Howard A. Stone

Synthetic, natural, and biological macromolecules are ubiquitous in aquatic environments, affecting industrial processes such as filtration and posing ecological risks even at trace levels. Here, exploiting particle-polymer interactions under flow, we report a microfluidic platform for ultrasensitive monitoring of macromolecules in water. Fixed obstacles act as "collectors" enriching macromolecules, while injected micro/nanoparticles serve as "detectors" binding to those accumulated at the surface. The resulting ordered particle accumulation is visualized to assess macromolecule contamination using a standard microfluidic setup without additional assays or specialized modules. Quantitative analyses show well-calibrated concentration-intensity relationships, with parts-per-billion-level detection limits and subminute response times. Transitions in accumulation patterns under varying macromolecular properties further allow substance identification. Compared with existing methods, our approach demonstrates superior performance across sensitivity, selectivity, efficiency, and cost. The platform provides an alternative for water quality assessment and enables broader applications such as capture of micro/nanoplastics.

The spread of a pulse of solute in a pressure-driven channel flow is well described for a wide range of Newtonian flows for which the viscosity and diffusivity are constants. Over many decades, various extensions have been suggested for the dispersion in pressure-driven non-Newtonian channel flows. While many theoretical studies have examined the effect of shear-rate-dependent viscosity on dispersion for a variety of non-Newtonian constitutive models, the solute diffusivity has invariably been treated as a constant. This assumption, however, is in contrast to the expectation that the diffusivity of a colloidal particle is inversely related to the viscosity, e.g. recall the Stokes–Einstein relation. We account for this coupling of transport coefficients – viscosity and diffusivity – by assuming a generalised form of the Stokes–Einstein equation, inspired by the recognition that the viscosity is now a field, although only transport transverse to the main flow direction is relevant because of the common assumptions of Taylor–Aris dispersion. Thus, we derive a general formula for axial dispersion in steady, pressure-driven shear-rate-dependent flows in uniform channels. In particular, we apply our general relation to calculate the Taylor–Aris dispersion coefficient for steady flows of a shear-thinning Carreau fluid and a viscoelastic Phan-Thien–Tanner fluid. Finally, we highlight new theoretical questions raised by this transport situation, where the underlying diffusivity is also a (tensorial) field related to variations in viscosity.

The coalescence and breakup of drops are classic examples of flows that feature singularities. The behaviour of viscoelastic fluids near these singularities is particularly intriguing – not only because of their added complexity, but also due to the unexpected responses they often exhibit. In particular, experiments have shown that the coalescence of viscoelastic sessile drops can differ significantly from that of their Newtonian counterparts, sometimes resulting in a sharply distorted interface. However, the mechanisms driving these differences in dynamics, as well as the potential influence of the contact angle are not fully known. Here, we study two different flow regimes effectively induced by varying the contact angle and demonstrate how that leads to markedly different coalescence behaviours. We show that the coalescence dynamics is effectively unaltered by viscoelasticity at small contact angles. The Deborah number, which is the ratio of the relaxation time of the polymer to the time scale of the background flow, scales as $\theta ^3$ for $\theta \ll 1$ , thus rationalising the near-Newtonian response. On the other hand, it has been shown previously that viscoelasticity dramatically alters the shape of the interface during coalescence at large contact angles. We study this large contact angle limit using two-dimensional numerical simulations of the equation of motion. We show that the departure of the coalescence dynamics from the Newtonian case is a function of the Deborah number and the elastocapillary number, which is the ratio between the shear modulus of the polymer solution and the characteristic stress in the fluid.

The coalescence and breakup of drops are classic examples of flows that feature singularities. The behaviour of viscoelastic fluids near these singularities is particularly intriguing – not only because of their added complexity, but also due to the unexpected responses they often exhibit. In particular, experiments have shown that the coalescence of viscoelastic sessile drops can differ significantly from that of their Newtonian counterparts, sometimes resulting in a sharply distorted interface. However, the mechanisms driving these differences in dynamics, as well as the potential influence of the contact angle are not fully known. Here, we study two different flow regimes effectively induced by varying the contact angle and demonstrate how that leads to markedly different coalescence behaviours. We show that the coalescence dynamics is effectively unaltered by viscoelasticity at small contact angles. The Deborah number, which is the ratio of the relaxation time of the polymer to the time scale of the background flow, scales as $\theta ^3$ for $\theta \ll 1$ , thus rationalising the near-Newtonian response. On the other hand, it has been shown previously that viscoelasticity dramatically alters the shape of the interface during coalescence at large contact angles. We study this large contact angle limit using two-dimensional numerical simulations of the equation of motion. We show that the departure of the coalescence dynamics from the Newtonian case is a function of the Deborah number and the elastocapillary number, which is the ratio between the shear modulus of the polymer solution and the characteristic stress in the fluid.

