Lyfe: learning to learn better

It is difficult to exactly define when research in micro/nanoscale heat transfer was initiated in the heat transfer community worldwide. There was no single paper that really started the field and there wasn’t any single inspiring lecture such as the famous “There is Plenty of Room at the Bottom,” that Richard Feynman gave in 1959. The physics and chemistry community had been looking at some microscopic aspects of phase change and transport phenomena for several decades. In fact, a few publications in microscale heat transfer that appeared in this journal can be traced back to the 1960s. However, it is clear that by the end of the 1980s, there was an increasing interest within the heat transfer, as well as the engineering community at large, for studying heat and mass transfer at small length and time scales. This was partly driven by the tremendous success of microelectronics and a clear vision that devices were going to become smaller and faster, and that future developments would require an extensive knowledge base for the continuously shrinking time and length scales that would reach tens of nanometers within decades. This also coincided with the invention of a wide variety of experimental tools such as the scanning tunneling and force microscopes and femtosecond lasers, which provided immediate access to phenomena at nanoscales. The ability to make microdevices using MEMS technology became standardized at around the same time. In addition, computing power became readily available such that molecular dynamic and stochastic simulations of micro/nanoscale thermal phenomena became feasible. All these factors put together led to an intense influx and genesis of new theoretical concepts, experimental techniques, and device designs, which led to a remarkable pace of progress. It is perhaps fair to say that over the past decade, micro/nanoscale heat transfer has been the most exciting and active area of research within the heat transfer community, and has also attracted the most amount of research funding. What is most heartening is that the participation has been highly collaborative and interdisciplinary, involving researchers from mechanical, chemical and electrical engineering, as well as physics, chemistry and materials science. From the very start, research in this field had an international flavor, which led to several focused workshops and symposia involving people from around the world, but mainly from the USA, Asia, and Europe. The number of sessions in conferences as well as the number of publications in this and other scientific journals increased very rapidly. Several books were written on various micro/nanoscale heat transfer topics. New courses on this topic were started at both undergraduate and graduate levels. In fact, perhaps the most positive outcome has been the birth of a new breed of young researchers for whom the boundaries of traditional disciplines have already been torn down. To reflect upon our rapid progress of the past decade, to define the state of the art today, and to look ahead towards a very promising and perhaps even more progressive future, we decided to put together this special issue on micro/nanoscale heat and mass transfer. The issue contains a few review papers on topics that are mature and have sufficiently well-established research content. By no means do they cover all the topics of micro/nanoscale heat and mass transfer. The purpose of the review papers is to capture the past progress in a field and present new challenges and questions for the future. We hope these papers will serve the community well, in particular, for those who want to enter the field as a new researcher and get abreast with the state of the art of the field. The issue also contains several regular publications that report new research covering various topics in micro/nanoscale heat and mass transfer. While it is easy and reassuring for us to discuss our research achievements over the past decade, we must continue to ask some hard and sometimes unnerving questions about the field. Why is micro/nanoscale heat and mass transfer important? What impact has it made or, more importantly, can it make? Why is it attracting so much new research funding? Is it just a fad or will it last? Does it have any scientific depth? Is there a roadmap for the future? While we cannot obviously answer all these questions at once, the question of why one should miniaturize is relevant and worth some thought. Two reasons come to mind. The first is a scientific one whereas the second is more technological. The fundamental length scales of nature that are involved in heat and mass transfer happen to be at micro and nanoscales. For example, the wavelength and mean free path of electrons and phonons in solids, mean free path of molecules in gases at atmospheric pressure, and the range of intermolecular forces in liquids generally lie in the length scale range of 100 nm or below. If one can probe and understand phenomena at these length scales, it may be possible to control and engineer heat and mass transfer in unprecedented ways. The second key reason is directly related to the fact that we are able to manufacture and manipulate systems at length scales that only a few decades ago were unthinkable. It is worth noting that the commercial success of microelectronics lies not only in the fact that one can fabricate sub-micrometer devices, but more so in the low cost of manufacturing integrated systems using parallel lithographic techniques. If the same manufacturing techniques can be exploited for thermal engineering, it would be possible to design and manufacture thermal micro/nanodevices that are not only inexpensive but also have better performance than their larger counterparts. In fact, if one could combine both these aspects—nanoscale fundamental control and low-cost microfabrication—it seems reasonable to expect commercial devices and systems with new functionality and high performance. It is the dual vision of generating scientific knowledge and enabling commercial technology that ought to stimulate further research in this field. We hope this special issue will play a positive role for achieving this dual vision.

