Freezing Of Water Droplets Research Articles

An improved approach for measuring immersion freezing in large droplets over a wide temperature range

Immersion freezing (ice nucleation by particles immersed in supercooled water) is a key process for forming ice in mixed-phase clouds. Immersion freezing experiments with particles in microliter-sized (millimeter-sized) water droplets are often applied to detecting very small numbers of ice nucleating particles (INPs). However, the application of such large droplets remains confined to the detection of INPs active at temperatures much higher than the homogeneous freezing limit, because of artifacts related to freezing of water droplets without added INPs at temperatures of −25 °C or higher on a supporting substrate. Here I report a method for measuring immersion freezing in super-microliter-sized droplets over a wide temperature range. To reduce possible artifacts, droplets are pipetted onto a thin layer of Vaseline and cooled in a clean booth. In the Cryogenic Refrigerator Applied to Freezing Test (CRAFT) system, freezing of pure (Milli-Q) water droplets are limited at temperatures above −30 °C. An intercomparison of various techniques for immersion freezing experiments with reference particles (Snomax and illite NX) demonstrates that despite the use of relatively large droplets, the CRAFT setup allows for evaluating the immersion freezing activity of the particles over almost the entire temperature range (about −30 °C to 0 °C) relevant for mixed-phase cloud formation.

Effect of Surface Energy on Freezing Temperature of Water.

Previous studies have found that superhydrophobic surfaces are effective in delaying freezing of water droplets. However, the freezing process of water droplets on superhydrophobic surfaces depends on factors such as droplet size, surface area, roughness, and cooling rate. The role of surface energy, independent of any other parameters, in delaying freezing of water is not understood. Here, we have used infrared-visible sum frequency generation spectroscopy (SFG) to study the freezing of water next to solid substrates with water contact angles varying from 5° to 110°. We find that the freezing temperature of water decreases with increasing surface hydrophobicity only when the sample volume is small (∼10 μL). For a larger volume of water (∼300 μL), the freezing temperature is independent of surface energy. For water next to the surfaces with contact angle ≥54°, we observe a strong SFG peak associated with highly coordinated water. This research sheds new light on understanding the key factors in designing new anti-icing coatings.

Ice-dependent liquid-phase convective cells during the melting of frozen sessile droplets containing water and multiwall carbon nanotubes

The melting of frozen water droplets is a fundamental and ubiquitous process and the study of the transport processes occurring within the system during phase change is essential to understanding the forces that govern it. Multiwall carbon nanotubes (MWCNTs) can be added to liquid systems prior to crystallization in order to modify the properties of the phase change process and of the liquid or solid systems themselves. In this study, the melting behavior of frozen sessile droplets composed of water and 50ppm of either functionalized or non-functionalized aqueous MWCNTs is investigated. Droplets are thawed from their base on a hydrophobic substrate set to temperatures between 1 and 30°C. Tracking of MWCNT clusters during melting shows convective fluid motion occurring within the liquid melt at temperatures above 5°C. This circulation is contingent on the presence of the ice phase above. The internal fluid dynamics are attributed predominantly to thermocapillary effects as a result of temperature-induced surface tension gradients along the air/liquid interface in the melt. Further, the melting times of MWCNT-containing systems were longer than pure water samples. These results highlight new and important mechanisms driving the melting processes within water droplet systems.

Theory and experiments on the ice–water front propagation in droplets freezing on a subzero surface

An approximate theory is presented that describes the propagation of the ice–water front that develops in droplets of water that are deposited on a planar surface at a temperature below the melting point of ice. This theory is compared with experimental observation of the time evolution of this front. These experiments were performed by freezing water droplets directly on a block of dry ice, and to examine the effects of the thermal conductivity of a substrate during the freezing process. Such droplets were also deposited on a glass plate and on a copper plate placed on dry ice. The temperature at the base of these droplets, and the dependence of the freezing time on their size were also obtained experimentally, and compared with our analytic theory. These experiment can be readily performed by physics undergraduate students, and reveal that the usual assumption of constant temperature at the base of the droplets cannot be implemented in practice.