From diabetes to malaria, altered blood flow contributes to poor clinical outcomes. Heterogeneity in red blood cell (RBC) properties within and across individuals has hindered our ability to establish the multiscale mechanisms driving pathological flow dynamics in such diseases. To address this, we develop microfluidic platforms to measure RBC properties and flow dynamics in the same blood samples from patients with sickle cell disease (SCD). We find that effective blood viscosity across individuals is explained by the proportion of stiff RBCs, exhibiting qualitative similarities to rigid-particle suspensions, despite considerable mechanical heterogeneity. By combining simulations with spatially resolved measurements of cell dynamics, we show how features of emergent rheology are governed by spatiotemporal cell organization, via margination at intermediate oxygen tensions, and localized jamming caused by spatial hematocrit variations under hypoxia. Our work defines the suspension physics underlying pathological blood flow in SCD and, more broadly, emergent rheology in heterogeneous particle suspensions.

The separation of colloidal particles from fluids is essential to ensure a safe global supply of drinking water, yet in the case of microscopic particles, it remains a highly energy-intensive process when using traditional filtration methods. Water cleaning through diffusiophoresis, spontaneous colloid migration in chemical gradients, effectively circumvents the need for physical filters, representing a promising alternative. This separation process is typically realized in internal flows, where a cross-channel electrolyte gradient drives particle accumulation at walls, with colloid separation slowly increasing in the streamwise direction. However, the maximum separation efficiency, achieved sufficiently downstream as diffusiophoretic migration (driving particle accumulation) is balanced by Brownian motion (inducing diffusive spreading), has not yet been characterized. In this work, we develop an asymptotic theory to predict colloid separation in this limit, deriving expressions for the water recovery, defined as the fraction of clean water that can be obtained from the suspension. We find that the mechanism by which the chemical permeates in the channel and the reaction kinetics governing its dissociation into ions play key roles in the process. Moreover, we identify four distinct regimes in which separation is controlled by different scaling laws involving Damköhler and Péclet numbers, which measure the ratios of reaction kinetics to ion diffusion and diffusiophoresis to Brownian motion, respectively. We also confirm the scaling of one of these regimes using microfluidic experiments where separation is driven by CO2 gradients. Our results shed light on pathways toward new, more efficient separations and are also applicable to quantify colloidal accumulation in the presence of chemical gradients in more general situations.

The deformation and rupture of a lipid vesicle due to the forced normal approach of an inclusion are essential for optimizing the design of magnetic giant unilamellar vesicles (GUV) [Malik et al., Nanoscale 17, 13720 (2025)NANOHL2040-336410.1039/D5NR00942A], with implications for active colloid-membrane interactions and cellular-scale chemical delivery. Here, we investigate vesicles propelled by a force-driven rigid inclusion and reveal a robust elastohydrodynamic mechanism: the inclusion outpaces the vesicle, sustaining a thinning film that drains symmetrically and self-similarly, largely independent of the initial shape. For soft membranes and small inclusions, the coupling drives a monotonic tension increase that can exceed the lysis tension. Evaluating the maximal tension over a delivery distance, we map an operating window in a vesicle reduced area and size relative to the inclusion.

Surface tension gradients of air–liquid–air films play a key role in governing the dynamics of systems such as bubble caps, foams, bubble coalescence and soap films. Furthermore, for common fluids such as water, the flow due to surface tension gradients, i.e. Marangoni flow, is often inertial, due to the low viscosity and high velocities. In this paper, we consider the localised deposition of insoluble surfactants onto a thin air–liquid–air film, where the resulting flow is inertial. As observed by Chomaz (2001 J. Fluid Mech . 442 , 387–409), the resulting governing equations with only inertia and Marangoni stress are similar to the compressible gas equations. Thus, shocks are expected to form. We derive similarity solutions associated with the development of such shocks, where the mathematical structure is closely related to the Burgers equation. It is shown that the nonlinearity of the surface tension isotherm has an effect on the strength of the shock. When regularisation mechanisms are included, the shock front can propagate and late-time similarity solutions are derived. The late-time similarity solution due to regularisation by capillary pressure alone was found by Eshima et al. (2025 Phys. Rev. Lett. 134 , 214002). Here, the regularisation mechanism is generalised to include viscous extensional stress.

