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

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

Surface Engineering for Phase Change Heat Transfer: A Review

Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiencies at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930's to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoveries

Contactless Transport and Mixing of Liquids on Self-Sustained Sublimating Coatings

The controlled handling of liquids and colloidal suspensions as they interact with surfaces, targeting a broad palette of related functionalities, is of great importance in science, technology, and nature. When small liquid volumes need to be processed in microfluidic devices, contamination on the solid-liquid interface and loss of liquid due to adhesion on the transport channels are two very common problems that can significantly alter the process outcome, e.g. the chemical reaction efficiency, or the purity and the final concentrations of a suspension. It is therefore no surprise that both levitation and minimal contact transport methods including non wetting surfaces have been developed to minimize the interactions between liquids and surfaces. Here we demonstrate contactless surface levitation and transport of liquid drops, realized by harnessing and sustaining the natural sublimation of a solid carbon dioxide-coated substrate to generate a continuous supporting vapor layer. The capability and limitations of this technique in handling liquids of extreme surface tension and kinematic viscosity values are investigated both experimentally and theoretically. The sublimating coating is capable of repelling many viscous and low surface tension liquids, colloidal suspensions, and non-Newtonian fluids as well, displaying outstanding omniphobic properties. Finally, we demonstrate how sublimation can be used for liquid transport, mixing and drop coalescence, with a sublimating layer coated on an underlying substrate with prefabricated channels, conferring omniphobicity with a simple physical approach, rather than a chemical one

Detergency and its implications for oil emulsion sieving and separation

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter--without any additional surface engineering or functionalization--to separate oil droplets from a surfactant stabilized emulsion with a flux of 11,000 L m$^{-2}$ hr$^{-1}$ bar$^{-1}$. We believe these findings can inform the design of future oil separation materials

Surface engineering for phase change heat transfer: A review

Owing to advances in micro- and nanofabrication methods over the last two decades, the degree of sophistication with which solid surfaces can be engineered today has caused a resurgence of interest in the topic of engineering surfaces for phase change heat transfer. This review aims at bridging the gap between the material sciences and heat transfer communities. It makes the argument that optimum surfaces need to address the specificities of phase change heat transfer in the way that a key matches its lock. This calls for the design and fabrication of adaptive surfaces with multiscale textures and non-uniform wettability. Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiency at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930s to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoverie

Large-area Coating and Patterning of Functional Nanocomposites: Design, Synthesis and Characterization

Polymer nanocomposite (NC) synthesis from processing of polymer/nanoparticle dispersions remains an active area of research today due to its ability to create nanomaterials using large-area techniques. The benefits of nanomaterials are well documented; however, specific applications remain limited due to their poor durability and scalability. Durability is the tendency of NCs to retain their desirable properties. Scalability is the ability of a given synthesis technique to be implemented at a manufacturing-level. So while large-area processing of superhydrophobic (SHPo) coatings from spray deposition of polymer/nanoparticle dispersions is well documented, most established processes require the use of organic solvents and fluorinated polymers, which raises issues of cost and safety, and in turn, limiting their commercial implementation and scalability. This thesis presents a methodology for generating a SHPo coating from a non-fluorinated, water-based dispersion, eliminating processing hazards. The durability of the SHPo NCs is a major problem impeding commercial implementation. Surfaces typically become compromised through fouling or mechanical failure. This thesis reports wettability studies on two classic, robust NCs. The effect of final composition of NCs on mechanical durability, wettability, and electrical conductivity is considered. The work also investigates failure modes of NCs undergoing mechanical strain and the associated effects on coating wettability. A methodology is presented for dramatically reducing the required filler concentration for achieving superhydrophobicity; ultimately, mechanical properties are enhanced. Surfaces with heterogeneous wettability—so-called patterned wettability (PW)—are also of importance. PW finds applications in pool boiling and lab-on-a-chip; however, previous work emphasized handling water (high surface tension). A methodology for synthesizing a PW coating capable of handling lower surface tension liquids is presented; applications are in combinatorial chemistry. Other PW surfaces—presented in this thesis—were also shown to play an important role in droplet impact and volumetric shaping. Finally, this thesis aims to develop large-area PW surfaces for pool boiling applications; previous work was non-scalable. The work demonstrates the potential of large-area wet processing of NCs for advancing many technologies that require efficient fluid management

