Ice Nucleation In Water Droplets Research Articles

Studying the Bulk and Contour Ice Nucleation of Water Droplets via Quartz Crystal Microbalances.

Due to the stochastic and time-dependent character of the ice embryo formation and growth (i.e., a process that can be analyzed statistically, but cannot be predicted precisely), the heterogeneous ice nucleation on atmospheric aerosols or macroscopic solid surfaces is still shrouded in mystery, regardless of the extremely active research and exponential progress within this scientific field. For instance, whether the icing appears from outside-in or inside-out is a subject of intense controversy, with practicability in designing passive icephobic coatings or improving the effectiveness of the cryopreservation technologies. Here, we propose an artful technique for quantitative analysis of the different modes of water freezing using super-nonwettable soot-coated quartz crystal microbalances (QCMs). To achieve this goal, a set of 5 MHz QCMs are loaded one at a time with a 50 μL droplet, whose bulk or contour solidification is detected in real-time. The obtained experimental results show that our sensor devices recognize explicitly if the ice nuclei form predominantly at the liquid–solid interface or spread along the droplet’s entire outer shell by triggering individual reproducible responses in terms of the direction of signal evolution in time. Our results may serve as a foundation for the future incorporation of QCM devices in different freezing assays, where gaining information about the ice adhesion forces and ice layer’s thickness is mandatory.

Ice nucleation in high alternating electric fields: Effect of electric field strength and frequency.

Icing is a severe problem for many technical systems such as aircraft or systems for high-voltage power transmission and distribution. Ice nucleation in water droplets is affected by several influencing factors like impurities or the liquid temperature, which have been widely investigated. However, although an electric field affects nucleation, this influence has been far less investigated and is still not completely understood. The present work is focused on the influence of high alternating electric fields on ice nucleation in sessile water droplets, which is examined for a systematic variation of the electric field frequency and strength. All experiments used to determine the influence of a single parameter like the electric field strength or frequency are performed with the same set of droplets to ensure well-defined conditions and a high repeatability of the procedure. For each parameter variation a large number of nucleation events is observed and analyzed. Droplet survival curves and the nucleation site density are used to analyze the experiments and to determine the influence of the electric field on ice nucleation. Especially for high electric field strengths, a significant influence on nucleation is observed. Some droplets freeze earlier, which leads to a higher median nucleation temperature. On the other hand, the lowest temperature required to freeze all droplets is almost constant compared to the reference case without an electric field. It is shown that not all droplets are affected by the electric field in the same way, but the influence of the electric field on ice nucleation is rather of singular nature. In addition, the frequency of the applied electric field has an impact on the nucleation behavior. The present experimental data quantitatively demonstrate the effect of an electric field on ice nucleation and improves our understanding of heterogeneous nucleation of supercooled water subjected to high alternating electric fields.

Probing the critical nucleus size for ice formation with graphene oxide nanosheets.

Water freezing is ubiquitous and affects areas as diverse as climate, the chemical industry, cryobiology and materials science. Ice nucleation is the controlling step in water freezing1-5 and has, for nearly a century, been assumed to require the formation of a critical ice nucleus6-10. But there has been no direct experimental evidence for the existence of such a nucleus, owing to its transient and nanoscale nature6,7. Here we report ice nucleation in water droplets containing graphene oxide nanosheets of controlled sizes and show that they have a notable impact on ice nucleation only above a certain size that varies with the degree of supercooling of the droplets. We infer from our experimental data and theoretical calculations that the critical size of the graphene oxide reflects the size of the critical ice nucleus, which in the case of sufficiently large graphene oxides sits on their surface and gives rise to ice formation behaviour consistent with classical nucleation theory. By contrast, when the graphene oxide size is smaller than that of the critical ice nucleus, pinning at the periphery of the graphene oxide deforms the ice nucleus as it grows. This gives rise to a much higher free-energy barrier for nucleation and suppresses the promoting effect of the graphene oxide11. The results provide experimental information on the existence and temperature-dependent size of the critical ice nucleus, which has previously only been explored theoretically and through simulations12-16. As pinning of a pre-critical nucleus at a nanoparticle edge is not specific to the ice nucleus on graphene oxides, we expect that our approach could be extended to probe the critical nuclei in other nucleation processes.