An experimental study of freezing of supercooled water droplet on solid surface

Results of experimental investigations of the freezing of immobile water droplet on an aluminum plate are presented. The process was studied with the aid of a high-speed photo camera. The freezing of supercooled water contained in the surface droplet proceeds in a few stages: (i) preliminary heating of water and nucleation of ice microcrystals, (ii) relatively fast formation of the ice–liquid system with a transition to the state of thermodynamic equilibrium near the freezing temperature, and (iii) slow process of complete freezing. The rate and duration of each stage and the time of delay between the moment of action upon the supercooled droplet and the onset of freezing are estimated. Processes of supercooled and nonsupercooled water solidification are compared.

Wetting hysteresis induced by temperature changes: Supercooled water on hydrophobic surfaces

The state and stability of supercooled water on (super)hydrophobic surfaces is crucial for low temperature applications and it will affect anti-icing and de-icing properties. Surface characteristics such as topography and chemistry are expected to affect wetting hysteresis during temperature cycling experiments, and also the freezing delay of supercooled water. We utilized stochastically rough wood surfaces that were further modified to render them hydrophobic or superhydrophobic. Liquid flame spraying (LFS) was utilized to create a multi-scale roughness by depositing titanium dioxide nanoparticles. The coating was subsequently made non-polar by applying a thin plasma polymer layer. As flat reference samples modified silica surfaces with similar chemistries were utilized. With these substrates we test the hypothesis that superhydrophobic surfaces also should retard ice formation. Wetting hysteresis was evaluated using contact angle measurements during a freeze–thaw cycle from room temperature to freezing occurrence at −7°C, and then back to room temperature. Further, the delay in freezing of supercooled water droplets was studied at temperatures of −4°C and −7°C. The hysteresis in contact angle observed during a cooling–heating cycle is found to be small on flat hydrophobic surfaces. However, significant changes in contact angles during a cooling–heating cycle are observed on the rough surfaces, with a higher contact angle observed on cooling compared to during the subsequent heating. Condensation and subsequent frost formation at sub-zero temperatures induce the hysteresis. The freezing delay data show that the flat surface is more efficient in enhancing the freezing delay than the rougher surfaces, which can be rationalized considering heterogeneous nucleation theory. Thus, our data suggests that molecular flat surfaces, rather than rough superhydrophobic surfaces, are beneficial for retarding ice formation under conditions that allow condensation and frost formation to occur.

Liquid crystal compound anti-ice surface

ABSTRACTIn some cold areas/regions, ice accumulation is harmful to aircrafts, highways and power lines. To overcome this challenge, many researchers have focused on developing anti-icing surfaces. In this paper, liquid crystal compound Cholest-5-en-3-ol(3β)-4-(2-propenyloxy)benzoate was synthesised, and a liquid crystal surface (LC surface) is prepared by heating the liquid crystal compound to 250°C and then cooling it. We determined ice-phobic properties of LC surface using differential scanning calorimetry (DSC) and polarised optical microscope (POM). A phenomenon that the freezing of water droplet on the LC surface is delayed was found. Compared with the two measurement methods, we obtained similar result that water freezing temperature was delayed nearly by 8°C on average on the LC surface. A process of ice/frost formation is observed using POM. The results displayed that the glass wafer without LC was covered completely by ice/frost, whereas on the LC surface at the same temperature no ice/frost was formed. Characteristics of LC surface were observed by field emission scanning electron microscope (FESEM) and Fourier transform infrared (FT-IR) imaging system. We suggest that the reason behind anti-ice surface is related to surface molecules and this is an important factor which may have an effect on anti-icing property.

