Lenan Zhang

Interactions between boiling bubbles are well known to influence departure dynamics and heat transfer, yet their role in governing nucleation stability, whether sites activate reproducibly, persist, and deactivate under changing thermal loads, remains poorly understood. Here we show that nucleation can be a collective process: neighboring sites at close spacings exhibit reduced variability and sustained activity, consistent with a non-local hydrodynamic shielding mechanism whereby neighboring bubbles slow the intervening flow, reducing convective heat removal and stabilizing vapor embryos. To isolate near-wall nucleation dynamics from bubble-scale vapor removal, we design surfaces comprising pairs of cavities, with intra-pair spacing tuned to the boundary layer scale and inter-pair separation to the departure diameter scale. While the former governs nucleation behavior, the latter governs collective vapor removal once sites are fully active, yielding transitions between excessive, promotive, and isolated departure regimes. Together these results establish a multiscale framework for designing robust, high-performance boiling surfaces.

Interactions between boiling bubbles are well known to influence departure dynamics and heat transfer, yet their role in governing nucleation stability, whether sites activate reproducibly, persist, and deactivate under changing thermal loads, remains poorly understood. Here we show that nucleation can be a collective process: neighboring sites at close spacings exhibit reduced variability and sustained activity, consistent with a non-local hydrodynamic shielding mechanism whereby neighboring bubbles slow the intervening flow, reducing convective heat removal and stabilizing vapor embryos. To isolate near-wall nucleation dynamics from bubble-scale vapor removal, we design surfaces comprising pairs of cavities, with intra-pair spacing tuned to the boundary layer scale and inter-pair separation to the departure diameter scale. While the former governs nucleation behavior, the latter governs collective vapor removal once sites are fully active, yielding transitions between excessive, promotive, and isolated departure regimes. Together these results establish a multiscale framework for designing robust, high-performance boiling surfaces.

Solar-driven water splitting offers a direct path to green hydrogen by using sunlight to split water into hydrogen and oxygen, enabling flexible energy storage and hydrogen fuel production. Yet, turning lab-scale demonstrations into widespread deployment requires overcoming intertwined fundamental and engineering challenges. This Voices piece gathers researchers from around the world who work on photocatalytic, photoelectrochemical, and biophotoelectrochemical water splitting to discuss bottlenecks, promising strategies, and the tools most helpful for advancing solar fuels production. They address design principles for materials and devices, emerging configurations and benchmarking concepts, pathways to commercialization, and how related catalytic processes and adjacent fields can guide sustainable hydrogen evolution. What becomes clear is that achieving scalable solar-driven hydrogen production will require integrated, multi-disciplinary efforts that bridge responsible discovery and deployment.

Photosynthetic biomanufacturing offers a sustainable route to generate valuable bioproducts by harnessing microorganisms such as algae to convert sunlight and carbon dioxide into biomass. A major barrier to efficient production is that light penetrates poorly into dense algal cultures, restricting photosynthesis to a thin surface layer and severely limiting the solar energy that can be utilized for algal growth and biomass production. Here, we present a material-based strategy to overcome this fundamental bottleneck by deploying bulk-scattering, index-matched optical fibers that redistribute sunlight uniformly throughout the culture volume. These fibers are made from amorphous hydrogels with a refractive index closely matched to that of algal media and contain scattering nanoparticles that redirect light to achieve volumetric illumination. When integrated into solar-powered algal systems, the fibers enable dense and sustained algal growth at 0.8 to 1.4 g L −1 over 2 mo of semicontinuous outdoor cultivation, resulting in volumetric biomass productivity of 0.15 g L −1 day −1 and photosynthetic efficiency of 1.4%, significantly higher compared to algal systems without fibers. This study demonstrates the transformative potential of optical modulation to the long-standing low productivity in dense algal culture, providing a scalable, sustainable, and efficient pathway for solar-driven biomanufacturing.

