Droplet Growth Rate Research Articles

Mechanisms of Secondary Spreading and Micro Droplet Formation on Steel

Abstract A new theory for secondary spreading based on the wetting theory of thin films is presented. It explains how micro droplets within the spreading zone and the primary droplet retain their shape, although connected by a thin electrolyte film and how humidity and salt concentration affect the growth rate of micro droplets. The trigger for secondary spreading, polarization or alkalization, is identified by using droplets of sodium hydroxide solution. Secondary spreading thus occurs on steel from pH 13.5 without corrosion or external polarization. The limiting pH value found explains why secondary spreading on steel only occurs when certain salts are used. 
The effect of the substrate is investigated by changing the microstructure of the steel. By comparing the sizes of micro droplets and micro structural phases and by scanning electron microscopy/energy-dispersive X-ray analysis measurements of the spreading zone, the existence of an electrolyte film connecting the micro droplets is supported. Ecorr potential profiles of secondary spreading droplets of sodium chloride solution on steel acquired by means of SKP are used to assess the contribution of secondary spreading to the total corrosion current, which is estimated to be low compared to that of the cathodic zone at the edge of the droplet

A multiphase flow model of water droplets dielectrophoretic-induced air dehumidification phenomena

Air humidity in indoor spaces plays a critical role in human comfort and health. Dehumidification systems are used for building humidity controls, but they can take significant energy consumption, especially in geographic locations with high outdoor humidity and warm climates. Consequently, there is a growing demand for innovative dehumidification processes that consume minimal energy. Dielectrophoretic air dehumidification represents one such promising approach. However, it has not garnered significant attention due to the absence of engineering models and simulation tools capable of evaluating its performance and limitations at large-scale airflows. A new numerical multiphase CFD model, which is also experimentally validated, is developed in a customized Reacting Foam solver based on OpenFOAM® version 9. The newly developed model seeks to decrease substantial energy consumption and lower costs by leveraging the dielectrophoretic phenomenon to regulate moisture levels in the air. The solver integrates a hybrid Eulerian-Lagrangian framework to track the droplet's trajectory and growth rate while solving the continuum equations for the moist air. An electrospray produces electrically charged droplets, which grow during their in-flight trajectories as water vapor condenses onto their surfaces. The role of electrostatic forces in promoting vapor condensation within a high-gradient electrical field is investigated, and the dielectrophoretic vapor nucleation process on charged water droplets is discussed. The CFD model was validated against results from the literature and from proof-of-concept experiments conducted by the authors, which showed a 2 % air dehumidification with a single electrospray and airflow rate of 5 cubic feet per minute. The simulation results indicated that augmenting the number of electrically charged spray droplets increased the dehumidification of the air to 25 %. The initial mean droplet diameter, the orientation of the injector and relative humidity significantly influence the assessment of dehumidification. Scaling up this approach to larger airflow volumes is identified as a potential future research direction.

Dropwise condensation performance of sprayable polymer/copper oxide composite coating

Even though dropwise condensation (DWC) has received significant attention for its potential to enhance heat transfer performance at a higher order than filmwise condensation (FWC), the scalability of durable DWC-promoting surfaces to sustain prolonged condensation remains challenging. The spray coating presents itself as a potential scalable method to deposit hydrophobic coating onto a metallic substrate. However, the conventional sprayable fluoropolymer exhibits poor adhesion to the substrate and thus requires a base coat before it can be deposited on a metallic substrate. To address this challenge, this study investigated dropwise condensation performance on a polytetrafluoroethylene/imide-based binder/copper oxide composite (PTFE/PI/CuO) coating that is deposited into a copper substrate using simple one-step spray coating and thermal annealing. The PTFE particles provide the necessary hydrophobic characteristics. At the same time, the polymer binder fills the pores between the particles, preventing droplet nucleation within the structure and minimizing the occurrence of flooding that can lead to undesirable droplet pinning. The PTFE/PI/CuO coating successfully promotes pinned-free dropwise condensation, leading to approximately two times higher heat transfer enhancement than filmwise condensation. Moreover, the formation of micro-scale hydrophilic CuO clusters at the top of the coating surface acts as nucleation sites that increase droplet growth rate and departure frequency without altering the droplet departure radius. As a result, PTFE/PI/CuO exhibits 1.4x times higher heat transfer over the PTFE/PI. Furthermore, a durability enhancement was obtained by adding sub-micron copper particles. While PTFE/PI coating failed after exposure to steam, no visible damage was observed from PTFE/PI/CuO, thus indicating enhanced durability. Moreover, the PTFE/PI/CuO composite also exhibited resistance against mechanical damage as its heat transfer performance remained similar even after being subjected to abrasion. Finally, the pull-off adhesion test revealed that PTFE/PI/CuO has higher adhesion strength than PTFE/PI, making it more suitable for dropwise condensation application.

