Drop Freezing Research Articles

Model simulations with COSMO-SPECS: impact of heterogeneous freezing modes and ice nucleating particle types on ice formation and precipitation in a deep convective cloud

Abstract. In deep convective clouds, heavy rain is often formed involving the ice phase. Simulations were performed using the 3-D cloud resolving model COSMO-SPECS with detailed spectral microphysics including parameterizations of homogeneous and three heterogeneous freezing modes. The initial conditions were selected to result in a deep convective cloud reaching 14 km of altitude with strong updrafts up to 40 m s−1. At such altitudes with corresponding temperatures below −40 ∘C the major fraction of liquid drops freezes homogeneously. The goal of the present model simulations was to investigate how additional heterogeneous freezing will affect ice formation and precipitation although its contribution to total ice formation may be rather low. In such a situation small perturbations that do not show significant effects at first sight may trigger cloud microphysical responses. Effects of the following small perturbations were studied: (1) additional ice formation via immersion, contact, and deposition modes in comparison to solely homogeneous freezing, (2) contact and deposition freezing in comparison to immersion freezing, and (3) small fractions of biological ice nucleating particles (INPs) in comparison to higher fractions of mineral dust INP. The results indicate that the modification of precipitation proceeds via the formation of larger ice particles, which may be supported by direct freezing of larger drops, the growth of pristine ice particles by riming, and by nucleation of larger drops by collisions with pristine ice particles. In comparison to the reference case with homogeneous freezing only, such small perturbations due to additional heterogeneous freezing rather affect the total precipitation amount. It is more likely that the temporal development and the local distribution of precipitation are affected by such perturbations. This results in a gradual increase in precipitation at early cloud stages instead of a strong increase at later cloud stages coupled with approximately 50 % more precipitation in the cloud center. The modifications depend on the active freezing modes, the fractions of active INP, and the composition of the internal mixtures in the drops.

Delay in the Freezing of Supercooled Water Drops on Superhydrophobic Surfaces of Silicone Rubber at Negative Temperatures

A method is proposed for fabricating textured superhydrophobic surfaces of silicone rubber with mechanical resistance toward liquid or freezing aqueous solutions. The anti-icing characteristics of silicone rubber samples that differ in the wetting characteristics and mechanical stability of their micro- and nanotextures are derived by analyzing the delays in the freezing of supercooled sessile water drops deposited on the sample surface. The longest delay in freezings are observed for sessile water drops on superhydrophobic surfaces prepared by laser texturing with subsequent application of a layer of a hydrophobic agent to consolidate the textural elements. Delay in freezings can be as long as tens of hours on such surfaces at T = −18°C. The prepared superhydrophobic surfaces exhibit greater anti-icing ability with respect to aqueous salt solutions than to deionized water.

Ice nucleation triggered by negative pressure

Homogeneous ice nucleation needs supercooling of more than 35 K to become effective. When pressure is applied to water, the melting and the freezing points both decrease. Conversely, melting and freezing temperatures increase under negative pressure, i.e. when water is stretched. This study presents an extrapolation of homogeneous ice nucleation temperatures from positive to negative pressures as a basis for further exploration of ice nucleation under negative pressure. It predicts that increasing negative pressure at temperatures below about 262 K eventually results in homogeneous ice nucleation while at warmer temperature homogeneous cavitation, i. e. bubble nucleation, dominates. Negative pressure occurs locally and briefly when water is stretched due to mechanical shock, sonic waves, or fragmentation. The occurrence of such transient negative pressure should suffice to trigger homogeneous ice nucleation at large supercooling in the absence of ice-nucleating surfaces. In addition, negative pressure can act together with ice-inducing surfaces to enhance their intrinsic ice nucleation efficiency. Dynamic ice nucleation can be used to improve properties and uniformity of frozen products by applying ultrasonic fields and might also be relevant for the freezing of large drops in rainclouds.

