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In the present study, we report novel methods to achieve accurate temperature measurements inside the water droplet at its supercooled state as well as during its freezing process. The temperature measurements were based on the molecular tagging thermometry technique. In order to maintain the nonfreezing state of the phosphorescent tracer complex aqueous solution at a subfreezing temperature, a double-layer temperature control container was designed and fabricated. Then, the calibration between the lifetime and temperature of the phosphorescent tracer complex aqueous solution from 7.5 °C to as low as −6.0 °C was carefully performed. Then, the ice fraction (f) was applied to revise the calibration curve for the ice–liquid mixture. The results indicated that the calibration curve for the pure liquid was suitable for the temperature measurements of the pure phosphorescent triplex solution at the supercooling state, while the revised calibration curve for the ice–liquid mixture was appropriate for the temperature measurements in the water droplet during its freezing stage. [ABSTRACT FROM AUTHOR]

In the present study, we carried out an experimental investigation of the impinging and freezing processes of a supercooled large water droplet on an ice surface. One high speed camera was used to measure the dynamic motions of the water droplet while two charge coupled device (CCD) cameras were adopted to obtain the images of the freezing process and the freezing morphologies, respectively. The effects of the water droplet temperature and the ice surface temperature on the impact and freezing processes of the water droplet were carefully evaluated. The results showed that the subcooling degree of the water droplet had an apparent influence not only on the spreading process but on the freezing morphology as well. When the subcooling temperature of the water droplet was high (e.g., Tw = –3.0 °C), a triangle cross-section profile formed. However, once the subcooling temperature of the water droplet was relatively low (e.g., Tw = –9.0 °C), the final ice morphology consisted of two parts: one part was a cone-like bead in the center while the other part was an ice ring at the periphery. Moreover, at the same water droplet temperature, raising the ice surface temperature led to an increase in the maximum spreading factor. [ABSTRACT FROM AUTHOR]

Highlights • Typical heterogeneous ice nucleation pattern was revealed under practical nocturnal cooling and freezing conditions. • Distinctive three-stage frosting process was found under subfreezing condition. • Ice nucleation was first initiated on the upper part of the windshield. • Surface roughness or contamination probably triggered an earlier ice nucleation. • The arrival of saturation point could approximately mark the initiation of icing. Abstract In the present study, we theoretically and experimentally investigated a typical and practical heterogeneous ice nucleation process on auto windshield under nocturnal radiative cooling and subfreezing conditions. During the experiment, a general class A passenger car was placed outdoors in a subfreezing environment (i.e., 44.14° N, 126.45° E) during nights in January, with a purpose to produce a nocturnal radiative cooling condition for supercooling the windshield to trigger the ice nucleation. The results revealed a distinctive three-stage frosting process: (I) inhomogeneous occurrence of incipient ice nucleation; (II) small-sized ice crystals grew into large-sized ones primarily based on the incipient nucleation sites, together with a continual formation of new nucleation sites and a merging between large-sized and small-sized crystals; (III) large-sized ice crystals coalesced into large-scale groups, tending to make a full ice coverage. Unlike the classical three-period frosting process, the present study exhibited an absence of the drop-wise condensation but with a presence of dendritic crystals nearly parallel to the supercooled surface. Besides, the incipient ice nucleation was first initiated on the upper part of the windshield, which then propagated downward with a further heat loss of the windshield due to the continual nocturnal radiation. Meanwhile, the probable presence of surface roughness or contamination was much more likely to trigger an earlier initiation of incipient heterogeneous ice nucleation. Additionally, the arrival of saturation point, rather than the nominal supersaturation state (i.e., the condensation nucleation point), could be expected to approximately mark the initiation of icing under a practical engineering condition. Higher relative humidity tended to induce an earlier occurrence of ice nucleation and produce a larger surface coverage. [ABSTRACT FROM AUTHOR]

In the present study, we experimentally investigated the impact and freezing processes of a water droplet on different cold cylindrical surfaces. The results showed that, during the impact process of the water droplet, the maximum contact diameter along azimuthal direction was generally larger than that along axial direction. Besides, during the water droplet recoiling process, the radius of the cylindrical surface was found to have an apparent effect on the spreading factor along azimuthal direction but a minor influence on that along axial direction. During the freezing process of the water droplet, the change of the temperature as well as the radius of the cylindrical surface did not result in a significant variation of the ice bead shape. [ABSTRACT FROM AUTHOR]