Two-dimensional (2D) materials hold significant promise for next-generation nanoelectronics while introducing critical thermal management challenges. The extreme thinness, anisotropic heat transport, and unique interfacial coupling make the thermal design principles of 2D materials-based electronics fundamentally different from that of bulk systems. In this Perspective, we discuss exciting opportunities that leverage recent advances in thermal science to unlock unprecedented thermal management capabilities, thereby providing new insights into the design of 2D materials-based electronics. We first provide an overview of key thermophysical properties of 2D materials that govern thermal management performance, including in-plane thermal conductivity, interfacial thermal conductance, and thermal expansion coefficient. Then, we not only highlight important physical phenomena distinct from bulk materials but more notably illustrate how the interplay among these thermophysical properties ultimately dictates the unique characteristics of heat dissipation and thermomechanical stress in 2D materials-based electronic devices. With both material- and device-level insights, we identify key thermal bottlenecks in existing 2D materials-based electronic devices and present a fully quantitative roadmap toward an electrical and thermal co-design strategy for substantially improved thermal management. Bridging thermal innovations to the device design, we envision this Perspective can foster next-generation thermal management technologies for reliable 2D materials-based electronics.

Scaling up water electrolysis to high current densities inevitably induces massive gas evolution at electrode interfaces, posing critical challenges to interfacial transport and system stability. While individual bubble dynamics have been extensively studied, the collective behaviors and critical transitions of densely populated bubbles under extreme conditions remain poorly understood. Here we present a synchronized acoustic–visual framework for characterizing hydrogen bubble evolution during alkaline electrolysis, enabling the segmentation of the polarization curve into four distinct regimes: nucleate whisper, resonant growth, cascade detachment, and turbulent film. This regime map constitutes an electrolytic gas evolution curve, an electrochemical analogue of the classical boiling curve. We identify a critical current density at which interfacial gas accumulation triggers a sharp performance inflection. Near this threshold, bubble dynamics exhibit scale-free acoustic energy distributions and clustered detachment behavior, signaling the emergence of self-organized criticality. Our findings establish a mechanistic foundation linking bubble evolution modes, dynamic acoustic fingerprints, and mass transport transitions under extreme electrolysis conditions, offering a new paradigm for monitoring, diagnosing, and optimizing high-rate gas-evolving electrochemical systems.

Salt creeping, the precipitation of salt crystals ahead of the liquid front of an evaporating salt solution, poses severe challenges to agriculture, buildings and structures, maritime field, and art conservation while holding significant promise for wastewater treatment and mineral extraction. Despite their critical role, insights into the key mechanisms of salt creeping remain elusive. Here, we leverage in situ X-ray microscopy to unravel the onset of salt creeping at a single-crystal level. Notably, we directly image the first salt crystal pinned on the solid–liquid interface, which penetrates the liquid meniscus and initiates a cascading crystallization process. New salt crystals precipitate from the extended meniscus created by the initial salt crystal. Combining X-ray imaging with thermodynamic analysis, we demonstrate that the formation of the first pinned crystal is associated with a critical contact angle of the liquid meniscus. This work elucidates the microscopic origin of salt creeping, shedding light on the effective manipulation of salt crystallization for various applications.

Droplets are prone to adhere or "pin" on solid surfaces which contain unavoidable micro- and nanoscale surface defects formed through chemical and topographical heterogeneity. To initiate droplet motion, potential energy gradients, surface energy gradients, or external energy input are needed. Here, in contrast to established wisdom, we show that properly designed surface heterogeneity can promote microdroplet self-transport without any external force or anisotropy. In the presence of topological defects, microdroplets can take advantage of contact line pinning to generate contact line and corresponding contact angle asymmetry, leading to spontaneous motion over distances 10-20 times larger than the droplet radius. The outcomes of this work present an alternative pathway for taking advantage of intrinsic surface heterogeneity to achieve droplet mobility in a range of applications, where passive droplet motion is desired.

A high-efficiency and sustainable approach produces green hydrogen with natural sunlight and seawater as the sole inputs.

