arXiv subjects

Detlef Lohse

Publications and source records attributed to Detlef Lohse.

At least 19 recordsLinked to original sources

arXiv2026

The prey-predator motion of the active droplets

We present a generic solution for the relative motion of two diffusiophoretic prey-predator particles of arbitrary chemical activity Ai and radius Ri, where i = 1;2 refers to the predator and prey, respectively. Using the Lorentz reciprocal theorem, we investigate the impact of radius ratios and chemical reaction rates on the interaction between the particles. Our study reveals four distinct regimes of interaction, similar to the findings of Nasouri & Ramin (Phys. Rev. Lett., vol. 124, 2020, pp. 168003) for particles of identical sizes: (i) the prey particle escapes; (ii) two particles reach a stable equilibrium distance; (iii) two particles reach an unstable equilibrium distance where the prey is captured below it and escapes above it; (iv) the prey particle is captured. Unlike previous work, we differentiate between regimes with near-field and far-field interactions, which can be either attractive or repulsive, and find that the boundary between (i, iii) and (ii, iv) represents the transition from the attraction to repulsion in the far field, given by jA1=A2j = (R2=R1)2, while the transition between (i, ii) and (iii, iv) represents the interaction transition for particles in close proximity. We also perform three-dimensional simulations of prey-predator systems, which validate our theoretical predictions. Our study extends the applicability of the Lorentz reciprocal theorem to prey-predator system and quantitatively obtains interaction regime transition curves by distinguishing near-field and far-field interactions.

cond-mat.soft↗

arXiv2026

Leidenfrost droplets: The roles of ambient humidity and internal droplet circulation

A volatile droplet gently deposited on a superheated substrate can sit on a thin film of its own vapour, which prevents contact between the drop and surface. This phenomenon is called the Leidenfrost effect. In this paper, through direct numerical simulations, we analyse characteristics of Leidenfrost water droplets with a single computational model over four decades of droplet radius, stitching together previous works in the limit of large and small droplets. Using the model, we show that the ambient humidity, an underappreciated factor in the Leidenfrost system, in combination with the flow in the drop has a significant impact on the geometry and drying kinetics. Our results imply the inadequacies of commonly made assumptions of a pure vapour phase and an isothermal droplet. When modelling large Leidenfrost droplets with an axisymmetric model, large discrepancies between experiments and the computational results occur. Through azimuthal stability analysis, we show that this is due to the unrealistic constraint of axisymmetry. This finding is supported by 3D simulations of a simplified model. Finally, some hypotheses are explored to account for the remaining discrepancies with experimental data.

physics.flu-dyn↗

arXiv2026

Hygroscopic hysteresis drives intermittent salt creeping

Salt creeping -- the precipitation of salt crystals away from an evaporating liquid interface along surrounding surfaces -- occurs across settings from geology and cultural-heritage weathering to inkjet printing and carbon sequestration. Yet why its dynamics are sometimes smooth and sometimes violently intermittent has remained unexplained. Here we investigate the confined evaporation of salt solutions from a capillary with unidirectional water loss and show that salt creeping is an intrinsically intermittent, out-of-equilibrium process. By systematically varying the initial salt concentration and the ambient relative humidity, we identify regimes in which crystal deposition on the outer capillary surface goes hand in hand with non-monotonic, intermittent dynamics. Time-resolved measurements reveal that these intermittent dynamics are sustained by episodic water imbibition into the growing salt structures on the outer surface of the capillary, which sets up a self-amplifying feedback between evaporation and crystallization. Combining experiments with a minimal theoretical model, we demonstrate that hysteresis between deliquescence and efflorescence concentrations is sufficient to generate oscillatory salt accumulation and intermittent dynamics. Hygroscopic hysteresis, in other words, is the switch that turns steady evaporation into intermittent creeping. Our results recast salt creeping as a relaxation oscillator, and point to the hysteretic phase change as a generic route to intermittency in evaporating multicomponent fluids.

cond-mat.soft↗

arXiv2026

Ultimate regimes in horizontal and internally heated convection

We derive asymptotic models for the ultimate regimes in horizontal convection (HC) and pure internally heated convection (IHC), in analogy with our recent (2024) extension of the ultimate-regime model for Rayleigh-Benard convection (RBC). To derive the corresponding models for HC and IHC, we combine turbulent boundary-layer relations with the exact dissipation balances for these two systems. For HC, the resulting scaling relations are consistent with the rigorous transport bound of Siggers et al. (2004). For pure IHC, they are consistent with the exact HC-IHC balance analogy of Wang et al. (2021) and with the rigorous bounds on the convective-flux asymmetry in the equal-temperature-plates configuration (Arslan et al 2021). The main difference between RBC and HC/IHC is that, in the latter two cases, the global kinetic-energy balance does not contain the additional response factor (dimensionless convective heat flux in HC or inverse bulk temperature in IHC), whereas it does in RBC. As a consequence, for fixed Pr, the ultimate-regime scaling exponent is 1/3 for both HC and IHC, rather than 1/2 as in RBC.

