52 research outputs found
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
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)
dropletsî—¸under conditions where partial solidification occurs
much faster than the natural droplet oscillationî—¸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
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