Receiving and use of streams of monodisperse ice granules for cleaning and deactivation of surfaces

The most generally useful methods for cleaning and processing of surfaces are the sand-jets and shot blasting jets. Installations of this kind are used for cleaning of corrosion surfaces, the oil-dirt deposits, paint coatings. However the use of these installations follows to high investment and operational expenditure, larger risk of operators disease, the negative affect for a environment. These problems can be solved with the use of new cleaning method through application of mono-disperse (identical by the size and the form) ice granules of 300 - 1000 microns, accelerated by air stream in the nozzle device to the speed of 10 - 100 m/s. In view of the extreme complexity of the receiving such particles by means of cooling and the subsequent freezing of water drops are necessary additional experimental researches. For study of thermal processes of receiving mono-disperse ice granules the experimental installation was created and experiments on deactivation and cleaning of surfaces with pollution of various types are made. Experiments showed that by means of a stream of the accelerated ice granules it is rather successfully possible to delete oil-dirt deposits, outdated paint coats and rust. Besides, efficient deactivation of radioactive surfaces is possible. The coefficient deactivation of γ activity is highest.

Winter Precipitation Liquid–Ice Phase Transitions Revealed with Polarimetric Radar and 2DVD Observations in Central Oklahoma

AbstractObservations and analysis of an ice–liquid phase precipitation event, collected with an S-band polarimetric KOUN radar and a two-dimensional video disdrometer (2DVD) in central Oklahoma on 20 January 2007, are presented. Using the disdrometer measurements, precipitation is classified either as ice pellets or rain/freezing rain. The disdrometer observations showed fast-falling and slow-falling particles of similar size. The vast majority (>99%) were fast falling with observed velocities close to those of raindrops with similar sizes. In contrast to the smaller particles (<1 mm in diameter), bigger ice pellets (>1.5 mm) were relatively easy to distinguish because their shapes differ from the raindrops. The ice pellets were challenging to detect by looking at conventional polarimetric radar data because of the localized and patchy nature of the ice phase and their occurrence close to the ground. Previously published findings referred to cases in which ice pellet areas were centered on the radar location and showed a ringlike structure of enhanced differential reflectivity ZDR and reduced copolar correlation coefficient ρhv and horizontal reflectivity ZH in PPI images. In this study, a new, unconventional way of looking at polarimetric radar data is introduced: slanted vertical profiles (SVPs) at low (0°–1°) radar elevations. From the analysis of the localized and patchy structures using SVPs, the polarimetric refreezing signature, reflected in local enhancement in ZDR and reduction in ZH and ρhv, became much more evident. Model simulations of sequential drop freezing using Marshall–Palmer DSDs along with the observations suggest that preferential freezing of small drops may be responsible for the refreezing polarimetric signature, as suggested in previous studies.

Local ice formation via liquid water growth in slowly ascending humid aerosol/liquid water layers observed with ground‐based lidars and radiosondes

AbstractObservations with lidars at Wuhan (30.5°N, 114.4°E), China, from 2010 to 2013 captured nine cases of slowly ascending humid aerosol/liquid water layers that occurred at altitudes of ~2–4 km in winter. Each of them was almost transparent initially with the backscatter ratio far less than 7.0 and depolarization ratio less than 0.03. With a slow ascent, the layer developed into a nearly opaque liquid cloud layer and then ice crystals abruptly formed at the upper edge of the cloud layer with very high liquid water content. The ice crystals likely came from water drop freezing. The freezing temperatures estimated from radiosonde measurements were −3 to −8°C. For two available long‐lived (>16 h) cases, the layer was observed to always lie just below an inversion layer. The ice development on the layer was followed by rainfall.

Redistribution of ice nuclei between cloud and rain droplets: Parameterization and application to deep convective clouds

AbstractIn model studies of aerosol‐dependent immersion freezing in clouds, a common assumption is that each ice nucleating aerosol particle corresponds to exactly one cloud droplet. In contrast, the immersion freezing of larger drops—“rain”—is usually represented by a liquid volume‐dependent approach, making the parameterizations of rain freezing independent of specific aerosol types and concentrations. This may lead to inconsistencies when aerosol effects on clouds and precipitation shall be investigated, since raindrops consist of the cloud droplets—and corresponding aerosol particles—that have been involved in drop‐drop‐collisions. Here we introduce an extension to a two‐moment microphysical scheme in order to account explicitly for particle accumulation in raindrops by tracking the rates of selfcollection, autoconversion, and accretion. This provides a direct link between ice nuclei and the primary formation of large precipitating ice particles. A new parameterization scheme of drop freezing is presented to consider multiple ice nuclei within one drop and effective drop cooling rates. In our test cases of deep convective clouds, we find that at altitudes which are most relevant for immersion freezing, the majority of potential ice nuclei have been converted from cloud droplets into raindrops. Compared to the standard treatment of freezing in our model, the less efficient mineral dust‐based freezing results in higher rainwater contents in the convective core, affecting both rain and hail precipitation. The aerosol‐dependent treatment of rain freezing can reverse the signs of simulated precipitation sensitivities to ice nuclei perturbations.

