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1. Electrowetting-based coalescence of droplets during dropwise condensation of humid air

   Wikramanayake, Enakshi; Bahadur, Vaibhav (July 2019, Summer Heat Transfer Conference)

   Dropwise condensation yields higher heat transfer coefficients by avoiding the thermal resistance of the condensate film, seen during filmwise condensation. This work explores further enhancement of dropwise condensation heat transfer through the use of electrowetting to achieve faster droplet growth via coalescence of the condensed droplets. Electrowetting is a well understood microfluidic technique to actuate and control droplets. This work shows that AC electric fields can significantly enhance droplet growth dynamics. This enhancement is a result of coalescence triggered by various types of droplet motion (translation of droplets, oscillations of three phase line), which in turn depends on the frequency of the applied AC waveform. The applied electric field modifies droplet condensation patterns as well as the roll-off dynamics on the surface. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation rates under the influence of electric fields. It is noted that this study deals with condensation of humid air, and not pure steam. Results show that increasing the voltage magnitude and frequency increases droplet growth rate and overall condensation rate. Overall, this study reports more than a 30 % enhancement in condensation rate resulting from the applied electric field, which highlights the potential of this concept for condensation heat transfer enhancement. 

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2. Accelerated freezing due to droplet pinning on a nanopillared surface

   https://doi.org/10.1063/1.5048933  

   Bohm, Rachel; Haque, Mohammad_Rejaul; Qu, Chuang; Kinzel, Edward_C; Betz, Amy_Rachel (December 2018, AIP Advances)

   The freezing process is significantly influenced by environmental factors and surface morphologies. At atmospheric pressure, a surface below the dew and freezing point temperature for a given relative humidity nucleates water droplets heterogeneously on the surface and then freezes. This paper examines the effect of nanostructured surfaces on the nucleation, growth, and subsequent freezing processes. Microsphere Photolithography (MPL) is used to pattern arrays of silica nanopillars. This technique uses a self-assembled lattice of microspheres to focus UV radiation to an array of photonic jets in photoresist. Silica is deposited using e-beam evaporation and lift-off. The samples were placed on a freezing stage at an atmospheric temperature of 22±0.5°C and relative humidities of 40% or 60%. The nanopillar surfaces had a significant effect on droplet dynamics and freezing behavior with freezing accelerated by an order of magnitude compared to a plain hydrophilic surface at 60% RH where the ice bridges need to cover a larger void for the propagation of the freezing front within the growing droplets. By pinning droplets, coalescence is suppressed for the nanopillared surface, altering the size distribution of droplets and accelerating the freezing process. The main mechanism affecting freezing characteristics was the pinning behavior of the nanopillared surface. 

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3. Hydrophilic reentrant SLIPS enabled flow separation for rapid water harvesting

   https://doi.org/10.1073/pnas.2209662119  

   Guo, Zongqi; Boylan, Dylan; Shan, Li; Dai, Xianming (September 2022, Proceedings of the National Academy of Sciences)

   Water harvesting from air is desired for decentralized water supply wherever water is needed. When water vapor is condensed as droplets on a surface the unremoved droplets act as thermal barriers. A surface that can provide continual droplet-free areas for nucleation is favorable for condensation water harvesting. Here, we report a flow-separation condensation mode on a hydrophilic reentrant slippery liquid-infused porous surface (SLIPS) that rapidly removes droplets with diameters above 50 μm. The slippery reentrant channels lock the liquid columns inside and transport them to the end of each channel. We demonstrate that the liquid columns can harvest the droplets on top of the hydrophilic reentrant SLIPS at a high droplet removal frequency of 130 Hz/mm 2 . The sustainable flow separation without flooding increases the water harvesting rate by 110% compared to the state-of-the-art hydrophilic flat SLIPS. Such a flow-separation condensation approach paves a way for water harvesting. 

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4. Electrowetting-assisted droplet coalescence and roll-off for condensation heat trasnfer

   Wikramanayake, Enakshi D.; Bahadur, Vaibhav (December 2019, Proceedings of the International Heat and Mass Transfer Conference)

   Dropwise condensation heat transfer is significantly higher than filmwise condensation heat transfer due to the absence of the thermal resistance associated with the condensed water film. This study uses electrowetting to enhance coalescence and roll-off of condensed droplets, with the objective of enhancing the condensation rate. Coalescence enhancement is achieved by electric field-driven droplet motion such as translation of droplets, and oscillations of the three-phase line. Experiments are conducted to study early-stage droplet growth dynamics, and steady state condensation under electrowetting fields. Results show that droplet growth and roll-off increases with the voltage and frequency of the applied AC field. AC electric fields are seen to be more effective than DC electric fields. The overall condensation rate depends on the roll-off size of droplets, frequency of roll-off events, and on the interactions of the rolled-off droplets with the remainder of the droplets. All these phenomena can be altered by the applied electric field. An analytical heat transfer model is developed which uses the measured droplet size distribution to estimate the surface heat flux. Overall, this study reports that electric fields can enhance the condensation rate by more than 30 %. 

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5. A new method for operating a continuous-flow diffusion chamber to investigate immersion freezing: assessment and performance study

   https://doi.org/10.5194/amt-13-6631-2020  

   Kulkarni, Gourihar; Hiranuma, Naruki; Möhler, Ottmar; Höhler, Kristina; China, Swarup; Cziczo, Daniel J.; DeMott, Paul J. (January 2020, Atmospheric Measurement Techniques)

   null (Ed.)

   Abstract. Glaciation in mixed-phase clouds predominantly occurs through theimmersion-freezing mode where ice-nucleating particles (INPs) immersedwithin supercooled droplets induce the nucleation of ice. Modelrepresentations of this process currently are a large source of uncertaintyin simulating cloud radiative properties, so to constrain these estimates,continuous-flow diffusion chamber (CFDC)-style INP devices are commonly usedto assess the immersion-freezing efficiencies of INPs. This study explored anew approach to operating such an ice chamber that provides maximumactivation of particles without droplet breakthrough and correction factorambiguity to obtain high-quality INP measurements in a manner thatpreviously had not been demonstrated to be possible. The conditioningsection of the chamber was maintained at −20 ∘C and water relative humidity (RHw) conditions of 113 % to maximize the droplet activation,and the droplets were supercooled with an independentlytemperature-controlled nucleation section at a steady cooling rate(0.5 ∘C min−1) to induce the freezing of droplets andevaporation of unfrozen droplets. The performance of the modified compactice chamber (MCIC) was evaluated using four INP species: K-feldspar,illite-NX, Argentinian soil dust, and airborne soil dusts from an arableregion that had shown ice nucleation over a wide span of supercooledtemperatures. Dry-dispersed and size-selected K-feldspar particles weregenerated in the laboratory. Illite-NX and soil dust particles were sampledduring the second phase of the Fifth International Ice Nucleation Workshop(FIN-02) campaign, and airborne soil dust particles were sampled from anambient aerosol inlet. The measured ice nucleation efficiencies of modelaerosols that had a surface active site density (ns) metric were higher but mostly agreed within 1 order of magnitude compared to results reported in the literature. 

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