Freezing of sessile water droplet for various contact angles

We present both modeling and experimental study of the freezing of a sessile water droplet on a cold surface with different contact angles. The numerical model consists of two parts. One is to determine the geometric profile of sessile droplets by the Young-Laplace equation, and the other is to solve the two-dimensional Stefan problem via the equivalent heat capacity method. An experimental setup is built to observe the freezing process, and the moving water–ice interface is recorded by a CCD camera. A good agreement is found between the experimental observations and the numerical simulations of the moving water–ice interface in a sessile droplet deposited on both hydrophilic and hydrophobic cold surfaces, suggesting that the present model can well describe the freezing process of sessile droplets. Based on the modeling and simulations, it is shown that for a given droplet the freezing time exponentially increases with increasing contact angle. Results of our study are helpful to better understand the anti-ice/-frost mechanism of the superhydrophobic surfaces.

Size determination of mixed liquid and frozen water droplets using interferometric out-of-focus imaging

We record simultaneously interferometric out-of-focus images and digital in-line holograms of liquid and frozen water droplets. We show that the analysis of speckle-like out-of-focus images allows a quantitative estimation of the size of the particles which is corroborated by numerical reconstruction of holograms recorded simultaneously. Interferometric out-of-focus imaging could be extended to the characterization of ice in clouds in the atmosphere.

Droplet freezing and ice adhesion strength measurement on super-cooled hydrophobic surfaces

ABSTRACTThis report investigates the freezing of water droplets on hydrophobic surfaces and reports a means to quantitatively measure the ice-adhesion strength at micro-patterned hydrophobic surfaces. Water droplets were placed on a super-cooled surface where they froze following the heterogeneous nucleation phase-transition. The freezing occurred with two distinct transitions: (a) an initial snapping transition freezing of the external shell of the droplet, and (b) a subsequent bulk freezing of the droplet with a characteristic formation of peaked apex protruding upwardly. To measure the ice-adhesion strength, frozen droplets were sheared from the surface using a Universal Materials Tester (UMT) measuring the resulting knock-off forces. Ice formation and adhesion was investigated on three variations of hydrophobic polydimethylsiloxane (PDMS) surfaces: regularly patterned with high and low surface densities of micro-pillar arrays and non-patterned flat surfaces. Patterning the surfaces was found to accelerate freezing transitions but decrease the measured ice-adhesion strength in correlation with a reduction of total interfacial contact area. However, a more concentrated local stress was observed for the less dense surface structures, indicating a potential area of concern for more fragile systems.

The microphysical pathway to contrail formation

AbstractA conceptual framework to predict microphysical and optical properties of contrail particles within a wingspan behind the source aircraft is developed. Results from two decades of contrail observations and numerical simulations are reviewed forming the basis of theoretical model development. The model utilizes cloud theory applied to the dynamics and thermodynamics of jet aircraft exhaust plumes in upper tropospheric conditions. Droplet nuclei include soot particles emitted from aircraft engines and atmospheric particles entrained into the plume. These precursor particles activate into copious homogeneously freezing water droplets as the plume relative humidity rises beyond liquid water saturation. A unimodal size spectrum of ice particles develops wherein ice particles grow to micrometer mean sizes. Contrail particle formation is analyzed over a wide range of soot emissions relating to conventional jet fuels as well as to alternative aviation fuels producing much less soot and volatile particle emissions. For current aviation fuels and propulsion technology, the number of contrail ice particles scales roughly in proportion to the number of emitted soot particles that act as water condensation nuclei despite their poor hygroscopicity. Close to the contrail formation threshold, only few plume particles can be water activated and freeze. Implications for effects of alternative fuels on contrails, an arena for future scientific exploration, are outlined.

