Freezing Of Water Droplets Research Articles

Modeling Water Droplet Freezing and Collision with a Solid Surface

Water droplets released from the sea surface represent one of the major causes of ice accretion on marine vessels. A one-dimensional model of the freezing of a spherical water droplet moving in cold air was developed. The crystallization model allows one to obtain an analytical solution if a uniform temperature distribution over the liquid’s core is assumed. The model was validated using STAR CCM+ Computational fluid dynamics (CFD) code. A collision of a partially frozen droplet with a solid wall assuming the plastic deformation of an ice crust was also considered. The ratio of the crust deformation to the crust thickness was evaluated. It was assumed that if this ratio were to exceed unity, the droplet would stick to the wall’s surface due to ice bridge formation caused by the water released from the droplet’s core.

The Portable Ice Nucleation Experiment (PINE): a new online instrument for laboratory studies and automated long-term field observations of ice-nucleating particles

Abstract. Atmospheric ice-nucleating particles (INPs) play an important role in determining the phase of clouds, which affects their albedo and lifetime. A lack of data on the spatial and temporal variation of INPs around the globe limits our predictive capacity and understanding of clouds containing ice. Automated instrumentation that can robustly measure INP concentrations across the full range of tropospheric temperatures is needed in order to address this knowledge gap. In this study, we demonstrate the functionality and capacity of the new Portable Ice Nucleation Experiment (PINE) to study ice nucleation processes and to measure INP concentrations under conditions pertinent for mixed-phase clouds, with temperatures from about −10 to about −40 ∘C. PINE is a cloud expansion chamber which avoids frost formation on the cold walls and thereby omits frost fragmentation and related background ice signals during the operation. The development, working principle and treatment of data for the PINE instrument is discussed in detail. We present laboratory-based tests where PINE measurements were compared with those from the established AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber. Within experimental uncertainties, PINE agreed with AIDA for homogeneous freezing of pure water droplets and the immersion freezing activity of mineral aerosols. Results from a first field campaign conducted at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) observatory in Oklahoma, USA, from 1 October to 14 November 2019 with the latest PINE design (a commercially available PINE chamber) are also shown, demonstrating PINE's ability to make automated field measurements of INP concentrations at a time resolution of about 8 min with continuous temperature scans for INP measurements between −10 and −30 ∘C. During this field campaign, PINE was continuously operated for 45 d in a fully automated and semi-autonomous way, demonstrating the capability of this new instrument to also be used for longer-term field measurements and INP monitoring activities in observatories.

A highly controlled fabrication of porous anodic aluminium oxide surface with versatile features by spatial thermo-anodization

Abstract Thermo-anodization technology has been considered as an effective means to improve the thermal, physical and chemical properties for metal alloy. In this work, we achieve a porous black layer on the surface of aluminium alloy through an environmentally friendly anodic oxidation process, with a high thermal emittance (0.96) and a high solar absorptivity (0.921). In addition, the black thermo-anodized coating layer shows a unique anti-corrosion property under UV irradiation. A hybrid hydrophobic surface has been facilitated through treating the thermo-anodized porous layer with silane. Moreover, an anti-icing feature can be realised that can effectively delay the freezing of water droplet on the surface of AA2024 aluminium alloy. As such, the specific anodic process of coating provides a simple method for improving the solar absorptivity and infrared emittance of aluminium alloys, enabling broad applications in aerospace engineering.

Approximate Models for Lateral Growth on Ice Crystal Surfaces during Vapor Depositional Growth

AbstractMeasurements show that after facets form on frozen water droplets, those facets grow laterally across the crystal surface leading to an increase in volume and surface area with only a small increase in maximum dimension. This lateral growth of the facets is distinctly different from that predicted by the capacitance model and by the theory of faceted growth. In this paper we develop two approximate theories of lateral growth, one that is empirical and one that uses explicit growth mechanisms. We show that both theories can reproduce the overall features of lateral growth on a frozen, supercooled water droplet. Both theories predict that the area-average deposition coefficient should decrease in time as the particle grows, and this result may help explain the divergence of some prior measurements of the deposition coefficient. The theories may also explain the approximately constant mass growth rates that have recently been found in some measurements. We also show that the empirical theory can reproduce the lateral growth that occurs when a previously sublimated crystal is regrown, as may happen during the recycling of crystals in cold clouds.

