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Lubricants, Volume 10, Issue 3 (March 2022) – 16 articles

Cover Story (view full-size image): Material design is instrumental in optimizing the tribological performance of high-end products. Nanostructured materials offer an innovative tool to reach the goal of reducing energy use, material depletion and maintenance costs by enhancing friction and wear behaviour. Molybdenum disulfide powder in its micrometric and nanometric forms and graphene nanoplatelets were studied as solid lubricants in two copper matrices: a dendritic and a spherical powder. A variety of techniques was employed, from micro-scratch testing to confocal laser and scanning electron microscopy. Wear test demonstrated that the composites produced by powder metallurgy combine optimal performance and functionality with their ability to operate in harsh environments, coupling low friction with reduced surface damage. View this paper.
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13 pages, 1653 KiB  
Article
On the Transient Effects at the Beginning of 3D Elastic-Plastic Rolling Contacts for a Circular Point Contact Considering Isotropic Hardening
by Michael Juettner, Marcel Bartz, Stephan Tremmel and Sandro Wartzack
Lubricants 2022, 10(3), 47; https://doi.org/10.3390/lubricants10030047 - 19 Mar 2022
Cited by 2 | Viewed by 2296
Abstract
In a three-dimensional transient simulation of the elastic–plastic rolling contact, transient effects can be observed at the beginning of the rolling until a stationary state is reached after rolling for a length of several times the contact radius. In most cases, the steady-state [...] Read more.
In a three-dimensional transient simulation of the elastic–plastic rolling contact, transient effects can be observed at the beginning of the rolling until a stationary state is reached after rolling for a length of several times the contact radius. In most cases, the steady-state regime is in focus of scientific investigations, whereas the transient effects are hardly considered. In the present work, those transient effects at the beginning of a frictionless rolling contact of a rigid sphere on an elastic–plastic plane are studied in detail. The analysis is limited to isotropic strain hardening. In particular, the changes of the contact pressure during rolling, as well as the plastic strain state and plastic deformations remaining after rolling are investigated. This is intended to get to the bottom of existing explanatory approaches from literature, which are based on the change in conformity. Beyond that, a more profound explanation of the transient effects is developed by identifying existing correlations. Full article
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29 pages, 7710 KiB  
Article
Effect of Preload on the Vibrations of EHL Angular Contact Ball Bearings: Theoretical and Experimental Results
by Hikmet Bal, Kerem Ateş, Tuncay Karaçay and Nizami Aktürk
Lubricants 2022, 10(3), 46; https://doi.org/10.3390/lubricants10030046 - 16 Mar 2022
Cited by 5 | Viewed by 3919
Abstract
The vibrations of a shaft in rotary mechanical systems supported by angular contact ball bearings are investigated theoretically and experimentally for various preloads in this paper. In the theoretical part of the study, a dynamic bearing model is presented, a rigid shaft supported [...] Read more.
The vibrations of a shaft in rotary mechanical systems supported by angular contact ball bearings are investigated theoretically and experimentally for various preloads in this paper. In the theoretical part of the study, a dynamic bearing model is presented, a rigid shaft supported by EHL angular contact bearing has been modelled as 5 DoF. Non-linear equations of motion are solved numerically by the Runge–Kutta method. In the second part of the study, an experimental setup that enables performing different operating cases has been designed to validate the theoretical results. Theoretical and experimental data are investigated and compared in both time and frequency domains and the results are compared. It is observed from both the theoretical and experimental studies that preload has a significant effect on the vibration behaviour. Results show that the increase in preload reduces the amplitude of the variable compliance frequencies of bearing, the natural frequency of system is shifted to a higher value, and using signal processing with an envelope spectrum gives better results in spectral analysis; small deviations occur between the theoretical and the experimental data due to modulation and noise from machine elements such as gear, motor, misalignment, waviness, etc. Therefore, the presented dynamic bearing model can be used with a reasonable accuracy to examine effect of preload on the vibration of shaft-bearing system. Full article
(This article belongs to the Special Issue Special Issue in Elastohydrodynamics: Remembering Ramsey Gohar)
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3 pages, 174 KiB  
Editorial
Grease
by Raj Shah, Mathias Woydt, Simon C. Tung and Andreas Rosenkranz
Lubricants 2022, 10(3), 45; https://doi.org/10.3390/lubricants10030045 - 16 Mar 2022
Cited by 2 | Viewed by 2450
Abstract
Grease is an extraordinarily complex lubricant with a complex material–property relationship, and to shed more light on its importance, we decided to launch the first Special Issue of “Lubricants” purely focusing on the most recent developmental trends of grease applications [...] Full article
(This article belongs to the Special Issue Grease)
8 pages, 7503 KiB  
Article
Evaluation of Writing Performance for Different Types of Ballpoint Pen Ink by Acoustic Emission Sensing
by Alan Hase
Lubricants 2022, 10(3), 44; https://doi.org/10.3390/lubricants10030044 - 15 Mar 2022
Cited by 3 | Viewed by 5569
Abstract
During contact, deformation, and fracture of surface asperities between the friction surfaces of materials, acoustic emission (AE) waves are generated as the strain energy is released. By detecting the AE waves during friction using an AE sensor, the state of friction, wear, and [...] Read more.