Supersonic separation benefiting the decarbonization of natural gas and flue gas

Supersonic separation utilizes the cryogenics created by gas expansion to purify the mixture. This method is a novel carbon capture scheme. In this work, the models of N2-CO2 (flue gas) and CH4-CO2 (natural gas) are established to calculate the condensation flow. The condensation behavior of CO2 in natural gas and flue gas is clarified respectively, and the difference between the two is compared for the first time. Under the same conditions, CO2 in flue gas reaches the limit non-equilibrium state first, forming non-equilibrium condensation with limit nucleation rate of 7.78 × 1020 m−3s−1 and maximum droplet growth rate of 8.19 × 10−3 m s−1. However, the maximum droplet growth rate in natural gas is only 2.22 × 10−3 m s−1, which implies that the interphase mass transfer rate in flue gas is larger during the CO2 condensation. In addition, the condensation behavior of CO2 in natural gas is more sensitive to the changes of inlet parameters. This work provides an invaluable contribution to the development of supersonic carbon capture.

Role of Surface Tension on Heat Feedback and Power from Energetic Composites.

Heat feedback to the unburned reaction interface is an important controlling factor of the velocity of the reaction front and power delivery. In this paper, we investigate the effect of agglomerate surface tension and its relationship to surface residence time and heat feedback on the combustion characteristics by Si addition to an Al/KClO4 composite. Macroscopic imaging demonstrates a significant increase in burn rate with the addition of Si despite the fact that Si/KClO4 has a slightly lower energy density than Al/KClO4. Microscopic imaging coupled with three-color pyrometry reveals that molten liquid forms and evolves into spherical droplets on the burning surface, which are subsequently ejected from the surface. We find that the addition of Si results in a small increase in droplet size and a negligible impact on droplet temperature. However, the droplet formation rate on the surface is slower, leading to a significantly longer surface residence time. This leads to enhanced conductive heat feedback to the unburnt materials, thereby increasing the burn rate and energy release rate. We attribute the decreased droplet growth rate to the lowered surface tension of the liquid mixture with Si addition. This study highlights the crucial role of agglomerate physical property (e.g., surface tension) in influencing the combustion behavior of energetic composites.

Numerical simulation of condensation flows of humid air in the high-pressure pneumatic pressure-reducing valve

The phenomenon of condensation and ice formation exists in the high-pressure air pressure-reducing valve (HPAPRV), severely impacting the pressure-reducing valve's aerodynamic performance and operational safety. The problem is that the phase change of humid air in a high-pressure supersonic flow is not fully understood. This study aims to elucidate the condensation effect in high-pressure supersonic flows and the influence of the inlet conditions on the behavior of the condensate flow. A computational humid air phase model has been developed to investigate the formation of massive droplets due to phase-change processes. The numerical approach is validated by comparing it with experimental data, demonstrating a strong correlation between the two. The condensation properties of humid air in HPAPRV are described in detail. The effects of the condensation properties of humid air at different inlet conditions are discussed in detail. The results show that an increase in the inlet pressure increases the level of supersaturation, which increases the nucleation rate and the droplet growth rate and promotes droplet production. Increasing the inlet temperature has little effect on the HPAPRV pressure-reducing performance and predicts the temperature range of water vapor condensation before and after the valve port. The liquid mass fraction can be reduced by 50.9% by increasing the inlet temperature by 30 K, from 283 K to 313 K. When the inlet temperature is below 283 K, water vapor condenses in front of the valve port. Decreasing the inlet humidity reduces the degree and extent of supersaturation, which decreases the nucleation rate and the liquid mass fraction. When low inlet humidity (under 10%), the condensation phenomenon did not almost occur inside the pressure-reducing valve.

