Showing papers on "Ohnesorge number published in 2021"

Initial spreading dynamics of a liquid droplet: The effects of wettability, liquid properties, and substrate topography

Abstract: The initial spreading of glycerol and silicon oil droplets on smooth, corrugated, and orthogonal surfaces is numerically investigated by an effective, sharp-interface modeling method. In this study, the temporal evolution of spreading radius during the initial phase is scaled by R/R0 = C(t/τi)α for inertial regime and R/R0 = C(t/τμ)α for the viscous regime. We focus on exploring how wettability, liquid properties, and substrate topography influence the exponent α and coefficient C. Instead of discussing the effects of density, viscosity, and surface tension separately, we use the Ohnesorge number Oh = μ/(ρD0γ)1/2 to unify the combined influence of liquid properties. The results show that in the inertial regime (Oh ≪ 1), α is determined by wettability and the capillary wave is observed to propagate along the droplet interface, whereas in the viscous regime (Oh ≫ 1), α is determined by Oh and no capillary wave is observed. Consequently, both qualitative (propagation of capillary wave) and quantitative (Ohnesorge number) criteria to distinguish the two distinct regimes are provided. Regarding the coefficient C, it is found to increase with the increasing hydrophilicity and decreasing Oh in the inertial regime. A larger C is also observed in orthogonal microgrooves with wider gap or narrower width. Besides, the hydrophobicity and hydrophilicity can be enhanced by the corrugated surfaces, inducing a higher and lower α on hydrophilic and hydrophobic corrugated surfaces, respectively. Meanwhile, some interesting phenomena are also observed, such as the faster contact line velocity on the inside of a single corrugation and the “stick-jump” advancing mode of the contact line on orthogonal surfaces.

On the physics of transient ejection from bubble bursting

Abstract: Using a dynamical scaling analysis of the flow variables and their evolution due to bubble bursting, here we predict the size and speed of ejected droplets for the whole range of experimental Ohnesorge and Bond numbers where ejection occurs. The transient ejection, which requires the backfire of a vortex ring inside the liquid to preserve physical symmetry, shows a delicate balance between inertia, surface tension and viscous forces around a critical Ohnesorge number, akin to an apparent singularity. Like in other natural phenomena, this balance makes the process extremely sensitive to initial conditions. Our model generalizes or displaces other recently proposed ones, impacting on, for instance, the statistical description of sea spray.

Contact line dynamics in curtain coating of non-Newtonian liquids

Abstract: The rheological characteristics of liquids play an important role in the flow near dynamic contact lines, where the deformation rates are extremely large. The liquid contact line dynamics and the free surface configuration in the curtain coating of polymer solutions were experimentally studied using visualization to determine the effect of two rheological characteristics: shear thinning and extensional thickening. The critical web speeds for heel formation, where the contact line on the moving substrate shifts upstream of the falling curtain, and air entrainment, where the contact line shifts downstream and air bubbles appear, were determined over a range of flow rates. The critical conditions were compared to the behavior observed for a Newtonian liquid. Moreover, the contact line dynamics were described by three dimensionless parameters: the Deborah number, the Ohnesorge number, and the ratio of the web speed to the liquid curtain velocity at the contact line.

Emission Performance of Ionic Liquid Electrospray Thruster for Micropropulsion

Abstract: It has been urgently required to develop ionic liquid electrospray thruster (ILET) for rapidly expanding micro-/nanosatellites. In this work, ILETs with high-density array emitters are fabricated t...

A numerical study of liquid compound filament contraction

Abstract: Droplets resulting from liquid filament contraction have been widely used in industrial processes. However, detailed investigations of liquid compound filament contraction processes are lacking in the literature. Therefore, this study provides a numerical investigation of the contraction of a two-layered compound filament. The simulations are based on an axisymmetric front-tracking method. It is found that because of the interfacial tension force, the initially long cylindrical filament contracts to a compound droplet without any breakup or breaks up into smaller droplets during contraction. Unlike simple filaments, the presence of the inner filament inside the compound filament results in a more complicated compound filament breakup process with various droplet types, e.g., simple droplets, single-core compound droplets, and multi-core compound droplets. We find that the inner filament breaks up into droplets, while the outer does not induce breakup. Such a breakup mode produces a multi-core compound droplet after contraction. In some cases, while the inner filament only contracts to a single droplet, its enclosing filament breaks up to produce simple droplets at each end. We also find a breakup mode that combines these two modes, where both the inner and outer filaments perform breakup. In addition, the breakup of the compound filament occurs via one of two mechanisms: end-pinching and necking. These breakup modes and mechanisms are affected by various parameters such as the inner and outer aspect ratios, the Ohnesorge number, the interfacial tension ratio, and the viscosity ratios. Based on these parameters, various regime diagrams of breakup and non-breakup are proposed.

