11 research outputs found
Dropwise Condensation of Low Surface Tension Fluids on Omniphobic Surfaces
Compared to the significant body of work devoted to surface engineering for promoting dropwise condensation heat transfer of steam, much less attention has been dedicated to fluids with lower interfacial tension. A vast array of low-surface tension fluids such as hydrocarbons, cryogens, and fluorinated refrigerants are used in a number of industrial applications, and the development of passive means for increasing their condensation heat transfer coefficients has potential for significant efficiency enhancements. Here we investigate condensation behavior of a variety of liquids with surface tensions in the range of 12 to 28 mN/m on three types of omniphobic surfaces: smooth oleophobic, re-entrant superomniphobic, and lubricant-impregnated surfaces. We demonstrate that although smooth oleophobic and lubricant-impregnated surfaces can promote dropwise condensation of the majority of these fluids, re-entrant omniphobic surfaces became flooded and reverted to filmwise condensation. We also demonstrate that on the lubricant-impregnated surfaces, the choice of lubricant and underlying surface texture play a crucial role in stabilizing the lubricant and reducing pinning of the condensate. With properly engineered surfaces to promote dropwise condensation of low-surface tension fluids, we demonstrate a four to eight-fold improvement in the heat transfer coefficient.National Science Foundation (U.S.). Graduate Research Fellowship ProgramNational Science Foundation (U.S.) (CAREER Award 0952564)MIT Energy Initiativ
Suppression of Frost Nucleation Achieved Using the Nanoengineered Integral Humidity Sink Effect
Inhibition
of frost formation is important for increasing efficiency
of refrigeration systems and heat exchangers, as well as for preventing
the rapid icing over of water-repellant coatings that are designed
to prevent accumulation of rime and glaze. From a thermodynamic point
of view, this task can be achieved by either increasing hydrophobicity
of the surface or decreasing the concentration of water vapor above
it. The first approach has been studied in depth, but so far has not
yielded a robust solution to the problem of frost formation. In this
work, we systematically explore how frost growth can be inhibited
by controlling water vapor concentration using bilayer coatings with
a porous exterior covering a hygroscopic liquid-infused layer. We
lay the theoretical foundation and provide experimental validation
of the mass transport mechanism that governs the integral humidity
sink effect based on this coating platform as well as reveal intriguing
sizing effects about this system. We show that the concentration profile
above periodically spaced pores is governed by the sink and source
concentrations and two geometrical parameters: the nondimensional
pore size and the ratio of the pore spacing to the boundary layer
thickness. We demonstrate that when the ratio of the pore spacing
to the boundary layer thickness vanishes, as for the nanoporous bilayer
coatings, the entire surface concentration becomes uniform and equal
to the concentration set by the hygroscopic liquid. In other words,
the surface concentration becomes completely independent of the nanopore
size. We identified the threshold geometrical parameters for this
condition and show that it can lead to a 65 K decrease in the nucleation
onset surface temperature below the dew point. With this fundamental
insight, we use bilayer coatings to nanoengineer the integral humidity
sink effect to provide extreme antifrosting performance with up to
a 2 h delay in nucleation onset at 263 K. The nanoporous bilayer coatings
can be designed to combine optimal antifrosting functionality with
a superhydrophobic water repelling exterior to provide coatings that
can robustly prevent frost, rime, and glaze accumulation. By minimizing
the required amount of antifreeze, this anti-icing method can have
minimal operational cost and environmental impact
Inhibition of Condensation Frosting by Arrays of Hygroscopic Antifreeze Drops
The formation of frost and ice can
have negative impacts on travel
and a variety of industrial processes and is typically addressed by
dispensing antifreeze substances such as salts and glycols. Despite
the popularity of this anti-icing approach, some of the intricate
underlying physical mechanisms are just being unraveled. For example,
recent studies have shown that in addition to suppressing ice formation
within its own volume, an individual salt saturated water microdroplet
forms a region of inhibited condensation and condensation frosting
(RIC) in its surrounding area. This occurs because salt saturated
water, like most antifreeze substances, is hygroscopic and has water
vapor pressure at its surface lower than water saturation pressure
at the substrate. Here, we demonstrate that for macroscopic drops
of propylene glycol and salt saturated water, the absolute RIC size
can remain essentially unchanged for several hours. Utilizing this
observation, we demonstrate that frost formation can be completely
inhibited in-between microscopic and macroscopic arrays of propylene
glycol and salt saturated water drops with spacing (<i>S</i>) smaller than twice the radius of the RIC (δ). Furthermore,
by characterizing condensation frosting dynamics around various hygroscopic
drop arrays, we demonstrate that they can delay complete frosting
over of the samples 1.6 to 10 times longer than films of the liquids