Gas bubble evolution plays a pivotal role in water electrolysis. Electrochemical gas evolution reactions operating at high current densities represent a promising future trend for improving efficiency. However, the lack of reliable bubble detection methods in electrochemical gas evolution systems, as well as the limited understanding of bubble behavior at different current levels, hinder a comprehensive understanding of the actual bubble dynamics under various voltage and current conditions. To address this challenge, a novel methodology that utilizes acoustic emission signals is proposed to evaluate bubble behavior in the electrolyzer. The dynamic variations in the amplitude and frequency of the acoustic emissions can be correlated with the different stages of bubble evolution. By combining this acoustic technique with high-speed optical imaging, the relationship between the acoustic signal variations and the characteristics of bubble release under different voltage and current levels can be unvealed. Furthermore, clustering algorithms from machine learning are employed to determine bubble acoustic characteristics under various conditions based on the acoustic variations, which allows for redefining the polarization curve. These findings significantly enhance the understanding of the actual bubble behavior under different voltage and current intensities, going beyond the traditional pool boiling curve-based approach. The results demonstrate that acoustic emission can serve as an effective and non-intrusive tool for monitoring bubble formation in gas evolution reactions, making it particularly valuable for applications in non-transparent electrolyzer cells. This facile operating approach offers a new perspective for studying and optimizing electrochemical gas evolution processes.

Leveraging the self-amplifying salt creeping and efflorescence effects, the salt crystals self-assemble to form a hierarchical porous salt evaporator, enabling passive liquid supply and efficient evaporation.

Bubbles play a ubiquitous role in electrochemical gas evolution reactions. However, a mechanistic understanding of how bubbles affect the energy efficiency of electrochemical processes remains limited to date, impeding effective approaches to further boost the performance of gas evolution systems. From a perspective of the analogy between heat and mass transfer, bubbles in electrochemical gas evolution reactions exhibit highly similar dynamic behaviors to them in the liquid-vapor phase change. Recent developments of liquid-vapor phase change systems have substantially advanced the fundamental knowledge of bubbles, leading to unprecedented enhancement of heat transfer performance. In this Review, we aim to elucidate a promising opportunity of understanding bubble dynamics in electrochemical gas evolution reactions through a lens of phase change heat transfer. We first provide a background about key parallels between electrochemical gas evolution reactions and phase change heat transfer. Then, we discuss bubble dynamics in gas evolution systems across multiple length scales, with an emphasis on exciting research problems inspired by new insights gained from liquid-vapor phase change systems. Lastly, we review advances in engineered surfaces for manipulating bubbles to enhance heat and mass transfer, providing an outlook on the design of high-performance gas evolving electrodes.

The presence of gas bubbles in electrochemical systems, such as water-splitting, significantly increases overpotential and diminishes energy efficiency. High-fidelity simulation holds significant promise to gain mechanistic understanding of bubble dynamics and guide high-performance electrolytic cell design. However, the extreme length scales involved into electrochemical gas evolving systems, from sub-nanometer dictated by the electrical double layer (EDL) to a few centimeters featured by the electrolytic cell, have posed a huge challenge to enable sufficient numerical accuracy and superior computational efficiency. As a result, state-of-the-art numerical approaches either can only simulate a nanoscale computational domain or neglect the electrochemical kinetics within the EDL, impeding an in-depth understanding of how bubbles could intervene with the electrochemical process. In this work, we demonstrate a full-field simulation approach of electrochemical gas evolving reactions that can capture the electrochemical kinetics of the EDL in a centimeter-scale electrolytical cell with the presence of micro-to-millimeter scale bubbles. To resolve all characteristic length scales, the EDL is geometrically decoupled from the electrolytical cell but physically coupled with the bulk electrolyte and gas bubble through a quasi-1D treatment. As a result, the Nernst-Planck-Poisson-Boltzmann model can be rigorously solved in both the EDL and the rest of the electrolytical cell with highly affordable computational cost. Taking hydrogen evolution reaction as an example, we validated our approach by comparing with a variety of existing numerical and experimental results. For the first time, the impact of bubbles on the increase of overpotential observed in experiments can be quantitatively confirmed through numerical simulation. More notably, compared to state-of-the-art high-fidelity simulations, our approach exhibits a remarkable computational efficiency, which reduced the computational time by a factor of 100,000. This work provides a viable solution to simulate electrochemical gas evolving reactions with highly desirable numerical accuracy and unprecedented computational efficiency, which can serve as an effective tool to understand bubble dynamics and guide the design of next-generation electrolytical cells.

