall first at a narrow range of heights above the floor, but most importantly, it must touch the floor before finishing its rolling time on the vertical wall. When the rolling time is shorter than the contact time, the vertical momentum is canceled due to a mechanical frustration condition. This behavior is explained considering a contact model, which agrees with the experimental observations. We argue that this concept could be relevant to the design of rolling shock dampers with many possible practical applications.

physics.class-ph↗

arXiv2025

Elastic Pseudoturbulence in Polymer Solutions

We study the effects of polymer additives on pseudoturbulence induced by a swarm of bubbles rising in a quiescent fluid. We find that, beyond a critical polymer concentration, the energy spectra of velocity fluctuations in bubble-induced turbulence decay more steeply with respect to the wavenumber $k$. This new scaling is significantly steeper than the classical $k^{-3}$ scaling observed for bubbles in Newtonian fluids; it is independent of the gas volume fraction in the inertial limit and occurs within the length scales between the bubble wake length and the bubble diameter. Furthermore, we provide strong evidence that the presence of polymers enhances the coherence of the flow, highlighting the significant role of polymer additives in modifying the characteristics of pseudoturbulence.

cond-mat.soft↗

arXiv2025

Gas bubble dynamics

The study of gas bubble dynamics in liquids is justified by the numerous applications and natural phenomena where this two-phase flow is encountered. Gas bubbles move as forces are applied to them; their dynamics are full of nuances that need to be addressed carefully. Since the mass of gas bubbles is practically negligible, in comparison to that of the surrounding liquid, their reaction to the fluid is controlled by the added mass acceleration and is thus impacted by all the forces arising from the fluid action. Furthermore, since their surface can be deformed by the same forces acting on them, their shape may change leading to changes in their resistance to move, the drag force, and therefore affecting their speed and their interaction with the surrounding flow which is often turbulent. The liquid rheology, as well as its surfactant content can also affect the bubble shape and motion as well. Understanding these issues, in addition to the effect of interactions with other bubbles, walls, and non-uniform flows, provides sufficient elements to model and predict bubble behavior through the solution of dynamic equations. In this review, we cover the key aspects of non-condensable gas bubble dynamics. We survey classical references on the subject and provide an overview of the main findings in the past 20 years. We conclude with a scope and suggestions for future research directions, with special attention to the dynamics of bubble in turbulence, in non-Newtonian fluid and/or in the presence of electrolytes.

physics.flu-dyn↗

arXiv2024

Helical locomotion in dilute suspensions

Motivated by the aim of understanding the effect of media heterogeneity on the swimming dynamics of flagellated bacteria, we study the rotation and swimming of rigid helices in dilute suspensions experimentally and theoretically. We first measure the torque experienced by, and thrust force generated by, helices rotating without translating in suspensions of neutrally buoyant particles with varying concentrations and sizes. Using the ratio of thrust to drag forces $\xi$ as an empirical proxy for propulsion efficiency, our experiments indicate that $\xi$ increases with the concentration of particles in the fluid, with the enhancement depending strongly on the geometric parameters of the helix. To rationalize these experimental results, we then develop a dilute theoretical approach that accounts for the additional hydrodynamic stress generated by freely suspended spheres around the helical tail. We predict similar enhancements in the drag coefficient ratio and propulsion at a given angular speed in a suspension and study its dependence on the helix geometry and the spatial distribution of the suspended spheres. These results are further reinforced by experiments on freely swimming artificial swimmers, which propel faster in dilute suspensions, with speed increases over $60 \%$ for optimal geometries. Our findings quantify how biological swimmers might benefit from the presence of suspended particles, and could inform the design of artificial self-propelled devices for biomedical applications.

