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This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

This manuscript presents an experimental effort to directly measure contact areas and the details behind these scaled experiments on a randomly rough model surface used in the “Contact Mechanics Challenge” (2017). For these experiments, the randomly rough surface model was scaled up by a factor of 1000× to give a 100 mm square sample that was 3D printed from opaque polymethylmethacrylate (PMMA). This sample was loaded against various optically smooth and transparent samples of PDMS that were approximately 15 mm thick and had a range in elastic modulus from 14 kPa to 2.1 MPa. During loading, a digital camera recorded contact locations by imaging the scattering of light that occurs off of the PMMA rough surface when it was in contact with the PDMS substrate. This method of illuminating contact areas is called frustrated total internal reflection and is performed by creating a condition of total internal reflection within the unperturbed PDMS samples. Contact or deformation of the surface results in light being diffusely transmitted from the PDMS and detected by the camera. For these experiments, a range of reduced pressure (nominal pressure/elastic modulus) from below 0.001 to over 1.0 was examined, and the resulting relative contact area (real area of contact/apparent area of contact) was found to increase from below 0.1% to over 60% at the highest pressures. The experimental uncertainties associated with experiments are discussed, and the results are compared to the numerical results from the simulation solution to the “Contact Mechanics Challenge.” The simulation results and experimental results of the relative contact areas as a function of reduced pressure are in agreement (within experimental uncertainties).

The “Contact Mechanics Challenge” posed to the tribology community by Muser and Dapp in 2015 detailed a 100 µm × 100 µm randomly rough surface with a root-mean-square gradient of unity, $${\bar{\text{g}}} = 1$$ . Many surfaces, both natural and synthetic, can be described as randomly rough, but rarely with a root-mean-square gradient as steep as $${\bar{\text{g}}} = 1$$ . The selection of such a challenging surface parameter was intentional, but potentially limiting for broad comparisons across existing models and theories which may be limited by small-slope approximations. In this manuscript, the root-mean-square gradients ( $${\bar{\text{g}}}$$ ) of the “Contact Mechanics Challenge” surface were produced on 1000 × scaled models such that there were three different surfaces for study with $${\bar{\text{g}}} = \, 0.2, \, 0.5$$ , and 1. In situ measurements of the real area of contact and contact area distributions were performed using frustrated total internal reflectance along with surface deformation measurements performed using digital image correlation. These optical in situ experiments used the scaled 3D-printed rough surfaces that were loaded into contact with smooth, flat, and elastic samples that were made from unfilled PDMS: (10:1) E* = 2.1 MPa Δγ = 4 mJ/m2; (20:1) E* = 0.75 MPa Δγ = 3 mJ/m2; (30:1) E* = 0.24 MPa Δγ = 2 mJ/m2. All of the loading was performed using a uniaxial load frame under force control. A Green’s function molecular dynamics simulation assuming the small-slope approximation was compared to all experimental data. These measurements reveal that decreasing root-mean-square gradient noticeably increases real area of contact area under conditions of “equal” applied load, but variations in the root-mean-square gradient did not significantly alter the contact patch geometry under conditions of nearly equal real area of contact. Including $${\bar{\text{g}}}$$ in the reduced pressure ( $$p = P /(E*{\bar{\text{g}}})$$ ) reduced the root-mean-square error between the simulation ( $${\bar{\text{g}}} = 1$$ ) and all experimental data for the relative area of contact as a function of reduced pressure over the entire range of surfaces, materials, and loads tested.

The oft discussed and fretted over environmental influences on hair have led to a popular consensus which suggests that elevated temperature and humidity lead to frizzier, wilder hair. However, few attempts at actually quantifying these effects have been made. Although frizziness is usually perceived visually, here the influence of variations in temperature and humidity on the tactile perception and friction of curly and straight hair were investigated. It is shown that changes in humidity may disproportionately affect perceived frizziness of curly hair by touch due to concurrent changes in the tactile friction.

