arXiv subjects

Detlef Lohse

Publications and source records attributed to Detlef Lohse.

At least 127 records · Page 7Linked to original sources

arXiv2021

Droplet plume emission during plasmonic bubble growth in ternary liquids

Plasmonic bubbles are of great relevance in numerous applications, including catalytic reactions, micro/nanomanipulation of molecules or particles dispersed in liquids, and cancer therapeutics. So far, studies have been focused on bubble nucleation in pure liquids. Here we investigate plasmonic bubble nucleation in ternary liquids consisting of ethanol, water, and trans-anethole oil which can show the so-called ouzo-effect. We find that oil (trans-anethole) droplet plumes are produced around the growing plasmonic bubbles. The nucleation of the microdroplets and their organization in droplet plumes is due to the symmetry breaking of the ethanol concentration field during the selective evaporation of ethanol from the surrounding ternary liquids into the growing plasmonic bubbles. Numerical simulations show the existence of a critical Marangoni number Ma (the ratio between solutal advection rate and the diffusion rate), above which the symmetry breaking of the ethanol concentration field occurs, leading to the emission of the droplet plumes. The numerical results agree with the experimental observation that more plumes are emitted with increasing ethanol-water relative weight ratios and hence Ma. Our findings on the droplet plume formation reveal the rich phenomena of plasmonic bubble nucleation in multicomponent liquids and help to pave the way to achieve enhanced mixing in multicomponent liquids in chemical, pharmaceutical, and cosmetic industries.

physics.flu-dyn↗

arXiv2021

The retraction of jetted slender viscoelastic liquid filaments

Long and slender liquid filaments are produced during inkjet printing, which can subsequently either retract to form a single droplet, or break up to form a primary droplet and one or more satellite droplets. These satellite droplets are undesirable since they degrade the quality and reproducibility of the print. Existing strategies for the suppression of satellite droplet formation include, among others, adding viscoelasticity to the ink. In the present work, we aim to improve the understanding of the role of viscoelasticity in suppressing satellite droplets in inkjet printing. We demonstrate that very dilute viscoelastic aqueous solutions (concentrations $\sim$ 0.003\% wt. polyethylene oxide (PEO), corresponding to nozzle Deborah number $De_{n}$ $\sim$ 3) can suppress satellite droplet formation. Furthermore, we show that, for a given driving condition, upper and lower bounds of polymer concentration exist, within which satellite droplets are suppressed. Satellite droplets are formed at concentrations below the lower bound, while jetting ceases for concentrations above the upper bound (for fixed driving conditions). Moreover, we observe that, with concentrations in between the two bounds, the filaments retract at velocities larger than the corresponding Taylor-Culick velocity for the Newtonian case. We show that this enhanced retraction velocity can be attributed to the elastic tension due to polymer stretching, which builds up during the initial jetting phase. These results shed some light on the complex interplay between inertia, capillarity, and viscoelasticity for retracting liquid filaments, which is important for the stability and quality of inkjet printing of polymer solutions.

physics.flu-dyn↗

arXiv2021

Diffusion-free scaling in rotating spherical Rayleigh-Bénard convection

Direct numerical simulations are employed to reveal three distinctly different flow regions in rotating spherical Rayleigh-Bénard convection. In the low-latitude region $\mathrm{I}$ vertical (parallel to the axis of rotation) convective columns are generated between the hot inner and the cold outer sphere. The mid-latitude region $\mathrm{II}$ is dominated by vertically aligned convective columns formed between the Northern and Southern hemispheres of the outer sphere. The diffusion-free scaling, which indicates bulk-dominated convection, originates from this mid-latitude region. In the equator region $\mathrm{III}$ the vortices are affected by the outer spherical boundary and are much shorter than in region $\mathrm{II}$. Thermally driven turbulence with background rotation in spherical Rayleigh-Bénard convection is found to be characterized by three distinctly different flow regions. The diffusion-free scaling, which indicates the heat transfer is bulk-dominated, originates from the mid-latitude region in which vertically aligned vortices are stretched between the Northern and Southern hemispheres of the outer sphere.\ These results show that the flow physics in rotating convection are qualitatively different in planar and spherical geometries. This finding underlines that it is crucial to study convection in spherical geometries to better understand geophysical and astrophysical flow phenomena.