Random distribution active site model for ice nucleation in water droplets

Analytical solutions and Monte Carlo simulation agree well with experimental ice nucleation temperature distributions for water droplets.

Role of the electric double layer in the ice nucleation of water droplets under an electric field

Figuring out the mechanism of ice nucleation on charged aerosols or in thunderstorms is of fundamental importance in atmospheric science. However, findings on whether the electric field promotes or suppresses heterogeneous ice nucleation are conflicting. In this work, we design an apparatus and test the influence of the electric field on ice nucleation by freezing a series of deionized water droplets resting on solid surfaces with an electric field perpendicular to the substrates. Results show that ice nucleation is obviously promoted under the electric field and is independent of the field direction. Theoretic analyses show that the promotion is due to the reduction of Gibbs free energy which can be partially rationalized by the electric field sustained in the electric double layer at the solid-water interface, with strength about two orders higher than that of the external electric field. Moreover, water-droplet deformation under the electric field is not expected to be the cause of the ice-nucleation promotion.

Sensitivity of liquid clouds to homogenous freezing parameterizations.

Water droplets in some clouds can supercool to temperatures where homogeneous ice nucleation becomes the dominant freezing mechanism. In many cloud resolving and mesoscale models, it is assumed that homogeneous ice nucleation in water droplets only occurs below some threshold temperature typically set at −40°C. However, laboratory measurements show that there is a finite rate of nucleation at warmer temperatures. In this study we use a parcel model with detailed microphysics to show that cloud properties can be sensitive to homogeneous ice nucleation as warm as −30°C. Thus, homogeneous ice nucleation may be more important for cloud development, precipitation rates, and key cloud radiative parameters than is often assumed. Furthermore, we show that cloud development is particularly sensitive to the temperature dependence of the nucleation rate. In order to better constrain the parameterization of homogeneous ice nucleation laboratory measurements are needed at both high (>−35°C) and low (<−38°C) temperatures.Key PointsHomogeneous freezing may be significant as warm as −30°CHomogeneous freezing should not be represented by a threshold approximationThere is a need for an improved parameterization of homogeneous ice nucleation

BINARY: an optical freezing array for assessing temperature and time dependence of heterogeneous ice nucleation

Abstract. A new optical freezing array for the study of heterogeneous ice nucleation in microliter-sized droplets is introduced, tested and applied to the study of immersion freezing in aqueous Snomax® suspensions. In the Bielefeld Ice Nucleation ARraY (BINARY) ice nucleation can be studied simultaneously in 36 droplets at temperatures down to −40 °C (233 K) and at cooling rates between 0.1 and 10 K min−1. The droplets are separated from each other in individual compartments, thus preventing a Wegener–Bergeron–Findeisen type water vapor transfer between droplets as well as avoiding the seeding of neighboring droplets by formation and surface growth of frost halos. Analysis of freezing and melting occurs via an automated real-time image analysis of the optical brightness of each individual droplet. As an application ice nucleation in water droplets containing Snomax® at concentrations from 1 ng mL−1 to 1 mg mL−1 was investigated. Using different cooling rates, a small time dependence of ice nucleation induced by two different classes of ice nucleators (INs) contained in Snomax® was detected and the corresponding heterogeneous ice nucleation rate coefficient was quantified. The observed time dependence is smaller than those of other types of INs reported in the literature, suggesting that the BINARY setup is suitable for quantifying time dependence for most other INs of atmospheric interest, making it a useful tool for future investigations.

Ice nucleation in water droplets on glass surfaces: From micro- to macro-scale

Ice nucleation encountered in engineering systems is often induced by solid/water interfaces. When classical nucleation theory is used to analyze ice nucleation in such systems, the uniformity of interfaces that contribute to ice nucleation must be carefully considered, because classical nucleation theory cannot be directly applied to non-uniform interfaces. In this study, to discuss the uniformity of ice nucleating activity of solid/water interfaces, ice nucleation in water droplets prepared on glass surfaces was investigated for various droplet sizes from micrometer to sub-millimeter. When the interfacial area between water and the glass surface was smaller than 1 × 10−10 m2, the ice nucleation temperature showed scatter of about 2 °C, suggesting uniformity of the interface. However, when the interfacial area was larger than 1 × 10−8 m2, the ice nucleation temperature showed large scatter, suggesting the ice nucleating activity was no longer uniform.

