Wettability of Graphene

Description
We investigated the effect of the background substrate on the wettability of graphene. We performed dynamic contact angle measurements and detailed graphene surface characterizations to demonstrate that the defects present in CVD grown and transferred graphene coatings result in unusually high contact angle hysteresis even on highly smooth substrates. The advancing contact angle measurements were observed to be independent of the number of layers of graphene and in good agreement with corresponding molecular dynamics simulations (see above) and theoretical calculations. The contact angle with a monolayer graphene coating on a substrate equaled the contact angle on highly ordered pyrolytic graphite.

Applications
Advanced functional coatings, microfluidics, heat transfer

People
Rishi Raj, Shalabh Maroo
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Texture and Wettability

Description
Controlling wettability by varying surface chemistry and roughness is of interest for a wide range of applications. It has been well known that droplets on textured hydrophilic, i.e., superhydrophilic, surfaces form thin-films with near zero contact angles. Here, we report an unexpected behavior where non-wetting droplets (see above) are formed by slightly heating textured superhydrophilic surfaces beyond the saturation temperature. This phenomenon is distinct from the widely researched Leidenfrost and offers an expanded parametric space for fabricating surfaces with desired temperature-dependent wettability.

Applications
Microfluidics, drag reduction, self-cleaning, water harvesting, anti-corrosion, thermal management

People
Solomon Adera, Rishi Raj, Ryan Enright

Surface structure enhanced microchannel flow boiling

Description
Description: Flow boiling is promising to cool high performance electronic devices. However dry-out of the liquid film on the channel wall due to flow instability can lead to significant temperature excursion and oscillation. We show that incorporating hydrophilic surface structures that are optimized to promote capillary wicking can effectively maintain the liquid film in the microchannel. As a result, the structures can suppress temperature oscillations and enhance the critical heat flux.

Applications
Thermal management, electronics cooling

People
Yangying Zhu, Dion Antao, Jay Sircar, Sameer Rao

Planar Solar Thermophotovoltaic Device

Description
Optical image of a planar solar thermophotovoltaic (STPV) device operating above 1000 °C, where thermal emission is well into the visible spectrum. These elevated temperatures are reached via concentrated solar power (CSP). Using vertically aligned multi-wall carbon nanotube forests, this high intensity light is converted to heat to drive narrow-band thermal emission from a photonic crystal surface towards a photovoltaic cell.

Applications
CSP, power generation, thermal management

People
Andrej Lenert, David Bierman, Colin Kelsall

Carbon Nanotubes for Capacitive Deionization (CDI)

Description
Capacitive deionization promises to be an energy-efficient water desalination technology. In CDI, a voltage is applied across high surface area electrodes. Salt ions adsorb onto the surface of the electrode in the electric double layer (EDL), while the desalinated water is then pumped through. Our research focuses on understanding the ion transport mechanisms in these porous electrodes. We are synthesizing vertically aligned carbon nanotube electrodes to create an idealized geometry for experimentally studying the diffusion and electrokinetics of ion transport.

Applications
Water desalination, supercapacitors

People
Heena Mutha

Optically Transparent Thermally Insulating Aerogels

Description
We have developed thermally super-insulating monolithic silica aerogels which we have tailored to achieve high optical transparency. The transparency of commercial silica aerogels has so far been limited to <85% % due to the scattering of low wavelengths from its relatively large pores, which results in significant energy losses. In our lab, we have optimized the silica aerogel fabrication recipe and processing conditions to achieve pore sizes of 2-50 nm. Carefully tailoring the nanostructure of our aerogel samples has allowed us to achieve solar-weighted transmittance greater than 97% for a 3 mm thick sample (shown in the image).

Applications
Concentrated Solar Power, Solar Water Heating, Industrial Process Heating, Windows

People
Sungwoo Yang, Lin Zhao, Elise Strobach, Bikram Bhatia

Thermal Management of GaN Electronics

Description
Gallium Nitride (GaN) electronics is one of the most exciting areas of innovation in the microelectronics industry and one of the greatest challenges in thermal management. We are using electro-thermal device simulation, thermal modeling, and micro-scale thermometry to understand the severity of the problem in RF amplifier and power switching applications. We are planning to develop advanced thermal management strategies based on high thermal conductivity solids and liquid-vapor phase change to enable record-breaking performance of GaN electronics.

Applications
Power electronics, RF/microwave power amplifiers (cellular telephone base stations, military radar and communications)

People
Kevin Bagnall, Lenan Zhang, Stephen Walsh, Prof. Tomás Palacios' Group (EECS, MIT)

PHUMP evaporator

Description
Cross sectional cut of a planar evaporator for a multiple-condenser loop-type heat pipe. Multiple wicking layers are combined into a single monolithic wicking structure. The sinter material, thickness, and particle size for each layer was chosen based on its combination of thermal conductivity, permeability, and capillary pressure.

Applications
Thermal Management

People
Arthur Kariya

Water nanodroplet on lipid monolayer

Description
Water nanodroplet on lipid monolayer simulated in a molecular dynamics simulation. Here, boundary water molecules (black circles) are detected and an elliptical curve fit (blue curve) is used to determine the contact angle. The monolayer is oriented such that its hydrophobic tails are pointing upward, hence the large contact angle.