Liquid film stability and contact line dynamics of emulsion liquid films in curtain coating process

Abstract: Previous studies reported that single-phase liquid curtains become more stable with increasing Ohnesorge number. Many liquid films used in the coating industry are types of emulsion samples. Despite this fact, the effect of multiphase liquids on the dynamics of the curtain breakup has not yet been considered. This study explores the stability of emulsion curtain coating via high-speed visualization. The critical condition at the onset of curtain breakup was determined by finding the flow rate below which the curtain broke. Curtain breakup was observed via a hole initiation within the curtain. The results reveal that curtain breakup dynamics is governed by the characteristic dynamic viscosity and the surface tension. The emulsion curtain stability, defined by Weber number, increases as Ohnesorge number rises, similar to the single-phase liquid (i.e., Newtonian and pure shear thinning) curtain stability. The critical web speed at which the contact line moves upstream of the curtain, a phenomenon called heel formation, and that at which air entrainment occurs, were determined for emulsion solutions at different flow rates. The results reveal that the surface tension increase delays the onset of air entrainment which could help to conduct faster curtain coating substrates with emulsion liquid films.

Bridge expansion after coalescence of two droplets in air: Inertial regime

Abstract: When two liquid droplets approach at negligible velocity in air, their coalescence spontaneously occurs by jump-to-contact instability and a connecting bridge joining the two facing interfaces at the nanoscale is created. We report experimental investigations of the expansion of this initial bridge by means of high-speed imaging. By considering droplets of water, polydimethylsiloxane or paraffin of a few hundred micrometers, we investigate regimes where inertia takes a major role. Depending on the Ohnesorge number Oh, the dynamics of the bridge differs a lot. For Oh ≈ 1, the initial flow is rapidly attenuated and the connecting bridge between the two droplets adopts a smooth parabolic shape. The maximum interface curvature and the minimum liquid pressure remain at the bridge centre. The expansion is thus caused by the capillary pressure that drives the fluid towards the center. At small Oh, in the inertial regime, the length of the initial bridge grows at constant speed and the bridge expansion can be described by the propagation of non-dispersive capillary wave packets. The central part of the bridge takes a cylindrical shape connected to the droplets by a narrow region of very large curvature. At the resolved scale, the interface exhibits slope discontinuities. By considering dihedral potential flows that result of the presence of the slope discontinuities, we show that the apparent angle made by the interface controls the flow-rate that enters the bridge and thus determines its radial expansion.

The characterization of particle number and distribution inside in-flight 3D printed droplets using a high speed droplet imaging system

Abstract: 3D bioprinting is an innovative and time-saving method to precisely generate cell-laden 3D structures for clinical and research applications. Ejected cell number and cell distribution are two key technical parameters for evaluation of the bioprinter performance. In this paper, a modified droplet imaging system is used to study cell-size fluorescent particle number and distribution within droplets ejected from a microvalve-based 3D bioprinter. The effects of droplet dispensing physics (dosing energy Ed), ink properties (Z number—the inverse of the Ohnesorge number and particle sedimentation velocity), and input particle concentration are considered. The droplet imaging system demonstrates a strong capability in analyzing bioprinting performance for seeded concentrations less than 3×106 particles/ml. The printed particle number increases near-linearly under increasing dosing energy and Z number. It was found that for 7

Curvature effect of electrowetting-induced droplet detachment

Abstract: Harnessing detachment of an aqueous droplet via electrowetting on a flat surface has been of considerable interest for potential practical applications, ranging from self-cleaning to novel optical and digital microfluidic devices, due to the wettability of the droplet on a solid substrate enhanced by applying an electric voltage between the droplet and the insulated substrate. However, a quantitative understanding of the detachment process and an accurate prediction on the thresholds of applied voltage for droplet detachment on curved surfaces are still lacking. In this paper, based on energy conservation, we derive a critical condition theoretically for electrowetting-induced droplet detachment from a hydrophobic curved surface. Furthermore, phase diagrams are constructed in terms of droplet volume, viscosity, the Ohnesorge number, friction coefficient at contact line, surface curvature, surface wettability, and electrowetting number. The deduced critical condition offers a general and quantitative prediction on when the detachment occurs, a criterion enabling us to gain more insights into how to accurately manipulate the electrowetting-induced detachment of an aqueous droplet on a curved surface. The results obtained in this paper also imply that the detachable regimes of the phase diagrams can be enlarged through increasing droplet volume and surface curvature and reducing liquid viscosity, friction coefficient, the Ohnesorge number, and wettability of substrates.