Evaluation of a New Droplet Growth Model for Small Droplets in Condensing Steam Flows

As the condensation phenomenon occurs in the low-pressure stages of steam turbines, an accurate modelling of the condensing flows is very crucial and has a significant impact on the development of highly efficient steam turbines. In order to accurately simulate condensing steam flows, it is essential to choose the right condensation model. Further research to enhance condensation models is of special importance because the outcomes of numerical studies of condensation models in recent years have not been entirely compatible with the experiments and there are still uncertainties in this area. Therefore, the main aim of this paper is to evaluate a proposed droplet growth model for modelling condensation phenomenon in condensing steam flows. The new model is derived to profit from the advantages of models based on the continuum approach for large droplets and those based on the kinetic theorem for small droplets, which results in the model being robust for a wide range of Knudsen numbers. The model is implemented into a commercial CFD tool, ANSYS Fluent 2022 R1, using UDFs. The results of the CFD simulations are validated against experimental data for linear cascades within the rotor and stator blade geometries of low-pressure steam turbine stages. The findings clearly demonstrate the superiority of the new model in capturing droplet growth, particularly for very small droplets immediately following nucleation. In contrast, widely used alternative droplet growth models tend to either underpredict or overpredict the droplet growth rate. This research significantly contributes to the ongoing efforts to enhance condensation modeling, providing a more accurate tool for optimizing the design and operation of low-pressure steam turbines, ultimately leading to a higher energy efficiency and a reduced environmental impact.

Nanorough Is Not Slippery Enough: Implications on Shedding and Heat Transfer.

Lowering droplet-surface interactions via the implementation of lubricant-infused surfaces (LISs) has received important attention in the past years. LISs offer enhanced droplet mobility with low sliding angles and the recently reported slippery Wenzel state, among others, empowered by the presence of the lubricant infused in between the structures, which eventually minimizes the direct interactions between liquid droplets and LISs. Current strategies to increase heat transfer during condensation phase-change relay on minimizing the thickness of the coating as well as enhancing condensate shedding. While further surface structuring may impose an additional heat transfer resistance, the presence of micro-structures eventually reduces the effective condensate-surface intimate interactions with the consequently decreased adhesion and enhanced shedding performance, which is investigated in this work. This is demonstrated by macroscopic and optical microscopy condensation experimental observations paying special attention at the liquid-lubricant and liquid-solid binary interactions at the droplet-LIS interface, which is further supported by a revisited force balance at the droplet triple contact line. Moreover, the occurrence of a condensation-coalescence-shedding regime is quantified for the first time with droplet growth rates one and two orders of magnitude greater than during condensation-coalescence and direct condensation regimes, respectively. Findings presented here are of great importance for the effective design and implementation of LISs via surface structure endowing accurate droplet mobility and control for applications such as anti-icing, self-cleaning, water harvesting, and/or liquid repellent surfaces as well as for condensation heat transfer.

Droplet Growth or Evaporation Does Not Buffer the Variability in Supersaturation in Clean Clouds