Humans are extraordinarily skilled in the tactile evaluation of, and differentiation between, surfaces. The chemical and mechanical properties of these surfaces are translated into tactile signals during haptic exploration by mechanoreceptors in our skin, which are specialized to respond to different types of temporal and mechanical stimulation. Describing the effects of measurable physical characteristics on the human response to tactile exploration of surfaces is of great interest to manufacturers of household materials so that the haptic experience can be considered during design, product development and quality control. In this study, methods from psychophysics and materials science are combined to advance current understanding of which physical properties affect tactile perception of a range of furniture surfaces, i.e., foils and coatings, thus creating a tactile map of the furniture product landscape. Participants’ responses in a similarity scaling task were analyzed using INDSCAL from which three haptic dimensions were identified. Results show that specific roughness parameters, tactile friction and vibrational information, as characterized by a stylus profilometer, a Forceboard, and a biomimetic synthetic finger, are important for tactile differentiation and preferences of these surface treatments. The obtained dimensions are described as distinct combinations of the surface properties characterized, rather than as ‘roughness’ or ‘friction’ independently. Preferences by touch were related to the roughness, friction and thermal properties of the surfaces. The results both complement and advance current understanding of how roughness and friction relate to tactile perception of surfaces.

Although knowledge surrounding the obstacles omnivorous consumers face when substituting meat products with plant-based alternatives has increased dramatically, less is known about their perspectives on plant-based alternatives to dairy products. Here, these perspectives are assessed in two survey-based studies. Study 1 (N = 175) adapts an existing scale (the 4Ns of meat consumption) to dairy products in an effort to identify similarities and differences between rationalizations for meat and dairy consumption. This 16-item scale quantifies four factors (Natural, Necessary, Normal, and Nice) describing common rationalizations for meat consumption. The results revealed that the 4Ns transfer well to the dairy category, and that endorsement of dairy products as Nice was the strongest predictor of dairy consumption, relative to the other 3Ns. This is further supported by evaluation of consumers own qualitative descriptions of why they do or do not consume meat/dairy products, where "taste" was the most frequently used word in both categories. Study 2 replicates the relationships between dairy 4Ns scores and reported dairy consumption found in Study 1 and builds upon these results by showing that 4N score could accurately categorize consumers as frequent (N = 192) or infrequent (N = 210) consumers of plant-based milk alternatives (PBMAs). Differences in consumers expectations for the sensory characteristics of cows milk and PBMAs are identified, and the impact of rationalization (total 4N score) on the likelihood of expected sensory attribute associations is described. The role of rationalization in shaping sensory expectations and impacting dietary choices, in particular resistance to adopting PBMAs, is discussed. Funding Agencies|FORMAS - Research Council for Environment, Agricultural Sciences, and Spatial Planning [2020-02839]

Climate change is altering consumer-plant interactions in ecosystems worldwide. How consumers alter their spatial distribution, grazing activities, and functional morphology in response to climate stress can determine whether their effects on plants intensify or relax. Few studies have considered multiple consumer response metrics to elucidate the mechanisms underpinning the resulting changes in consumer-plant interactions. Here, we tested how drought stress influences the interaction between the dominant consumer, the fungal-farming periwinkle snail Littoraria irrorate/i>, and a foundational plant, cordgrass Spartina alterniflora, in a southeastern US salt marsh. In a 4 mo field experiment, we maintained moderate snail densities in mesh control chambers and clear plastic climate chambers that simulated drought by elevating temperatures and drying soils. Monitoring revealed that snails more often congregated on cordgrass stems than leaves in climate chambers than in controls. Image analyses indicated that this behavioral shift corresponded to snails inflicting shorter, but more numerous, fungal-infested scars on cordgrass leaves, and causing less plant damage in climate chambers than controls. Coincident with their net reduction in grazing, snails maintained longer radulae, whose central teeth were blunter and lateral teeth were sharper, in climate chambers compared to controls. These results suggest that under drought, snail radulae may experience less frictional wear and that, at intermediate densities, snail-cordgrass interactions relax. Together with prior research showing that at high densities, snails can denude cordgrass during drought, we conclude that consumer density, behavior, and morphological responses must be integrated in predictions of how climate change will affect the direction, strength, and stability of consumer-plant interactions.

