Cold Droplets Research Articles

Flow structure near three phase contact line of low-contact-angle evaporating droplets

Flow structure near three phase contact line (TPCL) of evaporating liquids plays a significant role in liquid wetting and dewetting, liquid film evaporation, and boiling. Despite the wide focus it receives, the interacting mechanisms therein remain elusive and in specific cases, controversial. Here, we reveal the profile of internal flow and elucidate the dominating mechanisms near TPCL of evaporating droplets, using mathematical modeling, trajectory analysis, and infrared thermography. We indicate that for less volatile liquids such as butanol, the flow pattern is dominated by capillary flow. With increasing liquid volatility, e.g., alcohol, the effect of evaporation cooling, under conditions, induces interfacial temperature gradient with cold droplet apex and warm edge. The temperature gradient leads to Marangoni flow that competes with outwarding capillary flow, resulting in the reversal of interfacial flow and the formation of a stagnation point near TPCL. The spatiotemporal variations of capillary velocity and Marangoni velocity are further quantified by mathematically decomposing the tangential velocity of interfacial flow. The conclusions can serve as a theoretical base for explaining deposition patterns from colloidal suspensions and can be utilized as a benchmark in analyzing more complex liquid systems.

Standard technical specifications for methacholine chloride (Methacholine) bronchial challenge test (2023)

Standard technical specifications for methacholine chloride (Methacholine) bronchial challenge test (2023)

Cooling-driven coagulation

ABSTRACT Astrophysical gases such as the interstellar-, circumgalactic-, or intracluster-medium are commonly multiphase, which poses the question of the structure of these systems. While there are many known processes leading to fragmentation of cold gas embedded in a (turbulent) hot medium, in this work, we focus on the reverse process: coagulation. This is often seen in wind-tunnel and shearing layer simulations, where cold gas fragments spontaneously coalesce. Using 2D and 3D hydrodynamical simulations, we find that sufficiently large (≫cstcool), perturbed cold gas clouds develop pulsations which ensure cold gas mass growth over an extended period of time (≫r/cs). This mass growth efficiently accelerates hot gas which in turn can entrain cold droplets, leading to coagulation. The attractive inverse square force between cold gas droplets has interesting parallels with gravity; the ‘monopole’ is surface area rather than mass. We develop a simple analytic model which reproduces our numerical findings.

Numerical Study of Heat Transfer During Oblique Impact of a Cold Drop on a Heated Liquid Film

Abstract A three-dimensional study of a cold droplet impacting obliquely on a heated solid flat surface covered with a hot liquid layer has been performed. The drop Weber number, liquid film thickness, and drop impact angle are set to a range from 100 to 800, 0.1 to 0.4, and 0 deg to 60 deg, respectively. The interface evolution and thermal behavior of the drop impingement is well captured using coupled level set and volume of the fluid method (CLSVOF). The code is validated against previously published results both qualitatively and quantitatively. The results show that in the case of oblique drop impact, the crown dynamics and wall heat flux distribution exhibit an asymmetric pattern, with secondary droplets generated solely on the downstream side, as opposed to normal drop impact in which the secondary drops generated around the circumference of the crown. Based on heat flux values, two distinct region within the liquid film exist: (i) impact region around the impact point and (ii) undisturbed region far from the impact region characterized by the impact dynamics. A parametric analysis further reveals that for a moderate Weber number, asymmetric behavior increases as the drop impact angle increases, resulting in a reduction in heat transfer from the solid surface. However, for a drop impacting at an angle of 28 deg, increased asymmetry due to a increase in the Weber number results in significant cooling of the impact region. Furthermore, it is also found that a thinner liquid film promotes higher heat transfer from the solid surface, resulting in a higher wall heat flux.