Abstract Water vapor supersaturation in clouds is a random variable that drives activation and growth of cloud droplets. The Pi Convection–Cloud Chamber generates a turbulent cloud with a microphysical steady state that can be varied from clean to polluted by adjusting the aerosol injection rate. The supersaturation distribution and its moments, e.g., mean and variance, are investigated for varying cloud microphysical conditions. High-speed and collocated Eulerian measurements of temperature and water vapor concentration are combined to obtain the temporally resolved supersaturation distribution. This allows quantification of the contributions of variances and covariances between water vapor and temperature. Results are consistent with expectations for a convection chamber, with strong correlation between water vapor and temperature; departures from ideal behavior can be explained as resulting from dry regions on the warm boundary, analogous to entrainment. The saturation ratio distribution is measured under conditions that show monotonic increase of liquid water content and decrease of mean droplet diameter with increasing aerosol injection rate. The change in liquid water content is proportional to the change in water vapor concentration between no-cloud and cloudy conditions. Variability in the supersaturation remains even after cloud droplets are formed, and no significant buffering is observed. Results are interpreted in terms of a cloud microphysical Damköhler number (Da), under conditions corresponding to , i.e., the slow-microphysics regime. This implies that clouds with very clean regions, such that is satisfied, will experience supersaturation fluctuations without them being buffered by cloud droplet growth. Significance Statement The saturation ratio (humidity) in clouds controls the growth rate and formation of cloud droplets. When air in a turbulent cloud mixes, the humidity varies in space and time throughout the cloud. This is important because it means cloud droplets experience different growth histories, thereby resulting in broader size distributions. It is often assumed that growth and evaporation of cloud droplets buffers out some of the humidity variations. Measuring these variations has been difficult, especially in the field. The purpose of this study is to measure the saturation ratio distribution in clouds with a range of conditions. We measure the in-cloud saturation ratio using a convection cloud chamber with clean to polluted cloud properties. We found in clouds with low concentrations of droplets that the variations in the saturation ratio are not suppressed.

Secondary droplet activation during condensational growth in convective clouds and its detection from satellites

By acting as cloud condensation nuclei (CCN), aerosol particles play a key role in the climate system. The CCN can be activated into cloud droplets at the cloud base (i.e., primary activation), or above it (i.e., secondary activation). This study shows the conditions required for secondary activation during the condensational growth phase in convective clouds. It also proposes a methodology for detecting this secondary activation from satellites. Using a spectral bin adiabatic parcel model, we simulate the vertical profile of cloud microphysical properties and demonstrate how different aerosol size distributions and updraft velocities greatly affect the secondary activation initiation and the cloud properties. The secondary activation slows down the cloud drop effective radius (re) growth rate with increasing height, and decreasing temperature (T), due to the relatively larger population of smaller droplets in the cloud parcel. Therefore, the vertical profile of re growth with height is slower than the adiabatic rate when the secondary activation occurs.The proposed physical principle was verified by satellite-retrieved T-re profiles and adiabatic cloud drop number concentrations (Nd) over the Amazon region. The results obtained from this study can be utilized to identify the secondary droplet activation during the condensational growth phase, which can lead to an overestimation of the retrieved Nd as well as suppression of warm rain. This improves our knowledge and observational capabilities of the role of aerosol particles in the microphysics, dynamics, and precipitation behavior of convective clouds. Plain text summarySmall particles serve as sites for cloud droplets' condensation at the cloud base (primary droplet activation) or above it (secondary activation). Once a droplet is activated at the cloud base, the surrounding water vapor condenses on it and the droplet grows - this is called condensational growth. The cloud droplet number concentrations (Nd) can be obtained by the satellite-retrieved vertical growth rate of droplet size in the condensational growth phase. However, new small droplets formed during this phase (i.e., secondary activation), might cause an overestimate in the retrieved Nd. This study presents the physical principle of secondary activation during condensational growth. Using a model, we simulate the condensational growth of aerosol particles, which act as cloud condensation nuclei (CCN), for different particle size distributions and thermodynamic conditions (i.e., vertical velocities). Our simulations show that secondary activation slows the cloud droplet growth rate with height due to the larger competition for the available water vapor promoted by the new drops. Comparisons between satellite-retrieved Nd of convective clouds and in-situ measurements show that the satellite retrievals overestimate Nd when the secondary activation of droplets is neglected. Our findings improve the understanding of aerosols' role in the condensational growth of droplets in convective clouds.

