Heating, ventilation, air conditioning, and refrigeration (HVACR) systems are essential across residential, commercial, and industrial sectors but are critical energy consumers, and their refrigerant emissions and energy use present a major challenge to global sustainability goals. This state-of-the-art review moves beyond tracing the evolution of refrigerants to evaluate key international agreements, including the Kigali Amendment, and national regulations as primary drivers of the transition to sustainable refrigerants. It critically assesses current sustainable alternatives, including natural refrigerants and low-global warming potential synthetic options, such as hydrofluoroolefins, highlighting the trade-offs in their technical application, economic viability, safety, and infrastructure implications. Emerging solutions, including not-in-kind cooling technologies, innovative thermodynamic cycles, advanced materials, artificial intelligence, digitalization, and refrigerant lifecycle management, are examined to provide critical perspectives and guide future research, development, and deployment. By integrating regulatory, market, and technological insights, this review offers a comprehensive road map toward a sustainable, low-emission, and energy-efficient HVACR sector.
I embody the quintessential Californian spirit: Raised in Los Angeles, I spent weekends either at the beach or skiing at Big Bear, rode motorcycles, hiked the Sierras, and cherished the sounds of the Beach Boys and Creedence Clearwater Revival. Restless, undisciplined, and irreverent, I applied to only one college-UC Santa Barbara-mainly because that is where my high school friends were going. Although we like to think our intellect transcends culture, my childhood and teenage years were shaped by the rhythms of the 1960s: space flight, NASA, and virtually every episode of Star Trek (often watched multiple times). Unmoored, I entered college intending to major in biochemistry but drifted into chemistry while also immersing myself in the physics curriculum. I earned a PhD in chemical physics at Caltech, yet chose to work with a chemical engineer. When my thesis advisor was suddenly killed, I completed my dissertation at Xerox Palo Alto Research Center, working on solar cell materials alongside Cambridge-educated physicists. While my chemistry peers pursued academic postdocs, I went instead to IBM Yorktown Heights, dividing my time between silane reactor engineering and amorphous semiconductor physics. Would I ever become anchored? Here is the rest of the story that led to 43 years on the Berkeley faculty. Including motorcycles.
Free-energy surfaces (FESs) offer a unifying framework for understanding molecular-level structures, transformations, and thermodynamic stability. They distill the complexity of atomistic simulations into interpretable landscapes of metastable states, bridging molecular-level detail with macroscopic observables. This review provides researchers in molecular simulations, computational physical chemistry, and chemical engineering with a conceptual and practical guide to computing and interpreting FESs, from their statistical-mechanical foundations to modern machine learning approaches that are transforming the sampling, representation, and analysis of molecular systems.
Suspensions of particles dispersed in liquids are ubiquitous materials in industry and geophysics; relevant examples include cement and mud. At high particle concentration, in what is called a dense suspension, crowding induces multiscale interactions ranging from local, particle-level contact forces to macroscopic, system-spanning contact networks that dynamically evolve under applied shear. As the number of constraints on relative particle movement increases, the suspension viscosity rises, and eventually the material reaches a jammed state. In this review, we discuss frameworks developed to predict the rheological behavior of dense suspensions in the vicinity of jamming and describe the resulting flow-state diagram. Going beyond mean-field models, we discuss recent advances in understanding the contact network of spatially correlated particles. We also review recent developments for tailoring flow constraints at the particle level, both by particle geometry and by interactions induced by chemical bonds, which can be used to engineer the location and extent of different regimes in the flow-state diagram. We end with a set of issues and perspectives for future research, including possible ways to extend the current theoretical framework, apply simulations to suspensions comprising particles with more complex nonspherical or highly anisotropic shapes, and develop approaches to predict how molecular-scale details influence macroscopic flows.
Soaps are some of the oldest surfactants known to humans and remain indispensable in modern health, personal care, and sustainability agendas. Still, the simple chemistry of salts of fatty acids reveals a rich, multiscale landscape of self-assembly and flow behavior that controls soap processing and sensorial attributes. This review integrates phase behavior with rheological analysis to demonstrate how distinct microstructures, ranging from micellar to liquid-crystalline phases, possess unique rheological signatures. We further demonstrate how the coexistence and combination of these phases in a product such as a soap bar introduce additional levels of structural and rheological complexity. In addition, we highlight how linear viscoelastic analysis resolves relaxation hierarchies and how yield stress and thixotropic responses are inherent to the multiphase brick-and-mortar microstructures in products such as soap bars. The complex rheology of soap bars necessitates constitutive models that incorporate elasticity, plasticity, and time-dependent structure to predict extrusion or stamping performance. We also explore how the microstructure relates to cleaning efficacy, foam richness, lather dynamics, and rinse feel by coupling rheology measures with in-use dilution behavior. Finally, we discuss open questions, challenges, and future directions for developing more sustainable products.
