Search

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.

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.

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., Comment: 9 pages, 4 figures

In splat painting, a collection of liquid droplets is projected onto the substrate by imposing a controlled acceleration to a paint-loaded brush. To unravel the physical phenomena at play in this artistic technique, we perform a series of experiments where the amount of expelled liquid and the resulting patterns on the substrate are systematically characterized as a function of the liquid viscosity and brush acceleration. Experimental trends and orders of magnitude are rationalized by simple physical models, revealing the existence of an inertia-dominated flow in the anisotropic, porous tip of the brush. We argue that splat painting artists intuitively tune their parameters to work in this regime, which may also play a role in other pulsed flows, like violent expiratory events or sudden geophysical processes., Comment: 15 pages, 10 figures

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., Comment: 11 pages, 5 figures

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.

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.

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.

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.

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.

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.

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.

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.

Watercolor or aquarelle is one of the oldest painting methods in which pigments suspended in an aqueous liquid are deposited over a substrate, typically an absorbing material such as paper. The physical processes which lead to pattern formation have not been studied in any depth, in spite of being closely related to flows in other contexts. Identifying and understanding these processes is of imperative importance to restore and preserve watercolor paintings. Here, we report an experimental study of the effect of the pigment concentration and paper humidity on the pattern formation derived from evaporation of droplets of watercolor paintings. Optical analysis reveals the formation of color gradients, stratifications, flat regions, borders, dendritic shapes, and radial tips. We found that droplet evaporation on dry paper forms ring-shaped stains which resemble to the \emph{coffee-stain effect} regardless of the nature of the pigment. The mean pixel intensity of such deposits follows an exponential function that saturates at high concentration, while the thickness of the coffee ring increase for watercolor inks containing colloidal particles and does not change for non-colloidal . Our experiments reveal that water distribution on the paper surface, and not the volumetric absorption of water on the paper, determine the structural characteristics of watercolor stains. We show evidence that the cornerstone in the creation of complex patterns in watercolor paintings is driven by the coffee ring effect and imbibition processes. Our findings aim to serve as framework for further investigations of the complex processes involved in this ancient art form and could guide restoration processes needed to preserve the heritage value of historical watercolor artworks.

Bubbly drinks are surprisingly attractive. There is something about the nature of the these beverages that make them preferable among other choices. In this article we explore the physics involved in this particular kind of two-phase, mass-transfer-driven flows., Comment: Extended version of Zenit R and Rodr\'iguez-Rodr\'iguez J. The fluid mechanics of bubbly drinks. Physics Today, Vol. 71(11) pp. 44-50, November 2018

For low-Reynolds number shear-flows of neutrally-buoyant suspensions, the shear stress is often modeled using an effective viscosity that depends only on the solid fraction. As the Reynolds number ($Re$) is increased and inertia becomes important, the effective viscosity also depends on the Reynolds number itself. The current experiments measure the torque for flows of neutrally-buoyant particles in a coaxial-cylinder rheometer for solid fractions, $\phi$, from 10\% to 50\% and Reynolds numbers based on particle diameter from 2 to 1,000. For experiments for Reynolds of O(10) and solid fractions less than $30\%$, the effective viscosity increases with Reynolds number, in good agreement with recent numerical simulations found in the literature. At higher solid fractions over the same range of $Re$, the results show a decrease in torque with shear rate. For Reynolds numbers greater than 100 and lower solids concentrations, the effective viscosity continues to increase with Reynolds number. However, based on comparisons with pure fluid measurements the increase in the measured effective viscosity results from the transition to turbulence. The particles augment the turbulence by increasing the magnitude of the measured torques and causing the flow to transition at lower Reynolds numbers. For the highest solid fractions, the measurements show a significant increase in the magnitude of the torques, but the effective viscosity is independent of Reynolds number.