Discrepancy between Ice Particles and Ice Nuclei in Mixed Clouds: Critical Aspects

Measurements of ice crystal concentrations in mixed clouds tend to exceed ice nucleus concentrations measured in nearby clear air. This discrepancy is a source of uncertainty in climate change projections as the radiative properties of mixed phase clouds are largely determined by their liquid and ice water content. The ice enhancement process can sometimes depend on secondary ice production, which can occur through ice crystal fracture during sublimation, cloud drop shattering during freezing or following collision with ice particles. However, the discrepancy is observed even in mixed clouds where only primary ice nucleation processes occur. Several hypotheses have been suggested for the observed discrepancies. One factor could be the existence in clouds of pockets of high vapor supersaturation formed by droplet freezing or removal of small droplets by collision with larger droplets, associated with the fact that ice crystal concentration increases with water supersaturation. However, ice crystal concentrations are usually measured at near water saturation. Additional factors could be drop freezing during evaporation and activation of droplet evaporation residues. Here we suggest that a major factor could be underestimation of the contact freezing mode as it is not measured in experimental campaigns and seldom considered in nucleation models. Laboratory experiments give only incomplete answers to the important questions concerning the contact freezing mode, e.g. what fraction of the aerosol particles that come into contact with the droplet surface results in a freezing event and what is the influence of particle type and size, air temperature and relative humidity. As supercooled droplets grow or evaporate in mixed clouds, phoretic forces should play an important role in the collision efficiency between aerosol and droplets, and consequently in contact freezing. A further question is the possibility that aerosol, usually not active in deposition or condensation/immersion freezing, can trigger ice nucleation by colliding with supercooled droplets.

Solidification of supercooled water in the vicinity of a solid wall.

An experimental approach utilizing a Hele-Shaw cell for the investigation of the solidification of a supercooled liquid in contact with a solid wall is presented. The setup is based on an idea presented by Marín etal. [A. G. Marín et al., Phys. Rev. Lett. 113, 054301 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.054301], who investigated the planar freezing of a sessile drop without supercooling. This apparatus overcomes optical distortions present when observing the freezing of sessile drops, arising due to reflections and refraction of light on the drop surface. The facility is used to investigate the freezing process of water drops, supercooled down to -20^{∘}C, and to qualitatively demonstrate that the growth behavior is uninfluenced by the use of the Hele-Shaw cell. Different features during freezing, which are known for sessile water drops, are also observed with the Hele-Shaw cell. The growth morphology within the first phase of solidification is categorized according to the initial drop supercooling. Furthermore, freezing velocities within this phase are related to data available in the literature for the growth of single ice dendrites.

Effects of different temperature treatments on biological ice nuclei in snow samples

The heat tolerance of biological ice nucleation activity (INA) depends on their types. Different temperature treatments may cause varying degrees of inactivation on biological ice nuclei (IN) in precipitation samples. In this study, we measured IN concentration and bacterial INA in snow samples using a drop freezing assay, and compared the results for unheated snow and snow treated at 40 °C and 90 °C. At a measured temperature of −7 °C, the concentration of IN in untreated snow was 100–570 L−1, whereas the concentration in snow treated at 40 °C and 90 °C was 31–270 L−1 and 2.5–14 L−1, respectively. In the present study, heat sensitive IN inactivated by heating at 40 °C were predominant, and ranged 23–78% of IN at −7 °C compared with untreated samples. Ice nucleation active Pseudomonas strains were also isolated from the snow samples, and heating at 40 °C and 90 °C inactivated these microorganisms. Consequently, different temperature treatments induced varying degrees of inactivation on IN in snow samples. Differences in the concentration of IN across a range of treatment temperatures might reflect the abundance of different heat sensitive biological IN components.