Effect of Hydrophobicity on Silicone Film with Spherical Curvature Structure on Water Droplet Freezing Process

Effect of Hydrophobicity on Silicone Film with Spherical Curvature Structure on Water Droplet Freezing Process

Freezing of Nanofluid Droplets on Superhydrophobic Surfaces.

Droplet freezing on cold superhydrophobic surfaces has been studied extensively in recent years. However, previous works are mainly focused on studying water droplet freezing behavior; little work has been conducted to investigate the freezing dynamics of nanofluid droplets on superhydrophobic surfaces. In this work, freezing morphologies of water and nanofluid droplets on superhydrophobic surfaces with different roughnesses were compared and studied. The nanofluid droplets underwent a shape transition from spherical to flat plateau morphology, different from the frozen water droplets that exhibit a sharp cusp. The size of the flat plateau for the frozen nanofluid droplet increases with increasing nanoparticle concentration. The underlying mechanism of the morphology change during the freezing process was elucidated using COMSOL Multiphysics. Compared to the frozen water droplets, more air bubbles are trapped inside the frozen nanofluid droplets, which might be ascribed to the fast freezing speed of the nanofluid droplets. These results can provide important insights for many applications that require freezing of nanofluid droplets, such as material solidification, three-dimensional (3D) printing, as well as phase change enhancement.

Computational modelling of freezing of water droplet impacting on an ice surface

Abstract This study presents the computational modelling of freezing of water droplet on the ice surface. In this study, the transient fluid dynamics and heat transfer of the droplet during impact are considered. The governing equations for fluid flow are solved numerically using pressure–velocity coupling SIMPLE algorithm. The energy equation is modeled using an enthalpy-based formulation. The continuum surface flow model is used for modelling of surface tension. The model is endorsed with the experimental result from the literature by validating droplet spreading factor. The result shows that modelling of a droplet impacting on a surface with a suitable contact angle is very important. The results predicted from the present model are very much similar to the experimental results. Also, it is found that the high impact speed leads to more spreading of the droplet, which increases the surface area contact.

Numerical and experimental analysis of rapid solidification considering undercooling effect during water droplet impact on a substrate

Freezing of water droplets on a cold surface can involve rapid solidification and undercooling. In this numerical and experimental study, the role of rapid solidification considering the undercooling effect during the impact and spreading of a water droplet onto a cold substrate is investigated. Various attendant physical phenomena, such as free surface evolution, fluid flow, heat transfer, rapid solidification and undercooling at the moving solid-liquid front are accounted in the computational model. In the coupled thermo-fluidic-rapid solidification model, the free surface of the liquid droplet is tracked with the help of volume of fluid method, while the solid-liquid front is tracked with the help of rapid solidification kinetics. The rapid solidification model lets freezing to occur at nucleation temperature which is lower than the freezing point. Results of the rapid solidification model are compared with the conventional solidification model where freezing occurs as soon as the temperature reaches the equilibrium freezing point. A realtime imaging setup is used to experimentally measure droplet spreading and freezing behaviour. The numerical predictions are validated with the experimental results. The model successfully captures undercooling and recalescence. Interfacial heat transfer is analyzed with the help of droplet-substrate heat flux. Undercooling and rapid solidification significantly influence the heat flux evolution. In the end, the effect of substrate temperature and droplet impact velocity is described. The understanding developed in this study regarding the role rapid solidification on interfacial heat flux evolution can be useful for research on ice accretion in aircraft.