Ice nucleation by water-soluble macromolecules

Abstract. Cloud glaciation is critically important for the global radiation budget (albedo) and for initiation of precipitation. But the freezing of pure water droplets requires cooling to temperatures as low as 235 K. Freezing at higher temperatures requires the presence of an ice nucleator, which serves as a template for arranging water molecules in an ice-like manner. It is often assumed that these ice nucleators have to be insoluble particles. We point out that also free macromolecules which are dissolved in water can efficiently induce ice nucleation: the size of such ice nucleating macromolecules (INMs) is in the range of nanometers, corresponding to the size of the critical ice embryo. As the latter is temperature-dependent, we see a correlation between the size of INMs and the ice nucleation temperature as predicted by classical nucleation theory. Different types of INMs have been found in a wide range of biological species and comprise a variety of chemical structures including proteins, saccharides, and lipids. Our investigation of the fungal species Acremonium implicatum, Isaria farinosa, and Mortierella alpina shows that their ice nucleation activity is caused by proteinaceous water-soluble INMs. We combine these new results and literature data on INMs from fungi, bacteria, and pollen with theoretical calculations to develop a chemical interpretation of ice nucleation and water-soluble INMs. This has atmospheric implications since many of these INMs can be released by fragmentation of the carrier cell and subsequently may be distributed independently. Up to now, this process has not been accounted for in atmospheric models.

극지 해양 파이프라인 내부 유체의 온도별 영향 및 내부 충돌침식 분석

This paper describes thermal flow characteristic in various pipelines: straight pipeline and curved pipeline. In the Arctic and ocean area, pipelines are exposed to a extremely low temperature (0 ~ -40°C). In this situation, three-dimensional flow analysis should be analyzed to investigate thermal effects such as pressure drop, temperature change, velocity deficit and distribution change of liquid droplet of internal fluid. Also, due to freezing of water droplet, impingement erosion is expected in the curved pipeline. The stability of the pipelines can be influenced by impingement erosion. In this paper, multi-phase and multi-species analysis was introduced to analyze the flow characteristics and impingement erosion of Arctic and ocean pipelines.

Simulating the Freezing of Supercooled Water Droplets Impacting a Cooled Substrate

To study ice adhesion at the droplet scale, a strategy is presented to simulate the impact and solidification of a supercooled water droplet on a cooled substrate. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled volume-of-fluid and level-set method to capture the air–water interface, and an enthalpy-porosity method is used to capture the liquid–solid interface. Calibration of a simulation parameter is performed in order to match experimental data for different ideal surface types and surface temperatures. The simulation strategy successfully predicts the overall droplet response for several droplet impact conditions.

Synthesis of uniform hollow TiO2 and SiO2 microspheres via a freezing assisted reverse microemulsion-templated sol–gel method

Uniform titania and silica hollow microspheres of 300–500nm diameter with controlled wall thickness have been successfully synthesized using monodispersed micro-ice as the template generated by freezing water droplets in a reverse microemulsion at −40°C and −80°C and subsequent sol–gel reactions on the surface of the formed micro-ice. Furthermore, the growth mechanism of the hollow spheres by this facile and interesting technique was discussed in detail.

New insight into icing and de-icing properties of hydrophobic and hydrophilic structured surfaces based on core-shell particles.

Icing is an important problem, which often leads to emergency situations in northern countries. The reduction of icing requires a detailed understanding of this process. In this work, we report on a systematic investigation of the effects of geometry and chemical properties of surfaces on the formation of an ice layer, its properties, and thawing. We compare in detail icing and ice thawing on flat and rough hydrophilic and hydrophobic surfaces. We also show advantages and disadvantages of the surfaces of each kind. We demonstrate that water condenses in a liquid form, leading to the formation of a thin continuous water layer on a hydrophilic surface. Meanwhile, separated rounded water droplets are formed on hydrophobic surfaces. As a result of slower heat exchange, the freezing of rounded water droplets on a hydrophobic surface occurs later than the freezing of the continuous water layer on a hydrophilic one. Moreover, growth of ice on hydrophobic surfaces is slower than on the hydrophilic ones, because ice grows due to the condensation of water vapor on already formed ice crystals, and not due to the condensation on the polymer surface. Rough hydrophobic surfaces also demonstrate a very low ice adhesion value, which is because of the reduced contact area with ice. The main disadvantage of hydrophobic and superhydrophobic surfaces is the pinning of water droplets on them after thawing. Flat hydrophilic poly(ethylene glycol)-modified surfaces also exhibit very low ice adhesion, which is due to the very low freezing point of the water-poly(ethylene glycol) mixtures. Water easily leaves from flat hydrophilic poly(ethylene glycol)-modified surfaces, and they quickly become dry. However, the ice growth rate on poly(ethylene glycol)-modified hydrophilic surfaces is the highest. These results indicate that neither purely (super)hydrophobic polymeric surfaces, nor "antifreeze" hydrophilic ones provide an ideal solution to the problem of icing.