Abstract Sorption‐based atmospheric water harvesting (SAWH) offers a sustainable strategy to address the global freshwater shortage. However, obtaining sorbents with excellent performance over a wide relative humidity (RH) range and devices with fully autonomous water production remains challenging. Herein, magnesium chloride (MgCl 2 ) is innovatively converted into super hygroscopic magnesium complexes(MC), which can effectively solve the problems of salt deliquescence and agglomeration. The MC are then integrated with photothermal aerogels composed of sodium alginate and carbon nanotubes (SA/CNTs) to form composite aerogels, which showed high water uptake over a wide RH range, reaching 5.43 and 0.27 kg kg −1 at 95% and 20% RH, respectively. The hierarchical porous structure enables the as‐prepared SA/CNTs/MC to exhibit rapid absorption/desorption kinetics with 12 cycles per day at 70% RH, equivalent to a water yield of 10.0 L kg −1 day −1 . To further realize continuous and practical freshwater production, a fully solar‐driven autonomous atmospheric water generator is designed and constructed with two SA/CNTs/MC‐based absorption layers, which can alternately conduct the water absorption/desorption process without any other energy consumption. The design provides a promising approach to achieving autonomous, high‐performance, and scalable SAWH.

In this project, our MIT, Purdue, and GE team aims to design, model, fabricate, and test a novel high temperature, compact, and durable ceramic heat exchanger to be operated under high temperature and pressure conditions for aerospace applications.Our approach is grounded in introducing multiscale porosity, i.e., centimeter-scale channels embedded with micrometerscale channels, into the ceramic heat exchanger to significantly improve its heat transfer performance and mechanical strength while maintaining minimal pressure losses.We first developed high-fidelity thermal-fluid-mechanical model capable of precisely capturing the heat transfer rate, temperature profile, pressure drop, and mechanical stress throughout the entire heat exchanger design.Guided by our model, we identified the optimal design parameters for the SiC heat exchanger body and manifolds.Then, we established a completed fabrication procedure to create multiscale features in the ceramic heat exchanger, including co-extrusion, lamination, burnout, and sintering.We fabricated multiple heat exchanger bodies consisting of 6 6 and 3 3 centimeter-scale channels where each individual centimeter-scale channel comprises 625 crack-free microchannels with 90 m 90 m opening.Owing to the multiscale features, our fabricated heat exchanger bodies exhibited desirable mechanical strength with 156 MPa flexural strength under 1300 o C. Despite the demonstrated highly tailorable microscopic features and superior mechanical strength, we identified delamination due to the complex interaction among ceramic, polymer, and gas species can be a critical challenge to create fully defect-free heat exchanger, which requires further fundamental investigations in future works.To test the heat exchanger performance, we constructed a high-temperature and high-pressure experimental apparatus that can be safely operated under 400 o C and 4 bar.With insights gained from mechanistic modeling, material development, and detailed characterization, a cost model was finally developed to understand the market potential of the developed technology, where a cost of $43,000 o C/kW was envisioned.This project developed a transformative approach to high-performance heat exchanger design.The thermal-fluidmechanical design approach developed in this project can serve as a generic tool to guide the design of various heat-exchangers operated under high-temperature and high-pressure conditions.The material fabrication approach established in this project can be a useful guide for ceramic processing at extreme length scales.

Advances in two-dimensional (2D) devices require innovative approaches for manipulating transport properties. Analogous to the electrical and optical responses, it has been predicted that thermal transport across 2D materials can have a similar strong twist-angle dependence. Here, we report experimental evidence deviating from this understanding. In contrast to the large tunability in electrical transport, we measured an unexpected weak twist-angle dependence of interfacial thermal transport in MoS 2 bilayers, which is consistent with theoretical calculations. More notably, we confirmed the existence of distinct regimes with weak and strong twist-angle dependencies for thermal transport, where, for example, a much stronger change with twist angles is expected for graphene bilayers. With atomic simulations, the distinct twist-angle effects on different 2D materials are explained by the suppression of long-wavelength phonons via the moiré superlattice. These findings elucidate the unique feature of 2D thermal transport and enable a new design space for engineering thermal devices.