Applications
Tunable surface energy, electroporation, filtration, desalination

People
Shalabh Maroo, Jeremy Cho

Liquid Freezing Dynamics on Hydrophobic and Superhydrophobic Surfaces

Description
False color environmental scanning electron microscope (ESEM) images of water freezing on smooth and nanostructured hydrophobic surfaces. The image shows a larger droplet undergoing rapid freezing with notable protrusions.

Applications
Anti-icing surfaces

People
Nenad Miljkovic, Ryan Enright

Enhanced Nucleate Pool Boiling with Surfactants

Description
Surfactants are known to lower the surface tension of water, which is an important property in boiling. In addition to bubbles departing easier due to lower surface tension, surfactants can increase the amount of nucleation and improve boiling performance by up to 200%. In this photo, 0.133 mM of Triton X-114 in DI water is boiling on copper foil at 2.5 W/cm².

Applications
Nucleate pool boiling, industrial cooling, phase change desalination

People
Jeremy Cho

Advanced Thermo-Adsorptive Battery Climate Control System for Electric Vehicles

Description
Biporous zeolite and metal-oxide frameworks promise high water adsorption capacities at low relative pressures. Integrated in a modular thermo-adsorptive battery with enhanced heat and mass transfer characteristics, these materials are the basis of a novel climate-control system that could significantly increase the driving range of electric vehicles. Future implementations of the technology could provide significant energy savings and noise reduction for residential and commercial HVAC applications.

Applications
Automotive, residential and industrial HVAC systems

People
Sameer Rao, Hyunho Kim, Shankar Narayanan, Sungwoo Yang, Ian McKay

Nanofluid Volumetric Solar Absorbers

Description
Carbon-coated nano-sized (10-100 nm) solid particles are added to a liquid heat transfer fluid (VP-1) to volumetrically absorb concentrated solar radiation. These nanofluid absorbers promise increased performance over surface absorbers by minimizing temperature differences between the absorber and the fluid. Emissive losses are consequently reduced through a behavior referred to as "thermal trapping," which is essentially an engineered "greenhouse effect." Receiver-side efficiencies are predicted to exceed 35% when nanofluid volumetric receivers are coupled to a power cycle and optimized with respect to the optical thickness.

Applications
Concentrated Solar Power, Thermal Storage

People
Andrej Lenert
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Oriented Zeolite on Support

Description
An oriented layer of MFI zeolites on a silicon support. We are exploring the potential of zeolite-based membranes for their potential in separating fresh water from salt water more efficiently than current state-of-the-art polymeric reverse osmosis membranes.

Applications
Desalination, Gas Separation

People
Tom Humplik
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Planar Loop-type Heat Pipe

Description
Loop-type heat pipe with a fully planar evaporator, condenser and reservoir for the integration into an air-cooled heat sink. The planar geometry allows for the stacking of multiplue modular condensers to meet a variety of cooling needs with low thermal resistance.

Applications
Thermal Management

People
Arthur Kariya, Teresa Peters, Daniel Hanks, Martin Cleary

Liquid Meniscus Spreading in a Micropillar Array

Description
The liquid meniscus spreading in a micropillar array is captured using confocal microscopy. The interference pattern in the picture indicates the thickness of the liquid. The dynamics of the liquid front was investigated by studying the temporal evolution of the shape of the meniscus.

Applications
Lab-on-a-chip, Thermal management, Water harvesting

People
Rong Xiao
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Uni-directional Liquid Spreading of a Droplet

Description
A liquid droplet demonstrates uni-directional spreading behavior, where the propagation is primarily in one direction and pinning occurs in the other directions. This spreading behaviour is distinct in comparison with other engineered surfaces owing to the asymmetric geometry of the fabricated structures.

Applications
DNA microarrays, digital lab-on-a-chip, anti-fogging and fog-harvesting, inkjet printing, thin-film lubrication and thermal management

People
Kuang-Han (Hank) Chu, Rong Xiao Read more

Evaporation-Induced Cassie-to-Wenzel Droplet Transition

Description
A superhydrophilic surface at room temperature can take on the characteristics of a superhydrophobic surface by superheating the substrate and allowing droplets to reside in the non-wetting Cassie state on microstructured superhydrophilic surfaces due to an evaporation-induced force that counters the wetting force.

Applications
Thermal Management

People
Rishi Raj, Solomon Adera

High performance incandescent lighting

Description
A promising way to improve the efficiency of ILB is to tailor the emitted spectrum using interference filters that reflect the infrared energy back to the filament while transmitting the visible light. At DRL, we are using a planar tungsten incandescent emitter sandwiched between two interference filters optimized to reflected infrared and transmit visible light for a wide range of angles. The device currently shows close to 50% energy savings at a filament temperature of 2800 K and a threefold increase in the visible light output compared to a bare filament consuming an equal amount of power. Although significant energy efficiency improvement has been demonstrated, we are currently investigating (numerically and experimentally) several challenges such as minimizing filament evaporation and filter temperature while maximizing the geometrical view factor and luminous efficiency. We hope that this work will lead to an energy efficient incandescent light source that is also commercially viable.