A novel numerical approach for simulating low-pressure and high-pressure non-equilibrium condensation in real gases

This paper presents a novel numerical approach for simulating rapidly expanding condensing flows across a wide range of thermodynamic conditions, including low-pressure steam, high-pressure steam, and supercritical carbon dioxide. The condensation model considers the non-ideality of the gas during droplet nucleation, and a modified kinetic Hertz-Knudsen droplet growth rate model is proposed. Real gas properties in the supercritical, superheated, and metastable regions are obtained using an in-house Python code and incorporated into the solver via User-Defined Functions as external look-up tables. The numerical model is extensively tested to demonstrate its accuracy and validity, and the results exhibit excellent agreement with experimental data. The model has been implemented in a commercial CFD solver, making it suitable for a range of applications and fluids.

Moisture Absorbing and Water Self‐Releasing from Hybrid Hydrogel Desiccants

AbstractAtmospheric moisture is a valuable resource for fresh water and potentially sustainable energy. However, direct harvesting water from moisture (the vapor form) remains the most challenging. Hybrid desiccants made from hydrogels embedded with salt offer promise in absorbing moisture. However, the desorption process often requires additional energy to heat the samples. Here, poly(acrylic acid) (PAA) hydrogels embedded with lithium chloride are prepared, demonstratng simultaneous moisture absorption and self‐release of liquid water at room temperature with 50–90% relative humidity. The water self‐releasing process can be separated into two distinct stages: 1) surface release, where water droplets grow on the hydrogel surface due to differences in nucleation and diffusion rates, and 2) bulk release, triggered by the collapse of polymer chains, subsequently releasing water from the hydrogel network. Factors such as salt concentration, hydrogel crosslinking density, and film thickness are investigated to better understand the moisture absorption and water‐releasing processes. Moreover, hydrophobic domains are introduced onto the salt‐embedded PAA films, creating an edge effect that enhances the droplet growth rates. When the hydrophobic domains are patterned, the movement of released water can be guided, resulting in a threefold increase in water removal rate attributed to gravitational force.

Improved Heat Transfer Modeling of Moist Air Condensation on Hydrophobic Metallic Surface

For dropwise condensation of water over a textured surface, the condensation rate is limited by the conduction resistance of the droplet. Conduction models with varying levels of approximation have been reported in the literature. In the present study, the conduction resistance of a spherical-cap droplet is derived by regression analysis of data generated by 3D numerical simulation. Results show that conduction resistance is sensitive to the choice of contact angle and droplet radius. The proposed model has significant differences from reported models which are based on 1-D and 2-D approximations. An important source of discrepancy is traced to additional heat flux localized at the three-phase contact line. Equivalently, high local heat flux serves to lower thermal resistance between the vapor-liquid interface and the condensing wall. The prediction of the droplet growth rate using the proposed conduction model is in good agreement with the experiments reported in the literature. The improved conduction resistance correlation is incorporated in the model for a space- and time-resolved dropwise condensation cycle for vapor condensing from moist air. Simulations with the improved condensation model show close agreement with experimental data, justifying the utility of an improved correlation for conduction resistance.

Small-molecule autocatalysis drives compartment growth, competition and reproduction.

Sustained autocatalysis coupled to compartment growth and division is a key step in the origin of life, but an experimental demonstration of this phenomenon in an artificial system has previously proven elusive. We show that autocatalytic reactions within compartments-when autocatalysis, and reactant and solvent exchange outpace product exchange-drive osmosis and diffusion, resulting in compartment growth. We demonstrate, using the formose reaction compartmentalized in aqueous droplets in an emulsion, that compartment volume can more than double. Competition for a common reactant (formaldehyde) causes variation in droplet growth rate based on the composition of the surrounding droplets. These growth rate variations are partially transmitted after selective division of the largest droplets by shearing, which converts growth-rate differences into differences in droplet frequency. This shows how a combination of properties of living systems (growth, division, variation, competition, rudimentary heredity and selection) can arise from simple physical-chemical processes and may have paved the way for the emergence of evolution by natural selection.