Surface bubbles in the environment or engineering configurations, such as the ocean-atmosphere interface, sparkling wine, or during volcanic eruptions typically live on contaminated surfaces. A particularly common type of contamination is surface active agents (surfactants). We consider the effect of insoluble surfactant on jet drop formation by bubble bursting. Contrary to the observed trend that surfactants decrease the ejected drop radius for bubbles with precursor capillary waves, we find that surfactants increase the ejected drop radius for bubbles without precursor capillary waves—a regime characteristic of small bubbles. Consequently, the results have fundamental implications for understanding aerosol distributions in contaminated conditions. We find that the trend reversal is due to the effect of Marangoni stresses on the focusing of the collapsing cavity. We demonstrate quantitative agreement on the jet velocity and drop size between laboratory experiments and numerical simulations by using the measured surface tension dependence on surfactant concentration as the equation of state for the simulations.

Shocks due to hyperbolic partial differential equations (PDEs) appear throughout mathematics and science. The canonical example is shock formation in the inviscid Burgers' equation $\frac{\partial u}{\partial t}+u\frac{\partial u}{\partial x}=0$. Previous studies have shown that when shocks form for the inviscid Burgers' equation, for positions and times close to the shock singularity, the dynamics are locally self-similar and universal, i.e., dynamics are equivalent regardless of the initial conditions. In this paper, we show that, in fact, shock formation is self-similar and universal for general first-order strictly hyperbolic PDEs in one spatial dimension, and the self-similarity is like that of the inviscid Burgers' equation. An analytical formula is derived for the self-similar universal solution.

Rapid Room-Temperature Aerosol Dehydration (RTAD) is a novel, scalable drying technology for powderization and thermal stabilization of pharmaceutical drug products. Compared to conventional spray drying processes, typically using droplets of 10-200 micron in diameter generated by high-shear spraying, RTAD uses much smaller droplets with diameter 0.1 to 20 microns produced in modified liquid atomization processes. These fine droplets evaporate rapidly within 10-100 ms at room temperature conditions, thereby reducing drying-induced stresses for thermally sensitive biologics. In this study, we employed Green Fluorescent Protein (GFP) as a model biological molecule to optimize the RTAD system design and process parameters. We experimentally investigated the effects of droplet size, multiphase flow patterns in the drying chamber, and usage of polysorbate 20 as a model surfactant on GFP fluorescence after drying and powder reconstitution. The experiments demonstrated that the presence of surfactant in the formulation significantly influenced GFP fluorescence intensity, especially for smaller droplets. The numerical studies using Computational Fluid Dynamics simulations revealed that the intensity of GFP fluorescence in the produced dry powders was dependent on the patterns of multiphase flow in the drying chamber. Non-axisymmetric flows and closed circulating streamlines near the drying gas inlet resulted in considerably longer particle residence times and subjected GFP molecules to excess stress that negatively impacted the GFP fluorescence intensity. Through iterative optimization of the chamber design, process parameters and feedstock formulation, we achieved recovery of the GFP fluorescence intensity that exceeded 96% in the obtained dry powders. This work establishes GFP as a sensitive model biologic and its fluorescence intensity as a powerful tool to rapidly assess process efficiency and the ability to preserve bioactivity after dehydration. The study has broad implications for the design and scale-up of drying technologies, which can potentially transform the production of dry powder biopharmaceuticals.

The Weissenberg effect, or rod-climbing phenomenon, occurs in non-Newtonian fluids where the fluid interface ascends along a rotating rod. Despite its prominence, theoretical insights into this phenomenon remain limited. In earlier work, Joseph & Fosdick (1973, Arch. Rat. Mech. Anal. vol. 49, pp. 321–380) employed domain perturbation methods for second-order fluids to determine the equilibrium interface height by expanding solutions based on the rotation speed. In this work, we investigate the time-dependent interface height through asymptotic analysis with dimensionless variables and equations using the Giesekus model. We begin by neglecting inertia to focus on the interaction between gravity, viscoelasticity and surface tension. In the small-deformation scenario, the governing equations indicate the presence of a boundary layer in time, where the interface rises rapidly over a short time scale before gradually approaching a steady state. By employing a stretched time variable, we derive the transient velocity field and corresponding interface shape on this short time scale, and recover the steady-state shape on a longer time scale. In contrast to the work of Joseph and Fosdick, which used the method of successive approximations to determine the steady shape of the interface, we explicitly derive the interface shape for both steady and transient cases. Subsequently, we reintroduce small but finite inertial effects to investigate their interaction with viscoelasticity, and propose a criterion for determining the conditions under which rod climbing occurs. Through numerical computations, we obtain the transient interface shapes, highlighting the interplay between time-dependent viscoelastic and inertial effects.