The concept of multiscale modeling embodies the idea that a comprehensive description of a material will require an understanding over multiple time and length scales. A multiscale model requires that descriptions at all levels be consistent with each other, which can be particularly demanding for advanced materials and complex fluids. For crystalline materials, emerging modeling approaches have married smalland intermediate-scale descriptions in a highly effective manner, but challenges remain at long time and length scales. For soft materials, such as polymers or liquid crystals, modeling techniques have adopted a more or less systematic coarse-graining approach, in which atomic and molecular details are gradually blurred as one seeks to describe longer length scales. This approach presents its own brand of challenges. And, in spite of rapid advances, entire classes of materials, including amorphous glasses, foams, and gels, have resisted attempts to describe their structure and dynamics over long and relevant length and time scales. This issue of MRS Bulletin covers some areas of materials modeling in which enormous advances have been made, but which continue to raise intriguing questions and formidable challenges.

The stepped spillway design is characterized by an increase in the rate of energy dissipation on the chute associated with a reduction of the size of the downstream energy dissipation system. This study presents a thorough investigation of the air–water flow properties in skimming flows with a focus on the turbulent characteristics. New measurements were conducted in a large-size facility (θ = 22°; step height, h = 0.1 m) with several phase-detection intrusive probes. Correlation analyses were applied to estimate the integral turbulent length and time scales. The skimming flow properties presented some basic characteristics that were qualitatively and quantitatively in agreement with previous air–water flow measurements in skimming flows. Present measurements showed some relatively good correlation between turbulence intensities Tuand turbulent length and time scales. These measurements also illustrated large turbulence levels and large turbulent time and length scales in the intermediate region between the spray and bubbly flow regions.

The regulation of cell signaling occurs through a complex set of coupled processes occurring over multiple length and time scales. Computational modeling approaches have been applied to dissect this complexity over these various time and length scales, which range from the molecular level to the cell and tissue level, but these approaches have not focused heavily on the regulation or roles of phosphatases. Because of the clear importance of phosphatases in cell signaling, significant opportunities exist to expand our understanding of the regulation of cell signaling in metabolism and other cell regulatory processes by focusing modern computational approaches on phosphatases and the processes they regulate. The aim of this chapter is to provide a brief review of some computational modeling approaches that have been usefully applied to study the regulation of signaling, mainly by kinases, over a range of length and time scales and to describe opportunities to apply similar approaches for understanding signaling regulation by phosphatases. Some specific examples of key relevance to metabolism are described.

The association of ionizable polymers strongly affects their motion in solutions, where the constraints arising from clustering of the ionizable groups alter the macroscopic dynamics. The interrelation between the motion on multiple length and time scales is fundamental to a broad range of complex fluids including physical networks, gels, and polymer-nanoparticle complexes where long-lived associations control their structure and dynamics. Using neutron spin echo and fully atomistic, multimillion atom molecular dynamics (MD) simulations carried out to times comparable to that of chain segmental motion, the current study resolves the dynamics of networks formed by suflonated polystryene solutions for sulfonation fractions 0 ≤ f ≤ 0.09 across time and length scales. The experimental dynamic structure factors were measured and compared with computational ones, calculated from MD simulations, and analyzed in terms of a sum of two exponential functions, providing two distinctive time scales. These time constants capture confined motion of the network and fast dynamics of the highly solvated segments. A unique relationship between the polymer dynamics and the size and distribution of the ionic clusters was established and correlated with the number of polymer chains that participate in each cluster. The correlation of dynamics in associative complex fluids across time and length scales, enabled by combining the understanding attained from reciprocal space through neutron spin echo and real space, through large scale MD studies, addresses a fundamental long-standing challenge that underline the behavior of soft materials and affect their potential uses.