Evaporation of Leidenfrost droplet on thin soluble liquid bath with thermal non-equilibrium effect

Leidenfrost droplet evaporation on a liquid bath exhibits unique features such as ultra-low resistance to sample transition and low-temperature operation; however, the physical mechanisms responsible for these phenomena are incompletely understood. Droplet size and temperature are two key parameters influencing Leidenfrost droplet evaporation. We report herein the thermal non-equilibrium process of an FC-72 droplet over a thin oil layer. We show that the Leidenfrost droplet radius follows the power law R(t) ∼ (1 − t/τ)n, where τ is the characteristic droplet lifetime and n ranges from 0.63 to 0.91. Based on experimental results and theoretical predictions, the remarkable nonmonotonic variation of droplet temperature departs from the saturation-temperature assumption. For lower oil superheating, a cold (subcooled) droplet can sustain evaporation until it disappears. For higher oil superheating, the droplet goes through both subcooled and superheating stages. This phenomenon is well described by sensible heat absorption and release throughout droplet evaporation. These results are helpful for applications such as drug delivery, wherein a cold droplet can float on a liquid bath, thereby extending the lifetime of the biological sample in a high-temperature environment via a localized, low-temperature system.

Salt Enrichment and Dynamics in the Interface of Supercooled Aqueous Droplets.

The interconversion reaction of NaCl between the contact-ion pair (CIP) and the solvent-separated ion pair (SSIP) as well as the free-ion state in cold droplets has not yet been investigated. We report direct computational evidence that the lower is the temperature, the closer to the surface the ion interconversion reaction takes place. In supercooled droplets the enrichment of the subsurface in salt becomes more evident. The stability of the SSIP relative to the CIP increases as the ion-pairing is transferred toward the droplet's outer layers. In the free-ion state, where the ions diffuse independently in the solution, the number density of Cl- shows a broad maximum in the interior in addition to the well-known maximum in the surface. In the study of the reaction dynamics, we find a weak coupling between the interionic NaCl distance reaction coordinate and the solvent degrees of freedom, which contrasts with the diffusive crossing of the free energy barrier found in bulk solution modeling. The H2O self-diffusion coefficient is found to be at least an order of magnitude larger than that in the bulk solution. We propose to exploit the enhanced surface ion concentration at low temperature to eliminate salts from droplets in native mass spectrometry ionization methods.

The airflow and cooling characteristics in the airway of NHBD lung with hypothermic ventilation

Hypothermic ventilation through trachea can cool Non-heart beating donor (NHBD) lungs noninvasively in vivo, thus avoiding warm ischemic damage. However, previous experimental studies with large lungs have shown poor cooling effect in bronchial terminal of NHBD lungs. To investigate the airflow and cooling characteristics of this process, a physiologically realistic upper airway model was established. In addition, droplet spraying was considered to enhance the heat transfer during the cooling process. The results show that the irregular geometry of realistic airway will lead to non-uniformity distribution of internal flow and temperature, and the local cooling effect in different regions is related to the velocity distribution. The secondary flow at the bifurcation can forcibly strengthen the heat exchange between the cold air and the hot air in the nearby low-speed area. Increasing the ventilation flow will strengthen the turbulence and heat exchange in the airway, so as to improve the overall cooling effect. The synergistic effect of cold air and droplet spraying can enhance the cooling effect in the airway. And large droplets (i.e. 10 μm) are conducive to the cooling enhancement of the central airway, while small droplets (i.e. 2 μm) are conducive to the cooling of the distal bronchus.

Pancake Jumping of Sessile Droplets.

Rapid droplet shedding from surfaces is fundamentally interesting and important in numerous applications such as anti‐icing, anti‐fouling, dropwise condensation, and electricity generation. Recent efforts have demonstrated the complete rebound or pancake bouncing of impinging droplets by tuning the physicochemical properties of surfaces and applying external control, however, enabling sessile droplets to jump off surfaces in a bottom‐to‐up manner is challenging. Here, the rapid jumping of sessile droplets, even cold droplets, in a pancake shape is reported by engineering superhydrophobic magnetically responsive blades arrays. This largely unexplored droplet behavior, termed as pancake jumping, exhibits many advantages such as short interaction time and high energy conversion efficiency. The critical conditions for the occurrence of this new phenomenon are also identified. This work provides a new toolkit for the attainment of well‐controlled and active steering of both sessile and impacting droplets for a wide range of applications.

