Comparative study on immersion freezing utilizing single-droplet levitation methods

Abstract

Abstract. Immersion freezing experiments were performed utilizing two distinct single-droplet levitation methods. In the Mainz vertical wind tunnel, supercooled droplets of 700 µm diameter were freely floated in a vertical airstream at constant temperatures ranging from −5 to −30 ∘C, where heterogeneous freezing takes place. These investigations under isothermal conditions allow the application of the stochastic approach to analyze and interpret the results in terms of the freezing or nucleation rate. In the Mainz acoustic levitator, 2 mm diameter drops were levitated while their temperature was continuously cooling from +20 to −28 ∘C by adapting to the ambient temperature. Therefore, in this case the singular approach was used for analysis. From the experiments, the densities of ice nucleation active sites were obtained as a function of temperature. The direct comparison of the results from two different instruments indicates a shift in the mean freezing temperatures of the investigated drops towards lower values that was material-dependent. As ice-nucleating particles, seven materials were investigated; two representatives of biological species (fibrous and microcrystalline cellulose), four mineral dusts (feldspar, illite NX, montmorillonite, and kaolinite), and natural Sahara dust. Based on detailed analysis of our results we determined a material-dependent parameter for calculating the freezing-temperature shift due to a change in cooling rate for each investigated particle type. The analysis allowed further classification of the investigated materials to be described by a single- or a multiple-component approach. From our experiences during the present synergetic studies, we listed a number of suggestions for future experiments regarding cooling rates, determination of the drop temperature, purity of the water used to produce the drops, and characterization of the ice-nucleating material. The observed freezing-temperature shift is significantly important for the intercomparison of ice nucleation instruments with different cooling rates.

TinyLev acoustically levitated water: Direct observation of collective, inter-droplet effects through morphological and thermal analysis of multiple droplets

“…28 The oblate spheroidal shape, however, is characteristic of natural atmospheric droplet shapes, specifically of freely falling raindrops. 5,15,37 Moreover, as the droplet is surrounded solely by air, the conduction of the latent heat released from the drop during freezing is also similar to atmospheric conditions. 5 The average initial temperatures of the droplets, around 17.5 °C, are lower than the room temperature of 21.5 °C and decrease moving downward: from 17.94 ± 0.49…”

“…Levitational systems often have weaker airflows compared to free-fall levitation techniques so as not to destabilize the droplet. 5 The airflow created in the system used for this study is at the velocity limit: occasionally droplets were destabilized by the strength of this airflow and the experiment was restarted. Images of the droplets were taken with a Canon EOS 60D DSLR camera (18.0megapixel CMOS sensor) equipped with an MP-E 65 mm f/2.8 1-5x macro lens.…”

“…Therefore, particles with greater surface areas have the potential for more active sites for ice nucleation, which means the probability of freezing is proportional to particle surface area. 5 In fact, dendritic freezing of supercooled droplets is estimated to have a greater than 90% probability of occurring on the surface. 16 Therefore, the coldest droplet with the highest surface area (position 3) should see the fastest nucleation times.…”

“…This includes the formation of tropospheric ice particles as precursors to rain or as part of the cloud microstructure, as both can affect the wear on exposed parts of aircraft. [1][2][3][4][5] Unlike heterogeneous nucleation, where initial dendritic growth will occur at the solidliquid interface from which freezing then proceeds [6][7][8][9][10] , the homogeneous nucleation of atmospheric water begins by creating a shell of ice dendrites, usually over the entire air-liquid interface, before freezing towards the droplet center. 1,[11][12][13][14][15][16] Figure 1 shows the different stages of droplet cooling in the context of changes in surface temperature over time.…”

“…21 Using these types of levitation, single-droplet studies have been conducted for water, including examinations of nucleation and freezing 3,14,16,20 , structure and surface tension 22,23 , evaporation 24,25 , and gas-liquid interfacial flow 26 . Moreover, these studies have moved beyond the physicochemical properties and crystallization dynamics of water and examine immersion freezing techniques 5 , multi-component analyses [27][28][29] , gas hydrate formation 30,31 , and new tools and analytical methods such as trace analysis and breakdown spectroscopy. [32][33][34] Building upon this extensive base of research, a logical next step is to examine multiple levitated water droplets simultaneously, as lone droplets rarely exist in nature.…”