Improving the tactile aesthetics of products that can be described as touch intensive is an increasing priority within many sectors, including the furniture industry. Understanding which physical characteristics contribute to the haptic experience of a surface, and how, is therefore highly topical. It has earlier been shown that both friction and topography affect tactile perception. Thus, two series of stimuli have been produced using standard coating techniques, with systematic variation in (physical) friction and roughness properties. This was achieved through appropriate selection of matting agents and resins. The stimuli sets were then evaluated perceptually to determine the extent to which discrimination between pairs of surfaces followed the systematic materials variation. In addition to investigating the role of the physical properties in discrimination of the surfaces, their influence on perceived pleasantness and naturalness was also studied. The results indicate that changes in tactile perception can be understood in terms of friction and roughness, and that varying the matting agents (topography) and resins (material properties) in the coatings provide the controlling factors for furniture applications. Perceived pleasantness is associated with low friction and smoother topography, whilst perceived naturalness is found to be described by an interaction between tactile friction and the average maximum peak height of the surface features. Graphic Abstract

Fluid instabilities limit the ability of features to hold their shape in many types of 3D printing as liquid inks solidify into written structures. By 3D printing directly into a continuum of jammed granular microgels, these instabilities are circumvented by eliminating surface tension and body forces. However, this type of 3D printing process is potentially limited by inertial instabilities if performed at high speeds where turbulence may destroy features as they are written. Here, we design and test a high-speed 3D printing experimental system to identify the instabilities that arise when an injection nozzle translates at 1 m/s. We find that the viscosity of the injected material can control the Reynold’s instability, and we discover an additional, unanticipated instability near the top surface of the granular microgel medium.

The influence of thickener and additive interactions on grease lubricating performance is examined. Polypropylene and lithium complex thickened (Li-complex) greases were tested both as neat greases ...

This paper describes the experimental methodology used to study the contact mechanics of a rigid, rough surface and a compliant, nominally flat surface using digital image correlation (DIC). The rough surface was produced by 3-D printing PMMA and the flat surface was produced with transparent PDMS (silicone rubber). The deformation of the speckled top surface (contact) of the PDMS was measured via DIC viewed through the transparent media. Four different PDMS formulations with moduli ranging from 64 to 2120 kPa were used in the experiment program to cover a wide range of modulus normalized loads. The deformation of the contact surface and depth of penetration versus normalized load were measured. The results were overlaid with previous measurements of contact area and complemented them extremely well. Additionally, it was shown that scaling laws associated with such contact mechanics problems extend many length scales.

Fluoropolymers have unique mechanical, chemical, and tribological properties (low friction coefficients) but their use as solid lubricants is inhibited by high wear rates (1–5×10−4 mm3/Nm). The addition of certain types of α-alumina has been shown to reduce the wear rate of PTFE by over three orders of magnitude, but due to its extremely high molecular weight PTFE cannot be screw injection molded. However, PFA, a perfluorinated copolymer of tetrafluoroethylene (TFE) and a perfluorinated alkylvinyl ether (PAVE), can be. Teflon® PFA 340 samples with various weight fractions of α-alumina (0%, 5%, 7.5%, 10%) were injection molded, and samples from each mold were wear tested against stainless steel (P=6.3 MPa, v=50.8 mm/s). Experiments showed that the friction behavior of the PFA 340-α alumina composite was very close to that of both unfilled PFA 340 and PTFE-α alumina composites. The wear rate of unfilled PFA 340 was 1.4×10−4 mm3/Nm, and dropped to 4.0×10−8 mm3/Nm for the PFA-α alumina composites. Just as in the case of PTFE-α alumina composites, these PFA composites generated brown-colored tribofilms on both the polymer and metal surfaces, which were indicative of tribochemical changes. ATR-IR and FTIR spectra of each surface showed evidence for the generation of perfluorinated carboxylate salts and waters of hydration. This spectral similarity between PTFE and PFA 340 samples shows that the same tribological mechanism found in PTFE-α alumina composites is responsible for ultralow wear in PFA-α alumina composites as well.

Polytetrafluoroethylene (PTFE) filled with certain alumina additives has wear rates over four orders of magnitude lower than unfilled PTFE. The mechanisms for this wear reduction have remained a mystery. In this work, we use a combination of techniques to show that porous, nanostructured alumina microfillers (not nanofillers) are critical for this wear reduction. The microscale alumina particles break during sliding into nanoscale fragments. X-ray microtomography, transmission electron microscopy and infrared spectroscopy reveal nanoscale alumina fragments accumulated in the tribofilms. Tribochemically generated carboxylate endgroups bond to metal species in the transfer film and to alumina fragments in the surface of the polymer composite. These mechanically reinforced tribofilms create robust sliding surfaces and lead to a dramatic reduction in wear.

