Elsevier

Current Applied Physics

Volume 31, November 2021, Pages 74-86
Current Applied Physics

On the dynamics of contact line freezing of water droplets on superhydrophobic carbon soot coatings

https://doi.org/10.1016/j.cap.2021.07.015Get rights and content

Highlights

  • The soot delays the freezing time by 188 s and downshifts the freezing point by 7–10 °C.

  • The soot particle size, roughness and degree of non-wettability do not affect its icephobicity.

  • The porosity, thickness and chemical bonding of the soot control the freezing process.

  • A new hybrid “contour” freezing mode is detected during the freezing temperature assays.

Abstract

Despite the opportunity to manipulate the water freezing via superhydrophobic materials, their commercial use for passive icing protection is still questioned, since the combined effect of surface morphology, air cushion arrangement, roughness, chemistry and film thickness on the icephobic properties of a given non-wettable solid remains unexplored. This article addresses the existing research gaps by studying the ice nucleation dynamics at the contact line of various superhydrophobic soot-based surfaces, potentially applicable in cryobiology for enhancing the existing cryopreservation technologies. We examine the freezing time and freezing temperature of water droplets settled on three groups of soot coatings with divergent morphochemical features, adjusted by modifying the samples with alcohol, fluorocarbon and/or silver hydrogen fluoride. Our results demonstrate the appearance of a new “contour” freezing mode, where the droplet shell crystallizes simultaneously with the contact interface, whilst the soot's chemical bonds along with some of its physical characteristics govern the ice formation.

Introduction

The water's vapor-liquid-solid phase transitions are ubiquitous in nature and likely many of us have observed freshly budding dew on the village houses' windows or beautifully-shaped icicles pendant from the city gutters. However, the water crystallization to ice/frost causes performance issues to a wide range of facilities such as telecommunication towers, automobile roads, aircraft, high-voltage power lines, wind turbines, heat exchanging devices, power and chemical plants, solar panels and off-shore oil platforms [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. Moreover, the intracellular icing upon cryopreservation of cells, tissues and organs is recognized as one of the major drawbacks for the successful long-term storage of living matter [[11], [12], [13]]. For instance, the intracellular freezing alters the osmotic equilibrium with the already frozen extracellular environment, leading to osmotic shocks, cell membrane rupture and lethal end [13].

Since the beginning of 21st century, the vital interest in the fabrication of liquid-repellent materials has paved the path to the establishment of “passive” anti-icing systems employing the virtue of hierarchically roughened and chemically non-polar surfaces (coatings), known in the scientific literature as superhydrophobic [[14], [15], [16], [17], [18]]. The superhydrophobicity renders quasi-frictionless character of any surface-of-interest [19,20], and if freezing rain occurs, the kinetic energy of the supercooled water droplets transforms into surface energy (and vice versa) with negligible losses, triggering out-of-plane repulsive forces and limiting the liquid-solid contact time to a few milliseconds [21]. Hence, the heat transfer at the solid-liquid interface is reduced and the droplets bounce-off the surface prior to freezing [19]. The extreme water-repellency is beneficial likewise in delaying the freezing time and lowering the freezing temperature of static droplets adhering to a substrate due to the small surface fraction in contact with the liquid, minimizing the heat transfer rate [[22], [23], [24]]. Additionally, the presence of a complex air-liquid-solid interfacial area supports spherical shape of the unavoidably freezing sessile droplets and creates mechanical stresses that disrupt the ice bonds, leading to ~80–90% lower thermal energy required to melt the ice [25,26]. Finally, if the icing takes place via water vapor condensation, the nascency of ice embryos and the interdroplet frosting is hampered by tailoring the chemistry and topography of the superhydrophobic materials [[27], [28], [29]].

On the other hand, the specific environmental and operating conditions are unsurmountable factors governing the ice formation and thus, the design of genuinely icephobic surfaces is highly controversial [[14], [15], [16], [17],[30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. The scientific argumentation behind such a statement includes solid evidences that if the size of the particles composing the surface-of-interest exceeds a critical threshold value, the water repellency persists, but the icing probability increases exponentially [42]. It is demonstrated also that the ice adhesion depends on the configuration of the interfacial cracks [33,34], the surface topography/roughness [32,[43], [44], [45]] and the contact angle hysteresis [30,46]. In particular, the energy barrier for heterogeneous nucleation decreases and accelerates the water-ice transitions, when the surface roughness curvature (the concave pits that are a byproduct of the roughening) is larger than the critical nucleus' radius rc [41]. This clearly explains why a surface with а nanometer-scale roughness, commensurable or smaller than rc, provides nearly one order of magnitude longer freezing time compared to a counterpart with roughness values greater than rc [32]. Moreover, the high ambient humidity at negative temperatures may trigger condensation frosting along the surface protrusions in a random manner [41], hence, enhancing the ice adhesion forces and compromising the potential on-ground and in-flight applications of the non-wettable coatings [14,40,47]. Furthermore, it is revealed that the condensation rate on some superhydrophobic surfaces is boosted due to the existence of a large surface area or mechanical interlocking of the ice/frost within the micro-nanostructures [38], so the growing sub-cooled condensates are held in the “sticky” Wenzel state [16], preventing their complete removal by external forces and even destroying the roughened surface profile [14]. It is pertinent to mention that if the supercooled droplets' impact velocity during freezing rain events is high enough, the water could remove the texture-trapped air and imbibe into the rough structure [48]. Lastly, the protective film's thickness and the air plastron's convection are found to strongly interfere with the anti-icing performance of the water-repellent coatings by determining the heat transfer-mediated freezing delay [49,50], which has important implications in defining the probability of condensate halos formation too – a process regulated by the substrate's thermal conductivity [41] (presumably, in a composite substrate-coating-droplet system, the film's thermal resistance would influence the icing).