Superhydrophobic surfaces for extreme environmental conditions

Superhydrophobic surfaces for repelling impacting water droplets are typically created by designing structures with capillary (antiwetting) pressures greater than those of the incoming droplet (dynamic, water hammer). Recent work has focused on the evolution of the intervening air layer between droplet and substrate during impact, a balance of air compression and drainage within the surface texture, and its role in affecting impalement under ambient conditions through local changes in the droplet curvature. However, little consideration has been given to the influence of the intervening air-layer thermodynamic state and composition, in particular when departing from standard atmospheric conditions, on the antiwetting behavior of superhydrophobic surfaces. Here, we explore the related physics and determine the working envelope for maintaining robust superhydrophobicity, in terms of the ambient pressure and water vapor content. With single-tier and multitier superhydrophobic surfaces and high-resolution dynamic imaging of the droplet meniscus and its penetration behavior into the surface texture, we expose a trend of increasing impalement severity with decreasing ambient pressure and elucidate a previously unexplored condensation-based impalement mechanism within the texture resulting from the compression, and subsequent supersaturation, of the intervening gas layer in low-pressure, humid conditions. Using fluid dynamical considerations and nucleation thermodynamics, we provide mechanistic understanding of impalement and further employ this knowledge to rationally construct multitier surfaces with robust superhydrophobicity, extending water repellenc y behavior well beyond typical atmospheric conditions. Such a property is expected to find multifaceted use exemplified by transportation and infrastructure applications where exceptional repellency to water and ice is desired.ISSN:0027-8424ISSN:1091-649

Imparting Icephobicity with Substrate Flexibility

Ice accumulation hinders the performance of, and poses safety threats for, infrastructure both on the ground and in the air. Previously, rationally designed superhydrophobic surfaces have demonstrated some potential as a passive means to mitigate ice accretion; however, further studies on material solutions that reduce impalement and the contact time for impacting supercooled droplets (high viscosity) and can also repel droplets that freeze during surface contact are urgently needed. Here we demonstrate the collaborative effect of substrate flexibility and surface micro/nanotexture on enhancing both icephobicity and the repellency of viscous droplets (typical of supercooled water). We first investigate the influence of increased viscosity (spanning from 0.9 to 1078 mPa·s using water–glycerol mixtures) on impalement resistance and the droplet–substrate contact time after impact. Then we examine the effect of droplet partial solidification on recoil and simulate more challenging icing conditions by impacting supercooled water droplets (down to −15 °C) onto flexible and rigid surfaces containing ice nucleation promoters (AgI). We demonstrate a passive mechanism for shedding partially solidified (recalescent) dropletsunder conditions where partial solidification occurs much faster than the natural droplet oscillationwhich does not rely on converting droplet surface energy into kinetic energy (classic recoil mechanism). Using an energy-based model (kinetic–elastic–capillary), we identify a previously unexplored mechanism whereby the substrate oscillation and velocity govern the rebound process, with low areal density and moderately stiff substrates acting to efficiently absorb the incoming droplet kinetic energy and rectify it back, allowing droplets to overcome adhesion and gravitational forces, and recoil. This mechanism applies for a range of droplet viscosities, spanning from low- to high-viscosity fluids and even ice slurries, which do not rebound from rigid superhydrophobic substrates. For a low-viscosity fluid, i.e., water, if the substrate oscillates faster than the droplet spreading and retraction, the action of the substrate is decoupled from the droplet oscillation, resulting in a reduction in the droplet–substrate contact time