Охлаждение парогазовой смеси испаряющимися каплями воды

A model of thermal relaxation in the flow of a hot vapor–gas mixture and cold water droplets is presented. Numerical modeling of vapor and gas cooling during heating and evaporation of droplets has been carried out. Approximation expressions are obtained for the cooling time of vapor and gas in a given temperature range depending on the initial radius of droplets, the mass fraction of droplets, and the initial composition of the vapor-gas mixture.

Fluidics of an immiscible drop impact onto a hot oil film

Droplet rapid phase transition during its impact over a hot immiscible oil film is studied experimentally, which is of paramount importance in fire extinguishing, forensics, internal combustion engines, and various accident scenarios in the industries. The impact of a cold methanol droplet on a hot mustard oil film and its repercussions are analyzed from high-speed images for a temperature range relevant from the thermodynamic and heat transfer perspective. The interfacial temperature of the hot oil film has been varied through the saturation temperature, Leidenfrost point, superheat limit, and critical temperature, for methanol. A regime map has been proposed to characterize fluidic observations on Weber number and non-dimensional average temperature plane. Violent cavity crater explosions have been observed above a critical Weber number ranging 70–80 for average interfacial temperature higher than the critical point of methanol due to direct contact between the droplet and base of the petridish.

An Abrasion Resistant TPU/SH-SiO2 Superhydrophobic Coating for Anti-Icing and Anti-Corrosion Applications

As a passive anti-icing strategy, properly designed superhydrophobic coatings can demonstrate outstanding performances. However, common preparation strategies for superhydrophobic coatings often lead to environmental pollution, high energy-consumption, high-cost and other undesirable issues. Besides, the durability of superhydrophobic coating also plagues its commercial application. In this paper, we introduced a facile and environment-friendly technique for fabricating abrasion-resistant superhydrophobic surfaces using thermoplastic polyurethane (TPU) and modified SiO2 particles (SH-SiO2). Both materials are non-toxicity, low-cost, and commercial available. Our methodology has the following advantages: use of minimal amounts of formulation, take the most streamlined technical route, and no waste material. These advantages make it attractive for industrial applications, and its usage sustainability can be promised. In this study, the mechanical stability of the superhydrophobic surface was evaluated by linear wear test. It is found that the excellent wear resistance of the superhydrophobic coating benefits from the characteristics of raw materials, the preparation strategy, and the special structure. In anti-icing properties test, the TPU/SH-SiO2 coating exhibits the repellency to the cold droplets and the ability to extend the freezing time. The electrochemical corrosion measurement shows that the as-prepared superhydrophobic surface has excellent corrosion resistance that can provide effective protection for the bare Q235 substrates. These results indicate that the TPU/SH-SiO2 coating possesses good abrasion resistance and has great potential in anti-corrosion and anti-icing applications.

Modeling study on sessile water droplet during freezing with the consideration of gravity, supercooling, and volume expansion effects

Droplet freezing phenomenon widely exists in many fields, including aerospace, power production, and cryopreservation, etc. Considering the effects of supercooling, gravity and volume expansion, a theoretical model of droplet freezing is developed. A good agreement is found between the predicted results of freezing front radius and height and experiments for both hydrophilic and hydrophobic surfaces. The developed model also shows good performance in predicting freezing times. The average prediction deviation is 7.63%, and more than 93% simulation results show the deviation within ±15%. Gravity has a more obvious influence on final freezing times with the increase of cold plate temperature, contact angle, and droplet volume. For the droplet freezing process under various contact angles and cold plate temperatures, the fastest average temperature change rates inside water droplet are -2.48 °C/s and -1.72 °C/s, respectively. This study is beneficial for the better understanding of the droplet solidification as well as the optimization of refrigeration and defrosting technologies.