Low-pressure steam dropwise condensation on durable PFA-coated horizontal tube: Droplet dynamics in active region

The performance of low-pressure steam condensers is vital to the thermal energy utilization efficiency of the power cycle, refrigeration, heat pump, and seawater desalination systems. Dropwise condensation demonstrates better heat transfer performance than traditional filmwise condensation, and its performance is determined by the condensate droplet dynamic behaviors on hydrophobic surfaces. However, the droplet dynamics behavior on the horizontal tube has not been adequately investigated. In this study, a durable PFA-coated copper tube is achieved to sustain stable dropwise condensation at the vapor pressure ranging from 5 to 100 kPa. By optical observation of condensation on the horizontally oriented tube, the droplet dynamic characteristics and the influence of vapor pressure on the droplet dynamics are examined. We show that an ‘active droplet region’ (ADR), which is characterized by frequent droplet shedding and surface renewal promoting more active nucleation, growth, and departure of droplets, exists along the tube circumference. As a result of the varying droplet dynamics, a skew-normal distribution of sliding droplet numbers presents along the tube circumference. The droplets departing within ADR account for 80% of the total departure droplets. Then, the effect of steam pressure on the droplet dynamics behavior in ADR is studied systematically. The average sweeping frequency of droplets under steam pressure of 100 kPa increases by -122% more than that under steam pressure of 5 kPa. The growth rate of droplets and migration speed improve with steam pressure. Thus, the evolution rate of the transient size distribution becomes rapid as the vapor pressure increases. Taking vapor pressure and circumferential angle into consideration, a semi-empirical model for the prediction of condensate droplet departure radius on a horizontal tube is proposed, of which the standard deviation can be reduced to less than 15%. These findings and insights into droplet dynamic behaviors on PFA-coated hydrophobic horizontal tube are of great significance for the novel surface design strategy for the enhancement of dropwise condensation heat transfer.

VISUALIZATION OF DROPWISE CONDENSATION OF WATER VAPOR FROM EXPERIMENTS AND SIMULATION

Hydrophobic surfaces on which water vapor condensation is encountered in the form of droplets rather than a film is the preferred mode of condensation because of the large heat transfer coefficient and water production rate. The droplet pattern appearing on the surface depends on its orientation and the hydrophobicity characteristics. In the present work, patterns of dropwise condensation of vapor on various engineered surfaces were recorded from experiments and compared with simulations under various conditions of subcooling for pure vapor as well as moist air. Overall, condensation patterns reveal that droplet sliding frequency is higher for surfaces with large contact angles and small contact angle hysteresis. The growth rate of droplets of water vapor present in moist air is smaller than for pure vapor condensation.

The Growth Rate of DNA Condensate Droplets Increases with the Size of Participating Subunits.

Liquid-liquid phase separation (LLPS) is a common phenomenon underlying the formation of dynamic membraneless organelles in biological cells, which are emerging as major players in controlling cellular functions and health. The bottom-up synthesis of biomolecular liquid systems with simple constituents, like nucleic acids and peptides, is useful to understand LLPS in nature as well as to develop programmable means to build new amorphous materials with properties matching or surpassing those observed in natural condensates. In particular, understanding which parameters determine condensate growth kinetics is essential for the synthesis of condensates with the capacity for active, dynamic behaviors. Here we use DNA nanotechnology to study artificial liquid condensates through programmable star-shaped subunits, focusing on the effects of changing subunit size. First, we show that LLPS is achieved in a 6-fold range of subunit size. Second, we demonstrate that the rate of growth of condensate droplets scales with subunit size. Our investigation is supported by a general model that describes how coarsening and coalescence are expected to scale with subunit size under ideal assumptions. Beyond suggesting a route toward achieving control of LLPS kinetics via design of subunit size in synthetic liquids, our work suggests that particle size may be a key parameter in biological condensation processes.