The dynamics of self-propelled colloidal particles is strongly influenced by their environment through hydrodynamic and, in many cases, chemical interactions. We develop a theoretical framework to describe the motion of confined active particles by combining the Lorentz reciprocal theorem with a Galerkin discretisation of surface fields, yielding an equation of motion that efficiently captures self-propulsion without requiring an explicit solution for the bulk fluid flow. Applying this framework, we identify and characterise the long-time behaviours of a Janus particle near rigid, permeable and fluid–fluid interfaces, revealing distinct motility regimes, including surface-bound skating, stable hovering and chemo-hydrodynamic reflection. Our results demonstrate how the solute permeability and the viscosity contrast of the surface influence a particle’s dynamics, providing valuable insights into experimentally relevant guidance mechanisms for autophoretic particles. The computational efficiency of our method makes it particularly well suited for systematic parameter sweeps, offering a powerful tool for mapping the phase space of confined active particles and informing high-fidelity numerical simulations.

This paper is associated with a video winner of a 2024 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion,

Multiplexed methods for nucleic acid detection are immensely challenging to deploy outside of laboratory settings. Conversely, field-deployable methods are limited to low levels of multiplexing. During the COVID-19 pandemic, we developed Streamlined Highlighting of Infections to Navigate Epidemics (SHINE), a sensitive and deployable CRISPR-based technology for nucleic acid detection. Here, we introduce microfluidic SHINE (mSHINE) which enables >100-plex nucleic acid detection using a highly portable microfluidic manifold. The manifold directs a diluted sample into individual reaction chambers, each of which contains lyophilized SHINE reagents and a microscopic stir bar or bead for mixing. Samples can be loaded using a syringe by hand, greatly simplifying the testing process. A subsequent sealing step allows for >100 SHINE reactions to proceed independently and in parallel. We demonstrate that mSHINE has equal sensitivity to SHINE, allowing for highly multiplexed pathogen detection in ≤ 1 hour. In addition, mSHINE can detect single-nucleotide variants, including mutations associated with drug susceptibility. mSHINE shifts the paradigm of laboratory-based multiplexed nucleic acid testing, greatly benefiting patients and public health.

A small amount of wetting liquid is placed about the point of contact between a solid sphere and a planar solid wall. It is well known in the colloid science literature that the leading-order approximation for the adhesive force between the two surfaces is independent of the liquid volume. This independence breaks down when the sum of the contact angles on the two surfaces is close to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>180</mml:mn> <mml:mrow> <mml:mo>∘</mml:mo> </mml:mrow> </mml:msup> </mml:math> . We identify the pertinent distinguished limit in this scenario, linking the proximity to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>180</mml:mn> <mml:mrow> <mml:mo>∘</mml:mo> </mml:mrow> </mml:msup> </mml:math> to the liquid volume. We address the adhesion problem at that limit, where the meridional curvature becomes comparable to the azimuthal curvature. Our key result is a closed-form approximation to the adhesive force, which does depend upon the liquid volume. We quantify the condition of negligible gravity in the context of the distinguished limit.

Single-chain nanoparticles (SCNPs) are a class of materials formed by the intramolecular cross-linking and collapse of single polymer chains. Because their morphology dictates suitability for specific applications, such as nanoscale reactors and drug delivery vehicles, understanding how to control or tailor morphologies is of interest. Here, we investigate how the morphology of SCNPs depends on both precursor chain attributes, such as linker fraction and backbone stiffness, and an imposed shear flow. Using coarse-grained molecular dynamics simulations, we generate an ensemble of structures from 10,800 unique SCNPs, some formed under quiescent conditions and some in shear flow--the latter of which has not been previously studied. We then characterize morphologies by analysis of a three-dimensional embedding space obtained through unsupervised learning of the simulated structures. This reveals how SCNP morphology depends on dimensionless parameters, related to precursor-chain attributes and shear rate, and offers insight into their relative influence. Interestingly, we find that shear rate has comparable influence to the degree of polymerization and the blockiness of reactive sites. Furthermore, shear, which can be externally controlled independent of precursor chain synthesis, can have persistent effects on morphology, such as enhancing compaction of SCNPs based on stiff chains. This work provides guidelines for designing SCNPs with targeted characteristics based on five dimensionless variables and illustrates the utility of machine learning in analyzing SCNPs formed across a range of conditions.