A many-body dissipative particle dynamics with energy conservation study of droplets icing on microstructure surfaces

Droplets icing has important applications in real life. The icing process of droplets on microstructure is explored based on the MDPDE method in this study. Firstly, the correctness of the heat transfer model was verified by one-dimensional heat conduction simulation, and then the feasibility of the phase change model was verified by investigating the icing process of droplets. The influence of cold surface temperature, droplet volume and contact angle on freezing time of droplets was discussed, and it was found that the temperature of cold surfaces had a greater effect on freezing. We finally explored the influence of different microstructure surfaces on the icing of droplets, and results showed that the presence of microstructures greatly enhanced the anti-icing effect of the surface. In our research, the contact angle is a relatively large factor affecting the icing of droplets. In addition, it was discovered that the droplet had the strongest ability to delay freezing on the surface of triangle microstructures with a contact angle of 157.1°.

A Thermo-Fluid Model of Droplet Evaporation and Pressure Variation in Venturi Liquid-Gas Mixers

An analytical model has been developed to determine how effectively the temperature of a flowing hot gas can be reduced through the introduction of cold liquid droplets. The hot gas flows through a venturi-shaped mixer where the cold liquid is introduced into the mixer at the throat, vaporizes, and mixes with the hot gas resulting in a cooler gas temperature at the exit of the mixer. The cold liquid droplet residence time, evaporation time, and mixing with the hot gas determine the extent to which the hot gas temperature can be reduced. The model is used to predict the achievable gas cooling of hot hydrogen, nitrogen, and oxygen with cryogenic droplet coolant of the same species. Temperature and pressure along the mixer for these different fluids are calculated, and trends are presented using a non-dimensional number analogous to the Damkohler number in reaction chemistry. The model can be used to predict the minimum mixer length required to achieve specified amount of hot gas cooling or to assess the cooling efficacy of a mixer of given length for various hot gas and cold liquid flow rates.

An energy model of droplet impingement on an inclined wall under isothermal and non-isothermal environments

The study of spray-wall interaction is of great importance to understand the dynamics that occur during fuel impingement onto the chamber wall or piston surfaces in internal combustion engines. It is found that the maximum spreading length of an impinged droplet can provide a quantitative estimation of heat transfer and energy transformation for spray-wall interaction. Furthermore, it influences the air-fuel mixing and hydrocarbon and particle emissions at combusting conditions. In this paper, an analytical model of different droplet-wall impingement conditions is developed in terms of βm (dimensionless maximum spreading length, the ratio of maximum spreading length to initial droplet diameter) to understand the detailed impinging dynamic process. These conditions are grouped as: a single diesel droplet impinging on the wall with different inclined angles (α); cold wall - heated droplet and heated wall - cold droplet impingement when inclined angle of the wall is 0°, respectively. The analytical model is built up based on the energy conservation that considers kinetic energy, gravitation energy, and surface energy before impingement, as well as viscous dissipation, gravitation energy, adhesion energy, deformation energy, and heat energy after impingement. The experimental work of diesel droplet impinging on an inclined wall is performed at a certain range of the Weber number (We of 33 to 420) with various inclined angles (α of 0° to 45°), while for inclined angle is 0°, droplet and wall temperature are varied from 25°C to 150°C to study the effects of the inclined angle and temperature on the temporal evolution of the post-impingement characteristics (i.e. droplet spreading length, dynamic contact angle). The analytical model is validated and evaluated at the aforementioned experimental operating points. The validated model can be employed to predict maximum spreading length of the droplet impinged on the wall. It is further utilized to determine the transition from capillary regime to kinetic regime, then to viscous regime at different inclined angle of the wall.