Ice accretion by spraying supercooled droplets is not dependent on wettability and surface free energy of substrates

Three different characterization tests namely, ice adhesion strength, drop freezing time and a newly developed ice accretion method were applied to surfaces which were commonly used in anti-icing research such as aluminum, stainless steel, copper, styrene butadiene rubber, polypropylene and polytetrafluoroethylene in order to investigate the effect of their water wettability and surface free energies on their anti-icing performances. It was found that ice adhesion strengths increased and drop freezing times decreased with the increase of the total surface free energies and decrease of the water contact angles on these surfaces similar to previously published reports however, no direct relationship was found between the ice accretion results and solid wettability, surface free energies and the reasons were discussed in terms of heterogeneous nucleation mechanisms.

Convective transport and scavenging of peroxides by thunderstorms observed over the central U.S. during DC3

AbstractOne of the objectives of the Deep Convective Clouds and Chemistry (DC3) field experiment was to determine the scavenging of soluble trace gases by thunderstorms. We present an analysis of scavenging of hydrogen peroxide (H2O2) and methyl hydrogen peroxide (CH3OOH) from six DC3 cases that occurred in Oklahoma and northeast Colorado. Estimates of H2O2 scavenging efficiencies are comparable to previous studies ranging from 79 to 97% with relative uncertainties of 5–25%. CH3OOH scavenging efficiencies ranged from 12 to 84% with relative uncertainties of 18–558%. The wide range of CH3OOH scavenging efficiencies is surprising, as previous studies suggested that CH3OOH scavenging efficiencies would be <10%. Cloud chemistry model simulations of one DC3 storm produced CH3OOH scavenging efficiencies of 26–61% depending on the ice retention factor of CH3OOH during cloud drop freezing, suggesting ice physics impacts CH3OOH scavenging. The highest CH3OOH scavenging efficiencies occurred in two severe thunderstorms, but there is no obvious correlation between the CH3OOH scavenging efficiency and the storm thermodynamic environment. We found a moderate correlation between the estimated entrainment rates and CH3OOH scavenging efficiencies. Changes in gas‐phase chemistry due to lightning production of nitric oxide and aqueous‐phase chemistry have little effect on CH3OOH scavenging efficiencies. To determine why CH3OOH can be substantially removed from storms, future studies should examine effects of entrainment rate, retention of CH3OOH in frozen cloud particles during drop freezing, and lightning‐NOx production.

A ZDR Column Detection Algorithm to Examine Convective Storm Updrafts

AbstractObservations and recent high-resolution numerical model simulations indicate that liquid water and partially frozen hydrometeors can be lofted considerably above the environmental 0°C level in the updrafts of convective storms owing to the warm thermal perturbation from latent heating within the updraft and to the noninstantaneous nature of drop freezing. Consequently, upward extensions of positive differential reflectivity (i.e., ZDR ≥ 1 dB)—called ZDR columns—may be a useful proxy for detecting the initiation of new convective storms and examining the evolution of convective storm updrafts. High-resolution numerical simulations with spectral bin microphysics and a polarimetric forward operator reveal a strong spatial association between updrafts and ZDR columns and show the utility of examining the structure and evolution of ZDR columns for assessing updraft evolution. This paper introduces an automated ZDR column algorithm designed to provide additional diagnostic and prognostic information pertinent to convective storm nowcasting. Although suboptimal vertical resolution above the 0°C level and limitations imposed by commonly used scanning strategies in the operational WSR-88D network can complicate ZDR column detection, examples provided herein show that the algorithm can provide operational and research-focused meteorologists with valuable information about the evolution of convective storms.

New particle-dependent parameterizations of heterogeneous freezing processes: sensitivity studies of convective clouds with an air parcel model

Abstract. Based on the outcome of laboratory results, new particle-dependent parameterizations of heterogeneous freezing were derived and used to improve and extend a two-dimensional spectral microphysics scheme. They include (1) a particle-type-dependent parameterization of immersion freezing using the numbers of active sites per mass, (2) a particle-type and size-resolved parameterization of contact freezing, and (3) a particle-type-dependent description of deposition freezing. The modified microphysical scheme was embedded in an adiabatic air parcel model with entrainment. Sensitivity studies were performed to simulate convective situations and to investigate the impact of ice nuclei concentrations and types on ice formation. As a central diagnostic parameter, the ice water fraction (IWF) was selected, which is the relation of the ice water content to the total amount of water in the condensed form. The following parameters were varied: initial aerosol particle number size distributions, types of ice nucleating particles, final temperature, and the fractions of potential ice nucleating particles. Single and coupled freezing processes were investigated. The results show that immersion freezing seems to be the most efficient process. Contact freezing is constrained by the collision kernel between supercooled drops and potential ice nucleating particles. The importance of deposition freezing lies in secondary ice formation; i.e., small ice particles produced by deposition nucleation trigger the freezing of supercooled drops by collisions. Thus, a broader ice particle spectrum is generated than that by immersion and contact freezing. During coupled immersion–contact and contact–deposition freezing no competition was observed, and both processes contribute to cloud ice formation but do not impede each other. As already suggested in the literature, mineral dust particles seem to be the most important ice nucleating particles. Biological particles are probably not involved in significant ice formation. The sensitive parameters affecting cloud properties are temperature, aerosol particle composition and concentration, and particle size distribution.