Numerical simulations and experiments on droplet coalescence dynamics over a liquid–air interface: mechanism and effect of droplet-size/surface-tension

Abstract: Present study is on partial/complete coalescence dynamics of a droplet (surrounded by air) over a horizontal pool of the same liquid Experimental and numerical studies are presented for both isopropanol and glycerol droplet of a constant diameter Numerical study is presented in more detail for the isopropanol droplet to study the effect of diameter ( $$D=0035-67 mm$$ ) and surface tension coefficient ( $$\gamma =2-200 mN/m$$ ) on the coalescence dynamics For partial coalescence of an isopropanol droplet and complete coalescence of a glycerol droplet, excellent agreement is demonstrated between our numerically and experimentally obtained interface dynamics; and a qualitative discussion on the mechanism of the partial and complete coalescence is presented Three regimes of partial coalescence − viscous, inertio-capillary and gravity − proposed in the literature for a liquid-liquid system are presented here for the present liquid-air system while studying the effect of diameter of the isopropanol droplet Probably for the first time in the literature, our numerical study presents a flow and vorticity dynamics based quantitative evidence of the coalescence-mechanism, analogy with a freely vibrating Spring-Mass-Damper System, the gravity regime for a liquid-gas system, and the effect of surface tension coefficient $$\gamma$$ based coalescence dynamics study The associated novel $$\gamma$$ based droplet coalescence regime map presents a critical Ohnesorge number $$Oh_{c}$$ and critical Bond number $$Bo_{c}$$ for a transition from partial to full coalescence; and such critical values are also presented for the transition under effect of the droplet diameter The critical values based transition boundaries, obtained separately for the varying D and varying $$\gamma$$ , are demonstrated to be in excellent agreement with a correlation reported in the literature

Electrohydrodynamic Instability of a Cylindrical Interface: Effect of the Buoyancy Thermo-Capillary in Porous Media

Abstract: Electrohydrodynamics (EHD) instability of a vertical cylindrical interface is tackled in the present study. The interface separates two viscous, homogeneous, porous, incompressible, and dielectric fluids which totate about the common cylindrical axis with different uniform angular velocities. A uniform axial electric field acts upon the considered system. Additionally, the influence of heat transfer is incorporated into the buoyancy term as well as the surface tension parameter, giving rise to the thermo-capillary effect. In this context, the viscous potential theory as well as the standard normal modes analysis are employed. The distributions of temperature, pressure, and velocity fields are evaluated. The linear stability approach resulted in an exceedingly complicated transcendental dispersion relation. The non-dimensional analysis revealed some physical Ohnesorge, Darcy, Rayleigh, Prandtle and Weber numbers. Actually, the dispersion relation has no closed form solution. Consequently, a numerical technique is utilized to display the stability profile. The relation between the growth rate and the wavenumber of the surface waves is constructed. The influences of various physical parameters on the stability profile are illustrated. It is found that the Ohnesorge number plays a dual role in the stability configuration.

Experimental Study on Droplet Splash and Receding Breakup on a Smooth Surface at Atmospheric Pressure.

Abstract: Droplet impact on a smooth solid surface at atmospheric pressure was experimentally studied and physically interpreted. A particular emphasis of the study is on the effects of liquid viscosity on the transition between droplet deposition (or droplet spreading without breakup) and droplet disintegration (including droplet splash and receding breakup). Specifically, the critical Weber number separating droplet deposition from droplet disintegration decreases and then increases with increasing Ohnesorge number (Oh). The splash in the low-Oh region and the receding breakup in the high-Oh region were analyzed qualitatively based on the unbalanced forces acting on the rim of the spreading or receding liquid film. A semiempirical correlation of droplet deposition/disintegration thresholds is proposed and well fits the experimental results from previous and present studies over a wide range of liquid viscosity.

