Elucidating the effects of microbubble pinch-off dynamics on mammalian cell viability.

In the biotechnology industry, ensuring the health and viability of mammalian cells, especially Chinese Hamster Ovary (CHO) cells, plays a significant role in the successful production of therapeutic agents. These cells are typically cultivated in aerated bioreactors, where they encounter fluid stressors from rapidly deforming bubbles. These stressors can disrupt essential biological processes and potentially lead to cell death. However, the impact of these transient, elevated stressors on cell viability remains elusive. In this study, we first employ /cgqamicrofluidics to expose CHO cells near to bubbles undergoing pinch-off, subsequently collecting and assaying the cells to quantify the reduction in viability. Observing a significant impact, we set out to understand this phenomenon. We leverage computational fluid dynamics and numerical particle tracking to map the stressor field history surrounding a rapidly deforming bubble. Separately, we expose CHO cells to a known stressor level in a flow constriction device, collecting and assaying the cells to quantify the reduction in viability. By integrating the numerical data and results from the flow constriction device experiments, we develop a predictive model for cell viability reduction. We validate this model by comparing its predictions to the earlier microfluidic results, observing good agreement. Our findings provide critical insights into the relationship between bubble-induced fluid stressors and mammalian cell viability, with implications for bioreactor design and cell culture protocol optimization in the biotechnology sector.

Enrichment of Scavenged Particles in Jet Drops Determined by Bubble Size and Particle Position.

When small bubbles rupture in a contaminated water source, the resulting liquid jet breaks up into droplets that can aerosolize solid particulates such as bacteria, viruses, and microplastics. Particles collected on the bubble surface have the potential to become highly concentrated in the jet drops, dramatically increasing their impact. It has been assumed that only particles small enough to fit within a thin microlayer surrounding the bubble can be transported into its influential top jet drop. Yet here, we demonstrate that not only can larger particles be transported into this jet drop, but also that these particles can exceed previous enrichment measurements. Through experiments and simulations, we identify the prerupture location of the liquid that develops into the top jet drop and model how interfacial rearrangement combines with the bubble size, particle size, and the angular distribution of particles on the bubble surface to set the particle enrichment.

Universal non-monotonic drainage in large bare viscous bubbles.

Bubbles will rest at the surface of a liquid bath until their spherical cap drains sufficiently to spontaneously rupture. For large film caps, the memory of initial conditions is believed to be erased due to a visco-gravitational flow, whose velocity increases from the top of the bubble to its base. Consequently, the film thickness has been calculated to be relatively uniform as it thins, regardless of whether the drainage is regulated by shear or elongation. Here, we demonstrate that for large bare bubbles, the film thickness is highly nonuniform throughout drainage, spanning orders of magnitude from top to base. We link the film thickness profile to a universal non-monotonic drainage flow that depends on the bubble thinning rate. These results highlight an unexpected coupling between drainage velocity and bubble thickness profiles and provide critical insight needed to understand the retraction and breakup dynamics of these bubbles upon rupture.

Microscale heterogeneous pore occupancy with variable background resistance.

Fluid flow through porous media often involves the spontaneous displacement of one immiscible fluid by another. On the level of adjacent channels, a pore doublet model can estimate the relative flow rates in the two channels and the final fluid occupancy of the pair. Pore doublet models nominally capture the dominant roles of capillarity and viscosity, by describing the flow using hydraulic circuit theory. However, this approach neglects the more complex fluid flow in the region where the immiscible fluids meet, and therefore the applicability of this model for low aspect-ratio pores commonly seen in natural media is questionable. We show that for large channel aspect ratios, the numerical results replicate the traditional pore doublet models; however, as the aspect ratio approaches unity, the accuracy of these models break down. We quantify the effect of complex flow on the final fluid occupancy, which we link to the excess energy dissipation rate from the flow near the immiscible interface. Our results indicate that a minimal 1D pore doublet model may be sufficient to model global dynamics, including the fraction of residual oil that remains trapped in a reservoir, where the pore diameter and length are comparable.

Aerosol agitation: Quantifying the hydrodynamic stressors on particulates encapsulated in small droplets.