Surface bubbles in the environment or engineering configurations, such as the ocean-atmosphere interface, sparkling wine, or during volcanic eruptions typically live on contaminated surfaces. A particularly common type of contamination is surface active agents (surfactants). We consider the effect of insoluble surfactant on jet drop formation by bubble bursting. Contrary to the observed trend that surfactants decrease the ejected drop radius for bubbles with precursor capillary waves, we find that surfactants increase the ejected drop radius for bubbles without precursor capillary waves - a regime characteristic of small bubbles. Consequently, the results have fundamental implications for understanding aerosol distributions in contaminated conditions. We find that the trend reversal is due to the effect of Marangoni stresses on the focusing of the collapsing cavity. We demonstrate quantitative agreement on the jet velocity and drop size between laboratory experiments and numerical simulations by using the measured surface tension dependence on surfactant concentration as the equation of state for the simulations. *Jun Eshima and Tristan Aurégan contributed equally to this work.

We study the one-dimensional capillary-driven retraction of a finite, planar liquid sheet in the asymptotic regime where both the Ohnesorge number $\mathrm{Oh}$ and the initial length-to-thickness ratio $l_0/h_0$ are large. In this regime, the fluid domain decomposes into two regions: a thin-film region governed by one-dimensional mass and momentum equations, and a small tip region near the free edge described by a self-similar Stokes flow. Asymptotic matching between these regions yields an effective boundary condition for the thin-film region, representing a balance between viscous and capillary forces at the free edge. Surface tension drives the thin-film flow only through this boundary condition, while the local momentum balance is dominated by viscous and inertial stresses. We show that the thin-film flow possesses a conserved quantity, reducing the equation of thickness to heat equation with time-dependent boundary conditions. The reduced problem depends on a single dimensionless parameter $\mathcal{L} = l_0 / (4 h_0 \mathrm{Oh})$. Numerical solutions of the reduced model agree well with previous studies and reveal that the sheet undergoes distinct retraction regimes depending on $\mathcal{L}$ and a dimensionless time after rupture $T$. We derive asymptotic approximations for the thickness profile, velocity profile, and retraction speed during the early and late stages of retraction. At early times, the retraction speed grows as $T^{1/2}$, while at late times it decays as $1/T^2$. An intermediate regime arises for very long sheets ($\mathcal{L} \gg 1$). During this phase, the retraction speed approaches the Taylor-Culick value. When $T \approx \mathcal{L}$, the speed undergoes fast deceleration from the Taylor-Culick speed to late-time asymptotics.

ABSTRACT During migration through the female reproductive tract, sperm undergo physiological changes known as capacitation, including a motility transition termed hyperactivation. Hyperactivation is essential for various aspects of fertilization, particularly effective migration within the tract. However, how hyperactivation facilitates this migration remains elusive. Here, we profiled bull sperm hyperactivation in Newtonian and complex fluids near microfluidic surfaces, mimicking generic swimming conditions in the tract. We identified three swim gaits: wandering (persistent random walks), circling, and an intriguing circling-and- wandering mode marked by stochastic transitions between the two. All gaits exhibit diffusive behavior over long time scales, with wandering showing a tenfold higher diffusivity than circling, and the diffusivity of circling-and-wandering falling in between. We found that while wandering sperm scatter from convex and concave surfaces, circling sperm become trapped around pillars, highlighting the distinctive nature of each phase. Additionally, stochastic simulations of swimming in porous media showed that as the geometrical complexity of the environment increases, circling-and-wandering outperforms either strategy alone in spreading through the media. Our findings suggest that while wandering promotes exploration and circling supports local exploitation, circling-and-wandering combines the strengths of both strategies by balancing exploration and exploitation to adapt motility, enhance migration, and potentially improve target search.

We use experiments and theory to elucidate the size effect in capillary breakup rheometry, where prestretching in the viscocapillary stage causes the apparent relaxation time to be consistently smaller than the actual value. We propose a method accounting for both the experimental size and the finite extensibility of polymers to extract the actual relaxation time. A phase diagram characterizes the expected measurement variability and delineates scaling law conditions. The results refine capillary breakup rheometry for viscoelastic fluids and advance the understanding of breakup dynamics across scales.