Characteristics of icing without icicle on insulators under natural conditions

Icing on insulators seriously threatens the safe and stable operation of transmission lines. Most of the existing research has focused on the flashover characteristics of ice-covered insulators, lacking attention to the icing process on insulators. In order to reveal the mechanism of insulator icing, and lay the foundation for further establishment of insulator ice flashover model, this paper studies the characteristics of insulator icing without icicle. The characteristics of icing processes including the collision and freezing of water droplets, the generation and flowing of water film are analysed. On this basis, a three-dimensional numerical calculation model of insulator icing without icicle is established by using boundary element method, and the expressions of the local icing weight and thickness on the insulator surface are derived through the droplet local collision efficiency and freezing efficiency. Moreover, the time-varying characteristics of icing parameters are simulated. And the related icing tests are conducted on two kinds of insulators with different structures at Xuefeng Mountain. The results show that the distribution of collided water droplets and water film on the insulator surface will change with the increasing ice weight. Under natural conditions, compared with the aerodynamic insulator LDQ-100, the insulator LXY-120 with a smaller skirt diameter and a larger inclination angle can have a faster icing rate. However, at the insulator edge, the insulator LDQ-100 with a thinner shed has a greater icing rate of thickness. Compared with the test results, the simulation error of the model in this paper is within 15%.

Influence of electric field on the ice‐coating process of insulators with a different dielectric surface

Studying the influence of electric field on the icing process of insulators with various dielectric surfaces is of great importance to the design of external insulation. This study established a finite element model to simulate the distribution of electric field around various insulator strings. Energised and non-energised icing tests were conducted to obtain the influence rules of electric field on the insulator icing process. The results of water droplets freeze experiment provided microscopic explanation of icing tests. Research results indicate that electric field and surface dielectrics have influence on the appearance, density and mass of ice on insulators. In microcosmic view, water droplets on super-hydrophobic surface correspond to longer freezing time and smaller average diameters, which leads to the better anti-icing character of super-hydrophobic insulator. Different types of ice branches appear in the water droplets freezing process on various dielectric surfaces. Owing to the polarisation effect, electric field can inhibit the formation and growth of ice branch, which contributes to different anti-icing characters of various dielectric surfaces in electric field. The effect of dielectric surface on insulator icing process mainly act at the initial period, surface coatings of insulators are not suggested to be applied in heavily icing area.

A multiple level-set approach for modelling containerless freezing process

We propose a multiple level-set model to represent the physics governing the three-phase solidification problem. The model couples thermal characteristics of the freezing front with the dynamics of droplet interface. To deal with liquid, solid, and gas phases, two distinct level-sets are used. The liquid-gas interface which is represented by a level-set moves with an external velocity field that is obtained from the Navier-Stokes equations. The solid-liquid interface, on the other hand, evolves according to the freezing rate of the liquid. The solid-liquid level-set is comprised of an active and passive part. The active segment of the level-set evolves based on temperature gradients and the latent heat of fusion. The passive segment, however, is merely utilized to impose an angle at the tri-junction point. We propose a Hamilton-Jacobi type equation to impose constant or variable angles at the tri-junction point. In order to consider the effect of volume expansion we modify the continuity, and the energy equation. Importantly, in the case of expansion during solidification we can capture the pointy shape on top of the freezing droplet. For validation we compare numerical results with the analytical Stefan problem with and without the density expansion. In addition, we use experimental results of water droplet freezing, available in the literature, to examine the accuracy of the freezing rate, and the droplet morphological.

Mesoscopic Dynamical Model of Ice Crystal Nucleation Leading to Droplet Freezing.

We present a numerical model to study the dynamic behaviors and heat conduction of freezing liquid droplets based on the MDPDE method (many-body dissipative particle dynamics with energy conservation configurations). In this model, the freezing processes involved in cooling, recalescence, and nucleation are considered. A new scaling method was developed to connect the mesoscopic MDPDE coefficients and macrothermal conductivity. The freezing of water droplets on cold surfaces with different wettabilities was simulated. Both the evolution of temperature and ice–liquid interface movement showed close agreement with the experimental data. We discuss the formation of a pointy tip on the top of an ice-drop and nucleation and growth during the recalescence stage. The rapid expansion of the recalescence region and the growth of the solid-phase region were calculated numerically, and this showed that the nuclei distribution of the two processes were completely different. The MDPDE model can not only predict the freezing time and shape deformation of ice-drops but also the nuclei formation and crystal growth during solidification. This study provides a useful tool for deicing material design.