During contact, deformation, and fracture of surface asperities between the friction surfaces of materials, acoustic emission (AE) waves are generated as the strain energy is released. By detecting the AE waves during friction using an AE sensor, the state of friction, wear, and lubrication between the friction surfaces can be measured and evaluated with high sensitivity. In this study, in order to establish a method for evaluating the writing performance of ballpoint pens by AE sensing, the measurement method was examined, and AE signal waveforms were analyzed. It was found that AE sensing can detect phenomena that do not appear as a change in frictional force during writing. In addition, frequency analysis of the AE signal waveforms revealed that differences in writing performance depending on the ink type of the ballpoint pen can be evaluated and interpreted. Full article
(This article belongs to the Special Issue Acoustic Emission Techniques in Wear Monitoring II)
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12 pages, 2256 KiB  
Article
In Silico Approaches for Some Sulfa Drugs as Eco-Friendly Corrosion Inhibitors of Iron in Aqueous Medium
by Soukaina Alaoui Mrani, Nadia Arrousse, Rajesh Haldhar, Abdellatif Ait Lahcen, Aziz Amine, Taoufiq Saffaj, Seong-Cheol Kim and Mustapha Taleb
Lubricants 2022, 10(3), 43; https://doi.org/10.3390/lubricants10030043 - 15 Mar 2022
Cited by 13 | Viewed by 2673
Abstract
This paper addresses the prediction of the adsorption behavior as well as the inhibition capacity of non-toxic sulfonamide-based molecules, also called sulfa drugs, on the surface of mild steel. The study of the electronic structure was investigated through quantum chemical calculations using the [...] Read more.
This paper addresses the prediction of the adsorption behavior as well as the inhibition capacity of non-toxic sulfonamide-based molecules, also called sulfa drugs, on the surface of mild steel. The study of the electronic structure was investigated through quantum chemical calculations using the density functional theory method (DFT) and the direct interaction of inhibitors with the iron (Fe) metal surface was predicted using the multiple probability Monte Carlo simulations (MC). Then, the examination of the solubility and the environmental toxicity was confirmed using a chemical database modeling environment website. It was shown that the presence of substituents containing heteroatoms able to release electrons consequently increased the electron density in the lowest unoccupied and highest occupied molecular orbitals (LUMO and HOMO), which allowed a good interaction between the inhibitors and the steel surface. The high values of EHOMO imply an ability to donate electrons while the low values of ELUMO are related to the ability to accept electrons thus allowing good adsorption of the inhibitor molecules on the steel surface. Molecular dynamics simulations revealed that all sulfonamide molecules adsorb flat on the metal surface conforming to the highly protective Fe (1 1 0) surface. The results obtained from the quantum chemistry and molecular dynamics studies are consistent and reveal that the order of effectiveness of the sulfonamide compounds is P7 > P5 > P6 > P1 > P2 > P3 > P4. Full article
(This article belongs to the Special Issue Corrosion and Tribocorrosion Behavior of Metals and Alloys)
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20 pages, 7516 KiB  
Article
High-Performance Ni-SiC Coatings Fabricated by Flash Heating
by Peter Renner, Ajinkya Raut and Hong Liang
Lubricants 2022, 10(3), 42; https://doi.org/10.3390/lubricants10030042 - 14 Mar 2022
Cited by 4 | Viewed by 2412
Abstract
In this research, a novel flash heating coating application technique was utilized to create Ni-SiC coatings on carbon steel substrates with SiC contents much higher than is achievable using certain conventional coating techniques. Hardness profiles showed that the coatings improved the substrate by [...] Read more.