with equivalent volume. The significant delay in onset of ice nucleation
achieved by dispensing propylene glycol in drops rather than in films
is likely due to uniform dilution of the drops driven by thermocapillary
flow. This transport mode is absent in the films, leading to faster
dilution, and with that facilitated homogeneous nucleation, near the
liquid–air interface
Inhibition of Condensation Frosting by Arrays of Hygroscopic Antifreeze Drops
The formation of frost and ice can
have negative impacts on travel
and a variety of industrial processes and is typically addressed by
dispensing antifreeze substances such as salts and glycols. Despite
the popularity of this anti-icing approach, some of the intricate
underlying physical mechanisms are just being unraveled. For example,
recent studies have shown that in addition to suppressing ice formation
within its own volume, an individual salt saturated water microdroplet
forms a region of inhibited condensation and condensation frosting
(RIC) in its surrounding area. This occurs because salt saturated
water, like most antifreeze substances, is hygroscopic and has water
vapor pressure at its surface lower than water saturation pressure
at the substrate. Here, we demonstrate that for macroscopic drops
of propylene glycol and salt saturated water, the absolute RIC size
can remain essentially unchanged for several hours. Utilizing this
observation, we demonstrate that frost formation can be completely
inhibited in-between microscopic and macroscopic arrays of propylene
glycol and salt saturated water drops with spacing (<i>S</i>) smaller than twice the radius of the RIC (δ). Furthermore,
by characterizing condensation frosting dynamics around various hygroscopic
drop arrays, we demonstrate that they can delay complete frosting
over of the samples 1.6 to 10 times longer than films of the liquids
with equivalent volume. The significant delay in onset of ice nucleation
achieved by dispensing propylene glycol in drops rather than in films
is likely due to uniform dilution of the drops driven by thermocapillary
flow. This transport mode is absent in the films, leading to faster
dilution, and with that facilitated homogeneous nucleation, near the
liquid–air interface
“Insensitive” to Touch: Fabric-Supported Lubricant-Swollen Polymeric Films for Omniphobic Personal Protective Gear
The use of personal protective gear
made from omniphobic materials that easily shed drops of all sizes
could provide enhanced protection from direct exposure to most liquid-phase
biological and chemical hazards and facilitate the postexposure decontamination
of the gear. In recent literature, lubricated nanostructured fabrics
are seen as attractive candidates for personal protective gear due
to their omniphobic and self-healing characteristics. However, the
ability of these lubricated fabrics to shed low surface tension liquids
after physical contact with other objects in the surrounding, which
is critical in demanding healthcare and military field operations,
has not been investigated. In this work, we investigate the depletion
of oil from lubricated fabrics in contact with highly absorbing porous
media and the resulting changes in the wetting characteristics of
the fabrics by representative low and high surface tension liquids.
In particular, we quantify the loss of the lubricant and the dynamic
contact angles of water and ethanol on lubricated fabrics upon repeated
pressurized contact with highly absorbent cellulose-fiber wipes at
different time intervals. We demonstrate that, in contrast to hydrophobic
nanoparticle coated microfibers, fabrics encapsulated within a polymer
that swells with the lubricant retain the majority of the oil and
are capable of repelling high as well as low surface tension liquids
even upon multiple contacts with the highly absorbing wipes. The fabric
supported lubricant-swollen polymeric films introduced here, therefore,
could provide durable and easy to decontaminate protection against
hazardous biological and chemical liquids
Microscale Mechanism of Age Dependent Wetting Properties of Prickly Pear Cacti (<i>Opuntia</i>)
Cacti
thrive in xeric environments through specialized water storage
and collection tactics such as a shallow, widespread root system that
maximizes rainwater absorption and spines adapted for fog droplet
collection. However, in many cacti, the epidermis, not the spines,
dominates the exterior surface area. Yet, little attention has been
dedicated to studying interactions of the cactus epidermis with water
drops. Surprisingly, the epidermis of plants in the genus <i>Opuntia,</i> also known as prickly pear cacti, has water-repelling
characteristics. In this work, we report that surface properties of
cladodes of 25 taxa of <i>Opuntia</i> grown in an arid Sonoran
climate switch from water-repelling to superwetting under water impact
over the span of a single season. We show that the old cladode surfaces
are not superhydrophilic, but have nearly vanishing receding contact
angle. We study water drop interactions with, as well as nano/microscale
topology and chemistry of, the new and old cladodes of two <i>Opuntia</i> species and use this information to uncover the
microscopic mechanism underlying this phenomenon. We demonstrate that
composition of extracted wax and its contact angle do not change significantly
with time. Instead, we show that the reported age dependent wetting
behavior primarily stems from pinning of the receding contact line
along multilayer surface microcracks in the epicuticular wax that
expose the underlying highly hydrophilic layers