Recovering tiny nanoscale features using a general optical imaging system is challenging because of poor signal to noise ratio. Rayleigh scattering implies that the detectable signal of an object of ...size d illuminated by light of wavelength λ is proportional to d
/λ
, which may be several orders of magnitude weaker than that of additive and multiplicative perturbations in the background. In this article, we solve this fundamental issue by introducing the regularized pseudo-phase, an observation quantity for polychromatic visible light microscopy that seems to be more sensitive than conventional intensity images for characterizing nanoscale features. We achieve a significant improvement in signal to noise ratio without making any changes to the imaging hardware. In addition, this framework not only retains the advantages of conventional denoising techniques, but also endows this new measurand (i.e., the pseudo-phase) with an explicit physical meaning analogous to optical phase. Experiments on a NIST reference material 8820 sample demonstrate that we can measure nanoscale defects, minute amounts of tilt in patterned samples, and severely noise-polluted nanostructure profiles with the pseudo-phase framework even when using a low-cost bright-field microscope.
Flexible thermoelectric generators (f-TEGs) are promising solutions to power supply for wearable devices. However, the high fabrication costs and low output power density of conventional f-TEGs limit ...their applications. Here, we present a bulk-material-based f-TEG featuring multifunctional copper electrodes for heat concentration and dissipation and fabrics for comfort and heat-leakage reduction. When worn on the forehead, our f-TEG’s maximum output power density (based on the device’s area) reaches 48 μW/cm2 at a wind speed of 2 m/s and an ambient temperature of 15°C. A light-emitting diode (LED) powered by our f-TEG headband with 100 pairs of thermoelectric pillars can illuminate a paper for reading in a dark room at 17.5°C without an external heat sink or forced convection at the cold side. This work provides a general design approach for high-performance f-TEGs at a low cost. The device-level perspectives fill the critical knowledge gap between state-of-the-art material innovations and practical thermoelectric applications.
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•A mushroom-like f-TEG achieves high output power density on human skin•The copper electrodes also serve as heat concentrators and spreaders and spacers•An analytical model is developed to predict an f-TEG’s energy-harvesting performance
Xu et al. report an f-TEG featuring a mushroom-like structure that achieves high output power density on human skin without external heat sink via thermal design. The work provides an example of the design process of f-TEGs and paves the pathway toward scalable fabrication of low-cost and high-performance f-TEGs.
Environmental scanning electron microscopy (ESEM) is a powerful technique that enables imaging of diverse specimens (e.g., biomaterials, chemical materials, nanomaterials) in a hydrated or native ...state while simultaneously maintaining micro‐to‐nanoscale resolution. However, it is difficult to achieve high signal‐to‐noise and artifact‐free secondary electron images in a high‐pressure gaseous environment due to the intensive electron‐gas collisions. In addition, nanotextured substrates can mask the signal from a weakly scattering sample. These drawbacks limit the study of material dynamics under extreme conditions and correspondingly our understanding in many fields. In this work, an imaging framework called Quasi‐Newtonian ESEM is proposed, which introduces the concepts of quasi‐force and quasi‐work by referencing the scattering force in light–matter interactions, to break these barriers without any hardware changes. It is shown that quasi‐force is a more fundamental quantity that has a more significant connection with the sample morphology than intensity in the strongly scattering regime. Experimental and theoretical studies on the dynamics of droplet condensation in a high‐pressure environment (up to 2500 Pa) successfully demonstrate the effectiveness and robustness of the framework and that the overwhelmed signal of interest in ESEM images can be reconstructed through information stored in the time domain, i.e., frames captured at different moments.
This work introduces an electron imaging framework called quasi‐Newtonian environmental scanning electron microscopy (ESEM) that outperforms conventional ESEM in characterizing materials dynamics in high‐pressure gaseous environments.
Solar desalination holds significant promise for the water-energy nexus. Recent advances in passive solar desalination using thermal localization show great potential for high-efficiency freshwater ...production, which is particularly beneficial for areas without well-established water and energy infrastructure. However, there is a significant knowledge gap between laboratory scale innovation and commercial adoption. In this review, we discuss two critical factors - water production and reliability - which, if addressed systematically, could enable high-performance thermally-localized solar desalination systems. We show that optimizing heat and mass transfer of the entire device and recycling the latent heat of condensation are important to enhance total water production. Meanwhile, we discuss the potential of novel system architectures and fluid flow engineering to enable anti-fouling and robust desalination devices. In addition, we present techno-economic analysis that highlights the balance between water production, reliability, and cost. A criterion for economic feasibility is provided by comparing the price of desalinated water with commercially available bottle and tap water, which provides a roadmap for future development of solar desalination technologies.