In the present study, we report for the first time the detailed dynamic motions of a water droplet impacting on an ice surface. Besides, the effects of the initial height of the water droplet and the ice surface temperature on the impact and freezing processes of the water droplet were experimentally investigated. During the experiment, an ice surface was generated first and then kept at a desired temperature by a constant temperature bath circulator. After that, a deionized water droplet was deposited onto the ice surface and its impact and freezing processes were recorded. The results showed that, during the impact process of the water droplet, once the water droplet reached its maximum contact diameter, the contact line of the water droplet was pinned on the ice surface without recoiling. Besides, at the same initial height of the water droplet, the decrease of the ice surface temperature resulted in the reduction of the maximum spreading factor and an apparent increase of the height of the ice bead. Moreover, once the droplet initial height was increased, the maximum spreading factor increased while the height of the ice bead reduced significantly. [ABSTRACT FROM AUTHOR]

In the present study, we report for the first time the observations of the successive freezing processes of water droplets on an ice surface. During the experiment, an ice surface was generated first and then kept at a desired temperature by a semi-conductor cooler. After that, three deionized water droplets were deposited onto the ice surface successively and their freezing processes were recorded. The results show that both ice surface temperature and the initial height of the water droplet have significant effects on not only the droplet freezing time, but the contact diameter, angle, and height of the ice bead as well. When the initial height of the water droplet was relatively small, the subsequent water droplet could only cover a portion of the ice bead formed by the previous droplet. Besides, under the same initial height of the water droplet, the decrease of the ice surface temperature could result in an obvious reduction of the contact diameter of the ice bead and an apparent increase of the ice bead angle. [ABSTRACT FROM AUTHOR]

The present study experimentally investigated the impact process of a water droplet on a cold surface with different inclined angles. The inclined angle of the surface was changed from 0.0° to 60.0°, while the surface temperature was varied from room temperature 21.0 °C to −15.0 °C. During the impact process of the water droplet, the inclined angle of the surface was found to have an apparent influence on the spreading time and displacements of the foremost and rearmost points. Besides, once the inclined angle of the surface was high enough (e.g., α = 60.0°), the water droplet could not keep as a single droplet all the time during its impact process. It split into two smaller droplets and produced satellite droplets at the same time. In addition, during the freezing process of the water droplet, the inclined angle of the surface could affect the final geometry of the ice bead. [ABSTRACT FROM AUTHOR]

• A supercooled large droplet impacting on a freezing water film was studied. • The evolution characteristics of the crown generated by SLD were analyzed. • The dendritic ices significantly affected the formation and growth of crowns. • The relationship between secondary droplets number and water film thickness was studied. In the present study, we report for the first time the detailed dynamic processes of a supercooled large droplet (SLD) impacting on a freezing water film at different freezing moments. At the beginning of the experiment, the deionized water film was first set to a desired temperature and then triggered to freeze by further cooling. Then, a supercooled large droplet was released to impact the freezing water film at different freezing moments. Its impact and splashing processes were recorded by a high-speed camera. Besides, under room-temperature conditions, the impinging and splashing processes of a water droplet on the water film with different thicknesses were also measured. The results showed that, at the early stage of the water film freezing process, the dendritic ices could be pushed out of the water film surface due to the impact of the supercooled large droplet. Besides, the dendritic ices significantly affected the formation and growth of crowns, which could lead to the earlier crown breaks or the formation of the irregular crowns. Moreover, compared to its counterpart of the room-temperature droplet case, the evolution characteristics of the crown generated by the supercooled large droplet case were significantly different in terms of the crown top diameter, the crown base diameter, the crown height, the crown angle, and the crown lifetime. In addition, the correlations between the number of the secondary droplets and the dimensionless water film thickness were obtained and compared as for the room-temperature droplet cases and the supercooled large droplet cases. [ABSTRACT FROM AUTHOR]

The present study experimentally investigated the effects of droplet size and surface temperature on the impact, freezing, and melting processes of a water droplet on an inclined cold surface. The temperature of the inclined surface was changed from room temperature 20.0 °C to −14.0 °C while water droplets of three sizes ( D 0 = 2.21, 2.92, and 3.31 mm) were tested. The results show that the increase of droplet size led to the increases of spreading time, spreading maximum diameter, gliding maximum diameter, and maximum displacement of foremost point. Besides, during the relaxation phase, as the decrease of surface temperature, the formost point retracted a longer distance and the spreading factor decreased more smoothly. In addition, test plate temperature was found to have an effect on the freezing time, air bubble sizes, and frost accretion. At last, the melting processes of the ice beads under different conditions were also illustrated and compared. [ABSTRACT FROM AUTHOR]