Lower respiratory tract infections originate from multiple aerosol sources, varying from droplets erupting from bursting bubbles in a toilet or those produced by human speech. A key component of the aerosol-based infection pathway-from source to potential host-is the survival of the pathogen during aerosolization. Due to their finite-time instability, pinch-off processes occurring during aerosolization have the potential to rapidly accelerate the fluid into focused regions of these droplets, stress objects therein, and if powerful enough, disrupt biological life. However, the extent that a pathogen will be exposed to damaging hydrodynamic stressors during the aerosolization process is unknown. Here we compute the probability that particulates will be exposed to a hydrodynamic stressor during the generation of droplets that range in size from one to 100 microns. For example, particulates in water droplets less than 5 µm have a 50% chance of being subjected to an energy dissipation rate in excess of 1011 W/m3, hydrodynamic stresses in excess of 104 Pa, and strain rates in excess of 107 s-1, values known to damage certain biological cells. Using a combination of numerical simulations and self-similar dynamics, we show how the exposure within a droplet can be generally predicted from its size, surface tension, and density, even across different aerosolization mechanisms. Collectively, these results introduce aerosol agitation as a potential factor in pathogen transmission and implicate the pinch-off singularity flow as setting the distribution of hydrodynamic stressors experienced within the droplet.

A new wrinkle on liquid sheets: Turning the mechanism of viscous bubble collapse upside down.

Viscous bubbles are prevalent in both natural and industrial settings. Their rupture and collapse may be accompanied by features typically associated with elastic sheets, including the development of radial wrinkles. Previous investigators concluded that the film weight is responsible for both the film collapse and wrinkling instability. Conversely, we show here experimentally that gravity plays a negligible role: The same collapse and wrinkling arise independently of the bubble's orientation. We found that surface tension drives the collapse and initiates a dynamic buckling instability. Because the film weight is irrelevant, our results suggest that wrinkling may likewise accompany the breakup of relatively small-scale, curved viscous and viscoelastic films, including those in the respiratory tract responsible for aerosol production from exhalation events.

Shooting Rubber Bands: Two Self-Similar Retractions for a Stretched Elastic Wedge.

Stretching and shooting rubber bands is a familiar experience for both children and adults, yet the initial dynamics are so quick that they are generally missed. When a cut elastic strip is stretched from its end and suddenly released, the dynamics depend on a balance of stretching and inertia. However, when a rubber band is stretched, a region of high curvature is created and it is unclear how this curvature affects the dynamics. Here, we demonstrate that during the retraction of a circular rubber band, a wavelength develops at the rear which increases in size as time progresses. Through a combination of experiments and modeling, we investigate the speed at which the back of the elastic retracts and observe a self-similar shape that depends on stretching, inertia, and bending. These retraction dynamics illuminate how a rubber band can pass by a thumb when discharged without hitting it.

Surface coatings including fingerprint residues can significantly alter the size and shape of bloodstains.

When conducting a blood pattern analysis (BPA) the size, shape, distribution, and location of bloodstains found at a crime scene may be critical in forming a hypothesis as to what transpired during a bloody event. Prior studies have demonstrated that the size and shape of a bloodstain on a smooth surface are determined from impact dynamics and to a lesser degree by the target material itself. Yet, these studies have relied on clean surfaces, and it is unclear whether the presence of microscopic coatings and residues could significantly alter the size or shape of the dried stain. Here, in the present work, experiments are conducted to demonstrate that various coatings, such as the sebaceous residue from a latent fingerprint, can dramatically alter the size and shape of the stain from the moment of impact through the drying process. These experiments also highlight that a drop impacting a tilted superhydrophobic-coating glass substrate can cause the blood drop to completely recoil without leaving a stain. Relying on a combination of high-speed and time-lapse photography, the specific stages in the stain evolution responsible for the deviations from the current models are identified. At a relatively low impact velocity, the stain sizes on the coated glass surfaces were 35-72% smaller than on the clean glass surface. At a higher impact velocity, the stains on the coated surfaces were not only smaller, but also contained drop spatter around the primary stain that was not observable in the absence of the microscopic coatings. The reduction in bloodstain size did not appreciably change when a chemical was added to deactivate the anticoagulant and allow the blood to clot.

A simple fluid dynamic model of renal pelvis pressures during ureteroscopic kidney stone treatment.

Ureteroscopy is an endoscopic kidney stone removal procedure which increases the internal pressure in the renal pelvis, the kidney's urinary collecting system. Elevated renal pelvic pressure may result in systemic absorption of irrigation fluid and urine, which can increase the risk of postoperative fever and sepsis. Urologists have investigated the effects of various surgical parameters on the renal pelvic pressure. However, it still remains unknown which surgical parameter has the most dominant effect on the renal pelvic pressure over time. Here we develop a physical model that computes the renal pelvic pressure as a function of time based on parameters that can be varied during ureteroscopy. The model is developed by applying pipe network analysis to the regions of the urinary tract that are involved in a representative ureteroscopic procedure. Our model unifies the findings of the previously published studies on this topic; an ex-vivo porcine study and an in-vivo human study. Furthermore it allows simulation of surgical procedures based on various techniques. Our simulation demonstrates that the two strong regulators of renal pelvis pressure during ureteroscopy are the size of the gap between ureteroscope and ureteral access sheath and the frequency and duration of ureteroscope withdrawal.