This review summarizes recent advances in passive thermally-localized solar desalination and provides a roadmap for more efficient, reliable, and commercially feasible solar desalination technologies.
•Modeling framework of thermally-localized multistage solar stills (TMSS) is developed.•Heat and mass transfer in the TMSS are analyzed in detail.•Optimization strategies for the TMSS are ...presented.•Ultrahigh solar-thermal cumulative efficiency over 700% is predicted.
Seawater desalination is a promising solution to global water shortage. Commercially available desalination technologies typically require large installations which can be impractical for developing regions without well-developed infrastructure. Passive solar desalination promises a viable solution, but can suffer from low efficiencies. Recent advances in the thermal design of small-scale solar desalination systems have demonstrated the potential for high-efficiency solar desalination in portable systems. In particular, the concept of a thermally-localized multistage solar still (TMSS) – which combines localized heating of a capillary flow with condensation heat recycling – has been experimentally demonstrated very recently and achieved over 100% solar-thermal cumulative efficiency. However, a fundamental understanding of the heat and mass transfer, efficiency limits and optimization strategies are missing in the literature. This work presents a modeling framework that evaluates the thermal and vapor transport in a model TMSS system with varying device configuration and predicts its solar desalination efficiency. We demonstrate that an ultrahigh solar-thermal cumulative efficiency, many times higher than that of conventional solar stills, can be achieved by optimizing the number of stages and device geometry. Specifically, our modeling shows that the efficiency of the capillary fed TMSS is limited by the dissipation of thermal energy to the environment during condensation and significant gains in efficiency can be achieved by minimizing this loss. This work provides insights into physical processes critical for thermally-localized portable solar distillation which could lead to high-performance desalination or water purification technologies.
•We developed a mesoscopic approach for describing the nanoscale liquid-vapor interfacial statics and dynamics.•Mesoscopic predictions of density profile, interfacial thickness and the surface ...tension were found to agree very well with the NIST recommended data and molecular theories of capillarity.•The mesoscopic approach can resolve the nanoscale liquid-vapor interfacial transport phenomena for real fluids at an ultrahigh resolution with affordable computation costs.
Nanoscale liquid-vapor interfacial transport phenomena are of great significance to a variety of applications including evaporation, condescension, boiling and micro/nano-fluidics. In this work, we propose a mesoscopic approach to describe the nanoscale liquid-vapor interfacial statics and dynamics by combining the pseudopotential multiphase lattice Boltzmann method, the theorem of corresponding states and the principle of dynamic similarity. We demonstrate that our mesoscopic predictions of density profile, interfacial thicknesses and surface tensions for planar liquid-vapor interfaces of various real fluids agree very well with the NIST recommended data and molecular theories of capillarity in a very wide temperature range. We quantify the size effects of nano-bubbles and nano-droplets on the surface tension of water under a saturation temperature of 100°C. We show that the surface tensions of nano-bubbles decrease while the surface tensions of nano-droplets increase with increasing size, and the predictions from static cases and dynamic cases are consistent. The mesoscopic approach proposed in this paper could well resolve nanoscale liquid-vapor interfacial phenomena for various real fluids with high resolution, high accuracy and affordable computation cost in a very wide temperature range, which paves the way for quantitative investigations of nanoscale liquid-vapor interfacial transport for real fluids in practical applications.
Bubble growth and departure are ubiquitous phenomena in gas-evolving reactions, which govern the overall energy and mass transport. However, an in-depth understanding of the relationship between ...bubble dynamics and the electrochemical processes, in particular, the wettability effect on a gas-evolving porous electrode remains elusive. Here, we report the bubble dynamics and overpotential observed during alkaline water splitting on a polytetrafluoroethylene (PTFE) deposited nickel porous electrode. A slight decrease in hydrophilicity induced a drastic transition of bubble dynamics and a significant increase of the transport overpotential. We show that the porous electrode transitioned from a liquid-filled state to a gas-filled state when varying the wettability, which changed the bubble departure sizes and bubble coverage. As a result, there were substantial changes of the transport overpotential. Our work elucidates the fundamental relationship between wettability and water splitting characteristics, which provides a practical scenario for structuring the electrode for gas-evolving reactions.