Air film evolution during droplet impact onto a solid surface

Abstract: Recent years see increasing studies of air entrapment during droplet impacting on a solid surface with many results. The dynamics of trapped air film during a droplet impact on a solid surface is investigated in this work by the phase field method in combination with a dynamic contact angle (DCA) model. The DCA model is established experimentally by capturing the droplet dynamics in analogy to the entrapped air evolution. By using the DCA model as the input, the simulation can accurately reproduce the experimental results. The effects of droplet viscosity and surface tension on the dynamics of the air film are then studied, and three possible regimes are identified, demarcated by an effective Ohnesorge number (Ohe). Regime 1 is the case where no daughter droplet is generated and the air bubble is always attached to the substrate, corresponding to the classical case at a high Ohe number (Ohe > 0.073). Regime 3 is a newly discovered regime in this work where a daughter droplet is generated and the air bubble is always detached from the substrate, corresponding to a low Ohe number (Ohe < 0.019) due to combined strong surface tension and vortex effects. Regime 2 is for moderate Ohe numbers where a daughter droplet is generated and the air bubble can either detach from or attach to the substrate. Different from conventional thought that the detachment in this regime is decided by a static contact angle, the DCA plays a leading role in determining the volume ratio of the daughter droplet to the gas bubble, and the combined effects determine the fate of the bubble. Such finding provides better insight on the entrapped air dynamics upon droplet impacting on a solid surface, an area of high engineering importance.

Capillary-Driven Rise of Well-Wetting Liquid on the Outer Surface of Cylindrical Nozzles.

Abstract: Well-wetting liquids exiting small-diameter nozzles in the dripping regime can partially rise up along the outer nozzle surfaces. This is problematic for fuel injectors and other devices such as direct-contact heat and mass exchangers that incorporate arrays of nozzles to distribute liquids. We report our experimental and numerical study of the rising phenomenon for wide ranges of parameters. Our study shows that the interplay of three dimensionless numbers (the Bond number, the Weber number, and the Ohnesorge number) governs the capillary-driven rise dynamics. In general, as the flow rate or the viscosity increases, the capillary-driven rise height over each dripping period becomes smaller. We identify liquid flow rates below which the temporal evolution of the meniscus positions can be well approximated by a quasistatic model based on the Young-Laplace equation. Our analysis reveals two critical Bond numbers that give nozzle sizes, which correspond to the maximum meniscus rise and the onset of capillary-driven rise cessation. These critical Bond numbers are characterized as a function of the contact angle, regardless of the fluid type. Our study leads to a more efficient and optimized nozzle design in systems using wetting liquids by reducing both the risks of contamination and high pressure drop in such devices.

Extensional Magnetorheology of Viscoelastic Human Blood Analogues Loaded with Magnetic Particles.

Abstract: This study represents a pioneering work on the extensional magnetorheological properties of human blood analogue fluids loaded with magnetic microparticles. Dynabeads M-270 particles were dispersed in Newtonian and viscoelastic blood analogue fluids at 5% wt. Capillary breakup experiments were performed, with and without the influence of an external magnetic field aligned with the flow direction. The presence of the particles increased the viscosity of the fluid, and that increment was larger when embedded within a polymeric matrix. The application of an external magnetic field led to an even larger increment of the viscosity of the working fluids, as the formation of small aggregates induced an increment in the effective volume fraction of particles. Regarding the liquid bridge stability, the Newtonian blood analogue fluid remained as a Newtonian liquid exhibiting a pinch-off at the breakup time in any circumstance. However, in the case of the viscoelastic blood analogue fluid, the presence of the particles and the simultaneous application of the magnetic field enhanced the formation of the beads-on-a-string structure, as the Ohnesorge number remained basically unaltered, whereas the time of the experiment increased due to its larger viscosity, which resulted in a decrease in the Deborah Number. This result was confirmed with fluids containing larger concentrations of xanthan gum.

Simulation of impulsively induced viscoelastic jets using the Oldroyd-B model

Abstract: Understanding the physics of viscoelastic liquid jets is relevant to jet-based printing and deposition techniques. In this paper we study the behaviour of jets induced from viscoelastic liquid films, using the mechanical impulse provided by a laser pulse to actuate jet formation. We present direct numerical simulations of viscoelastic liquid jets solving the two-phase flow problem, accounting for the Oldroyd-B rheology. We describe how the jet extension time and length are controlled by the Deborah number (ratio of the elastic and inertia-capillary time scales), the viscous dissipation described by the Ohnesorge number (ratio of the viscous-capillary and inertia-capillary time scales), as well as the ratio of laser impulse energy to the energy required to create free surface during jet formation and propagation. Using the droplet ejection laser threshold energy of a Newtonian liquid, we investigate the influence of increasing viscoelastic effects. We show that viscoelastic effects can modify the effective drop size at the tip of the jet, while the maximum jet length increases with increasing Deborah number. Using the simulations, we identify a high-Deborah-number regime, where the time of maximum jet extension can be described as depending on the Ohnesorge number and laser energy. The observed asymptotic relationships are in good agreement with experiments performed at much higher Deborah numbers.