• The impact process of a supercooled water droplet onto a cold SLIPS was studied. • The freezing process of a supercooled water droplet onto a cold SLIPS was studied. • The melting process of a supercooled water droplet onto a cold SLIPS was studied. • The effects of the droplet supercooling degree and the SLIPS temperature were tested. • The freezing process could happen during the supercooled water droplet recoiling process. In the present study, we report for the first time the observations of the impact, freezing, and melting processes of a supercooled water droplet onto a cold slippery liquid-infused porous surface (SLIPS). A parameter study of the droplet supercooling degree and the SLIPS temperature was carefully performed. Moreover, the impact process of a water droplet onto the SLIPS under the room temperature conditions was also measured for comparisons. The results showed that the impact kinematics of a water droplet on a SLIPS under the room temperature conditions was significantly different from those of the supercooled water droplet impinging onto a cold SLIPS. Under the same water droplet temperature conditions, the spreading factors rapidly increased to their maximum values at a similar rate, regardless of the change of the SLIPS temperature. Besides, under the same SLIPS temperature conditions, the maximum spreading factor of the water droplet at room temperature was significantly greater than those of the supercooled water droplet cases. When the temperatures of the water droplet and SLIPS were relatively high, freezing process started after the recoiling process had already finished for a certain period of time. However, when the temperatures of the water droplet and SLIPS were relatively low, freezing process could happen during the supercooled water droplet recoiling process. In addition, under the same water droplet temperature conditions, the decrease of the SLIPS temperature could result in the increase of the average energy dissipation rate of the water droplet. [ABSTRACT FROM AUTHOR]

• The impact and freezing processes of two water droplets on an ice surface was studied. • The effects of the ice surface temperature, Weber number, and time lag were investigated. • The time lag played an important role on the evolution of the central uprising sheet. • The time lag had apparent influence on the final ice bead morphology. In the present study, we report the detailed impact and freezing processes of two water droplets on an ice surface. The effects of the ice surface temperature, Weber number, and droplet time lag on the impact and freezing processes were experimentally investigated. The results showed that the time lag between the two water droplets played an important role on the formation and evolution of the central uprising sheet. As the increase of the droplet time lag, the increase rate of the spreading factor at the early stage of the droplet spreading process decreased. In addition, the droplet time lag was found to have apparent influence on the final ice bead morphology. Under the same Weber number and ice surface temperature conditions, the increase of the time lag could lead to the increase of the number of the fingering-like structures. Moreover, both droplet Weber number and ice surface temperature had apparent effects on the maximum radial distance of the primary ice beads. As the decrease of the ice surface temperature, the primary ice bead height increased apparently. [Display omitted] [ABSTRACT FROM AUTHOR]

In the present study, we report the observations of the impact and freezing processes of a water droplet on different cold concave surfaces. During the experiment, the concave surface was put into a cover in which pressurized Argon gas was injected at the same time with a purpose to minimize the desublimation of the vapor. Then, the surface temperature was cooled down to a desirable value by a constant temperature bath circulator. After that, a deionized water droplet was released to impinge the concave surface and its impact and freezing processes were recorded by two cameras. The results showed that, during the impact process of the water droplet, the maximum contact diameter along azimuthal direction was generally smaller than that along axial direction. In addition, during the water droplet recoiling process, the residue of a ring appeared on the cold concave surface and the radius of the concave surface was found to have a minor effect on the spreading factor along axial direction. During the freezing process of the water droplet, the change of the temperature as well as the radius of the concave surface did not lead to an apparent variation of the ice bead shape. [ABSTRACT FROM AUTHOR]

In the present study, we experimentally investigated the impact and freezing processes of a water droplet on different cold spherical surfaces. During the experiment, the spherical surface was put into a cover in which pressurized Argon gas was injected with a purpose to minimize the desublimation of the vapor. Then, the surface temperature was cooled down to a desirable value by a constant temperature bath circulator. After that, a deionized water droplet was released to impinge the spherical surface and its impact and freezing processes were recorded. The results showed that, during the recoiling process of the water droplet, the spreading factors at low surface temperature cases were larger than that of the room temperature case, regardless of the radius of the spherical surface. In addition, the radius of the spherical surface had an apparent effect on the spreading factor when the surface temperature was relatively low (eg. T w = −9.5 and −14.0 °C). During the freezing process of the water droplet, the change of the temperature as well as the radius of the spherical surface did not lead to an apparent variation of the ice bead shape. [ABSTRACT FROM AUTHOR]

The present study experimentally investigated the impact and freezing processes of a water droplet on different inclined cold surfaces. The surface temperature was changed from room temperature 24.0 °C to −15.0 °C. The materials of the surfaces included red copper, quartz glass, carbon fiber reinforced composite (CFRC), and glass fiber reinforced composite (GFRC). During the impact process of the water droplet, the surfaces were found to have a minor influence on the spreading time and gliding time but an apparent effect on the equilibrium time. Besides, these surfaces significantly affected the spreading maximum diameter, gliding maximum diameter, and maximum displacement of foremost point of the water droplet. In addition, once the equilibrium state of water droplet was reached, the contact diameter of the water droplet depended heavily on the type of the surfaces. As for the freezing process, the cold surfaces affected not only the freezing time of the water droplets, but the shape of the ice beads as well. [ABSTRACT FROM AUTHOR]