physics.flu-dyn↗

arXiv2026

The effects of salinity and inclination on the morphology of melting ice

The salinity of water and the slope of ice significantly influence the melt rate and surface morphology of ice, both highly relevant in the context of glacier and iceberg melting in oceanic environments. In this study, we conducted experiments on vertical and sloped ice blocks melting in quiescent saline water. Through the use of fringe projection profilometry, we measured the morphology of the ice's front face. In particular, we combine the spatio-temporal phase shifting and orthogonal sampling moire methods. The far field salinity in the experiments ranged from 0 g/kg to 35 g/kg, and angles were between -18{\deg} and 50{\deg}. The ice block sizes were 32 cm $\times$ 23 cm $\times$ 12 cm high, wide, and long respectively, leading to Ra = $\mathcal{O}(10^7)$. We identified and classify five surface morphologies and regimes arising from the flow regimes imposed by salinity and inclination, namely scalloped, channelized, top-melting, bottom-melting, and incurved. The channelized morphology consists of vertical channels carved along the ice surface, whose development originates from a Rayleigh--B\'enard type instability, and which are enhanced by bubbles released from the melting ice and rising along the interface. The scalloped regime is characterize by a rough dimpled pattern commonly referred to as scallops. We observe that increasing the salinity leads to scallops that are smaller, shallower, and more uniform in size. Additionally, a salinity dependence of the melt rate is found, showing a non-monotonic behavior, while the inclination angle shows little influence on the overall melt rate.

physics.flu-dyn↗

arXiv2026

Transition from classical to ultimate melting

Melting is omnipresent in nature and technology, with applications ranging from metallurgy, biology, food science, and latent thermal energy storage to oceanography, geophysics, and climate science, and occurring on all scales from sub-millimeter to global scales. The key objective is to understand the rate at which an object melts as a function of its size and of the ambient conditions. To achieve this it is important to be able to extrapolate from small scale experiments and observations to large or even global scales. This is done by scaling laws. However, these are only meaningful if there is no transition from one scaling relation to another one. Here we show, however, that for both fixed and freely-advected melting objects immersed in a turbulent flow a melting transition does exist, namely from slow melting at the small scales to fast melting at the large scales. We do so by controlled melting experiments and corresponding direct numerical simulations, covering four orders of magnitude in scale. The transition corresponds to the transition from a laminar-type boundary layer around the melting object to a turbulent-type boundary layer, i.e., from so-called classical turbulence to ultimate turbulence, with its enhanced transport properties. Our results thus provide a quantitative understanding of the flow physics of the melting process and thereby enable a better extrapolation and prediction of melt rates on large scales such as relevant in geophysics, oceanography, and climate science.

physics.flu-dyn↗

arXiv2026

Settling of chiral particles in a turbulent flow

Chiral particles are experimentally investigated while settling inwater with various turbulence intensity levels. The locations and orientations of the particles are tracked over time, allowing the close investigation of the particles' settling dynamics. The generated turbulent flow is measured using laser Doppler anemometry (LDA), and the turbulence strength varies between experiments in the range $0 \leq Re_\lambda \leq 250$. Starting with quiescent particle settling, the chiral particle's orientation dynamics are studied, revealing a preferred alignment and a strong translation-rotation coupling. The particle chirality determines the preferred rotation direction, though the alignment and translation-rotation coupling gradually vanish with increasing turbulence. We identify multiple settling modes for the chiral particles, which are characterised by the evolution of the rotation angles. Finally, a theoretical model assuming a simplified chiral particle in Stokes flow clarifies the emergence of each settling mode.