physics.flu-dyn↗

arXiv2021

Boundary layers in turbulent vertical convection at high Prandtl number

Many environmental flows arise due to natural convection at a vertical surface, from flows in buildings to dissolving ice faces at marine-terminating glaciers. We use three-dimensional direct numerical simulations of a vertical channel with differentially heated walls to investigate such convective, turbulent boundary layers. Through the implementation of a multiple-resolution technique, we are able to perform simulations at a wide range of Prandtl numbers $Pr$. This allows us to distinguish the parameter dependences of the horizontal heat flux and the boundary layer widths in terms of the Rayleigh number $Ra$ and Prandtl number $Pr$. For the considered parameter range $1\leq Pr \leq 100$, $10^6 \leq Ra \leq 10^9$, we find the flow to be consistent with a 'buoyancy-controlled' regime where the heat flux is independent of the wall separation. For given $Pr$, the heat flux is found to scale linearly with the friction velocity $V_\ast$. Finally, we discuss the implications of our results for the parameterisation of heat and salt fluxes at vertical ice-ocean interfaces.

physics.flu-dyn↗

arXiv2021

Heat transfer in turbulent Rayleigh-Bénard convection within two immiscible fluid layers

We numerically investigate turbulent Rayleigh-Bénard convection within two immiscible fluid layers, aiming to understand how the layer thickness and fluid properties affect the heat transfer (characterized by the Nusselt number $Nu$) in two-layer systems. Both two- and three-dimensional simulations are performed at fixed global Rayleigh number $Ra=10^8$, Prandtl number $Pr=4.38$, and Weber number $We=5$. We vary the relative thickness of the upper layer between $0.01 \le α\le 0.99$ and the thermal conductivity coefficient ratio of the two liquids between $0.1 \le λ_k \le 10$. Two flow regimes are observed: In the first regime at $0.04\leα\le0.96$, convective flows appear in both layers and $Nu$ is not sensitive to $α$. In the second regime at $α\le0.02$ or $α\ge0.98$, convective flow only exists in the thicker layer, while the thinner one is dominated by pure conduction. In this regime, $Nu$ is sensitive to $α$. To predict $Nu$ in the system in which the two layers are separated by a unique interface, we apply the Grossmann-Lohse theory for both individual layers and impose heat flux conservation at the interface. Without introducing any free parameter, the predictions for $Nu$ and for the temperature at the interface well agree with our numerical results and previous experimental data.

physics.flu-dyn↗

arXiv2021

The Leidenfrost effect as a directed percolation phase transition

Volatile drops deposited on a hot solid can levitate on a cushion of their own vapor, without contacting the surface. We propose to understand the onset of this so-called Leidenfrost effect through an analogy to non-equilibrium systems exhibiting a directed percolation phase transition. When performing impacts on superheated solids, we observe a regime of spatiotemporal intermittency in which localized wet patches coexist with dry regions on the substrate. We report a critical surface temperature, which marks the upper bound of a large range of temperatures in which levitation and contact coexist. In this range, with decreasing temperature, the equilibrium wet fraction increases continuously from zero to one. Also, the statistical properties of the spatio-temporally intermittent regime are in agreement with that of the directed percolation universality class. This analogy allows us to redefine the Leidenfrost temperature and shed light on the physical mechanisms governing the transition to the Leidenfrost state.

physics.flu-dyn↗

arXiv2021

Electroconvective instability in water electrolysis: an evaluation of electroconvective patterns and their onset features

In electrochemical systems, an understanding of the underlying transport processes is required to aid in their better design. This includes knowledge of possible near-electrode convective mixing that can enhance measured currents. Here, for a binary acidic electrolyte in contact with a platinum electrode, we provide evidence of electroconvective instability during electrocatalytic proton reduction. The current-voltage characteristics indicate that electroconvection, visualized with a fluorescent dye, drives current densities larger than the diffusion transport limit. The onset and transition times of the instability do not follow the expected inverse-square dependence on the current density, but, above a bulk-reaction-limited current density are delayed by the water dissociation reaction. The dominant size of the electroconvective patterns is also measured and found to vary as the diffusion length scale, confirming previous predictions on the size of electroconvective vortices.