A spherical capsule (radius $R$ ) is suspended in a viscous liquid (viscosity $\mu$ ) and exposed to a uniaxial extensional flow of strain rate $E$ . The elasticity of the membrane surrounding the capsule is described by the Skalak constitutive law, expressed in terms of a surface shear modulus $G$ and an area dilatation modulus $K$ . Dimensional arguments imply that the slenderness $\epsilon$ of the deformed capsule depends only upon $K/G$ and the elastic capillary number ${Ca}=\mu R E/G$ . We address the coupled flow–deformation problem in the limit of strong flow, ${Ca}\gg 1$ , where large deformation allows for the use of approximation methods in the limit $\epsilon \ll 1$ . The key conceptual challenge, encountered at the very formulation of the problem, is in describing the Lagrangian mapping from the spherical reference state in a manner compatible with hydrodynamic slender-body formulation. Scaling analysis reveals that $\epsilon$ is proportional to ${Ca}^{-2/3}$ , with the hydrodynamic problem introducing a dependence of the proportionality prefactor upon $\ln \epsilon$ . Going beyond scaling arguments, we employ asymptotic methods to obtain a reduced formulation, consisting of a differential equation governing a mapping field and an integral equation governing the axial tension distribution. The leading-order deformation is independent of the ratio $K/G$ ; in particular, we find the approximation $\epsilon ^{2/3} {Ca}\approx 0.2753\ln (2/\epsilon ^2)$ for the relation between $\epsilon$ and $Ca$ . A scaling analysis for the neo-Hookean constitutive law reveals the impossibility of a steady slender shape, in agreement with existing numerical simulations. More generally, the present asymptotic paradigm allows us to rigorously discriminate between strain-softening and strain-hardening models.

The nucleolus is a multiphasic biomolecular condensate that facilitates ribosome biogenesis, a complex process involving hundreds of proteins and RNAs. The proper execution of ribosome biogenesis likely depends on the material properties of the nucleolus. However, these material properties remain poorly understood due to the challenges of in vivo measurements. Here, we use micropipette aspiration (MPA) to directly characterize the viscoelasticity and interfacial tensions of nucleoli within transcriptionally active Xenopus laevis oocytes. We examine the major nucleolar subphases, the outer granular component (GC) and the inner dense fibrillar component (DFC), which itself contains a third small phase known as the fibrillar center (FC). We show that the behavior of the GC is more liquid-like, while the behavior of the DFC/FC is consistent with that of a partially viscoelastic solid. To determine the role of ribosomal RNA in nucleolar material properties, we degrade RNA using RNase A, which causes the DFC/FC to become more fluid-like and alters interfacial tension. Together, our findings suggest that RNA underlies the partially solid-like properties of the DFC/FC and provide insights into how material properties of nucleoli in a near-native environment are related to their RNA-dependent function.

The thinning of liquid sheets and the resulting capillary waves due to surfactant deposition are relevant to understanding how bubbles burst, with implications for the environment, health, and industry. Here, a similarity solution is obtained, which describes the sheet thinning and capillary waves. The final rupture mechanism of a bubble is explored, suggesting that insoluble surfactant deposition alone does not cause finite-time rupture; instead, sufficient thinning may allow other physical mechanisms to do so. Comparisons to an existing experiment and suggestions for measurements are given.

Summary Background Pancreatitis is a poorly understood condition in the horse. The DGGR‐lipase assay has recently been validated for horses. Objectives To evaluate serum DGGR‐lipase concentrations in equids presented to an equine hospital in the United Kingdom with gastrointestinal disease. Study design Prospective descriptive. Methods Blood samples were obtained by convenience sampling of horses and donkeys presented for evaluation of gastrointestinal disease. Results Serum pancreatic ( DGGR ‐) lipase concentrations were measured in 205 equids with gastrointestinal disease, of which 147 survived, 47 were euthanised and 11 died. The median serum pancreatic lipase concentration in all animals was 17 U/L ( IQR 14–27; range 1–3484). The lipase concentration was categorised as normal in 124 animals (60.5%) and elevated in 81 (39.5%). There was a statistically significant difference in the disease category and pancreatic lipase concentration ( p &lt; 0.001), with colic cases having higher lipase concentrations than colitis and peritonitis cases. There was strong evidence ( p = 0.01) of an association between pain severity and lipase values, with higher lipase concentrations in horses with more severe pain. Of 12 horses with severely increased pancreatic lipase concentration (&gt;200 U/L), 3/12 had spontaneous nasogastric reflux and 6/10 had distended and/or thickened small intestine on abdominal ultrasonography; 7/12 survived to hospital discharge and 5/12 died or were euthanised. Main limitations We were unable to confirm the presence of pancreatitis in any of the horses with elevated serum DGGR‐lipase concentrations by post‐mortem examination or histopathology. Conclusions Some equids with gastrointestinal disease have increased serum pancreatic (DGGR‐) lipase concentrations, especially those with colic. This suggests that a degree of pancreatitis may be present in many colic cases, although this does not indicate causation.