In this research, a novel flash heating coating application technique was utilized to create Ni-SiC coatings on carbon steel substrates with SiC contents much higher than is achievable using certain conventional coating techniques. Hardness profiles showed that the coatings improved the substrate by as much as 121%, without affecting the substrate. Tribotests showed that the wear performance was improved by as much as 4.7× in terms of the wear rate (mm3/N·m) for the same coating when using an Al2O3 counterpart. Pure SiC coatings as a reference were also fabricated. However, the SiC coatings experienced elemental diffusion of Fe from the carbon steel substrate into the coating during fabrication. This occurred due to the increased heat input required for pure SiC to fuse to the substrate compared to the Ni-SiC coatings and resulted in decreased tribological performance. Diffusion of Fe into the coating weakened the coating’s hardness and reduced the resistance to wear. It was concluded that ceramic–metallic composite coatings can successfully be fabricated utilizing this novel flash heating technique to improve the wear resistance of ceramic counterparts. Full article
(This article belongs to the Special Issue Special Issue in Elastohydrodynamics: Remembering Ramsey Gohar)
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12 pages, 6900 KiB  
Article
Validation of a Multi-Scale Contact Temperature Model for Dry Sliding Rough Surfaces
by Jamal Choudhry, Andreas Almqvist and Roland Larsson
Lubricants 2022, 10(3), 41; https://doi.org/10.3390/lubricants10030041 - 9 Mar 2022
Cited by 3 | Viewed by 2503
Abstract
A multi-scale flash temperature model is validated against existing experimental work. The model shows promising results and proves itself to be a reliable tool for the accurate prediction of the flash temperature development between rough surfaces in sliding systems. Model predictions for the [...] Read more.
A multi-scale flash temperature model is validated against existing experimental work. The model shows promising results and proves itself to be a reliable tool for the accurate prediction of the flash temperature development between rough surfaces in sliding systems. Model predictions for the maximum flash temperatures as well as the bulk temperature fields were in very good agreement with the experimentally measured values. The model was also able to accurately predict the formation of hotspots as well as the temperature variations around the hotspots. From the model predictions, it is concluded that it is sufficient to only assess the flash temperatures on a small portion of the contact area and thus save both computational time and memory. Full article
(This article belongs to the Special Issue Surface Engineering for Wear Protection and Friction Reduction)
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14 pages, 3634 KiB  
Article
Lubricating Oil Consumption Measurement on Large Gas Engines
by Bernhard Rossegger, Albrecht Leis, Martin Vareka, Michael Engelmayer and Andreas Wimmer
Lubricants 2022, 10(3), 40; https://doi.org/10.3390/lubricants10030040 - 8 Mar 2022
Cited by 4 | Viewed by 4303
Abstract
Increasing the reliability of combustion engines while further reducing emissions and life cycle costs are the main drivers for optimizing lubricating oil consumption (LOC). However, in order to reduce the lube oil consumption of an engine, it is crucial to measure it accurately. [...] Read more.
Increasing the reliability of combustion engines while further reducing emissions and life cycle costs are the main drivers for optimizing lubricating oil consumption (LOC). However, in order to reduce the lube oil consumption of an engine, it is crucial to measure it accurately. Therefore, a LOC measurement device based on the use of the stable isotope deuterium has been developed. Previous publications have focused on the use of passenger car engines. This publication describes the first application of this newly developed method on a large gas engine. This is of particular interest as large-bore engines might show different oil consumption behavior, much higher LOC in gram per hour and the bigger oil reservoir need larger amounts of tracer. Additionally, a different type of fuel has an effect on oil consumption measurement as well, as presented in this paper. The results showed this method can be applied to large gas engines as well after conducting minor changes to the measurement setup. However, other than liquid fuels, the origin and isotopic composition of the natural gas has to be monitored. Ideally, gas from large storage is used for carrying out these measurements. Full article
(This article belongs to the Special Issue Tribology in Mobility)
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10 pages, 3946 KiB  
Article
Analysis of Grooves Used for Bearing Lubrication Efficiency Enhancement under Multiple Parameter Coupling
by Chao Wang, Mingkai Wang and Linbo Zhu
Lubricants 2022, 10(3), 39; https://doi.org/10.3390/lubricants10030039 - 7 Mar 2022
Cited by 2 | Viewed by 2784
Abstract
Adding axial groove structures to the surface of bearing inner ring is an effective way to enhance the bearing lubrication efficiency. In this paper, the angular contact ball bearing H7006C was taken as the research object, and through visual simulations and quantitative experiments, [...] Read more.