Green hydrogen production powered by photovoltaics provides a clean solution for energy conversion and storage. The key parameter of photovoltaic electrolysis (PV-EC) system is the solar-to-hydrogen (STH) efficiency. Previous studies have been mostly focused on improving the efficiency of each system individually using multi-junction PV materials, catalysts and electrolytes. However, the coupling between photovoltaic and electrochemical devices is still elusive to achieve high STH efficiency in a system level. In our study, the optimization strategies of PV-EC systems are demonstrated to approach thermodynamic limit (26.6%) for green hydrogen production. First, we list three design parameters for modeling, namely number of PV, number of EC and the area ratio between PV and EC. Then we model the crystalline silicon PV coupled with a real EC system. The STH efficiency varies under different PV and EC combinations. The STH efficiency map clearly defines three regions indicating the engineering space for PV-EC coupling, Tafel slope and area ratio. The optimization strategies presented in this study can lead to further advancements in coupled PV-EC systems with well-controlled performance for different conditions.

Acoustic deformation potential (ADP) quantifies carrier-acoustic phonon coupling and is essential for dissecting transport physics in thermoelectrics. Herein, we report the use of ultrafast spectroscopy of coherent acoustic phonons (CAPs) to characterize the ADP of thermoelectric materials, using ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$ as an example. The photon energy-dependent amplitudes of the CAP-induced oscillatory reflectance were used to determine the ADP coupling constant, agreeing well with that from first-principles calculations. This method relies on the transient Coulombic interaction between carriers and acoustic phonons, free of influence from other scattering channels. It is shown that the method is particularly feasible for the study of thermoelectric materials, because their common features of strong phonon anharmonicity and small band gaps make the measurement insensitive to the uncertainty of carrier diffusion coefficients, ensuring its accuracy.

Adsorption systems promise to address energy storage, water harvesting, and carbon capture, among other applications in energy and sustainability. Improving the kinetics of the sorbent layer is essential to enable substantial enhancement in the performance of such systems, but challenges remain owing to the highly tortuous and random distribution of adsorbents. Here, we present a boiling-assisted channel-templating (BACT) coating method to enhance the kinetics of sorbent layers. Driven by boiling-induced vapor flow, randomly distributed adsorbents become well aligned, approaching the theoretical minimum tortuosity of porous structures. Using water adsorption in zeolite-coated copper foam, we demonstrate 2× improvement in effective diffusivity. We develop design guidelines to control the porous structure. For AQSOA-Z02 coating, the BACT method was able to reduce tortuosity to 1.09. We show the impact of BACT on adsorptive systems where cyclability can be improved by ≈1.7× compared with state-of-the-art coatings. This work demonstrates a simple, low-cost, and scalable approach to enhance sorption kinetics.

Water vapor sorption is a ubiquitous phenomenon in nature and plays an important role in various applications, including humidity regulation, energy storage, thermal management, and water harvesting. In particular, capturing moisture at elevated temperatures is highly desirable to prevent dehydration and to enlarge the tunability of water uptake. However, owing to the thermodynamic limit of conventional materials, sorbents inevitably tend to capture less water vapor at higher temperatures, impeding their broad applications. Here, an inverse temperature dependence of water sorption in poly(ethylene glycol) (PEG) hydrogels, where their water uptake can be doubled with increasing temperature from 25 to 50 °C, is reported. With mechanistic modeling of water-polymer interactions, this unusual water sorption is attributed to the first-order phase transformation of PEG structures, and the key parameters for a more generalized strategy in materials development are identified. This work elucidates a new regime of water sorption with an unusual temperature dependence, enabling a promising engineering space for harnessing moisture and heat.