physics.flu-dyn↗

arXiv2021

Deformation and relaxation of viscous thin films under bouncing drops

Thin, viscous liquid films subjected to impact events can deform. Here we investigate free surface oil film deformations that arise due to the air pressure buildup under the impacting and rebouncing water drops. Using Digital Holographic Microscopy, we measure the 3D surface topography of the deformed film immediately after the drop rebound, with a resolution down to 20 nm. We first discuss how the film is initially deformed during impact, as a function of film thickness, film viscosity, and drop impact speed. Subsequently, we describe the slow relaxation process of the deformed film after the rebound. Scaling laws for the broadening of the width and the decay of the amplitude of the perturbations are obtained experimentally and found to be in excellent agreement with the results from a lubrication analysis. We finally arrive at a detailed spatio-temporal description of the oil film deformations that arise during the impact and rebouncing of water drops.

physics.flu-dyn↗

arXiv2021

Crown formation from a cavitating bubble close to a free surface

A rapidly growing bubble close to a free surface induces jetting: a central jet protruding outwards and a crown surrounding it at later stages. While the formation mechanism of the central jet is known and documented, that of the crown remains unsettled. We perform axisymmetric simulations of the problem using the free software program basilisk, where a finite-volume compressible solver has been implemented, that uses a geometric Volume-of-Fluid method (VoF) for the tracking of the interface. We show that the mechanism of crown formation is a combination of a pressure distortion over the curved interface, inducing flow focusing, and of a flow reversal, caused by the second expansion of the toroidal bubble that drives the crown. The work culminates in a parametric study with the Weber number, the Reynolds number, the pressure ratio and the dimensionless bubble distance to the free surface as control parameters. Their effects on both the central jet and the crown are explored. For high Weber numbers, we observe the formation of weaker "secondary crowns", highly correlated with the third oscillation cycle of the bubble.

physics.flu-dyn↗

arXiv2021

Bursting Bubble in a Viscoplastic Medium

When a rising bubble in a Newtonian liquid reaches the liquid-air interface, it can burst, leading to the formation of capillary waves and a jet on the surface. Here, we numerically study this phenomenon in a yield stress fluid. We show how viscoplasticity controls the fate of these capillary waves and their interaction at the bottom of the cavity. Unlike Newtonian liquids, the free surface converges to a non-flat final equilibrium shape once the driving stresses inside the pool fall below the yield stress. Details of the dynamics, including the flow's energy budgets, are discussed. The work culminates in a regime map with four main regimes with different characteristic behaviours.

physics.flu-dyn↗

arXiv2021

Air-cushioning effect and Kelvin-Helmholtz instability before the slamming of a disk on water

The macroscopic dynamics of a droplet impacting a solid is crucially determined by the intricate air dynamics occurring at the vanishingly small length scale between droplet and substrate prior to direct contact. Here we investigate the inverse problem, namely the role of air for the impact of a horizontal flat disk onto a liquid surface, and find an equally significant effect. Using an in-house experimental technique, we measure the free surface deflections just before impact, with a precision of a few micrometers. Whereas stagnation pressure pushes down the surface in the center, we observe a lift-up under the edge of the disk, which sets in at a later stage, and which we show to be consistent with a Kelvin-Helmholtz instability of the water-air interface.

physics.flu-dyn↗

arXiv2021

Marangoni instability triggered by selective evaporation of a binary liquid inside a Hele-Shaw cell

Interfacial stability is important for many processes involving heat and mass transfer across two immiscible phases. When this transfer takes place in the form of evaporation of a binary solution with one component being more volatile than the other, gradients in surface tension can arise. These gradients can ultimately destabilise the liquid-gas interface. In the present work, we study the evaporation of an ethanol-water solution, for which ethanol has a larger volatility. The solution is contained in a horizontal Hele-Shaw cell which is open from one end to allow for evaporation into air. A Marangoni instability is then triggered at the liquid-air interface. We study the temporal evolution of this instability by observing the effects that it has on the bulk of the liquid. More specifically, the growth of convective cells is visualized with confocal microscopy and the velocity field close to the interface is measured with micro-particle-image-velocimetry. The results of numerical simulations based on quasi 2D equations satisfactorily compare with the experimental observations, even without consideration of evaporative cooling, although this cooling can play an extra role in experiments. Furthermore, a linear stability analysis applied to a simplified version of the quasi 2D equations showed reasonably good agreement with the results from simulations at early times, when the instability has just been triggered and no coarsening has taken place. In particular, we find a critical Marangoni number below which a regime of stability is predicted.