Swimming microorganisms often have to propel in complex, non-Newtonian fluids. We carry out experiments with self-propelling helical swimmers driven by an externally rotating magnetic field in shear-thinning, inelastic fluids. Similarly to swimming in a Newtonian fluid, we obtain for each fluid a locomotion speed which scales linearly with the rotation frequency of the swimmer, but with a prefactor which depends on the power index of the fluid. The fluid is seen to always increase the swimming speed of the helix, up to 50% faster and thus the strongest of such type reported to date. The maximum relative increase for a fluid power index of around 0.6. Using simple scalings, we argue that the speed increase is not due to the local decrease of the flow viscosity around the helical filament but hypothesise instead that it originates from confinement-like effect due to viscosity stratification around the swimmer.

Fluid-suspended microorganisms have evolved different swimming and feeding strategies in order to cope with an environment dominated by viscous effects. For instance ciliated organisms rely on the collective motion of flexible appendices to move and feed. By performing a non-reciprocal motion, flexible filaments can produce a net propulsive force, or pump fluid, in the absence of inertia. Inspired by such fundamental concept, we propose a strategy to produce macroscopic pumping and mixing in creeping flow. We measure experimentally the net motion of a Newtonian viscous fluid induced by the reciprocal motion of a flapper. When the flapper is rigid no net motion is induced. In contrast, when the flapper is made of a flexible material, a net fluid pumping is measured. We quantify the effectiveness of this pumping strategy and show that optimal pumping is achieved when the length of the flapper is on the same order as the elasto-hydrodynamic penetration length. We finally discuss the possible applications of flexible impellers in mixing operations at low Reynolds numbers.

This fluid dynamics video is an entry for the Gallery of Fluid Motion for the 66th Annual Meeting of the Fluid Dynamics Division of the American Physical Society. We show the curious behaviour of a light ball interacting with a liquid jet. For certain conditions, a ball can be suspended into a slightly inclined liquid jet. We studied this phenomenon using a high speed camera. The visualizations show that the object can be `juggled' for a variety of flow conditions. A simple calculation showed that the ball remains at a stable position due to a Bernoulli-like effect. The phenomenon is very stable and easy to reproduce., Comment: Two videos

We conduct experiments with flexible swimmers to address the impact of fluid viscoelasticity on their locomotion. The swimmers are composed of a magnetic head actuated in rotation by a frequency-controlled magnetic field and a flexible tail whose deformation leads to forward propulsion. We consider both viscous Newtonian and glucose-based Boger fluids with similar viscosities. We find that the elasticity of the fluid systematically enhances the locomotion speed of the swimmer, and that this enhancement increases with Deborah number. Using Particle Image Velocimetry to visualize the flow field, we find a significant difference in the amount of shear between the rear and leading parts of the swimmer head. We conjecture that viscoelastic normal stresses lead to a net elastic forces in the swimming direction and thus a faster swimming speed., Comment: 14 pages, 7 figures, 3 videos

This is an entry for the Gallery of Fluid Motion of the 65st Annual Meeting of the APS-DFD (fluid dynamics video). This video shows the motion of levitated liquid droplets. The levitation is produced by the vertical vibration of a liquid container. We made visualizations of the motion of many droplets to study the formation of clusters and their stability., Comment: This entry contains videos for the Gallery of Fluid Motion of the 65st Annual Meeting of the APS-DFD

This is an entry for the Gallery of Fluid Motion of the 65th Annual Meeting of the APS-DFD (fluid dynamics video). This video shows an analysis of the 'accidental painting' technique developed by D.A. Siqueiros, a famous Mexican muralist. We reproduced the technique that he used: pouring layers of paint of different colors on top of each other. We found that the layers mix, creating aesthetically pleasing patterns, as a result of a Rayleigh-Taylor instability. Due to the pigments used to give paints their color, they can have different densities. When poured on top of each other, if the top layer is denser than the lower one, the viscous gravity current undergoes unstable as it spread radially. We photograph the process and produced slowed-down video to visualize the process., Comment: There are videos included

The fluid dynamics video show a gas stream which is injected into a packed bed immersed in water and fluid dynamcis video present the dynamics involved. The refractive index of the water an the packed bed are quite similar and the edges of the spherical particles can be seen. Two distinctive regimens can be observed. The first one, for low air flow rates, which is characterized by the percolation of the air thought the interstitial space among particles. And the second one, for high air flow rates, which is characterized by the accumulation of air inside the packed bed without percolation, it can be observed that the bubble pull apart the particles apart. Furthermore, for the first case the position of the particles remains constant while for the second one a circulation of particles is induced by the bubbles flow.