physics.flu-dyn↗

arXiv2025

Chiral Particles in Taylor-Couette Turbulence

This work investigates chiral particles, which break mirror symmetry, in turbulent Taylor--Couette flow. These particles generally display a translation-rotation coupling moving through a quiescent fluid. Here we performed experiments using large chiral particles (typical size \unit{5}{mm}) in turbulent Taylor--Couette flow, for Reynolds numbers $9\cdot10^3 \leq \text{Re} \leq 1.5 \cdot 10^5$. The density-matched chiral particles are studied in a dilute regime $(\phi = 1.7 \cdot 10^{-4})$, where their location and orientation are tracked over time to investigate the particle-fluid coupling. We investigate whether the translation-rotation coupling observed at low Reynolds numbers is still observable over the measured high Reynolds numbers, using the tracked location and orientation. Similarly, we verify whether the chiral particles display a preferred location or orientation, and whether the left-handed and right-handed particles show different rotation statistics. The location data show that the chiral particles closely follow the structure of Taylor vortices. Hence, the orientation data and rotation data of the chiral particles are split between the Taylor vortices and particle chiralities. The results show no difference in rotation and orientation dynamics between chiralities. Rather, the particle dynamics are flow-dominated, where the flow vorticity determines the specific particle dynamics.

physics.flu-dyn↗

arXiv2025

Stood-up drop to measure receding contact angles

The wetting behavior of drops on natural and industrial surfaces is determined by the advancing and receding contact angles. They are commonly measured by the sessile drop technique, also called goniometry, which doses liquid through a solid needle. Consequently, this method requires substantial drop volumes, long contact times, tends to be user-dependent, and is difficult to automate. Here, we propose the stood-up drop (SUD) technique as an alternative to measure receding contact angles. The method consists of depositing a liquid drop on a surface by a short liquid jet, at which it spreads radially forming a pancake-shaped film. Then the liquid retracts, forming a spherical cap drop shape (stood-up drop). At this quasi-equilibrium state, the contact angle ($\theta_\text{SUD}$) closely resembles the receding contact angle measured by goniometry. Our method is suitable for a wide variety of surfaces from hydrophilic to hydrophobic, overcoming typical complications of goniometry such as needle-induced distortion of the drop shape, and it reduces user dependence. We delineate when the receding contact angle can be obtained by the stood-up method using Volume-of-Fluid (VoF) simulations that systematically vary viscosity, contact angle, and deposited drop volume. Finally, we provide simple scaling criteria to predict when the stood-up drop technique works.

physics.flu-dyn↗

arXiv2025

Settling dynamics of an oloid: experiments and simulations

This study presents a combined experimental and computational investigation of an oloid shaped particle settling in a quiescent fluid. The oloid, a unique convex shape with anisotropic geometry, provides a distinctive model for exploring how a particle's shape and orientation affect its settling dynamics. The settling oloids are tracked experimentally for Galileo numbers $48 \leq \text{Ga} \leq 5.4 \cdot 10^3$, using two particle sizes ($D_{\text{eq}}$ = 21.6 mm, and $D_{\text{eq}}$ = 10.8 mm). The density ratio between the particle and fluid $\Gamma$ = $\frac{\rho_p}{\rho_f}$ ranges from $1.11 \leq \Gamma \leq 1.30$ in the experiments. Computationally, the Galileo numbers $10 \leq \text{Ga} \leq 100$ are simulated, with $\Gamma = 2$. The experimental findings and numerical results are in good agreement, and give a consistent idea of the oloid settling dynamics. Our results indicate two distinct falling modes for the oloid, separated by Galileo number. The stable mode is characterised by a preferential orientation, with a rotation around the vertical axis, whereas the tumbling mode has randomly distributed orientation and rotation statistics. We characterise the falling velocity, orientation, and rotation dynamics of the oloids over a range of Galileo numbers. Additionally, the influence of the initial orientation is revealed to determine the rotation dynamics at low Galileo numbers.

physics.flu-dyn↗

arXiv2025

Hysteresis in the freeze-thaw cycle of emulsions and suspensions

Freeze-thaw cycles can be regularly observed in nature in water and are essential in industry and science. Objects present in the medium will interact with either an advancing solidification front during freezing or a retracting solidification front, i.e., an advancing melting front, during thawing. It is well known that objects show complex behaviours when interacting with the advancing solidification front, but the extent to which they are displaced during the retraction of the solid-liquid interface is less well understood. To study potential hysteresis effects during freeze-thaw cycles, we exploit experimental model systems of oil-in-water emulsions and polystyrene (PS) particle suspensions, in which a water-ice solidification front advances and retracts over an individual immiscible (and deformable) oil droplet or over a solid PS particle. We record several interesting hysteresis effects, resulting in non-zero relative displacements of the objects between freezing and thawing. PS particles tend to migrate further and further away from their initial position, whereas oil droplets tend to return to their starting positions during thawing. We rationalize our experimental findings by comparing them to our prior theoretical model of Meijer, Bertin & Lohse, Phys. Rev. Fluids (2025), yielding a qualitatively good agreement. Additionally, we look into the reversibility of how the droplet deforms and re-shapes throughout one freeze-thaw cycle, which will turn out to be remarkably robust.