We investigate the dynamics of close-contact melting (CCM) on ‘gas-trapped’ hydrophobic surfaces, with specific focus on the effects of geometrical confinement and the liquid–air meniscus below the liquid film. By employing dual-series and perturbation methods under the assumption of small meniscus deflections, we obtain numerical solutions for the effective slip lengths associated with velocity $\lambda$ and temperature $\lambda _t$ fields, across various values of aspect ratio $\Lambda$ (defined as the ratio of the film thickness $h$ to the structure’s periodic length $l$ ) and gas–liquid fraction $\phi$ . Asymptotic solutions of $\lambda$ and $\lambda _t$ for $\Lambda \ll 1$ and $\Lambda \gg 1$ are derived and summarised for different surface structures, interface shapes and $\Lambda$ , which reveal a different trend of $\lambda$ for $\Lambda \ll 1$ and depending on the presence of a meniscus. In the context of constant-pressure CCM, our results indicate that longitudinal grooves can enhance heat transfer under the effects of confinement and a meniscus when $\Lambda \lesssim 0.1$ and $\phi \lt 1 - 0.5^{2/3} \approx 0.37$ . For gravity-driven CCM, the parameters of $l$ and $\phi$ determine whether the melting rate is enhanced, reduced or nearly unaffected. We construct a phase diagram based on the parameter matrix $(\log _{10} l, \phi )$ to delineate these three regimes. Lastly, we derive two asymptotic solutions for predicting the variation in time of the unmelted solid height.

We study theoretically and experimentally the propagation of two bubbles in a Hele-Shaw cell under a uniform background flow. We consider the regime where the bubbles are large enough to be flattened by the cell walls into a pancake-like shape, but small enough such that each bubble remains approximately circular when viewed from above. In a system of two bubbles of different radii, if the smaller bubble is in front, it will be overtaken by the larger bubble. Under certain circumstances, the bubbles may avoid collision by rolling over one another while passing. We find that, for a given ratio of the bubble radii, there exists a critical value of a dimensionless parameter (the Bretherton parameter) above which the two bubbles will never collide, regardless of their relative size and initial transverse offset, provided they are initially well separated in the direction of the background flow. Additionally, we determine the corrections to the bubble shape from circular for two bubbles aligned with the flow direction. We find that the front bubble flattens in the flow direction, while the rear bubble elongates. These shape changes are associated with changes in velocity, which allow the rear bubble to catch the bubble in front even when they are of the same size.

Multiphase displacement is important in oil recovery, microfluidics, and CO${}_{2}$ capture. We study viscous oil trapping in microfluidic devices with sinusoidal pockets during water invasion. Varying capillary number (Ca), viscosity ratios, and pore geometries reveals that higher oil viscosity and water velocities increase oil trapping due to transition from meniscus displacement to viscous fingering. We find that trapping dynamics at high Ca are geometry independent. Our three-dimensional model based on the long-wave approximation predicts some experimental observations, such as increased oil retention at higher Ca and viscosity ratios, and the characteristic interfacial shape of trapped oil.

Molecular rotors are molecules that form twisted conformations upon photoexcitation, with their fluorescence relaxation time serving as a measure of viscosity. They have been used to estimate the viscosity of biological and other membranes, but yield higher values than more common methods. We show that the rotor's relaxation time is influenced by a combination of membrane viscosity and interleaflet friction, and rationalize the existing discrepancy among methods.