physics.flu-dyn↗

arXiv2024

Low Reynolds number pumping near an elastic half space

Previous studies on peristalsis, the pumping of fluid along a channel by wave-like displacements of the channel walls, have shown that the elastic properties of the channel and the peristaltic wave shape can influence the flow rate. Motivated by the oscillatory flow of cerebrospinal fluid along compliant perivascular spaces, we consider a prescribed wave motion of a single boundary which pumps fluid at small Reynolds number near an elastic half space. We investigate the relationship between flow rate and elastic deformation as a function of the fluid and solid properties. We consider transverse and longitudinal motion of the driving peristaltic wave. We find that a transverse peristaltic wave produces net forward flow and induces elastic motion in which all material points oscillate uniformly. Conversely, a longitudinal peristaltic wave produces a net backward flow and drives elastic motion which is nonuniform in the elastic solid. We use dimensional values relevant to the flow of cerebrospinal fluid and find agreement with in vivo velocity data.

physics.flu-dyn↗

arXiv2024

Velocity fluctuations for bubbly flows at small Re

We experimentally investigate the effect of Reynolds number (Re) on the turbulence induced by the motion of bubbles in a quiescent Newtonian fluid at small Re. The energy spectra, $E(k)$, is determined from the decaying turbulence behind the bubble swarm obtained using particle image velocimetry (PIV). We show that when Re $\sim$ $O$(100), the slope of the normalized energy spectra is no longer independent on the gas volume fraction and the $k^{-3}$ subrange is significantly narrower, where $k$ is the wavenumber. This is further corroborated using second-order longitudinal velocity structure function and spatial correlation of the velocity vector behind the bubble swarm. On further decreasing the bubble Reynolds number ($O(1) < $ Re $ < O(10)$), the signature $k^{-3}$ of the energy spectra for the bubble induced turbulence is replaced by $k^{-5/3}$ scaling. Thus, we provide experimental evidence to the claim by \citet{mazzitelli2003effect} that at low Reynolds numbers the normalized energy spectra of the bubble induced turbulence will no longer show the $k^{-3}$ scaling because of the absence of bubble wake and that the energy spectra will depend on the number of bubbles, thus non-universal.

physics.flu-dyn↗

arXiv2023

The Fluid Mechanics of Splatter Painting

In splatter painting, a collection of liquid droplets is projected onto the substrate by imposing a controlled acceleration to a paint-loaded brush. This work aims at unraveling the physical phenomena at play in this widespread artistic technique. By characterizing the kinematics of the splattering action (brush flicking or tapping), we identify the main force driving liquid out of the tip of the brush (hair bundle). For real or simplified brushes, the amount of liquid expelled is measured for various liquid viscosities and imposed accelerations, among other parameters. Experimental trends are successfully captured by a physical model that reproduces two distinguished limits: an inertia-dominated flow and a viscous-dominated flow in an anisotropic porous medium with parallel pores. Beyond splatter painting, our analysis provides a framework to describe mechanical drying strategies for fibrous materials or porous structures.

physics.flu-dyn↗

arXiv2023

Asymmetry of motion: vortex rings crossing a density gradient

Vortex rings are critical for thrust production underwater. In the ocean, self-propelled mesozooplankton generate vortices while swimming within a weakly stratified fluid. While large-scale biogenic transport has been observed during vertical migration in the wild and lab experiments, little focus has been given to the evolution of induced vortex rings as a function of their propagation direction relative to the density gradient. In this study, the evolution of an isolated vortex ring crossing the interface of a stable two-layer system is examined as a function of its translation direction with respect to gravity. The vortex ring size and position are visualized using Planar Induced Fluorescence (PLIF) and the induced vorticity field derived from Particle Image Velocimetry (PIV) is examined. It is found that the production of baroclinic vorticity significantly affects the propagation of vortex rings crossing the density interface. As a result, any expected symmetry between vortex rings traveling from dense to light fluids and from light to dense fluids breaks down. In turn, the maximum penetration depth of the vortex ring occurs in the case in which the vortex propagates against the density gradient due to the misalignment of the pressure and density gradients. Our results have far-reaching implications for the characterization of local ecosystems in marine environments.

physics.flu-dyn↗

arXiv2022

The presence of surfactants controls the stability of bubble chains in carbonated drinks