Trapping a Hot Drop on a Superhydrophobic Surface with Rapid Condensation or Microtexture Melting.

A water drop can bounce upon impacting a superhydrophobic surface. However, on certain superhydrophobic surfaces, a water drop will stick rather than bounce if it is sufficiently hot. Here, we aim to better understand the mechanisms that can lead to this bouncing-sticking transition. Specifically, we model two potential mechanisms in which a superhydrophobic surface could trap a sufficiently hot drop within milliseconds: melting of microtextured wax and condensation of the vapor within the superhydrophobic texture. We then test these mechanisms through systematic drop impact experiments in which we independently vary the substrate and drop temperatures on a waxy superhydrophobic Nasturtium leaf. We find that, whenever the surface or the drop is above a microtexture-melting temperature, the drop sticks. Below this temperature, a critical temperature threshold for bouncing can be predicted and controlled by considering the relative timescales between condensation growth and drop residence time. We envision that these results can provide insight into the design of a new class of superhydrophobic surfaces to act as a rapid thermal fuse to prevent drops that exceed a critical temperature from bouncing onto a thermally sensitive target.

Quantifying the potential for bursting bubbles to damage suspended cells.

Bubbles that rise to the surface of a cell suspension can damage cells when they pop. This phenomenon is particularly problematic in the biotechnology industry, as production scale bioreactors require continuous injection of oxygen bubbles to maintain cell growth. Previous studies have linked cell damage to high energy dissipation rates (EDR) and have predicted that for small bubbles the EDR could exceed values that would kill many cells used in bioreactors, including Chinese Hamster Ovary (CHO) cells. However, it's unclear how many cells would be damaged by a particular bursting bubble, or more precisely how much volume around the bubble experiences these large energy dissipation rates. Here we quantify these volumes using numerical simulations and demonstrate that even though the volume exceeding a particular EDR increases with bubble size, on a volume-to-volume basis smaller bubbles have a more significant impact. We validate our model with high-speed experiments and present our results in a non-dimensionalized framework, enabling predictions for a variety of liquids and bubble sizes. The results are not restricted to bubbles in bioreactors and may be relevant to a variety of applications ranging from fermentation processes to characterizing the stress levels experienced by microorganisms within the sea surface microlayer.

Capillary induced twisting of Janus cylinders.

At the appropriate length scales, capillary forces exerted by a liquid in contact with a compliant solid can cause the solid's deformation. Capillary forces are also able to align particles with discrete wettabilities - or Janus particles - at liquid interfaces. Their amphiphilic properties enable Janus particles to orient themselves at liquid interfaces such that both of their surfaces are facing their preferred fluid. However, it is unclear how to spontaneously obtain varying degrees of rotational alignment. Here we extend ideas of elasto-capillarity to modulate rotational alignment by connecting amphiphilic Janus cylinders in an antisymmetric configuration. As the Janus cylinders rotate they cause a twisting deformation of rod. We develop both a mathematical model and a physical macroscale setup to relate the angle of twist to the elastic and interfacial properties, which can be used to tune the extent of alignment of Janus particles at air-water interfaces. We additionally extend our analysis to calculate the twist profile on a compliant element with a distributed capillary torque.

Heat exchange between a bouncing drop and a superhydrophobic substrate.

The ability to enhance or limit heat transfer between a surface and impacting drops is important in applications ranging from industrial spray cooling to the thermal regulation of animals in cold rain. When these surfaces are micro/nanotextured and hydrophobic, or superhydrophobic, an impacting drop can spread and recoil over trapped air pockets so quickly that it can completely bounce off the surface. It is expected that this short contact time limits heat transfer; however, the amount of heat exchanged and precise role of various parameters, such as the drop size, are unknown. Here, we demonstrate that the amount of heat exchanged between a millimeter-sized water drop and a superhydrophobic surface will be orders of magnitude less when the drop bounces than when it sticks. Through a combination of experiments and theory, we show that the heat transfer process on superhydrophobic surfaces is independent of the trapped gas. Instead, we find that, for a given spreading factor, the small fraction of heat transferred is controlled by two dimensionless groupings of physical parameters: one that relates the thermal properties of the drop and bulk substrate and the other that characterizes the relative thermal, inertial, and capillary dynamics of the drop.

Macrotextured spoked surfaces reduce the residence time of a bouncing Leidenfrost drop.