physics.flu-dyn↗

arXiv2021

Strong alignment of prolate ellipsoids in Taylor-Couette flow

We report on the mobility and orientation of finite-size, neutrally buoyant prolate ellipsoids (of aspect ratio $Λ=4$) in Taylor-Couette flow, using interface resolved numerical simulations. The setup consists of a particle-laden flow in between a rotating inner and a stationary outer cylinder. We simulate two particle sizes $\ell/d=0.1$ and $\ell/d=0.2$, $\ell$ denoting the particle major axis and $d$ the gap-width between the cylinders. The volume fractions are $0.01\%$ and $0.07\%$, respectively. The particles, which are initially randomly positioned, ultimately display characteristic spatial distributions which can be categorised into four modes. Modes $(i)$ to $(iii)$ are observed in the Taylor vortex flow regime, while mode ($iv$) encompasses both the wavy vortex, and turbulent Taylor vortex flow regimes. Mode $(i)$ corresponds to stable orbits away from the vortex cores. Remarkably, in a narrow $\textit{Ta}$ range, particles get trapped in the Taylor vortex cores (mode ($ii$)). Mode $(iii)$ is the transition when both modes $(i)$ and $(ii)$ are observed. For mode $(iv)$, particles distribute throughout the domain due to flow instabilities. All four modes show characteristic orientational statistics. We find the particle clustering for mode ($ii$) to be size-dependent, with two main observations. Firstly, particle agglomeration at the core is much higher for $\ell/d=0.2$ compared to $\ell/d=0.1$. Secondly, the $\textit{Ta}$ range for which clustering is observed depends on the particle size. For this mode $(ii)$ we observe particles to align strongly with the local cylinder tangent. The most pronounced particle alignment is observed for $\ell/d=0.2$ around $\textit{Ta}=4.2\times10^5$. This observation is found to closely correspond to a minimum of axial vorticity at the Taylor vortex core ($\textit{Ta}=6\times10^5$) and we explain why.

physics.flu-dyn↗

arXiv2021

Stability of Respiratory-Like Droplets under Evaporation

Pathogens contained in airborne respiratory droplets have been seen to remain infectious for periods of time that depend on the ambient temperature and humidity. In particular, regarding the humidity, the empirically least favorable conditions for the survival of viral pathogens are found at intermediate humidities. However, the precise physico-chemical mechanisms that generate such least-favorable conditions are not understood yet. In this work, we analyze the evaporation dynamics of respiratory-like droplets in air, semi-levitating them on superhydrophobic substrates with minimal solid-liquid contact area. Our results reveal that, compared to pure water droplets, the salt dissolved in the droplets can significantly change the evaporation behaviour, especially for high humidities close to and above the deliquesence limit. Due to the hygroscopic properties of salt, water evaporation is inhibited once the salt concentration reaches a critical value that depends on the relative humidity. The salt concentration in a stable droplet reaches its maximum at around 75% relative humidity, generating conditions that might shorten the time in which pathogens remain infectious.

physics.flu-dyn↗

arXiv2021

Surface nanodroplet-based nanoextraction from sub-milliliter volumes of dense suspensions

Cleaner analytic technique for quantifying compounds in dense suspension is needed for wastewater and environment analysis, chemical or bio-conversion process monitoring, biomedical diagnostics, food quality control among others. In this work, we introduce a green, fast, one-step method called nanoextraction for extraction and detection of target analytes from sub-milliliter dense suspensions using surface nanodroplets without toxic solvents and pre-removal of the solid contents. With nanoextraction, we achieve a limit of detection (LOD) of 10^(-9) M for a fluorescent model analyte obtained from a particle suspension sample. The LOD lower than that in water without particles 10^(-8) M, potentially due to the interaction of particles and the analyte. The high particle concentration in the suspension sample thus does not reduce the extraction efficiency, although the extraction process was slowed down up to 5 min. As proof of principle, we demonstrate the nanoextraction for quantification of model compounds in wastewater slurry containing 30 wt% sands and oily components (i.e. heavy oils). The nanoextraction and detection technology developed in this work may be used as fast analytic technologies for complex slurry samples in environment industrial waste, or in biomedical diagnostics.