Bubbles appear when a carbonated drink is poured in a glass. Very stable bubble chains are clearly observed in champagne, showing an almost straight line from microscopic nucleation sites from which they are continuously formed. In some other drinks such as soda, such chains are not straight (not stable). Considering pair interactions for spherical clean bubbles, bubble chains should not be stable which contradicts these observations. The aim of this work is to explain the conditions for bubble chain stability. For this purpose, experiments and direct numerical simulation are conducted. The bubble size as well as the level of interface contamination are varied, to match the range of parameters in typical drinks. Both factors are shown to affect the bubble chain stability. The transition from stable to ustable behavior results from the reversal of the lift force, which is induced by the bubble wake. A criteria based on the production of vorticity at the bubble surface is proposed to identify the conditions of transition from stable to unstable bubble chains. Beyond carbonated drinks, understanding bubble clustering has impact in many two-phase problems of current importance.

physics.flu-dyn↗

arXiv2022

Viscoelastic levitation

The effects of viscoelasticity have been shown to manifest themselves via symmetry breaking. In this investigation, we show a novel phenomenon that arises from this idea. We observe that when a dense sphere is rotated near a wall (the rotation being aligned with the wall-normal direction and gravity), it levitates to a fixed distance away from the wall. Since the shear is larger in the gap (between the sphere and the wall) than in the open side of the sphere, the shear-induced elastic stresses are thus asymmetric, resulting in a net elastic vertical force that balances the weight of the sphere. We conduct experiments, theoretic models, and numerical simulations for rotating spheres of various sizes and densities in a Boger-type fluid. In the small Deborah number range, the results are collapsed into a universal trend by considering a dimensionless group of the ratio of elastic to gravitational forces.

physics.flu-dyn↗

arXiv2022

Hydrodynamic interaction of a bubble pair in viscoelastic shear-thinning fluids

We experimentally investigate the interaction between a pair of bubbles ascending in a stagnant viscoelastic shear-thinning fluid. In particular, we focus on the effect of bubble size, across the velocity discontinuity, on the bubble-bubble interaction. Compared to the drafting-kissing-tumbling (DKT) behavior in Newtonian fluid, bubbles in the viscoelastic shear-thinning fluid exhibit, what we call, drafting-kissing-dancing (DKD) phenomenon. In the dancing phase, the bubble pair repeatedly interchange their relative leading and trailing positions as they rise to the free surface. To gain further insights, the flow fields around the bubble pair interaction are obtained using particle image velocimetry (PIV). From the experimental results, we suggest that the elasticity, deformability, and negative wake are responsible for such an interaction between the bubble pair, thus revealing the fundamental physics of bubble clustering often observed in non-Newtonian fluids.

physics.flu-dyn↗

arXiv2021

Force and torque-free helical tail robot to study low Reynolds number microorganism swimming

Helical propulsion is used by many microorganisms to swim in viscous-dominated environments. Their swimming dynamics are relatively well understood, but detailed study of the flow fields and actuation mechanisms are still needed to realize wall effects and hydrodynamic interactions. In this letter, we describe the development of an autonomous swimming robot with a helical tail that operates in the Stokes regime. The device uses a battery-based power system with a miniature motor that imposes a rotational speed to a helical tail. The speed, direction, and activation are controlled electronically using an infrared remote control. Since the robot is about 5 centimeters long, we use highly viscous fluids to match the Reynolds number to be $\text{Re} \lessapprox 0.1$. Measurements of swimming speeds are conducted for a range of helical wavelengths, $\lambda$, head geometries and rotation rates, $\omega$. We provide comparisons of the experimental measurements with analytical predictions derived from resistive force theory. This force and torque-free neutrally-buoyant swimmer mimics the swimming strategy of bacteria more closely than previously used designs and offers a lot of potential for future applications.

cond-mat.soft↗

arXiv2020

Fractal dimension and topological invariants as methods to quantify complexity in Yayoi Kusama's paintings