An Experimental Study on the Heat Transfer of Traveling Airborne Water Droplets in Cold Environment

AbstractThis paper presents experimental results on rapid freezing of water droplets injected into a low temperature environment. A heat balance method was applied to determine the ratio of the water droplets frozen at the collection after the airborne time. The experimental results show that rapid freezing of water droplets could be achieved within three seconds of airborne time. Droplet size distribution of the frozen water droplets after collection was estimated. Heat transfer during the airborne time was calculated with consideration of the droplet size distribution. At attempt was taken to compare the heat transfer obtained with some previous studies on heat transfer of spherical objects in air. The research results show that droplet size distribution is important for the prediction of heat transfer of water droplets traveling in air. The results presented in this study contribute to the understanding of heat transfer of water droplets injected into a low temperature air.

Conical shape of frozen water droplets

When a small water droplet is deposited on a cold surface, the freezing process transforms the droplet so that its upper portion attains a conical shape. Such a transformation occurs because the density of ice is lower than the density of water. Here, we assume that the freezing process in its final stage becomes self-similar, so the freezing front has a concave spherical shape, with a center that coincides with the vertex of the forming cone. This assumption gives a vertex angle of 65°, corresponding to a density ratio of 0.917, in a good agreement with experimental data.

Electric effect during the fast dendritic freezing of supercooled water droplets.

An electrical phenomenon consisting of two alternating voltage peaks of up to 6 V amplitude was observed during the rapid dendritic freezing phase of supercooled water droplets in the millimeter size range with supercoolings ΔT in the range of 5 to 20 K. For correlation of the dendritic freezing front with the electric potential, a fast recording oscilloscope was combined with a high-speed camera operating at up to 5000 frames per second. The strength of the effect is roughly proportional to the supercooling and dendritic freezing speed. Furthermore, during the subsequent second freezing phase, which is much slower than the dendritic, a qualitatively different electric potential evolution of similar magnitude has been found which resembles the well-investigated Workman-Reynolds freezing potential (WRFP). The experiments show clear evidence that the first rapid dendritic freezing stage significantly influences direction and amount of the electric potential during the second slow freezing stage. Compared to the WRFP, which takes place for much smaller supercoolings of ΔT ≪ 5 K, the evolution of the presented dendritic freezing potential occurs about 10(4) times faster, is about 10 times smaller in view of the maximum voltage, and shows similar break off concentrations but remarkably does not vanish at low foreign ion concentrations. This phenomenon has direct relevance to atmospheric freezing processes of the Earth, other planets, and satellites.

Experimental study on frosting/defrosting characteristics of microgrooved metal surfaces

Effect of microgroove geometry on the frost formation and frost melt-water drainage is experimentally investigated on a number of aluminum surfaces. Condensation and frost formation processes are also examined on a set of microgrooved copper surfaces for which the dimensions of the microgroove geometry are varied in a cyclic manner. These metal surfaces are studied because of their technical importance as working materials and are fabricated by two different techniques, photolithography and wet etching, respectively. The morphology, distribution and growth pattern of the condensed and frozen water droplets are found to be considerably different on the microgrooved surfaces from that on the flat baseline surfaces. While the amount of frost melt-water retention on the flat surfaces is found to increase in the subsequent refrost cycles and is highest in the last frost cycle, the microgrooved aluminum surfaces consistently exhibit improved frost-water drainage in all frost cycles and under different operating conditions. Findings of this study will be useful in designing microgrooved metal surfaces operating under frosting/defrosting conditions to have improved frost-water drainage properties.