High throughput cryopreservation of cells by rapid freezing of sub-μl drops using inkjet printing--cryoprinting.

We have successfully used inkjet printing to cryopreserve 3T3 mouse fibroblast cells and human neuroprogenitor cells (NPCs) derived from human embryonic stem cells (hESCs). Sessile drops of volume 114 nl were formed by printing cell suspensions containing dimethyl sulphoxide (DMSO) as a cryoprotection agent (CPA) at rates in the region 100 Hz-20 kHz, from individual droplets of 380 pl. After printing and a freeze/thaw cycle (with a minimum 24 hours hold period at liquid N2 temperature), 3T3 cells showed an average viability of >90% with CPA concentration <0.8 M at all drop deposition rates. This is a significantly lower CPA concentration than normally used with conventional cryopreservation methods. Cell viability shows a small variation with the polymer substrates used, with the best results obtained using a polyimide substrate. The viability of 3T3 cells after 2 months storage at liquid nitrogen temperature was slightly reduced compared to the cells held for 24 hours but there was no significant further deterioration after 4 or 6 months storage. The viability of NPCs after an identical freeze/thaw cycle were only 55% but this is comparable with conventional cryopreservation methods that use much higher CPA concentrations. A parallel series of experiments printing cells onto substrates held at 195 K or directly into liquid N2 showed considerable variation in cell survival rate with drop deposition rate. Cell suspensions required higher levels of CPA than when printing followed by freezing. At low deposition rates a combination of DMSO and polyethylene glycol (PEG) was needed to allow cell viability after freezing. These results show that inkjet printing provides a practical high throughput method for the cryopreservation of cells with lower CPA concentrations than are required for current low volume cryopreservation methods.

Effect of the accretion by water drops on the melting of snowflakes

Melting of solid hydrometeors plays an important role in the formation of precipitation. A two-moment bin microphysics scheme was developed to simulate the melting process corresponding with laboratory observations. The following physical processes are taken into consideration: collision—coalescence between the solid and liquid precipitation elements; melting; condensation/deposition; freezing of supercooled drops and re-freezing of partly melted ice particles. Completely melted particles transfer to the water drop category and shedding occurs only when the diameter of the melted solid hydrometeors exceeds 0.8cm. Numerical experiments were made by using a one-dimensional kinematic model to investigate how the accretion by water drops affects melting.Results of the numerical experiments show that an increase of the mass of snowflakes and change of their shape due to riming significantly increase the depth of melting layer. In the presence of water drops within a melting layer, its depth is controlled by two competing processes: increased melted ice particle fall speed acts to increase melting layer depth while vapor depositional growth combined with an increase of liquid fraction due to collision–coalescence growth decreases the depth of the melting layer. If the liquid water is relatively small (0.1gkg−1 or less), the effects of collision are almost negligible within the melting layer. If the liquid water content is relatively large (about 1gkg−1), roughly half of the liquid water content on the surface of the snowflakes forms due to the collision coalescence process, the other half due to melting.An earlier published relation between melting fraction and the mass of snowflake (Szyrmer and Zawadzki (1999) was investigated but our results showed that the melting fraction hardly depends on the mass of the snowflakes in a relatively wide size range. With relatively modest to high amounts of water present, our results show an even larger spread and discrepancy compared to the previous relationship that used spherical rather than oblate spheroid shaped snow. Finally, precipitation formation of a real case with partial snow melting and particle re-freezing was simulated. The type of the surface precipitation produced by the model agreed well with surface observations, especially compared to a similar bulk microphysics scheme that produced far more freezing rain that contradicted the surface observations.