D214 固体・水界面での氷の核生成に対する接触面積の影響(OS-11:凝固・融解を伴う伝熱現象とその応用(4))

To discuss the uniformity of ice nucleating activity of solid/water interfaces, ice nucleation in water droplets prepared on glass surfaces was investigated for various droplet sizes from micrometer to sub-millimeter. When the interfacial area between water and the glass surface was larger than 1×10^<-8> m^2, the ice nucleation temperature showed large scatter, suggesting that the ice nucleating activity was no longer uniform. When the classical nucleation theory (CNT) is used to analyze surface-induced ice nucleation, the uniformity of interfaces that contribute to ice nucleation must be carefully considered, because CNT cannot be directly applied to non-uniform interfaces.

Immersion mode heterogeneous ice nucleation by an illite rich powder representative of atmospheric mineral dust

Abstract. Atmospheric dust rich in illite is transported globally from arid regions and impacts cloud properties through the nucleation of ice. We present measurements of ice nucleation in water droplets containing known quantities of an illite rich powder under atmospherically relevant conditions. The illite rich powder used here, NX illite, has a similar mineralogical composition to atmospheric mineral dust sampled in remote locations, i.e. dust which has been subject to long range transport, cloud processing and sedimentation. Arizona Test Dust, which is used in other ice nucleation studies as a model atmospheric dust, has a significantly different mineralogical composition and we suggest that NX illite is a better surrogate of natural atmospheric dust. Using optical microscopy, heterogeneous nucleation in the immersion mode by NX illite was observed to occur dominantly between 246 K and the homogeneous freezing limit. In general, higher freezing temperatures were observed when larger surface areas of NX illite were present within the drops. Homogenous nucleation was observed to occur in droplets containing low surface areas of NX illite. We show that NX illite exhibits strong particle to particle variability in terms of ice nucleating ability, with ~1 in 105 particles dominating ice nucleation when high surface areas were present. In fact, this work suggests that the bulk of atmospheric mineral dust particles may be less efficient at nucleating ice than assumed in current model parameterisations. For droplets containing ≤2 × 10−6 cm2 of NX illite, freezing temperatures did not noticeably change when the cooling rate was varied by an order of magnitude. The data obtained during cooling experiments (surface area ≤2 × 10−6 cm2) is shown to be inconsistent with the single component stochastic model, but is well described by the singular model (ns(236.2 K ≤ T ≤ 247.5 K) = exp(6.53043 × 104− 8.2153088 × 102T + 3.446885376T2 − 4.822268 × 10−3T3). However, droplets continued to freeze when the temperature was held constant, which is inconsistent with the time independent singular model. We show that this apparent discrepancy can be resolved using a multiple component stochastic model in which it is assumed that there are many types of nucleation sites, each with a unique temperature dependent nucleation coefficient. Cooling rate independence can be achieved with this time dependent model if the nucleation rate coefficients increase very rapidly with decreasing temperature, thus reconciling our measurement of nucleation at constant temperature with the cooling rate independence.

Homogeneous Freezing of Water Starts in the Subsurface

Molecular dynamics simulations of homogeneous ice nucleation in extended aqueous slabs show that freezing preferentially starts in the subsurface. The top surface layer remains disordered during the freezing process. The subsurface accommodates better than the bulk the increase of volume connected with freezing. It also experiences strong electric fields caused by oriented surface water molecules, which can enhance ice nucleation. Our computational results shed new light on the experimental controversy concerning the bulk vs surface origin of homogeneous ice nucleation in water droplets. This has important atmospheric implications for the microphysics of formation of high altitude clouds.

Atmospheric science in the laboratory: Progress 1991–1994

Laboratory study of atmospheric phenomena provides a controlled, steady environment which is usually difficult to obtain in the turbulent atmosphere. Boundary conditions for cloud formation such as homogeneous ice nucleation of water droplets near −40°C and cloud droplet formation on hygroscopic particles at less than a few percent above water saturation were first appreciated by careful laboratory experiments many years ago. These ideas are foundation‐stones of our understanding of atmospheric clouds. Such ideas came from careful laboratory studies of the behavior of particles under well controlled conditions.