Towards modeling spatiotemporal processes in metal–organic frameworks

Computational modeling of metallic materials across various length and time scales has been on the rise since the advent of efficient, fast computing machines. From atomistic methods like molecular statics and dynamics at the nanoscale to continuum mechanics modeled by finite element methods at the macroscale, various techniques have been established that describe and predict the mechanics of materials. Many recent technologies, however, fall into a gap between length scales (referred to as mesoscales), with microstructural features on the order of nanometers (thereby requiring full atomistic resolution) but large representative volumes on the order of micrometers (beyond the scope of molecular dynamics). There is an urgent need to predict material behavior using scale-bridging techniques that build up from the atomic level and reach larger length and time scales. To this end, there is extensive ongoing research in building hierarchical and concurrent scale-bridging techniques to master the gap between atomistics and the continuum, but robust, adaptive schemes with finite-temperature modeling at realistic length and time scales are still missing. In this thesis, we use the quasicontinuum (QC) method, a concurrent scale-bridging technique that extends atomistic accuracy to significantly larger length scales by reducing the full atomic ensemble to a small set of representative atoms, and using interpolation to recover the motion of all lattice sites where full atomistic resolution is not necessary. We develop automatic model adaptivity by adding mesh refinement and adaptive neighborhood updates to the new fully nonlocal energy-based 3D QC framework, which allows for automatic resolution to full atomistics around regions of interest such as nanovoids and moving lattice defects. By comparison to molecular dynamics (MD), we show that these additions allow for a successful and computationally efficient coarse graining of atomistic ensembles while maintaining the same atomistic accuracy. We further extend the fully nonlocal QC formulation to finite temperature (termed hotQC) using the principle of maximum entropy in statistical mechanics and averaging the thermal motion of atoms to obtain a temperature-dependent free energy using numerical quadrature. This hotQC formulation implements recently developed optimal summation rules and successfully captures temperature-dependent elastic constants and thermal expansion. We report for the first time the influence of temperature on force artifacts and conclude that our novel finite-temperature adaptive nonlocal QC shows minimal force artifacts and outperforms existing formulations. We also highlight the influence of quadrature in phase space on simulation outcomes. We study 3D grain boundaries in the nonlocal hotQC framework (previously limited to single-crystals) by modeling coarse-grained symmetric-tilt grain boundaries in coincidence site lattice (CSL) based bicrystals. We predict relaxed energy states of various Σ-boundaries with reasonable accuracy by comparing grain boundary energies to MD simulations and outline a framework to model polycrystalline materials that surpasses both spatial and temporal limitations of traditional MD.

The basic time (and length) scales governing the physical transport and mixing processes in aquatic environments are briefly reviewed in an ecohydrodynamic perspective. Such time scales are: the molecular diffusion time, T d, the falling particle time, T f, the mixing time, T m, the advection time, T a, and the Kolmogorov (or viscous) time, T k. For large water bodies, two more time scales can be formulated based on the Coriolis frequency, f c, and the Kibel frequency, f k. These time scales form several spectral windows, which correspond to the scales of external forcing or of intrinsic mechanisms, determine the hydrodynamic processes that may significantly interact with the various populations of the aquatic communities, and govern the dynamics of the aquatic system. Motions at the time scales of the weather of the aquatic environment are resonant with the ecosystem dynamics and impose to the ecosystem certain length scales through the process of ecohydrodynamic adjustment. Knowledge of such characteristic time scales facilitates the selection of appropriate strategies for sampling environmental quantities and satisfying the frequency sampling requirements.