The aim of this study was to evaluate whether smooth surfaces varying in surface chemistry could be perceptually distinguished with the sense of touch. A set of ten glass surfaces was prepared which varied systematically in terms of the molecular composition of a thin coating of low topography. The contact angle, contact angle hysteresis, and surface energy were evaluated as objective physical parameters characterizing each coating. Additionally, the interaction forces between a human finger and the different coatings were quantified and compared in terms of tactile friction coefficients. The surfaces were evaluated psychophysically in terms of perceived similarities and were then ranked according to pleasantness. The participants could perceptually distinguish between surfaces varying in surface chemistry and a primary and secondary perceptual dimension were identified as sufficient to distinguish them. The primary dimension correlates with surface free energy, but both tactile friction and surface energy contribute to this dimension depending on whether the coatings are organic or inorganic. The secondary dimension could not be identified explicitly in terms of a physical quantity but is discussed in terms of recent developments in the literature. Coated glass is characterized by high friction coefficient upon interaction with a human finger as well as significant hysteresis in the stroking directions (lower applied load and higher friction in the backward stroke). Despite the complexity of the tribology, pleasantness can be clearly linked to it, where low friction (high contact angle) materials receive a higher ranking.

The wear and friction behavior of ultralow wear polytetrafluoroethylene (PTFE)/α-alumina composites first described by Burris and Sawyer in 2006 has been heavily studied, but the mechanisms responsible for the 4 orders of magnitude improvement in wear over unfilled PTFE are still not fully understood. It has been shown that the formation of a polymeric transfer film is crucial to achieving ultralow wear on a metal countersurface. However, the detailed chemical mechanism of transfer film formation and its role in the exceptional wear performance has yet to be described. There has been much debate about the role of chemical interactions between the PTFE, the filler, and the metal countersurface, and some researchers have even concluded that chemical changes are not an important part of the ultralow wear mechanism in these materials. Here, a “stripe” test allowed detailed spectroscopic studies of PTFE/α-alumina transfer films in various stages of development, which led to a proposed mechanism which accounts ...

Measurements of surface deformations as part of the “Contact Mechanics Challenge” were collected using digital image correlation (DIC). For these experiments, a scaled version (1000×) of the periodic and random roughness surface provided for the “Contact Mechanics Challenge” was used. A 100 mm × 100 mm scale replica of the surface, approximately 10 mm thick, was 3D-printed using an opaque polymethylmethacrylate and pressed into contact against flat, transparent polydimethylsiloxane (PDMS) sheets with dead weight loads. Four different formulations of PDMS were used, and the resulting elastic moduli ranged from 64 kPa to 2.1 MPa. The DIC technique was used in situ to measure the deformation of the PDMS surface at each load increment from 22.5 to 450 N. Surface deformations in and out of contact were measured across the entire apparent area of contact and overlaid with the measurements of contact area to provide a complete description of the surface profile during loading. A direct comparison between these experiments and the simulations regarding the gap within the contact at a reduced pressure of 0.164 agrees to within ±10% when normalized to the maximum gap.

Biotribology and tribocorrosion are often not included in numerical or computational modeling efforts to predict wear because of the apparent complexity in the geometry, the variability in removal rates, and the challenge associated with mixing time-dependent removal processes such as corrosion with cyclic material removal from wear. The lollipop is an accessible bio-tribocorrosion problem that is well known but underexplored scientifically as a tribocorrosion process. Stress-assisted dissolution was found to be the dominant tribocorrosion process driving material removal in this system. A model of material removal was described and approached by lumping the intrinsically time-dependent process with a mechanically driven process into a single cyclic volumetric material removal rate. This required the collection of self-reported wear data from 58 participants that were used in conjunction with statistical analysis of actual lollipop cross-sectional information. Thousands of repeated numerical simulations of material removal and shape evolution were conducted using a simple Monte Carlo process that varied the input parameters and geometries to match the measured variability. The resulting computations were analyzed to calculate both the average number of licks required to reach the Tootsie Roll® center of a Tootsie Roll® pop, as well as the expected variation thereof.

Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many prac- tical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of mag- nitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 9 10 -6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1-10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 9 10 -7 mm 3 /(Nm).

In this manuscript, a simple binary model is devised that describes the wear behavior of two blocks coupled under a constant, dynamically partitioned normal load. In this simple system, the frictional force is reacted by two independent springs and the blocks are allowed to move and wear independently based on system dynamics and kinematics. The only coupling between the blocks occurs through the partitioning of the applied normal load, which uses a pair of springs in parallel to model elasticity. This system is found to preferentially wear one of the blocks until two disparately unique conditions of steady wear are reached in the system: (1) a condition in which the partitioning of the load between the blocks yields equal wear and thus steady partitioning of the load and (2) a condition in which the pair of blocks go to zero wear by having one block not sliding but carrying all of the load and the other block completely slipping but carrying none of the load. These “Janus blocks,” the simplest of binary spring–block systems, begin life in a nominally identical state and then their behavior bifurcates, producing runaway or irregular wear. The onset of this instability can initiate from any differences in load partitioning, spring constants, friction coefficient, or wear rates (no matter how small).

Using neutron reflectivity, the electro-responsive structuring of the non-halogenated ionic liquid (IL) trihexyl(tetradecyl)phosphonium-bis(mandelato)borate, [P6,6,6,14][BMB], has been studied at a ...

For four decades the NIH Guidelines have served as the framework for the oversight of research with recombinant and synthetic nucleic acid molecules. IBCs are a vital component in that system of oversight as the entity that ensures the safe conduct of this type of research at the local level. IBC review also provides a public window into the procedures utilized by institutions to manage biohazards, thereby protecting human health, animal health, and the environment. It is both because of their biosafety responsibilities, as well as the manner in which they offer transparency to the public, that IBCs play a central role in fostering public trust in the safety of the life science research enterprise.

PTFE/α-alumina composites are well known to exhibit very low wear rates compared to unfilled PTFE and various other PTFE-matrix composites. The improved wear life of these composites is attributed in part to the formation of a uniform protective transfer film on the metal countersurface. It is postulated that the retention of transferred material and the recirculation of third bodies between the transfer film and running surface of the polymer composite are necessary for the maintenance of low wear within this tribological system. The accumulation of these third bodies was observed in reciprocating sliding tests on countersamples prescribed with aligned roughness. Wear performance of the polymer composite was tested as a function of the between the sliding direction and the aligned roughness of the countersample, ranging from parallel to perpendicular to the sliding direction. The wear rate of roughness oriented with the sliding direction was 300 times higher than roughness perpendicular to the sliding direction, revealing the importance of surface morphology and third body retention.

Institutions that receive National Institutes of Health (NIH) funding for research involving recombinant or synthetic nucleic acid molecules are required, as a term and condition of their funding, to comply with the NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules (NIH Guidelines) (NIH, 2013). Under the NIH Guidelines, institutions must establish and register an Institutional Biosafety Committee (IBC) with the NIH. The IBC is then responsible for reviewing and approving research projects subject to the NIH Guidelines. The IBC review of projects involving recombinant or synthetic nucleic acid molecules is critical to ensuring that such research is conducted in a safe and responsible manner. In 2006, staff from NIH began conducting educational site visits to institutions that had an IBC registered with NIH. The purpose of these site visits is to assist IBCs with their institutional programs of oversight for recombinant or synthetic nucleic molecules. Based on our findings, the site visit program has been beneficial to institutional biosafety programs. The information gathered during the site visits has allowed NIH to tailor its educational materials to help institutions address their oversight challenges. Additionally, since NIH's visits are primarily educational in nature, we have been able to foster a positive environment in which IBC members and staff feel comfortable reaching out to NIH for advice and assistance.