Adding axial groove structures to the surface of bearing inner ring is an effective way to enhance the bearing lubrication efficiency. In this paper, the angular contact ball bearing H7006C was taken as the research object, and through visual simulations and quantitative experiments, systematic analyses and discussions were carried out to find out the relationship between groove-enhancing performance and working conditions. Firstly, simulation models of standard bearing and groove-added bearing were established. By combining the Volume of Fluid (VOF) model, the enhancing mechanism of grooves was found. Secondly, the trend of groove-enhancing performance with the change of rotation speed was studied through simulations and quantitative experiments. On this basis, through multiple parameter coupling, the effects of oil supply amount and nozzle diameter on the groove performance were discussed. The results show that the bottom oil layer is the key for grooves to achieve the lubrication efficiency enhancement, and its distribution can reflect the groove-enhancing performance. The groove width that best adapts to the change of working conditions increases with the increase of oil supply amount and nozzle diameter. To maintain the stability of bottom oil layer, the nozzle diameter should be larger than the groove width. This research is of great significance to the application of grooves in the lubrication efficiency enhancement of high-speed bearing. Full article
(This article belongs to the Special Issue Advances in Bearing Lubrication and Thermal Sciences)
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19 pages, 2148 KiB  
Article
Improving the Endoprosthesis Design and the Postoperative Therapy as a Means of Reducing Complications Risks after Total Hip Arthroplasty
by Valentin L. Popov, Aleksandr M. Poliakov and Vladimir I. Pakhaliuk
Lubricants 2022, 10(3), 38; https://doi.org/10.3390/lubricants10030038 - 4 Mar 2022
Cited by 2 | Viewed by 2687
Abstract
One of the most high-tech, efficient and reliable surgical procedures is Total Hip Arthroplasty (THA). Due to the increase in average life expectancy, it is especially relevant for older people suffering from chronic joint disease, allowing them to return to an active lifestyle. [...] Read more.
One of the most high-tech, efficient and reliable surgical procedures is Total Hip Arthroplasty (THA). Due to the increase in average life expectancy, it is especially relevant for older people suffering from chronic joint disease, allowing them to return to an active lifestyle. However, the rejuvenation of such a severe joint disease as osteoarthritis requires the search for new solutions that increase the lifespan of a Total Hip Replacement (THR). Current trends in the development of this area are primarily focused on the creation of new materials used in THR and methods for their processing that meet the requirements of biocompatibility, long-term strength, wear resistance and the absence of an immune system response aimed at rejection. This study is devoted to the substantiation of one of the possible approaches to increase the reliability and durability of THR, based on the improvement of the implant design and postoperative rehabilitation technology, potentially reducing the risk of complications in the postoperative period. Full article
(This article belongs to the Special Issue Tribology of Biomaterials for Bone Regeneration)
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18 pages, 7915 KiB  
Article
Wear Property and Wear Mechanisms of High-Manganese Austenitic Hadfield Steel in Dry Reciprocal Sliding
by Quanshun Luo and Jingzhi Zhu
Lubricants 2022, 10(3), 37; https://doi.org/10.3390/lubricants10030037 - 4 Mar 2022
Cited by 10 | Viewed by 3066
Abstract
This paper reports the sliding wear properties and wear mechanisms of Hadfield high-Mn austenitic steel in a dry-sliding reciprocal tribotest against a WC counterpart. The associated wear mechanisms were studied through extensive characterisation of the obtained worn surface using analytical SEM, TEM, XRD [...] Read more.