In this video, the mechanism followed to disperse an oil phase in water using a Scaba impeller in a cylindrical tank is presented. Castor oil (viscosity = 500 mPas) is used and the Reynolds number was fixed to 24,000. The process was recorded with a high-speed camera. Initially, the oil is at the air water interface. At the beginning of the stirring, the oil is dragged into the liquid bulk and rotates around the impeller shaft, then is pushed radially into the flow ejected by the impeller. In this region, the flow is turbulent and exhibits velocity gradients that contribute to elongate the oil phase. Viscous thin filaments are generated and expelled from the impeller. Thereafter, the filaments are elongated and break to form drops. This process is repeated in all the oil phase and drops are incorporated into the dispersion. Two main zones can be identified in the tank: the impeller discharge characterized by high turbulence and the rest of the flow where low velocity gradients appear. In this region surface forces dominate the inertial ones, and drops became spheroidal.

Drops of water at room temperature were released in hot oil, which had a temperature higher than that of the boiling point of water. Initially, the drop temperature increases slowly mainly due to heat transfer diffusion; convective heat transfer is small because the motion takes place at a small Reynolds number. Once the drop reaches the bottom of the container, it sticks to the surface with a certain contact angle. Then, a part of the drop vaporizes: the nucleation point may appear at the wall, the interface or the bulk of the drop. The vapor expands inside the drop and deforms its interface. The way in which the vapor expands, either smooth or violent, depends on the location of the nucleation point and oil temperature. Furthermore, for temperatures close to the boiling point of water, the drops are stable (overheated); the vaporization does not occur spontaneously but it may be triggered with an external perturbation. In this case the growth of the vapor bubble is rather violent. Many visualization for different conditions will be shown and predictions of the growth rate will be discussed.

This is an entry for the Gallery of Fluid Motion of the 62st Annual Meeting of the APS-DFD (fluid dynamics videos). This video shows the formation of sharps cusps in the interface of a viscoelastic liquid with air considering a selective withdrawal device. We found that a wide variety of shapes can be observed, similar to those appearing in the rear of air bubbles ascending in non-Newtonian liquids., Comment: Changed content, instead of Faculta it should say Facultad. Instead of in general in particular it should say in particular. Instead of sharp cusps it should say non-axisymmetryc cusps

This is an entry for the Gallery of Fluid Motion of the 61st Annual Meeting of the APS-DFD (fluid dynamics videos). This video shows the collision and rebound of viscoelastic drops against a solid wall. Using a high speed camera, the process of approach, contact and rebound of drops of a viscoelastic liquid is observed. We found that these drops first splash, similar to what is observed in Newtonian colliding drops; after a few instants, the liquid recoils, recovering its original drop shape and bounce off the wall., Comment: Small manuscript that goes along with a video submission for the Gallery of Fluid Motion of the 61 anual meeting of the APS-DFD, November 2008

The climbing effect of a viscoelastic fluid when stirred by a spinning rod is well documented and known as Weissenberg effect(Wei et al, 2006). This phenomenon is related to the elasticity of the fluid. We have observed that this effect can appear when the fluid is stirred without a rod. In this work, a comparison of the flow around a spinning disk for a Newtonian and a non-Newtonian liquids is presented. The flow is visualized with ink and small bubbles as fluid path tracers. For a Newtonian fluid, at the center of the spinning disk, the fluid velocity is directed towards the disk (sink flow); on the other hand, for a viscoelatic liquid, a source flow is observed since the fluid emerges from the disk. The toroidal vortices that appear on top of the disk rotate in opposite directions for the Newtonian and non-Newtonian cases. Similar observations have been reported for the classical rod climbing flow (Siginer, 1984 and Escudier, 1984). Some authors have suggested that this flow configuration can be used to determine the elastic properties of the liquid (Escuider, 1984 and Joshep, 1973).