cond-mat.soft↗

arXiv2025

Impacting spheres: from liquid drops to elastic beads

A liquid drop impacting a non-wetting rigid substrate laterally spreads, then retracts, and finally jumps off again. An elastic solid, by contrast, undergoes a slight deformation, contacts briefly, and bounces. The impact force on the substrate - crucial for engineering and natural processes - is classically described by Wagner's (liquids) and Hertz's (solids) theories. This work bridges these limits by considering a generic viscoelastic medium. Using direct numerical simulations, we study a viscoelastic sphere impacting a rigid, non-contacting surface and quantify how the elasticity number ($El$, dimensionless elastic modulus) and the Weissenberg number ($Wi$, dimensionless relaxation time) dictate the impact force. We recover the Newtonian liquid response as either $El \to 0$ or $Wi \to 0$, and obtain elastic-solid behavior in the limit $Wi \to \infty$ and $El \ne 0$. In this elastic-memory limit, three regimes emerge - capillary-dominated, Wagner scaling, and Hertz scaling - with a smooth transition from the Wagner to the Hertz regime. Sweeping $Wi$ from 0 to $\infty$ reveals a continuous shift from materials with no memory to materials with permanent memory of deformation, providing an alternate, controlled route from liquid drops to elastic beads. The study unifies liquid and solid impact processes and offers a general framework for the liquid-to-elastic transition relevant across systems and applications.

cond-mat.soft↗

arXiv2025

Why and when merging surface nanobubbles jump

Gas bubble accumulation on substrates reduces the efficiency of many physicochemical processes, such as water electrolysis. For microbubbles, where buoyancy is negligible, coalescence-induced jumping driven by the release of surface energy provides an efficient pathway for their early detachment. At the nanoscale, however, gas compressibility breaks volume conservation during coalescence, suppressing surface energy release and seemingly disabling this detachment route. Using molecular dynamics simulations, continuum numerical simulations, and theoretical analysis, we show that surface nanobubbles with sufficiently large contact angles can nevertheless detach after coalescence. In this regime, detachment is powered by the release of pressure energy associated with nanobubble volume expansion. This finding thus establishes a unified driving mechanism for coalescence-induced bubble detachment across all length scales.

cond-mat.soft↗

arXiv2025

Upstream motion of oil droplets in co-axial Ouzo flow due to Marangoni forces

To explore the physicochemical hydrodynamics of phase-separating ternary liquids (Ouzo-type), a binary oil-ethanol mixture is introduced into a co-flowing stream of water. Oil droplets nucleate at the interface between the two liquids, leading to a larger oil droplet interacting with the ethanol-rich jet. Although buoyancy forces and hydrodynamic drag forces push the droplet in downstream direction, we observe an upstream motion. Using computational fluid dynamics simulations of a simplified model system, we identify the nucleation zone for oil droplets and uncover Marangoni forces to be responsible for the upstream motion of the droplet. A semi-analytical model allows us to identify the key parameters governing this effect. A general conclusion is that Marangoni stresses can reverse the motion of droplets through channels, where the surrounding liquid is a multi-component mixture. The insights from this work are not only relevant for channel flow, but more generally, for the physicochemical hydrodynamics of multiphase, multi-component systems.

physics.flu-dyn↗

arXiv2025

Holes in Sheets: Double-Threshold Rupture of Draining Liquid Films

Classical rupture is attributed to molecular (van der Waals) forces acting at nanometric thicknesses. Nonetheless, micron-thick liquid sheets routinely perforate far above the scale where these molecular forces act, yet the mechanism that selects opening versus healing has remained unclear. Using direct numerical simulations of a draining sheet with an entrained air bubble (cavity), we show that irreversible rupture occurs only when a deterministic double-threshold is crossed: (i) the outward driving (from airflow or inertia) is strong enough and (ii) the cavity is distorted enough. If either condition falls short, surface tension heals the cavity and the sheet reseals. The time for this process is set by the balance between inertia and viscosity -- fast for inertia-dominated sheets and slower for viscous ones. This double-threshold mechanism explains why micrometer-thick films perforate and offers practical control options -- driving strength and defect geometry -- for predicting and controlling breakup in spray formation processes, wave breaking, and respiratory films.