Liquid drops can bounce when they impact non-wetting surfaces. Recently, studies have demonstrated that the time that the bouncing drop contacts a superhydrophobic surface can be reduced by incorporating ridged macrotextures on the surface. Yet the existing models aimed at explaining this phenomenon offer incompatible predictions of the contact time when a drop impacts multiple intersecting macrotextures, or spokes. Furthermore, it is unclear whether the effects of the macrotexture on the drop hydrodynamics extend to non-wetting surfaces in which direct contact is avoided by a thin vapor layer. Here we demonstrate that the phenomenon observed for macrotextured, superhydrophobic surfaces extends to macrotextured, wettable surfaces above the Leidenfrost temperature. We show that the number of droplets and overall residence time both depend on the number of intersecting spokes. Finally, we compare and contrast our results with mechanistic models to rationalize various elements of the phenomenon.

Periodic jetting and monodisperse jet drops from oblique gas injection.

When air is blown in a straw or tube near an air-liquid interface, typically one of two behaviors is observed: a dimple in the liquid's surface, or a frenzy of sputtering bubbles, waves, and spray. Here we report and characterize an intermediate regime that can develop when a confined air jet enters the interface at an angle. This regime is oscillatory with a distinct characteristic frequency and can develop periodic angled jets that can break up into monodisperse aerosols. The underlying mechanisms responsible for this highly periodic regime are not well understood. Here we flow a continuous stream of gas through a tube near a liquid surface, observing both optically and acoustically the deformation of the liquid-air interface as various parameters are systematically adjusted. We show that the Kelvin-Helmholtz instability is responsible for the inception of waves within a cavity formed by the gas. Inertia, gravity, and capillary forces both shape the cavity and govern the frequency and amplitude of these gas-induced cavity waves. The flapping cavity focuses the waves into a series of periodic jets that can break up into droplets following the Rayleigh-Plateau instability. We present scaling arguments to rationalize the fundamental frequencies driving this system, as well as the conditions that bound the periodic regime. These frequencies and conditions compare well with our experimental results.

Capillary Displacement of Viscous Liquids.

When a capillary tube is brought into contact with a wetting liquid, surface tension forces overcome gravity and the liquid spontaneously rises into the tube until an equilibrium height is reached. The early viscous dynamics of the rise typically follow the well-known Lucas-Washburn law, which is independent of gravity and neglects the displaced fluid. Here we explore the early viscous dynamics when the properties of displaced fluid are significant. Using a combination of experiments and theory, we show how the characteristic behavior of the Lucas-Washburn law is modified when the viscosity of the displaced fluid is comparable to or exceeds the wetting fluid. Additionally, we find that the effects of gravity reshape the dynamics of the capillary rise not only in the late viscous regime but also in the early viscous regime.

Jet drops from bursting bubbles: How gravity and viscosity couple to inhibit droplet production.

When a bubble ruptures at a liquid surface the collapsing cavity produces a central jet that frequently breaks up into a series of droplets. Current experiment and theory predict that the production of jet drops will be limited by either viscous or gravitational effects. However, while there are a number of studies focusing on these two limiting cases, less is understood about the production of jet drops when both gravitational and viscous effects are significant. Here, we uncover the existence of an intermediate region where both gravitational and viscous effects play a critical role in jet-drop formation. We propose that the role of gravity is most important before rupture, and carry out simulations that demonstrate the importance of the equilibrium bubble shape in the production of jet drops.

Asymmetric capillary bridges between contacting spheres.

When a drop of liquid wets two identical solid spheres, the liquid forms a capillary bridge between the spheres to minimize surface energy. In the absence of external forces, these bridges are typically assumed to be axisymmetric, and the shape that minimizes surface energy can be calculated analytically. However under certain conditions, the bridge is axisymmetrically unstable, and migrates to a non-axisymmetric configuration. The goal of this paper is to characterize these non-axisymmetric capillary bridges. Specifically, we numerically calculate the shape of the capillary bridge between two contacting spheres that minimizes the total surface energy for a given volume and contact angle and compare to experiments. When the bridge is asymmetric, finite element calculations demonstrate that the shape of the bridge is spherical. In general, the bridge shape depends on both volume and contact angle, yet we find the degree of asymmetry is controlled by a single parameter.

Moving with bubbles: a review of the interactions between bubbles and the microorganisms that surround them.

Bubbles are ubiquitous in biological environments, emerging during the complex dynamics of waves breaking in the open oceans or being intentionally formed in bioreactors. From formation, through motion, until death, bubbles play a critical role in the oxygenation and mixing of natural and artificial ecosystems. However, their life is also greatly influenced by the environments in which they emerge. This interaction between bubbles and microorganisms is a subtle affair in which surface tension plays a critical role. Indeed, it shapes the role of bubbles in mixing or oxygenating microorganisms, but also determines how microorganisms affect every stage of the bubble's life. In this review, we guide the reader through the life of a bubble from birth to death, with particular attention to the microorganism-bubble interaction as viewed through the lens of fluid dynamics.