Transient multiphysics coupled model for multiscale droplet condensation out of moist air

As a key physical process, water vapor condensation has attracted significant attention because of its potential in engineering applications. The non-condensable gas in the surrounding vapor has a significant influence on condensation heat transfer. Considering as a crucial aspect, this work developed a transient multiphysics coupled solver to investigate droplet condensation in a moist air environment (considering dry air as the non-condensable gas). The current solver couples the time-dependent vapor-liquid phase-change heat transfer, mass transport of water vapor, and two-phase fluid flow. In contrast to the classical thermal resistance theory model, this solver can capture the dynamic and strong coupling characteristics during condensation comprehensively. The results demonstrate that for small-scale droplets, vapor condensation is driven by the coupled internal conduction-dominated heat transfer and external vapor diffusion. As the droplet grows and the contact angle increases, internal convection driven by the Marangoni effect becomes increasingly important. The enhanced fluid mixing inside the droplet can affect both the internal heat transfer and the external vapor diffusion. Because of the significant diffusion resistance, the droplet growth rates in a moist air environment are reduced up to 1-2 orders of magnitude compared with the case of pure steam. For large-scale droplets, the internal convection can increase the droplet growth rate up to 18.7%. Furthermore, the contact angle, the subcooling temperature, and the relative humidity have significant influences on droplet condensation in a moist air environment. This work not only promotes the mechanistic understanding of condensation heat transfer in a moist air ambient but also provides a flexible solver for vapor-liquid phase change problems.

Enhancing Recurrent Droplet Jumping Phenomena on Heterogeneous Surface Designs

AbstractCoalescence‐induced droplet jumping phenomena on superhydrophobic surfaces can significantly enhance their heat transfer performances by effectively removing droplets from the surfaces. However, understanding the ideal design for condensing surfaces is still challenging due to the complex nature of droplet dynamics associated with their nucleation, coalescing, and jumping mechanisms. The intrinsic dynamic nature of droplet behaviors suggests the use of hierarchical concave morphology to account for the different length scales associated with each transport phenomenon. The hierarchical morphology thereby enables heterogeneous wetting characteristics by realizing both microscale droplets on superhydrophobic surfaces and nanoscale pinning regions beneath the droplets by arresting liquid residues after droplet jumping. Heat transfer performances are further examined by extracting physically meaningful descriptors, such as nucleation sites, droplet growth rates, and droplet jumping frequency, showing 44% enhancements when droplet nucleation sites are designed in selective locations.

Long-Term Stability of Pickering Nanoemulsions Prepared Using Diblock Copolymer Nanoparticles: Effect of Nanoparticle Core Crosslinking, Oil Type, and the Role Played by Excess Copolymers.

A poly(N,N′-dimethylacrylamide) (PDMAC) precursor is chain-extended via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of diacetone acrylamide (PDAAM) to produce PDMAC77-PDAAM40 spherical nanoparticles. Post-polymerization core-crosslinking of such nanoparticles was performed at 20 °C, and the resulting covalently stabilized nanoparticles survive exposure to methanol. The linear and core-crosslinked nanoparticles were subjected to high-shear homogenization in turn in the presence of n-dodecane to form macroemulsions. Subsequent processing of these macroemulsions via high-pressure microfluidization produced nanoemulsions. When using the core crosslinked nanoparticles, the droplet diameter was strongly dependent on the copolymer concentration. This indicates that such nanoparticles remain intact under the processing conditions, leading to formation of genuine Pickering nanoemulsions with a z-average diameter of 244 ± 60 nm. In contrast, the linear nanoparticles undergo disassembly to afford molecularly dissolved diblock copolymer chains, which stabilize oil droplets of 170 ± 59 nm diameter. The long-term stability of these two types of n-dodecane-in-water nanoemulsions with respect to Ostwald ripening was examined using analytical centrifugation. When prepared at the same copolymer concentration, Pickering nanoemulsions stabilized by core-crosslinked nanoparticles proved to be significantly more stable than the nanoemulsion stabilized by the amphiphilic PDMAC77-PDAAM40 chains. Moreover, higher copolymer concentrations led to a significantly faster rate of droplet growth. This is attributed to excess copolymer facilitating the diffusion of n-dodecane through the aqueous phase. Finally, analytical centrifugation is used to assess the long-term stability of the analogous squalane-in-water nanoemulsions. These systems are much more stable than the corresponding n-dodecane-in-water nanoemulsions, regardless of whether the copolymer is adsorbed as sterically stabilized nanoparticles or surface-active chains.