Scaling considerations and sampling strategies in monitoring aquatic ecohydrodynamics

Background: Elucidation of the highly forward scattering of photons in random media such as biological tissue is crucial for further developments of optical imaging using photon transport models. We evaluated length and time scales of the photon scattering in three-dimensional media. Methods: We employed analytical solutions of the time-dependent radiative transfer, M-th order delta-Eddington, and photon diffusion equations (RTE, dEM, and PDE). We calculated the fluence rates at different source-detector distances and optical properties. Results: We found that the zeroth order dEM and PDE, which approximate the highly forward scattering to the isotropic scattering, are valid in longer length and time scales than approximately 10 / μ t ′ and 40 / μ t ′ v , respectively, where μ t ′ is the reduced transport coefficient and v the speed of light in a medium. The first and second order dEM, which approximate the highly forward-peaked phase function by the first two and three Legendre moments, are valid in the longer scales than approximately 4.0 / μ t ′ and 6.3 / μ t ′ v ; 2.8 / μ t ′ and 3.5 / μ t ′ v , respectively. The boundary conditions less influence the length scales, while they reduce the times scales from those for bulk at the longer length scale than approximately 4.0 / μ t ′ . Conclusion: Our findings are useful for constructions of accurate and efficient photon transport models. We evaluated length and time scales of the highly forward scattering of photons in various kinds of three-dimensional random media by analytical solutions of the radiative transfer, M-th order delta-Eddington, and photon diffusion equations.

Peering into Batteries: Electrochemical Insight Through In Situ and Operando Methods over Multiple Length Scales

This contribution considers recent developments in the computer modelling of amorphous polymeric materials. Progress in our capabilities to build models for the computer simulation of polymers from the detailed atomistic scale up to coarse-grained mesoscopic models, together with the ever-improving performance of computers, have led to important insights from computer simulations into the structural and dynamic properties of amorphous polymers. Structurally, chain connectivity introduces a range of length scales from that of the chemical bond to the radius of gyration of the polymer chain covering 2–4 orders of magnitude. Dynamically, this range of length scales translates into an even larger range of time scales observable in relaxation processes in amorphous polymers ranging from about 10−13 to 10−3 s or even to 103 s when glass dynamics is concerned. There is currently no single simulation technique that is able to describe all these length and time scales efficiently. On large length and time scales basic topology and entropy become the governing properties and this fact can be exploited using computer simulations of coarse-grained polymer models to study universal aspects of the structure and dynamics of amorphous polymers. On the largest length and time scales chain connectivity is the dominating factor leading to the strong increase in longest relaxation times described within the reptation theory of polymer melt dynamics. Recently, many of the universal aspects of this behaviour have been further elucidated by computer simulations of coarse-grained polymer models. On short length scales the detailed chemistry and energetics of the polymer are important, and one has to be able to capture them correctly using chemically realistic modelling of specific polymers, even when the aim is to extract generic physical behaviour exhibited by the specific chemistry. Detailed studies of chemically realistic models highlight the central importance of torsional dynamics in all relaxation processes in polymer materials. Finally, the interplay between local energetics, both intramolecular and intermolecular, and the local packing governs the glass transition in polymer melts.

