Mechanism of supercooled droplet freezing on surfaces

Abstract

understanding ice formation from supercooled water on surfaces is a problem of fundamental importance and general utility. superhydrophobic surfaces promise to have remarkable 'icephobicity' and low ice adhesion. Here we show that their icephobicity can be rendered ineffective by simple changes in environmental conditions. Through experiments, nucleation theory and heat transfer physics, we establish that humidity and/or the flow of a surrounding gas can fundamentally switch the ice crystallization mechanism, drastically affecting surface icephobicity. Evaporative cooling of the supercooled liquid can engender ice crystallization by homogeneous nucleation at the droplet-free surface as opposed to the expected heterogeneous nucleation at the substrate. The related interplay between droplet roll-off and rapid crystallization is also studied. overall, we bring a novel perspective to icing and icephobicity, unveiling the strong influence of environmental conditions in addition to the accepted effects of the surface conditions and hydrophobicity.

Supercooled Water Droplet Impacting Superhydrophobic Surfaces in the Presence of Cold Air Flow

“…The resulting shear and normal force of the air flow might lead to important changes in wetting dynamics of an impacting water drop. Furthermore, it significantly changes thermal transport properties of air-liquid interface by evaporation cooling [16]. However, quantitative analysis of temporal evolution of spreading diameter, illustrated in Figure 3, shows that the effect of incoming cold air flow has an insignificant effect on wetting dynamics on hydrophilic and hydrophobic surfaces which arise from strong crystallization process at solid-liquid interface combined with sever increase in contact line viscosity up to 8 and 4.3 fold at −30 • C and −20 • C , respectively [41,42].…”

“…As depicted in Figure 7C, for supercooling conditions, the scaling On the other hand, when droplet is under the effect of cold air flow it becomes an even further complicated process due to the presence of another type of nucleation mechanism such as homogeneous nucleation and crystallization induced nucleation from inside-out [52]. Due to the presence of forced convective evaporation cooling, which results in the presence of aforementioned nucleation mechanism, the gas-liquid interface temperature becomes even lower than air temperature, which is a function of air velocity and air humidity [16]. It was observed that, when relative humidity is negligible and the chance of superhydrophobicity degradation is small [53], which is the case in the present study, the effect of evaporation cooling becomes significant and homogeneous nucleation happens at the air-liquid interface.…”

“…Larger solid-liquid interfacial area of impacting drop causes a significant increase in the heat transfer cooling rate [15]. Furthermore, both induced homogeneous nucleation [16] (i.e., due to evaporation cooling) and heterogeneous nucleation (solid-liquid interaction) [17] are related to liquid interface temperature. In fact, in real atmospheric cold conditions, induced homogeneous nucleation can happen even above the critical temperature of homogeneous nucleation (i.e., −37 • C) [18,19] due to the presence of air flow which systematically increases the rate of evaporation cooling at low and high humidity environments [16].…”

“…It was reported that droplet contact time can be reduced by increasing the receding contact angle [25] or by switching recoiling dynamics from an axisymmetric retraction to a nonaxisymmetric one [23] up to about 50% (based on inertia-capillary time scale [26]: ρD 3 o /8σ 1/2 ) through breaking up the droplets and also through the pancake bouncing mechanism [27]. Although the aforementioned techniques were used to reduces droplet adhesion with a surface by minimizing droplet contact time or by shedding the droplet by airflow [16,28], the effect of stagnation air flow on an impacting water drop [29] whether in room or super cooling conditions has not been fully addressed yet. In the present study, the influence of stagnation flow on a spreading water droplet is evaluated through classical Homann flow approach [30].…”

“…Furthermore, both induced homogeneous nucleation [16] (i.e., due to evaporation cooling) and heterogeneous nucleation (solid-liquid interaction) [17] are related to liquid interface temperature. In fact, in real atmospheric cold conditions, induced homogeneous nucleation can happen even above the critical temperature of homogeneous nucleation (i.e., −37 • C) [18,19] due to the presence of air flow which systematically increases the rate of evaporation cooling at low and high humidity environments [16]. Therefore, gas-liquid interface temperature becomes as important as solid-liquid interface temperature (i.e., Gibbs free surface energy is related to interface temperature whether gas-liquid or solid-liquid) when evaluation of both nucleation mechanisms on wetting dynamics of an impacting supercooled water drop are taken into account.…”