Understanding the anti-icing behavior of superhydrophobic surfaces

Superhydrophobic surfaces (SHS) are promising nature-inspired materials in low-temperature-condition applications to resist frost, ice, snow and water drop freezing. However, other observations bring into question the advantage of using SHS. In particular, the water supercooling phenomenon and the lack of characterization methods make it difficult to understand icing phenomena on SHS. Here, the authors bring about clarity by measuring water kinetic freezing points on surfaces of various wettabilites through differential scanning calorimetry technique. It is shown that under relatively dry environment, the freezing delay on cold surfaces is attributed to the water-freezing-point depression, which is determined by the combined effect of surface chemistry and roughness. In addition, drop-freezing tests using a thermoelectric cooler under humid atmosphere showed that the freezing delay is dependent on the frosting, as well. Frost crystals growing on the cold surface will act as nucleus initiating drop freezing. The results of this study provide new insights into the mechanism responsible for the drop-freezing delay on SHS. This article contains supporting information that is available online.

Effect of Wettability on Sessile Drop Freezing: When Superhydrophobicity Stimulates an Extreme Freezing Delay

An increasing number of studies directed at supercooling water droplets on surfaces with different wettabilities have appeared in recent years. This activity has been stimulated by the recognition that water supercooling phenomena can be effectively used to develop methods for protecting outdoor equipment and infrastructure elements against icing and snow accretion. In this article, we discuss the nucleation kinetics of supercooled sessile water droplets on hydrophilic, hydrophobic, and superhydrophobic surfaces under isothermal conditions at temperatures of -8, -10, and -15 °C and a saturated water vapor atmosphere. The statistics of nucleation events for the ensembles of freezing sessile droplets is completed by the detailed analysis of the contact angle temperature dependence and freezing of individual droplets in a saturated vapor atmosphere. We have demonstrated that the most essential freezing delay is characteristic of the superhydrophobic coating on aluminum, with the texture resistant to contact with ice and water. This delay can reach many hours at T = -8 °C and a few minutes at -23 °C. The observed behavior is analyzed on the basis of different nucleation mechanisms. The dissimilarity in the total nucleation rate, detected for two superhydrophobic substrates having the same apparent contact angle of the water drop but different resistivities of surface texture to the contact with water/ice, is associated with the contribution of heterogeneous nucleation on external centers located at the water droplet/air interface.

A Dual-Polarization Radar Signature of Hydrometeor Refreezing in Winter Storms

AbstractPolarimetric radar measurements in winter storms that produce ice pellets have revealed a unique signature that is indicative of ongoing hydrometeor refreezing. This refreezing signature is observed within the low-level subfreezing air as an enhancement of differential reflectivity ZDR and specific differential phase KDP and a decrease of radar reflectivity factor at horizontal polarization ZH and copolar correlation coefficient ρhv. It is distinct from the overlying melting-layer “brightband” signature and suggests that unique microphysical processes are occurring within the layer of hydrometeor refreezing. The signature is analyzed for four ice-pellet cases in central Oklahoma as observed by two polarimetric radars. A statistical analysis is performed on the characteristics of the refreezing signature for a case of particularly long duration. Several hypotheses are presented to explain the appearance of the signature, along with a summary of the pros and cons for each. It is suggested that preferential freezing of small drops and local ice generation are plausible mechanisms for the appearance of the ZDR and KDP enhancements. Polarimetric measurements and scattering calculations are used to retrieve microphysical information to explore the validity of the hypotheses. The persistence and repetitiveness of the signature suggest its potential use in operational settings to diagnose the transition between freezing rain and ice pellets.

Solution coating around ice particles of incipient cirrus clouds

Kuhs et al. (1) report on the nanometer-scale roughness on the surface of laboratory vapor-deposited cubic ice, Ic, and state that such roughness on the surface of cirrus Ic ice particles can influence the surface reactivity, radiative properties, and water vapor uptake coefficient of incipient cirrus clouds. This assumes that the ice surface of cirrus particles is exposed to air. However, this is not the case when cirrus particles are formed by freezing aqueous aerosol drops. Freezing of aqueous drops with ∼20–40 wt% of solute is the mechanism discussed in literature studies mentioned by Kuhs et al., i.e., their refs. 20, 55, 58, and 59.