Anti-frosting/anti-icing property of nano-ZnO superhydrophobic surface on Al alloy prepared by radio frequency magnetron sputtering

Superhydrophobic (SHP) surfaces had promising application in anti-icing field. In this work, ZnO SHP surfaces were prepared on aluminum alloy by radio frequency (RF) magnetron sputtering. The structure, chemical composition, morphology, wettability of the SHP surfaces were investigated by using corresponding methods. The anti-frosting/anti-icing performance of the ZnO SHP surfaces were investigated. Morphology of surfaces showed that sputtering time affected the surface morphology of the ZnO surfaces. When the sputtering time was 15 min, the rough structure exhibited excellent anti-frosting/anti-icing property due to the exceptional superhydrophobicity and self-transfer behavior at subzero temperature, which composed by densely gathered nanoclusters. The freezing of water droplets was delayed for about 2 h, and frosting was delayed for approximately 5 h at the temperature of −10 °C. The SHP surfaces could efficiently enhance the anti-icing property in glaze ice. Meanwhile, the as-prepared ZnO SHP surfaces displayed a low ice-adhesion strength of 12 ± 4.7 kPa. However, the structure with large undulation and pore space reduced the anti-frosting/anti-icing property of the SHP surface. This work provided a direction for the design and preparation of anti-icing surfaces for power transmission lines.

FDTS‐Modified SiO2/rGO Wrinkled Films with a Micro‐Nanoscale Hierarchical Structure and Anti‐Icing/Deicing Properties under Condensation Condition

AbstractIcing accretion on the surface of an aircraft not only increases the weight but also causes changes in the surface shape, so there is an urgent need for anti‐icing/deicing without shutdown. Here, a graphene‐based film with light weight, super‐hydrophobicity, high electric heat, and durability, which has both anti‐icing properties and electrothermal deicing capability, is proposed. The film is prepared with 1H,1H,2H,2H‐perfluorodecyltrichlorosilane (FDTS)‐modified SiO2/rGO wrinkles, and it has a micro‐nanoscale hierarchical structure. Moreover, the superhydrophobic properties of the film can be controlled by adjusting the wrinkle size. Because of its good water repellency, the film can prevent and delay the freezing of water droplets at low temperatures. At −10 °C, compared to a smooth rGO film, the icing time of 10 µL supercooled water droplet on the micro‐nanoscale hierarchical structured film can be delayed by ≈8.3 times, and the ice adhension decreases greatly. In addition, the film exhibits high‐efficiency electrothermal properties. At −10 °C, under a low voltage of 15 V, the film can be heated rapidly to achieve rapid deicing and defrosting. The graphene‐based micro‐nanoscale hierarchical structure shows great potential in the field of sky anti‐icing and real‐time deicing.

Experimental study of the internal flow in freezing water droplets on a cold surface

The study of a freezing droplet is interesting in areas, where the understanding of build up of ice is important, for example, on wind turbines, airplane wings and roads. In this work, the main focus is to study the internal motion inside freezing water droplets using particle image velocimetry and to reveal if mechanisms such as natural convection and Marangoni convection have a noticeable influence on the flow within the droplet. The flow has successfully been visualized and measured for the first 25% of the total freezing time of the droplet when the velocity in the water is the highest and when the characteristic vortices can be seen. After this initial time period, the high amount of ice in the droplet scatters the PIV light sheet too much and the images retrieved are not suitable for analysis. Initially, it can be seen that the Marangoni effects have a large impact on the internal flow, but after about 15% of the total freezing time, the flow turns indicating increased effects of natural convection on the flow. Shortly after this time, almost no internal flow can be seen.Graphic abstract

Effect of nucleation and icing evolution on run-back freezing of supercooled water droplet