Intricate patterns in abstract art many times can be wrongly characterized as being complex. Complexity can be an indicator of the internal dynamic of the whole system, regardless of the type of system in question, including art creation. In this investigation, we use two different techniques to objectively quantify complexity in abstract images: the fractal dimension and the value of the Betti numbers. We first validate our technique by considering synthetic images with a random distribution of dots, to then apply it to a series of `Net obsession' paintings by Yayoi Kusama. Surprisingly, we found that although the fractal dimension of her works in this series is comparable to those by Jackson Pollock in his dripping period, which could indicate a high level of complexity, the value of the Betti numbers do show disconnectedness and not high complexity. This is intuitively in agreement with the visual assessment of such works.

nlin.PS↗

arXiv2020

Dynamics of a helical swimmer crossing viscosity gradients

We experimentally and theoretically study the dynamics of a low-Reynolds number helical swimmer moving across viscosity gradients. Experimentally, a double-layer viscosity is generated by superposing two miscible fluids with similar densities but different dynamic viscosities. A synthetic helical magnetically-driven swimmer is then made to move across the viscosity gradients along four different configurations: either head-first (pusher swimmer) or tail-first (puller), and through either positive (i.e. going from low to high viscosity) or negative viscosity gradients. We observe qualitative differences in the penetration dynamics for each case. We find that the swimming speed can either increase or decrease while swimming across the viscosity interface, which results from the fact that the head and the tail of the swimmer can be in environments in which the local viscosity leads to different relative amounts of drag and thrust. In order to rationalize the experimental measurements, we next develop a theoretical hydrodynamic model. We assume that the classical resistive-force theory of slender filaments is locally valid along the helical propeller and use it to calculate the swimming speed as a function of the position of the swimmer relative to the fluid-fluid interface. The predictions of the model agree well with experiments for the case of positive viscosity gradients. When crossing across a negative gradient, gravitational forces in the experiment become important, and we modify the model to include buoyancy, which agrees with experiments. In general our results show that it is harder for a pusher swimmer to cross from low to high viscosity, whereas for a puller swimmer it is the opposite. Our model is also extended to the case of a swimmer crossing a continuous viscosity gradient.

physics.flu-dyn↗

arXiv2019

Viscoelastic propulsion of a rotating dumbbell

Viscoelastic fluids impact the locomotion of swimming microorganisms and can be harnessed to devise new types of self-propelling devices. Here we report on experiments demonstrating the use of normal stress differences for propulsion. Rigid dumbbells are rotated by an external magnetic field along their axis of symmetry in a Boger fluid. When the dumbbell is asymmetric (snowman geometry), non-Newtonian normal stress differences lead to net propulsion in the direction of the smaller sphere. The use of a simple model allows to rationalise the experimental results and to predict the dependence of the snowman swimming speed on the size ratio between the two spheres.

physics.flu-dyn↗

arXiv2019

Some fluid mechanical aspects of artistic painting

Painting is a fluid mechanical process. The action of covering a solid surface with a layer of a viscous fluid is one of the most common human activities; virtually all man-made surfaces are painted to provide protection against the environment or simply for decoration. This process, in an industrial context, has been vastly studied and it is well understood. In case of artistic painting the purpose is different. Painters learn how to manipulate the non-uniform deposition of paint onto a surface, through lengthy empirical testing of the action and modifying the physical properties of the fluids, to create textures and patterns of aesthetic value. In this paper, an analysis of some notable painting techniques is presented from the point of view of fluid mechanics. In particular, we discuss the so-called `accidental painting' technique, originally devised by David A. Siqueiros, which is the result of a Rayleigh-Taylor instability. An analysis of several techniques used by Jackson Pollock is also presented, showing how he learned to carefully control the motion of viscous filaments to create his famous abstract compositions. We also briefly discuss how pattern and textures are produced in decalcomania and watercolor painting. These investigations indicate that it is possible to establish concrete scientific discussions among modern fluid mechanics, art, art history and conservation.

physics.flu-dyn↗