Droplet Splashing on Rough Surfaces

Abstract: When a droplet hits a surface fast enough, droplet splashing can occur: smaller secondary droplets detach from the main droplet during impact. While droplet splashing on smooth surfaces is by now well understood, the surface roughness also affects at which impact velocity a droplet splashes. In this study, the influence of the surface roughness on droplet splashing is investigated. By changing the root mean square roughness of the impacted surface, we show that the droplet splashing velocity is only affected when the droplet roughness is large enough to disrupt the spreading droplet lamella and change the droplet splashing mechanism from corona to prompt splashing. Finally, using Weber and Ohnesorge number scaling models, we also show that the measured splashing velocity for both water and ethanol on surfaces with different roughness and water-ethanol mixtures collapse onto a single curve, showing that the droplet splashing velocity on rough surfaces scales with the Ohnesorge number defined with the surface roughness length scale.

Electric and viscous correction for viscous potential flow analysis of electrohydrodynamic instability of an electrified leaky-dielectric jet

Abstract: The electric and viscous correction of viscous potential flow (EVCVPF) is developed for analyzing the electrohydrodynamic instability of an electrified leaky-dielectric viscous jet. The EVCVPF model is based on the viscous potential flow (VPF) and the viscous correction of VPF (VCVPF), proposed by Joseph and Wang [“The dissipation approximation and viscous potential flow,” J. Fluid Mech. 505, 365–377 (2004)]. The purpose is to resolve the discrepancy between the non-zero irrotational viscous and the electric tangent stresses. The power of the pressure correction is introduced to compensate the neglected viscous dissipation in the flow bulk in VPF, which is equal to the average power of the irrotational viscous and the electric tangent stresses. The model has been validated by comparing it to the exact normal-mode solution of the linearized Navier–Stokes equations (fully viscous flow, FVF). The energy budget is also performed to assist in understanding underlying mechanisms. Results show that EVCVPF is accurate for charged jets with low and moderate viscosities, i.e., the Ohnesorge number approximately Oh ≤ 0.1. The inaccuracy for highly viscous jets are the limitations of VPF itself. The electric field has less influence compared to the fluid viscosity. To achieve more accurate approximations, VCVPF and VPF are chosen for axisymmetric and non-axisymmetric modes under weak electric fields. EVCVPF is in remarkably good agreement with FVF under moderate and strong electric fields. In general, as VCVPF extends the applicability in fluid viscosity of VPF, EVCVPF further improves the adequacy when studying the electrohydrodynamic instability.

Electrohydrodynamics of dielectric droplet collision on different wettability surfaces

Abstract: In this article, we report the experimental and semi-analytical findings to elucidate the electrohydrodynamics (EHD) of a dielectric liquid droplet impact on superhydrophobic (SH) and hydrophilic surfaces. A wide range of Weber numbers (We) and electro-capillary numbers (Cae) are covered to explore the various regimes of droplet impact EHD. We show that for a fixed We ∼ 60, droplet rebound on a SH surface is suppressed with increase in electric field intensity (increase in Cae). At high Cae, instead of the usual uniform radial contraction, the droplets retract faster in an orthogonal direction to the electric field and spread along the direction of the electric field, inducing large electrical stresses at the liquid rim facing the electrodes. This prevents the accumulation of sufficient kinetic energy to achieve the droplet rebound phenomena. For certain values of We and Ohnesorge number (Oh), droplets exhibit somersault-like motion during rebound. Subsequently, we propose a semi-analytical model to explain the field induced rebound phenomenon on SH surfaces. Above a critical Cae ∼ 4.5, EHD instability causes a fingering pattern via evolution of a spire at the rim. Further, the spreading EHD on both hydrophilic and SH surfaces is discussed. On both wettability surfaces and for a fixed We, the spreading factor shows an increasing trend with increase in Cae. We have formulated an analytical model based on energy conservation to predict the maximum spreading diameter. The model predictions hold reasonably good agreement with the experimental observations. Finally, a phase map was developed to explain the post impact droplet dynamics on SH surfaces for a wide range of We and Cae.