A Qualitative Inter-Relation Between Drop Impacts on a Solid Surface and the Interaction of a Droplet Placed Inside a Superheated Pool

The phenomenon of drop impacts on a solid surface and the interaction of a cold droplet with a superheated liquid pool are both of great relevance in many practical and engineering situations. Most of the reported research in the literature on these phenomena focus on the impact of a droplet on a solid surface or a liquid surface and there are hardly any study on the interaction of a stationary cold droplet inside a hot pool of liquid, a phenomenon observed quite often. This study reports high speed flow visualization of a drop impact on a Teflon-coated solid surface at different liquid properties and impact Reynolds number. Simultaneously, high speed imaging of the interaction of a cold droplet with the superheated olive oil pool has also been carried out. The images show a striking resemblance between the flow dynamics of the two phenomena. A comparison between the two phenomena is made and suggestions are made for the possible underlying physics.

Cold condensing scrubbing method for fine particle reduction from saturated flue gas

The wet scrubbing method can be used for reducing particles and recover waste heat from flue gas. But its particle reduction performance is relatively poor, and an outward Stefan flow caused by droplet evaporation may be the main adverse cause. This work aims to improve the performance through developing a cold condensing scrubbing method, wherein an inward Stefan flow is created through water vapor condensing on cold droplets to prompt the particle capture from saturated flue gas. This method has barely been studied, except a few theoretical predictions. These predictions lack experimental verification, which has been performed in this work. The particle reduction efficiencies obtained in our experiments are much higher than previous predicted results, thereby increasing the possibility of application. Besides the work to confirm the inward Stefan flow is the dominant mechanism for a condensing droplet capturing fine particles, the contributions of a preceding scrubber and a subsequent demister are also explored to refine the mechanism knowledge system. Variable analysis on scrubbing temperature, liquid-to-gas ratio, and droplet size are addressed to create a valuable characteristic curve figure and to provide guidelines for engineering applications. Furthermore, the technical feasibility and the economic performance of the proposed method are discussed.

High Speed Imaging Reveals a Close Resemblance Between Drop Impacts on a Solid Surface and the Interaction of a Cold Droplet Placed Inside a Hot Liquid

The phenomenon of drop impacts on a solid surface and the interaction of a cold droplet with a hot liquid pool are both of great relevance in many practical and engineering situations. Most of the reported research in the literature on these phenomena focus either on the impact of a droplet on a solid surface or a liquid surface and there are hardly any study on the interaction of a stationary cold droplet with a hot pool of liquid. This study reports high speed flow visualization of a drop impact on a Teflon-coated solid surface at different liquid properties and impact Reynolds number. Simultaneously, high speed imaging of the interaction of a cold droplet with the hot olive oil pool has also been carried out with a high intensity continuous light source. The images show a striking resemblance between the flow dynamics of the two phenomena. A comparison between the two phenomena is made and suggestions are made for the possible underlying physics.

Single-phase heat transfer of multi-droplet impact on liquid film

Multi-droplet impact on a thin liquid film covering a heated wall is numerically studied using a three-dimensional model with an implement of a random disturbance subjected to Gaussian distribution. Results show that successive impact results in higher splashing threshold for the trailing droplet than the leading one due to radial flow inside the residual film. Single-phase heat transfer coefficient in the impinged region is remarkably higher than the undisturbed film region, caused by enhanced convective heat transfer. A heat transfer blind spot situated under the central liquid sheet in simultaneous impact is identified, at which relatively high local temperature is observed and heat is more difficult to be rejected due to flow stagnation there. Cooling of the heated wall in successive impact is mainly achieved by contacting of the trailing droplet with the heated wall, rather than mixing of cold droplets and warm film.

Cascade Freezing of Supercooled Water Droplet Collectives.

Surface icing affects the safety and performance of numerous processes in technology. Previous studies mostly investigated freezing of individual droplets. The interaction among multiple droplets during freezing is investigated less, especially on nanotextured icephobic surfaces, despite its practical importance as water droplets never appear in isolation, but in groups. Here we show that freezing of a supercooled droplet leads to spontaneous self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We put forth the explanation that, as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble, resulting in ice coverage of the nanotextured surface. Although cascade freezing is observed in a low-pressure environment, it introduces an unexpected pathway of freezing propagation that can be crucial for the performance of rationally designed icephobic surfaces.