Applications
Lighting, thermal radiation management

People
Arny Leroy, Bikram Bathia, Ognjen Ilic

Dropwise Condensation on a Nanostructured Superhydrophobic Surface

Description
Environmental scanning electron micrograph (ESEM) of water condensation on a nanostructured superhydrophobic surface showing both partially wetting and suspended droplets (P = 1200 ± 12 Pa, Ts = 282 ± 1.5 K). The nanostructured surface consists of an array of equidistant superhydrophobic silicon nanopillars with diameters, heights, and spacings of d = 300 nm, h = 6.1 µm, and l = 2 µm, respectively.

Applications
Dropwise condensation, atmospheric water harvesting, anti-wetting surfaces, high flux thermal management

People
Nenad Miljkovic, Ryan Enright Read more

Condenser with copper sinter

Description
A sintered copper wick is incorporated into the condenser of a LHP to stabilize the liquid-vapor interface using capillary forces. Using this design, multiple planar condensers can be vertically stacked in a loop heat pipe in spite of the uneven liquid pressures caused by gravity.

Applications
Thermal management

People
Daniel Hanks, Arthur Kariya, Teresa Peters, Martin Cleary

Jumping Droplet Condensation

Description
When droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump from the surface due to the release of excess surface energy. Here, we show that silanized copper oxide surfaces created via a simple fabrication method can achieve highly efficient jumping-droplet condensation heat transfer. We experimentally demonstrated a 25% higher overall heat flux compared to state-of-the-art hydrophobic condensing surfaces.

Applications
Enhanced condensation, anti-wetting surfaces

People
Daniel J. Preston, Nenad Miljkovic, Ryan Enright

Thermal Enhancement of High-Temperature Heat Transfer Fluids

Description
Improving the poor thermal properties of molten salt heat transfer fluids is crucial to increasing the efficiency of heat transfer at high temperatures, such as in concentrated solar power plants. Adding a small fraction of ceramic nanoparticles has been shown to significantly increase the specific heat capacity of molten salts and ultimately enhance the efficiency of concentrated solar power plants. We believe the underlying mechanism for the improvement in heat capacity is related to the liquid-solid adsorption energy at the nanoparticle interface and are using a number of characterization techniques to fully explore its thermal benefits.

Applications
Concentrated solar power, nuclear power, general thermal management

People
Matt Thoms

Liquid Evaporation on Superhydrophobic and Superhydrophilic Nanostructured Surfaces

Description
Environmental scanning electron microscope (ESEM) images of water evaporation from superhydrophilic and superhydrophobic nanostructured surfaces (P = 1400 ± 14 Pa, Ts = 283 ± 1.5 K). The nanostructured surfaces consist of an array of equidistant silicon nanopillars with diameter, height, and spacing of 300 nm, 7.5 µm, and 2 µm, respectively. Capillary forces generated by the receding meniscus on the hydrophilic surface result in liquid entrapment and 'kissing' pillars. These 'kissing' pillars allow for the formation of rare metastable pinned droplets with highly irregular contact lines.

Applications
Thermal management, elastocapillary micro/nanofabrication

People
Nenad Miljkovic, Ryan Enright Read more

Contact Angle Hysteresis on Structured and Superhydrophobic Surfaces

Description
Understanding the complexities associated with contact line dynamics on chemically heterogeneous and superhydrophobic surfaces are important for a wide variety of engineering problems. Through careful dynamic contact angle experiments and thermodynamic modeling, we demonstrated that contact line distortion (see above) is the key aspect that needs to be accounted for in the dynamic droplet models. On a surface comprised of discrete wetting defects on an interconnected less wetting area, the advancing contact angle was determined to be independent of the defects. Conversely, the receding contact angle was heavily influenced by defects.

Applications
Advanced functional coatings, microfluidics, condensation, anti-fogging and fog-harvesting, thin-film lubrication

People
Rishi Raj, Solomon Adera, Yangying Zhu, Ryan Enright
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Electrowetting-on-Dielectric Fluid Manipulation

Description
A MEMS vertical translation, or focusing, stage that uses electrowetting-on-dielectric (EWOD) as the actuating mechanism was developed. EWOD has the potential to eliminate solid-solid contact by actuating through deformation of liquid droplets placed between the stage and base to achieve stage displacement. Our EWOD stage is capable of linear spatial manipulation with resolution of 10μm over a maximum range of 130μm and angular deflection of approximately ±1°, comparable to piezoelectric actuators. We also developed a model that suggests a higher intrinsic contact angle on the EWOD surface can further improve the translational range, which was validated experimentally by comparing different surface coatings. The capability to operate the stage without solid-solid contact offers potential improvements for applications in micro-optics, actuators, and other MEMS devices.

Applications
Enhanced condensation, anti-wetting surfaces

People
Daniel J. Preston, Ariel Anders, Banafsheh Barabadi,Evelyn Tio, Yangying Zhu, DingRan Annie Dai