Linear stability analysis of a Newtonian ferrofluid cylinder surrounded by a Newtonian fluid

Abstract: We performed a linear stability analysis of a Newtonian ferrofluid cylinder surrounded by a Newtonian non-magnetic fluid in an azimuthal magnetic field. A wire is used at the centre of the ferrofluid cylinder to create this magnetic field. Isothermal conditions are considered and gravity is ignored. An axisymmetric perturbation is imposed at the interface between the two fluids and a dispersion relation is obtained allowing us to predict whether the flow is stable or unstable with respect to this perturbation. This relation is dependent on the Ohnesorge number of the ferrofluid, the dynamic viscosity ratio, the density ratio, the magnetic Bond number, the relative magnetic permeability and the dimensionless wire radius. Solutions to this dispersion relation are compared with experimental data from Arkhipenko et al. (Fluid Dyn., vol. 15, issue 4, 1981, pp. 477–481) and, more recently, Bourdin et al. (Phys. Rev. Lett., vol. 104, issue 9, 2010, 094502). A better agreement than the inviscid theory and the theory that only takes into account the viscosity of the ferrofluid is shown with the data of Arkhipenko et al. (Fluid Dyn., vol. 15, issue 4, 1981, pp. 477–481) and those of Bourdin et al. (Phys. Rev. Lett., vol. 104, issue 9, 2010, 094502) for small wavenumbers.

Effect of inertia on capillary-driven breakup of drops surrounded by another fluid

Abstract: We study the capillary-driven breakup of a slender drop suspended in a quiescent viscous fluid using direct numerical simulation. We focus on a parametric space comprising viscosity ratio and Ohnesorge number. While the large Ohnesorge number approximation of the problem has received experimental and theoretical attention over the years, the influence of inertia—at small Ohnesorge number—on the behavior of the slender drop is not well studied. We first validate our simulation results with previous experimental results at large viscosity ratios. We then consider the drop suspended in a quiescent fluid and systematically study the capillary-driven breakup of the drop at different Ohnesorge numbers and viscosity ratios. Our simulations reveal that the slender drop breaks up under all conditions, but the instability is transitional for some viscosity ratios. By considering both inertial and viscous effects in the ambient surrounding fluid, we show how the structure of the flow field is modified upon the introduction of inertia and how the viscosity of the surrounding fluid aids in vorticity diffusion. Finally, we extend the stability diagram for drops, which classifies them into asymptotically unstable and asymptotically stable states in a parametric space comprising viscosity ratio and Ohnesorge number. We finely probe the stability diagram and present a stability curve in the parametric space of viscosity ratio and Ohnesorge number.

On Liquid Viscosity Effects on Droplet Splash and Receding Breakup on a Smooth Solid Surface at Atmospheric Pressure

Abstract: Experimental and modelling study is presented for the effect of a wide range of liquid viscosities on the droplet impact on a smooth solid surface at atmospheric pressure. A non-monotonic variation of threshold between droplet deposition and splash was observed experimentally. Specifically, the critical Weber number separating deposition from splashing decreases and then increases with increasing the Ohnesorge number. The observations of splash in low viscosity region and receding breakup in high viscosity region were analyzed qualitatively from the perspectives of Kelvin-Helmholtz instability and Rayleigh-Taylor instability, respectively. Based on instability analysis for the viscosityinduced nonmonotonicity, a new semi-empirical correlation of droplet splashing thresholds is proposed by fitting experimental results from previous and present data and shows better performance than previous correlation formulas.

Delay of Leidenfrost point during drop impact of surfactant solutions.

Abstract: In this article, a novel method of increasing the dynamic Leidenfrost temperature is proposed through the addition of both anionic (SDS) and cationic (CTAB) surfactants to water droplets. We focus on understanding the hydrodynamics and thermal aspects of droplet impact Leidenfrost behaviour of surfactant solutions, and aim to delay the onset of the Leidenfrost regime. The effects of Weber number (We), Ohnesorge number (Oh) and surfactant concentration on dynamic Leidenfrost temperature were experimentally studied in details, covering a wide gamut of governing parameters. At a fixed impact velocity, increased with the increase of surfactant concentration. decreased with increase of impact velocity for all solutions of surfactant droplets at a fixed surfactant concentration. We proposed a scaling relationship for in terms of We and Oh. At temperatures (~ 400oC) considerably higher than , droplets exhibit trampoline like dynamics or central jet formation, associated with fragmentation, depending upon the impact velocity. Finally, a regime map of the different boiling regimes such as transition boiling, Leidenfrost effect, trampolining and explosive behaviour is presented as function of impact We and substrate temperature (Ts). The findings may hold strong implications in thermal management systems operating at high temperatures