Supercooled large droplet (SLD), which can cause run-back ridged ice, will affect the aerodynamic performance of aircraft and poses a serious threat to aircraft safety. However, there is little knowledge about the abnormal run-back icing mechanism, which is vital for the development of aircraft anti-icing technologies. The flow and freezing of supercooled droplet impinging on inclined surface can help us better understand the run-back icing of SLD, especially the coupling of water flow and phase change in the freezing process. This paper experimentally investigates the freezing behavior of supercooled water droplet impinging on inclined surface. By observing the processes of water flow, ice formation and growth with a high-speed camera, the overflow distance and the freezing time are recorded with different temperatures. Different from previous discoveries that droplet freezes as the form of two smaller droplets on an inclined surface, we found two new frozen morphologies under the condition of short nucleation time: ellipse and slender strip, when the supercooling is high and low, respectively. The overflow distance and freezing time will increase with the decrease of ice growth rate, when supercooling decreases. And the freezing time will increase dramatically when the supercooling is low enough. Finally, the mechanism of run-back freezing of droplet based on the nucleation and icing evolution is discussed. Droplet impact on the inclined surface will result in large overflow distance when it nucleates after its retraction stage.

An experimental investigation of the melting process of an ice bead on the smooth and micro-grooved surfaces under a hot shear flow

In the present study, we experimentally investigated the melting process of an ice bead on the smooth and micro-grooved surfaces under a hot shear flow. One smooth silicon surface and three micro-grooved silicon surfaces were fabricated and tested. A parameter study of the substrate surface temperature and the air flow speed was conducted. During the experiment, an ice bead was first formed from a freezing water droplet on a cold substrate surface. Then, the ice bead was exposed to a hot shear flow and its melting process was recorded by a CCD camera and an infrared camera simultaneously. As for the micro-grooved surfaces, the direction of the hot shear flow was parallel to the micro-grooves. The results showed that the melting process of the ice bead on the smooth and micro-grooved surfaces under a hot shear flow could be divided into three categories. The air flow speed, the surface temperature, and the type of the surface had a significant influence on which category the melting process of the ice bead belonged to. Besides, the presence of the micro-grooves was found to apparently affect the wetting length, the removable time, and the temperature along the centerline of the ice bead. In general, the micro-grooved surfaces were found to be more favorable for the ice bead melting process than the smooth surface.

Freezing and melting of sessile droplet on micro- and hierarchically-structured silicon surfaces

Water droplet freezing and melting on three micro- or hierarchically-structured silicon surfaces and a smooth silicon wafer were studied experimentally. The surface temperature was kept at −13.8 °C during freezing and increased gradually to room temperature after turning off the bath circulator. The freezing behaviors including the freezing delay, solidification front movement, and the droplet profile were obtained through the photographic method. The results show that the tiny black silicon particles on hierarchical structures facilitate a longer freezing delay. The solidification stage lasted for 5–17 s depending on the surface structures, during which the freezing rate was larger in the initial seconds. Droplet on three micro-structured surfaces formed a tip with the angle of 122 ± 2°, smaller than that on the smooth silicon which was 136 ± 3°. The melting onset temperature for samples 1–3 was about 4.4 °C while melting happened on smooth silicon needed a smaller degree of superheat.

Experimental investigation of the impact and freezing processes of a hot water droplet on an ice surface

Water droplet freezing on an ice surface is a common phenomenon and poses hazards to a lot of applications, including wind turbines, aircraft, and power transmission lines. Since the water droplet temperature is critical, many studies have been carried out to understand the influence of the water droplet temperature on both the impact and freezing processes of droplets on different surfaces. However, the past research studies mainly focused on supercooled water droplets, not on hot water droplets. For applications such as hot-water ice-drilling, the understanding of freezing of hot water droplets on an ice surface is necessary. In the present study, we report the detailed dynamic motions of a hot water droplet impacting on an ice surface. The impact and freezing processes of the hot water droplet on the ice surface are recorded by two cameras. The effects of the water droplet temperature and the ice surface temperature on the impact and freezing processes of the water droplet were experimentally investigated. The results showed that, at the same ice surface temperature, the increase of the water droplet temperature resulted in the increase of the maximum spreading factor, the reduction of the height of the ice bead, and the slight increase of the freezing time. In addition, during the droplet spreading process, the experimental results of the normalized contact diameter fitted well with the exponential model and the water droplet temperature was found to have an apparent influence on the lamella thickness.

Freezing of Supercooled Water Droplets Impinged on Cold, Micro-grooved Glass Surfaces

Freezing of Supercooled Water Droplets Impinged on Cold, Micro-grooved Glass Surfaces