Over the last decade, researchers have explored an intriguing polymer composite composed of granular polytetrafluoroethylene (PTFE) 7C and alumina particles. This material is extraordinary because a very small amount of alumina additive (

Objective Emergency airway situations are relatively rare events in pediatrics with most graduating residents having little exposure to intubate. Newer video technology offers the promise of reducing complications associated with intubation. This study proposes that video laryngoscopy (VL) should aid less skilled residents to intubate an infant mannequin with greater success and speed as compared with traditional direct laryngoscopy (DL). Methods Pediatric (PED) and emergency medicine (EM) residents were randomized in a prospective controlled study. A standard respiratory failure scenario was conducted using SimBaby with an uncomplicated airway. Residents who inadvertently performed esophageal intubation were made aware as part of the scenario and allowed to reattempt until successful. Results Sixty-nine residents voluntarily participated, 49% EM and 51% PED. Seven subjects in the DL group required multiple attempts (21%), compared with 6 subjects in the VL group (17%) (P = 0.718). Median time to intubation was 30 seconds (95% confidence interval [CI], 19-41 seconds) for DL and 39 seconds (95% CI, 36-42 seconds) for VL (P = 0.111). Comparison of programs revealed a 77% PED success rate versus 85% EM success rate (P = 0.578) and median time to intubation of 38 seconds (95% CI, 31-45 seconds) for PED compared with 32 seconds (95% CI, 23-41 seconds) for EM residents (P = 0.316). In a subanalysis, subjects successful at first attempt revealed a 13-second median difference (DL, 23 seconds [95% CI, 18-28 seconds] vs. VL, 36 seconds [95% CI, 29-43 seconds; P = 0.01). Conclusions In a simulated respiratory failure scenario involving residents, VL provided no additional success over DL with slightly longer time to intubation.

Introduction Obstetrics and gynecology residents benefit from providing care to diverse patient populations and increasing their awareness of the social determinants of health. Objectives To describe and evaluate an outpatient rotation for obstetrics and gynecology residents at a county jail. Methods A comprehensive curriculum incorporating Accreditation Council for Graduate Medical Education (ACGME) core competencies was designed for all first-year residents to rotate weekly at the local county jail during their 6-week ambulatory care block. Residents completed an anonymous online evaluation and wrote a reflective essay at the end of the rotation. Data for patient visits were tabulated. Results All 9 first-year residents completed the rotation and the evaluation. Seventy-eight percent of patient visits were for gynecologic services, predominantly family planning. Residents reported that the rotation overall was a positive experience, emphasizing the unique intersection between psychosocial issues and health care in the jail setting. Rotation objectives that satisfied the 6 ACGME competencies were met. Discussion Providing care to incarcerated women through a structured curriculum is a novel way to encourage obstetrics and gynecology residents to consider the social determinants of health and for residents to cultivate their counseling skills. The rotation also included a wide breadth and depth of clinical diagnoses and procedures. Obstetrics and gynecology residency programs should consider a curriculum in reproductive health for incarcerated women.

Background: The biological actions of NO depend critically on its concentration, which is very difficult to measure. Results: Recorded using the most sensitive detectors yet described, NO levels synthesized by activated brain neurons generally were in the picomolar range but varied regionally. Conclusion: NO operates physiologically at the subnanomolar concentrations that selectively target its guanylyl cyclase-linked receptors. Significance: Physiological NO levels are far lower than commonly supposed., Nitric oxide (NO) is a widespread signaling molecule with potentially multifarious actions of relevance to health and disease. A fundamental determinant of how it acts is its concentration, but there remains a lack of coherent information on the patterns of NO release from its sources, such as neurons or endothelial cells, in either normal or pathological conditions. We have used detector cells having the highest recorded NO sensitivity to monitor NO release from brain tissue quantitatively and in real time. Stimulation of NMDA receptors, which are coupled to activation of neuronal NO synthase, routinely generated NO signals from neurons in cerebellar slices. The average computed peak NO concentrations varied across the anatomical layers of the cerebellum, from 12 to 130 pm. The mean value found in the hippocampus was 200 pm. Much variation in the amplitudes recorded by individual detector cells was observed, this being attributable to their location at variable distances from the NO sources. From fits to the data, the NO concentrations at the source surfaces were 120 pm to 1.4 nm, and the underlying rates of NO generation were 36–350 nm/s, depending on area. Our measurements are 4–5 orders of magnitude lower than reported by some electrode recordings in cerebellum or hippocampus. In return, they establish coherence between the NO concentrations able to elicit physiological responses in target cells through guanylyl cyclase-linked NO receptors, the concentrations that neuronal NO synthase is predicted to generate locally, and the concentrations that neurons actually produce.