cond-mat.soft↗

arXiv2021

Off-center gravity induces large-scale flow patterns in spherical Rayleigh-Bénard

Inspired by the hemispherical asymmetry observed in the Earth's inner core, we perform direct numerical simulations to study the effect of the gravity center offset in spherical Rayleigh-Bénard convection. We find that even a minimal shift of the gravity center has a pronounced influence on the flow structures. When the gravity center is shifted towards the South, the co-latitudinal buoyancy component creates an energetic jet on the Northern side of the inner sphere that is directed towards the outer sphere. As a result, a large-scale meridional circulation is formed. However, surprisingly, the global heat flux is not affected by the changes in the large-scale flow organization induced by the gravity center offset. Our results suggest that the hemispherical core asymmetry is key to model the flow phenomena in the Earth's outer core and mantle.

physics.flu-dyn↗

arXiv2021

Higher-order meniscus oscillations driven by flow-focusing leading to bubble pinch off and entrainment in a piezo acoustic inkjet nozzle

The stability of high-end piezo-acoustic drop-on-demand (DOD) inkjet printing is sometimes compromised by the entrainment of an air bubble inside the ink channel. Here, bubble pinch-off from an acoustically driven meniscus is studied in an optically transparent DOD printhead as a function of the driving waveform. We show that bubble pinch-off follows from low-amplitude higher-order meniscus oscillations on top of the global high-amplitude meniscus motion that drives droplet formation. In a certain window of control parameters, phase inversion between the low and high frequency components leads to the enclosure of an air cavity and bubble pinch-off. Although phenomenologically similar, bubble pinch-off is not a result of capillary wave interaction such as observed in drop impact on a liquid pool. Instead, we reveal geometrical flow focusing as the mechanism through which at first, an outward jet is formed on the retracted concave meniscus. When the subsequent high-frequency pressure wave hits the now toroidal-shaped meniscus, it accelerates the toroidal ring outward resulting in the formation of an air cavity that can pinch off. The critical control parameters for pinch off are the pulse timing and amplitude. To cure the bubble entrainment problem, the threshold for bubble pinch-off can be increased by suppressing the high frequency acoustic waves through appropriate waveform design. The present work therefore aids the improvement of the stability of inkjet printers through a physical understanding of meniscus instabilities.

physics.flu-dyn↗

arXiv2021

An efficient phase-field method for turbulent multiphase flows

With the aim of efficiently simulating three-dimensional multiphase turbulent flows with a phase-field method, we propose a new discretization scheme for the biharmonic term (the 4th-order derivative term) of the Cahn-Hilliard equation. This novel scheme can significantly reduce the computational cost while retaining the same accuracy as the original procedure. Our phase-field method is built on top of a direct numerical simulation solver, named AFiD (www.afid.eu) and open-sourced by our research group. It relies on a pencil distributed parallel strategy and a FFT-based Poisson solver. To deal with large density ratios between the two phases, a pressure split method [1] has been applied to the Poisson solver. To further reduce computational costs, we implement a multiple-resolution algorithm which decouples the discretizations for the Navier-Stokes equations and the scalar equation: while a stretched wall-resolving grid is used for the Navier-Stokes equations, for the Cahn-Hilliard equation we use a fine uniform mesh. The present method shows excellent computational performance for large-scale computation: on meshes up to 8 billion nodes and 3072 CPU cores, a multiphase flow needs only slightly less than 1.5 times the CPU time of the single-phase flow solver on the same grid. The present method is validated by comparing the results to previous studies for the cases of drop deformation in shear flow, including the convergence test with mesh refinement, and breakup of a rising buoyant bubble with density ratio up to 1000. Finally, we simulate the breakup of a big drop and the coalescence of O(10^3) drops in turbulent Rayleigh-Bénard convection at a Rayleigh number of $10^8$, observing good agreement with theoretical results.

physics.flu-dyn↗