In contrast, recent reports hint at a direct correlation among the water and ice repellency [22,51,52], and apparently, any hydrophobic coating (despite the surface parameters) yields better icephobic properties in an outdoor (real-life) setting compared to a hydrophilic counterpart [53,54]. Besides, the creation of ultrafine roughnesses neutralizes the ice nucleation-promoting effect of the concave pits [55] and on superhydrophobic surfaces with low wetting hysteresis, the ice adhesion strength is up to 5.7 times lower than on a bare polished hydrophilic substrate [56]. Another relevant opinion expressed by Boreyko et al. is that the highly supersaturated conditions used for examining the condensation frosting in laboratory studies are somewhat extreme and normally, the air and surface temperatures are identical, and the humidity can fluctuate widely [28]. Logically, the impact of ambient water vapor on the anti-icing properties of a particular non-wettable coating should be less detrimental in the reality, but even at extremely cold and humid weather, the undesired frosting is successfully halted using out-of-plane dry zones [57]. Moreover, Esmeryan et al. show in pioneering research that the superhydrophobic carbon soot can control the freezing/thawing rates of human seminal fluids, thus, opening a fundamentally new page in the future development of cryopreservation technologies [58]. Namely, and as thoroughly explained by prof. David Pegg, if the cooling is sufficiently slow (naturally achievable with superhydrophobic contact interfaces [[22], [23], [24]]), the liquid water will leave the cells as the temperature falls, due to the hydraulic conductivity and the surface-to-volume ratio of the cells, inducing only innocuous extracellular icing [13]. “Perhaps the point has been made and the need now is for new methods that completely avoid the ice formation in multicellular organized systems” [13], a task that seems to be feasible using non-wettable soot [58]. Although the candle and rapeseed oil-generated soot coatings are biocompatible [59,60] and possess attractive icephobic features [54,58,[61], [62], [63], [64], [65], [66]], their practical applicability, especially in cryobiology, necessitates additional knowledge about the mechanisms of ice incipiency on these materials, lacking at the moment in the scientific literature.

This article aims to disseminate novel findings regarding the freezing time delay and freezing temperature depression of sessile water droplets residing on superhydrophobic carbon soot coatings differing by morphology, porosity, surface chemistry, roughness and thickness. The collected experimental results help to establish the pathways of water freezing on soot coated surfaces, while in the meantime elucidating the impact of overall physicochemical profile of the coating on its anti-icing behavior; tasks that have not been accomplished yet, but crucially important for the possible future integration of the icephobic soot in cryobiology and reproductive medicine. In addition, the newly gained information proofs the concept that at certain conditions, the freezing is initiated at the water droplet's outer shell and the ice-liquid phase boundary moves inwards in the slurry bulk, a phenomenon described by Wildeman et al. [67]. and Graeber et al. [68], and later adopted by Esmeryan et al. in explaining the efficient conservation of human semen via soot [58].

Section snippets

Materials

Short alkyl chain alcohols (99 wt % concentration) and fluorocarbon suspension (Grangers Performance Proofer) were purchased from Valerus Ltd., Bulgaria and Grangers International Ltd., UK. Crystalline AgHF2 was received from Alfa Aesar, Karlsruhe, Germany, while Zoya Organic & Natural, Bulgaria delivered the rapeseed oil. A commercially available plastic atomizer and microscope glass slides were supplied by “Shop for homemade cosmetics”, Plovdiv Bulgaria and Medical Technics Engineering, Sofia

Structure, morphology and thickness

Fig. 1, Fig. 2 illustrate the main particularities of the soot coatings in terms of structure, morphology and porosity.

In compliance with our previous research [58,61,62,66,69,70], the oxygen-deficient laminar diffusion flames promote the nascency of branched quasisquare soot aggregates without clearly identifiable nanoparticles, connected into large agglomerates with dimensions above 200 nm. The oxygen-rich flames produce quasispherical soot particles with an average size of 50 nm, forming

Conclusions

A video capturing of the freezing time and crystallization point of microliter water droplets, standing on a set of eight soot coatings, showed that the particle/aggregate size, surface roughness and degree of non-wettability did not affect the anti-icing efficiency of the soot. Undoubtedly, the air cushion and the film thickness managed the inception of ice embryos, but the most significant parameter in this regard was the chemical bonding of the soot, which tailored the interfacial thermal

Author contributions

Dr. Esmeryan conceived the idea to study the freezing time delay and freezing temperature depression of sessile water droplets residing on superhydrophobic carbon soot coatings with divergent surface profile. He planned, designed and implemented the experiments, except the surface characterization. Dr. Esmeryan processed and interpreted the data, contrived the scientific concept of the article and wrote its first and final versions. Dr. Castano conducted the SEM, XPS and AFM analyses, and

Declaration of competing interest

The authors declare that they have not known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was performed thanks to the financial support of Bulgarian National Science Fund under grant № KP-06-Н37/7/06.12.2019. The efforts of Mr. Yulian Fedchenko (Metal Vapor Lasers Laboratory at ISSP-BAS) dedicated to drawing the graphical abstract are much appreciated. Dr. Castano thanks Dr. Diana Galeano-Osorio and Mr. Santiago Vargas for helping with the SEM and XPS analyses. All authors acknowledge the VCU's Nanomaterials Core Characterization Facility for ensuring access to the

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