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•Drastic transition of bubble dynamics on wettable/non-wettable porous electrodes•Significant change of overpotential on wettable/non-wettable porous electrodes•Design guideline for high-performance porous gas-evolving electrodes
Electrochemical gas-evolving reactions play a crucial role in many industrial energy conversion and storage processes. The continuous gas production leads to the evolution of bubbles at the reaction sites, which further result in energy loss due to the increase of transport resistance. To enable high-performance electrochemical systems, bubble dynamics during gas-evolving reactions have attracted particular interest recently. Yet the fundamental relationship among gas-evolving electrode wettability, bubble dynamics, and overpotential has not been well understood. In this work, we investigate the bubble dynamics and the resulting overpotential in alkaline water splitting by engineering the wettability of a porous electrode. The insights gained from this study not only shed light on the fundamentals among electrode wettability, bubble dynamics, and overpotential but also provide design guidelines for porous electrodes to enable high-performance gas-evolving reactions.
Electrochemical gas-evolving reactions have been widely used for industrial energy conversion and storage processes. Bubbles generated on electrodes create extra transport resistance and induce undesired overpotential. Therefore, the fundamental understanding of bubble dynamics on gas-evolving electrodes is of particular importance. In this work, the relationship among porous electrode wettability, bubble dynamics, and overpotential was investigated. Distinct bubble growth and departure modes for wettable/non-wettable electrodes were observed, leading to a general design guideline for high-performance porous electrodes.
Boiling crisis due to bubble interactions Zhang, Lenan; Gong, Shuai; Lu, Zhengmao ...
International journal of heat and mass transfer,
January 2022, 2022-01-00, 20220101, Letnik:
182
Journal Article
Recenzirano
•A mechanistic and predictive theory for the boiling crisis, combining the thermo-fluidic interaction between bubbles and the stochastic interaction of nucleation sites, is proposed.•A dimensionless ...boiling crisis constant during the saturated pool boiling crisis is identified.•Quantitative and simultaneous predictions of the critical heat flux and the corresponding wall superheat are achieved.
The boiling crisis determines the maximum heat flux for the safe operation of boiling equipment, which is widely used in various applications including power generation, thermal management of electronics and water desalination. Here we present a mechanistic and predictive theory for the boiling crisis, combining the thermo-fluidic interaction between bubbles and the stochastic interaction of nucleation sites. Using Rayleigh and Poisson distributions, we demonstrate that the boiling crisis occurs when the population of isolated nucleation sites reaches the maximum. We identified a dimensionless boiling crisis constant 1/πe, which universally relates the bubble base diameter to the isolated nucleate site density during the saturated pool boiling crisis. This finding is supported by our direct numerical simulation as well as by previous numerical and experimental results. Combining the thermo-fluidic and stochastic interaction, quantitative and simultaneous predictions of the critical heat flux (CHF) and the corresponding wall superheat at the CHF were achieved, which agrees with existing experimental data. Our theory thus offers a new avenue for understanding the boiling crisis, and therefore can serve as a guideline for the future boiling enhancement design.
High-flux evaporators are important for various fundamental research and industrial applications. Understanding the heat loss mechanisms, especially the contribution of natural convection during ...evaporation is thus a ubiquitous process to predict and optimize the performance of evaporators. However, a comprehensive analysis on natural convection heat transfer, where the vertical Stefan flow due to evaporation couples with buoyancy driven convective flow has not been carefully considered. In this work, we developed a theoretical framework to elucidate the effect of Stefan flow on natural convection during evaporation. This theory incorporates the vertical Stefan flow into the conventional boundary layer theory. We found that a significant suppression of natural convection can be induced by a weak Stefan flow owing to the increase of boundary layer thickness. To understand this phenomenon, we discuss the governing mechanisms at different Stefan flow regimes. We provide a theoretical correlation to the overall heat transfer which includes both effects of the Stefan flow velocity and the buoyancy force. We finally predict the effect of natural convection on an evaporator at different operating temperatures. The heat loss from natural convection no longer monotonically increases with the superheat temperature due to the effect of Stefan flow suppression. As a result, there is an approximately 40% overestimation of the natural convection contribution at saturation temperature using conventional theory. This work improves the fundamental understanding of the natural convection during evaporation and can help guide future high-performance evaporator designs.
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