In recent years, research activity on the recovery technique known as low salinity waterflooding has sharply increased. The main motivation for field application of low salinity waterflooding is the improvement of oil recovery by acceleration of production (‘oil faster’) compared to conventional high salinity brine injection. Up to now, most research has focused on the core scale by conducting coreflooding and spontaneous imbibition experiments. These tests serve as the main proof that low salinity waterflooding can lead to additional oil recovery. Usually, it is argued that if the flooding experiments show a positive shift in relative permeability curves, field application is justified provided the economic considerations are also favorable. In addition, together with field pilots, these tests resulted in several suggested trends and underlying mechanisms related to low salinity water injections that potentially explain the additional recovery.While for field application one can rely on the core scale laboratory tests which can provide the brine composition dependent saturation functions such as relative permeability, they are costly, time consuming and challenging. It is desirable to develop predictive capability such that new candidates can be screened effectively or prioritized. This has not been yet achieved and would require under-pinning the underlying mechanism(s) of the low salinity response.Recently, research has intensified on smaller length scales i.e. the sub-pore scale. This coincides with a shift in thinking. In field and core scale tests the main goal was to correlate bulk properties of rock and fluids to the amount of oil recovered. Yet in the tests on the sub-pore scale the focus is on ruling out irrelevant mechanisms and understanding the physics of the processes leading to a response to low salinity water. Ultimately this should lead to predictive capability that allows to pre-select potential field candidates based on easily obtained properties, without the need of running time and cost intensive tests.However, low salinity waterflooding is a cooperative process in which multiple mechanisms acting on different length and time scales aid the detachment, coalescence, transport, banking, and eventual recovery of oil. This means investigating only one particular length scale is insufficient. If the physics behind individual mechanisms and their interplay does not transmit through the length scales, or does not explain the observed fast and slow phenomena, no additional oil may be recovered at core or field scale.Therefore, the mechanisms are not discussed in detail in this review, but placed in a framework on a higher level of abstraction which is ‘consistency across the scales’. In doing so, the likelihood and contribution of an individual mechanism to the additional recovery of oil can be assessed. This framework shows that the main uncertainty lies in how results from sub-pore scale experiments connect to core scale results, which happens on the length scale in between: the pore-network scale.On the pore-network scale two different types low salinity responses can be found: responses of the liquid-liquid or the solid-liquid interfaces. The categorization is supported by the time scale differences of the (optimal) response between liquid-liquid and solid-liquid interfaces. Differences in time scale are also observed between flow regimes in water-wet and mixed-wet systems. These findings point to the direction of what physics should be carried from sub-pore to core scale, which may aid in gaining predictive capability and screening tool development. Alternatively, a more holistic approach of the problems in low salinity waterflooding is suggested.

Active matter embraces systems that self-organize at different length and time scales, often exhibiting turbulent flows apparently deprived of spatiotemporal coherence. Here, we use a layer of a tubulin-based active gel to demonstrate that the geometry of active flows is determined by a single length scale, which we reveal in the exponential distribution of vortex sizes of active turbulence. Our experiments demonstrate that the same length scale reemerges as a cutoff for a scale-free power law distribution of swirling laminar flows when the material evolves in contact with a lattice of circular domains. The observed prevalence of this active length scale can be understood by considering the role of the topological defects that form during the spontaneous folding of microtubule bundles. These results demonstrate an unexpected strategy for active systems to adapt to external stimuli, and provide with a handle to probe the existence of intrinsic length and time scales.

In high-velocity open channel flows, the measurements of air-water flow properties are complicated by the strong interactions between the flow turbulence and the entrained air. In the present study, an advanced signal processing of traditional single- and dual-tip conductivity probe signals is developed to provide further details on the air-water turbulent level, time and length scales. The technique is applied to turbulent open channel flows on a stepped chute conducted in a large-size facility with flow Reynolds numbers ranging from 3.8 E+5 to 7.1 E+5. The air water flow properties presented some basic characteristics that were qualitatively and quantitatively similar to previous skimming flow studies. Some self-similar relationships were observed systematically at both macroscopic and microscopic levels. These included the distributions of void fraction, bubble count rate, interfacial velocity and turbulence level at a macroscopic scale, and the auto- and cross-correlation functions at the microscopic level. New correlation analyses yielded a characterisation of the large eddies advecting the bubbles. Basic results included the integral turbulent length and time scales. The turbulent length scales characterised some measure of the size of large vortical structures advecting air bubbles in the skimming flows, and the data were closely related to the characteristic air-water depth Y90. In the spray region, present results highlighted the existence of an upper spray region for C > 0.95 to 0.97 in which the distributions of droplet chord sizes and integral advection scales presented some marked differences with the rest of the flow.