Cascade Freezing of Supercooled Water Droplet Collectives

Surface icing affects the safety and performance of numerous processes in technology. Previous studies mostly investigated freezing of individual droplets. The interaction among multiple droplets during freezing is investigated less, especially on nanotextured icephobic surfaces, despite its practical importance as water droplets never appear in isolation, but in groups. Here we show that freezing of a supercooled droplet leads to spontaneous self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We put forth the explanation that, as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble resulting in ice coverage of the nanotextured surface. Although cascade freezing is observed in a low-pressure environment, it introduces an unexpected pathway of freezing propagation that can be crucial for the performance of rationally designed icephobic surfaces

Intrinsic water transport in moisture-capturing hydrogels

Moisture-capturing hydrogels have emerged as attractive sorbent materials capable of converting ambient humidity into liquid water. Recent works have demonstrated exceptional water capture capabilities of hydrogels, while simultaneously, exploring different strategies to accelerate water capture and release. However, on the material level, an understanding of the intrinsic transport properties of moisture-capturing hydrogels is currently missing, which hinders their rational design. In this work, we combine absorption and desorption experiments of macroscopic hydrogel samples in pure-vapor with models of water diffusion in the hydrogels to demonstrate the first measurements of the intrinsic water diffusion coefficient in hydrogel-salt composites. Based on these insights, we pattern hydrogels with micropores to significantly decrease the required absorption and desorption time by 19% and 72%, respectively, while reducing the total water capacity of the hydrogel by only 4%. Thereby, we provide an effective strategy towards hydrogel material optimization, with a particular significance in pure-vapor environments

3D-Printed Surface Architecture Enhancing Superhydrophobicity and Viscous Droplet Repellency

Macro-textured superhydrophobic surfaces can reduce droplet-substrate contact times of impacting water droplets, however, surface designs with similar performance for significantly more viscous liquids are missing, despite their importance in nature and technology such as for chemical shielding, food staining repellency, and supercooled (viscous) water droplet removal in anti-icing applications. Here, we introduce a deterministic, controllable and up-scalable method to fabricate superhydrophobic surfaces with a 3D-printed architecture, combining arrays of alternating surface protrusions and indentations. We show a more than threefold contact time reduction of impacting viscous droplets up to a fluid viscosity of 3.7mPa s, which equals 3.7 times the viscosity of water at room temperature, covering the viscosity of many chemicals and supercooled water. Based on the combined consideration of the fluid flow within and the simultaneous droplet dynamics above the texture, we recommend future pathways to rationally architecture such surfaces, all realizable with the methodology presented here.Comment: ACS Appl. Mater. Interfaces, Article ASAP, Publication Date (Web): November 19, 201

Intrinsic Water Transport in Moisture-Capturing Hydrogels

Moisture-capturing hydrogels have emerged as attractive sorbent materials capable of converting ambient humidity into liquid water. Recent works have demonstrated exceptional water capture capabilities of hydrogels while simultaneously exploring different strategies to accelerate water capture and release. However, on the material level, an understanding of the intrinsic transport properties of moisture-capturing hydrogels is currently missing, which hinders their rational design. In this work, we combine absorption and desorption experiments of macroscopic hydrogel samples in pure vapor with models of water diffusion in the hydrogels to demonstrate the first measurements of the intrinsic water diffusion coefficient in hydrogel–salt composites. Based on these insights, we pattern hydrogels with micropores to significantly decrease the required absorption and desorption times by 19% and 72%, respectively, while reducing the total water capacity of the hydrogel by only 4%. Thereby, we provide an effective strategy toward hydrogel material optimization, with a particular significance in pure-vapor environments.Schweizerischer Nationalfonds zur F?rderung der Wissenschaftlichen Forschung 10.13039/501100001711Peer Reviewe

Freezing Physics and Derived Surface Nano-Engineering for Spontaneous Deicing

Droplet freezing is important both in nature and in technology. In this thesis I investigate the fundamentals of freezing water droplets and derive design criteria for the development of intrinsically ice-repellent materials. Such icephobic surfaces could improve the performance and safety of a multitude of technical processes in energy and transport. This includes for example heat exchangers, where ice built-up reduces thermal transport, and airplane flight, where freezing of water on airfoils can result in catastrophic events. The thesis consists of three individual studies. In the first study we investigated how the environmental conditions during droplet freezing affect the freezing outcome. We found that evaporatively or convectively supercooled water droplets resting on solid substrate can self-remove during freezing. This phenomenon, which we termed self-dislodging, requires that the heat removal from the droplet’s free surface dominates the heat removal through the solid substrate. Consequently, the freezing front moves from the outside of the droplet towards the center and from the top to the bottom, resulting in a solid ice shell with an unsolidified core and an unfrozen droplet-substrate interface. We observed experimentally that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, due to the volumetric expansion associated with freezing, semi-frozen water is displaced towards the droplet-substrate interface lifting the freezing droplet away from the substrate. The combined effects of dewetting and lifting result in droplet self-removal. We found that the more the substrate is hydrophobic the more robust self-dislodging occurs. In the second study we examined how multiple water droplets interact during freezing in a low-pressure environment. Understanding droplet interactions during freezing is important as droplets do not appear in isolation, but always in groups. We found that the freezing of a supercooled droplet results in self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We suggest that as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble resulting in ice coverage of the solid surface. In the third study we introduced a controllable and upscalable method to fabricate superhydrophobic surfaces with a 3D-printed architecture for improved repellency of viscous liquids. We show a more than threefold contact time reduction of impacting viscous droplets up to a fluid viscosity of 3.7mPa s, which covers the viscosity of supercooled water down to -17 °C. Based on the combined consideration of the fluid flow within and the simultaneous droplet dynamics above the texture, we recommend future pathways to rationally architecture such surfaces that can repel supercooled water before it freezes and sticks to the surface. The three studies presented in this thesis address the topic of surface icing from three different angles, collaboratively covering a broad range of the problem. Only when taking into account the environmental conditions, freezing group dynamics and liquid solid interactions, robust icephobic surfaces can be designed in the future. With my thesis I contribute to this development process

Bericht : über d. Schuljahr ... / Quaestionum Ovidianarum

scripsit Gustavus GraeberErschienen: 1. 1881 - 2. 188

Spontaneous self-dislodging of freezing water droplets and the role of wettability.

Spontaneous removal of liquid, solidifying liquid and solid forms of matter from surfaces, is of significant importance in nature and technology, where it finds applications ranging from self-cleaning to icephobicity and to condensation systems. However, it is a great challenge to understand fundamentally the complex interaction of rapidly solidifying, typically supercooled, droplets with surfaces, and to harvest benefit from it for the design of intrinsically icephobic materials. Here we report and explain an ice removal mechanism that manifests itself simultaneously with freezing, driving gradual self-dislodging of droplets cooled via evaporation and sublimation (low environmental pressure) or convection (atmospheric pressure) from substrates. The key to successful self-dislodging is that the freezing at the droplet free surface and the droplet contact area with the substrate do not occur simultaneously: The frozen phase boundary moves inward from the droplet free surface toward the droplet-substrate interface, which remains liquid throughout most of the process and freezes last. We observe experimentally, and validate theoretically, that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, the droplet lifts from the substrate due to its incompressibility, density differences, and the asymmetric freezing dynamics with inward solidification causing not fully frozen mass to be displaced toward the unsolidified droplet-substrate interface. Depending on surface topography and wetting conditions, we find that this can lead to full dislodging of the ice droplet from a variety of engineered substrates, rendering the latter ice-free

Spontaneous self-dislodging of freezing water droplets and the role of wettability

ISSN:0027-8424ISSN:1091-649