We analyse the steady viscoelastic fluid flow in slowly varying contracting channels of arbitrary shape and present a theory based on the lubrication approximation for calculating the flow rate–pressure drop relation at low and high Deborah ( $De$ ) numbers. Unlike most prior theoretical studies leveraging the Oldroyd-B model, we describe the fluid viscoelasticity using a FENE-CR model and examine how the polymer chains’ finite extensibility impacts the pressure drop. We employ the low-Deborah-number lubrication analysis to provide analytical expressions for the pressure drop up to $O(De^4)$ . We further consider the ultra-dilute limit and exploit a one-way coupling between the parabolic velocity and elastic stresses to calculate the pressure drop of the FENE-CR fluid for arbitrary values of the Deborah number. Such an approach allows us to elucidate elastic stress contributions governing the pressure drop variations and the effect of finite extensibility for all $De$ . We validate our theoretical predictions with two-dimensional numerical simulations and find excellent agreement. We show that, at low Deborah numbers, the pressure drop of the FENE-CR fluid monotonically decreases with $De$ , similar to the previous results for the Oldroyd-B and FENE-P fluids. However, at high Deborah numbers, in contrast to a linear decrease for the Oldroyd-B fluid, the pressure drop of the FENE-CR fluid exhibits a non-monotonic variation due to finite extensibility, first decreasing and then increasing with $De$ . Nevertheless, even at sufficiently high Deborah numbers, the pressure drop of the FENE-CR fluid in the ultra-dilute and lubrication limits is lower than the corresponding Newtonian pressure drop.

The dynamics of self-propelled colloidal particles are strongly influenced by their environment through hydrodynamic and, in many cases, chemical interactions. We develop a theoretical framework to describe the motion of confined active particles by combining the Lorentz reciprocal theorem with a Galerkin discretisation of surface fields, yielding an equation of motion that efficiently captures self-propulsion without requiring an explicit solution for the bulk fluid flow. Applying this framework, we identify and characterise the long-time behaviours of a Janus particle near rigid, permeable, and fluid-fluid interfaces, revealing distinct motility regimes, including surface-bound skating, stable hovering, and chemo-hydrodynamic reflection. Our results demonstrate how the solute permeability and the viscosity contrast of the surface influence a particle's dynamics, providing valuable insights into experimentally relevant guidance mechanisms for autophoretic particles. The computational efficiency of our method makes it particularly well-suited for systematic parameter sweeps, offering a powerful tool for mapping the phase space of confined active particles and informing high-fidelity numerical simulations.

Many problems in elastocapillary fluid mechanics involve the study of elastic structures interacting with thin fluid films in various configurations. In this work, we study the canonical problem of the steady-state configuration of a finite-length pinned and flexible elastic plate lying on the free surface of a thin film of viscous fluid. The film lies on a moving horizontal substrate that drives the flow. The competing effects of elasticity, viscosity, surface tension and fluid pressure are included in a mathematical model consisting of a third-order Landau–Levich equation for the height of the fluid film and a fifth-order Landau–Levich-like beam equation for the height of the plate coupled together by appropriate matching conditions at the downstream end of the plate. The properties of the model are explored numerically and asymptotically in appropriate limits. In particular, we demonstrate the occurrence of boundary-layer effects near the ends of the plate, which are expected to be a generic phenomenon for singularly perturbed elastocapillary problems.

In diseases from diabetes to malaria, blood dynamics are significantly altered, resulting in poor clinical outcomes. However, the multiscale mechanisms that determine blood flow in the microcirculation in health and disease are undefined, largely owing to the difficulty in directly linking cell properties to whole-blood rheology. Here, we overcome these difficulties by developing a microfluidic platform to measure red blood cell properties and flow dynamics in the same blood samples from donors. We focus on sickle cell disease (SCD), a genetic disorder that causes red blood cells to stiffen in deoxygenated conditions, with disease pathology driven by oxygen-dependent blood rheology. Our linked cell and whole-blood measurements establish that increases in effective resistances in heterogeneous suspensions are driven by increases in the proportion of stiff cells, similar macroscopically to the behavior of rigid-particle suspensions. Furthermore, by combining simulations with spatially resolved measurements of cell dynamics, we show how the spatio-temporal organization of stiff and deformable cells determines blood rheology and drives disease pathophysiology. In the presence of deformable cells, the stiffened cells marginate towards channel walls, increasing effective wall friction. In fully deoxygenated conditions in which all cells are stiffened, significant heterogeneity in cell volume fraction along the direction of flow causes localized jamming, drastically increasing effective viscous flow resistance. Our work defines the relevant suspension physics required to understand pathological blood rheology in SCD and other diseases affecting red blood cell properties. More broadly, we reveal the multiscale processes that determine emergent rheology in heterogeneous particle suspensions.