physics.flu-dyn↗

arXiv2025

Transition to the ultimate regime of turbulent convection in stratified inclined duct flow

The stratified inclined duct (SID) flow provides a canonical model for sustained, buoyancy-driven exchange between two reservoirs of different density, and emerges as a new paradigm in geophysical fluid dynamics. Yet, the flow dynamics remain unclear in the highly turbulent regime; laboratory experiments can access this regime but they lack resolution, while direct numerical simulations (DNS) at realistically high Prandtl number $\mathrm{Pr}=7$ (for heat in water) have not achieved sufficiently high Reynolds numbers $\mathrm{Re}$. We conduct three-dimensional DNS up to $\mathrm{Re}= 8000$ and observe the transition to the so-called ultimate regime of turbulent convection as evidenced by the Nusselt number scaling $\mathrm{Nu} \sim \mathrm{Ra}^{1/2}$, i.e., considerably enhanced transport. At the transition the shear Reynolds number, a key parameter characterizing boundary layer (BL) dynamics, exceeds the threshold range of 420 for turbulent kinetic BLs with the emergence of logarithmic velocity profiles. The nature of the transition towards ultimate SID flow is of non-normal-nonlinear nature, i.e., subcritical and hysteretic, as typical for the transition to fully turbulent shear flows. Our work connects SID flow with the broader class of wall-bounded turbulent convection flows and gives insight into mixing in the vigorously turbulent regimes in oceanography and industry.

physics.flu-dyn↗

arXiv2025

Internal flow and concentration in neighbouring evaporating binary droplets and rivulets

In this paper, the evaporation of neighbouring multi-component droplets or rivulets - often found in applications such as inkjet printing, spray cooling, and pesticide delivery - is studied numerically and theoretically. The proximity induces a shielding effect that reduces individual evaporation rates and disrupts the symmetry of both the concentration profile and the flow field in the liquids. We examine how the symmetry of flow and concentration fields is affected by key parameters, namely the contact angle, the inter-droplet (or inter-rivulet) distance, and the magnitude of surface tension gradient forces (i.e. the Marangoni number). We focus on binary mixtures, such as water and 1,2-hexanediol, where only one component evaporates and evaporation is slow, thereby allowing simplifications to the governing equations. To manage the complexity of the full three-dimensional droplet problem, we begin with a two-dimensional model of neighbouring rivulets. Solving the complete transient equations for rivulets with pinned contact lines and fixed inter-rivulet distance reveals that the asymmetry - quantified by the position of the interfacial stagnation point of the flow - diminishes over time. Using a validated quasi-stationary model, we find, with increasing contact angle and inter-rivulet distance, that the stagnation point migrates closer to the centre, yet it remains unaffected by the Marangoni number. A simplified lubrication model applied to droplets shows similar dependencies on contact angle and distance, although here the stagnation point appears to vary with the Marangoni number. We attribute this dependence to the additional azimuthal flow in droplets, leading to a non-linear evolution of the concentration and therefore a non-trivial dependence of the symmetry on the Marangoni number.

physics.flu-dyn↗

arXiv2025

Hidden in plain sight: How evaporation impacts the pendant drop method

The surface tension of a liquid, which drives most free surface flows at small scales, is often measured with the pendant drop method due to its simplicity and reliability. When the drop is suspended in air, controlling the ambient temperature and humidity is usually an afterthought, resulting in evaporation of the drop during the measurement. Here, we investigate the effect of evaporation on the measured surface tension using experiments and numerical simulations. In the experiments, we measured the evolution of the droplet temperature, which can drastically reduce by ($\Delta T \approx 10 \degC$) due to evaporative cooling, and thereby altering the measured surface tension by more than 1 mN/m. This finding can be reproduced by numerical simulations, which additionally allows for controlled investigations of the individual influences of further effects on the pendant drop method, namely shape deformations by evaporation-driven flows in the gas-phase and in the liquid-phase including the resulting Marangoni flow. We provide a simple passive method to control the relative humidity without requiring additional instrumentation. Our findings are particularly pertinent to Marangoni flows which are driven by surface tension gradients, and which are consequently highly sensitive to measurement inaccuracies. We apply our method with different aqueous mixtures of glycerol and various diols. Our results and insights have implications for various applications, ranging from inkjet printing to agricultural sprays. Finally, we have meticulously documented our setup and procedure for future reference.

physics.flu-dyn↗