This paper reports the sliding wear properties and wear mechanisms of Hadfield high-Mn austenitic steel in a dry-sliding reciprocal tribotest against a WC counterpart. The associated wear mechanisms were studied through extensive characterisation of the obtained worn surface using analytical SEM, TEM, XRD and micro-hardness test. The tribotest revealed a coefficient of wear in the scale of 10−14 m3·N−1·m−1 and a coefficient of friction of 0.5–0.6. The steel encountered severe plastic deformation beneath the worn surface leading to a gradient of hardness profile, including the extreme hardening of the worn surface up to HV0.1 8.2 GPa. Despite the severe deformation and significant strain hardening, the steel still retained its austenitic structure without any detectable martensite. The combined surface and cross-sectional microscopic observations and extensive analysis of XRD peak breadth revealed the formation of nano-heterogeneous microstructure including nano-laminate, nanotwins and nanocrystalline beneath the worn surface. Spalling wear was found as the predominant wear mechanism. The spalling wear was caused by the embrittlement of the extremely hardened and nanocrystallised worn surface. Tribo-oxidation was also observed in the resultant wear debris. Full article
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18 pages, 1246 KiB  
Review
Increase Service Life for Rail Wheel Bearings—A Review of Grease Lubrication for This Application
by Hannes Allmaier
Lubricants 2022, 10(3), 36; https://doi.org/10.3390/lubricants10030036 - 3 Mar 2022
Cited by 7 | Viewed by 4266
Abstract
Billions of rolling bearings (RB) are in use today in a broad and diverse range of applications. In the mobility sector, RB help to reduce friction losses and increase efficiency. In rail applications, the rail wheel rolling bearing is a critical component, which [...] Read more.
Billions of rolling bearings (RB) are in use today in a broad and diverse range of applications. In the mobility sector, RB help to reduce friction losses and increase efficiency. In rail applications, the rail wheel rolling bearing is a critical component, which requires a strict maintenance schedule. In this literature review, grease lubrication in RB is reviewed and potential ways to improve the service life of greases in RB are discussed with special emphasis on the application as rail wheel bearing. Understanding the discussed fundamental lubrication processes is the key to increase the service life of the rail wheel bearings and might provide a basis for future work that aims to make maintenance of these bearings condition-based (condition-based maintenance). This review is primarily intended for R&D professionals from rail (and related) industry and others being interested in a rather brief, but fundamental, overview of this subject. Full article
(This article belongs to the Special Issue Special Issue in Elastohydrodynamics: Remembering Ramsey Gohar)
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21 pages, 9602 KiB  
Article
Physico-Mechanical Properties of Metal Matrix Self-Lubricating Composites Reinforced with Traditional and Nanometric Particles
by Marco Freschi, Alessia Arrigoni, Oskari Haiko, Luca Andena, Jukka Kömi, Chiara Castiglioni and Giovanni Dotelli
Lubricants 2022, 10(3), 35; https://doi.org/10.3390/lubricants10030035 - 2 Mar 2022
Cited by 7 | Viewed by 3338
Abstract
Innovative nanostructured materials offer the possibility of enhancing the tribological performance of traditional materials like graphite and molybdenum disulfide (MoS2). In this study, the scratch resistance of two different copper powders, dendritic and spherical, and their composites with traditional MoS2 [...] Read more.
Innovative nanostructured materials offer the possibility of enhancing the tribological performance of traditional materials like graphite and molybdenum disulfide (MoS2). In this study, the scratch resistance of two different copper powders, dendritic and spherical, and their composites with traditional MoS2, nanometric MoS2, and graphene nanoplatelets was investigated. Metal powder metallurgy was employed to produce composite materials with 5 wt% and 10 wt% of each solid lubricant. A ball milling step was employed to grind and mix the matrix copper powder with the lubricants. The use of a cold press combined with the sintering in inert atmosphere at 550 °C limited the oxidation of the copper and the degradation of the solid lubricants. The so-produced materials were characterized through a variety of techniques such as micro-indentation hardness, electrical resistivity, contact angle wettability, X-ray diffraction, Raman scattering, and scanning electron microscopy. Moreover, micro-scratch tests were performed on both pure copper and composite materials for comparing the apparent scratch hardness and friction coefficients. The scratches were examined with confocal laser scanning microscopy (CLSM), to identify the evolution of the damage mechanisms during the formation of the groove. The results highlighted the important difference between the dendritic and spherical copper powders and demonstrated a way to improve wear behavior thanks to the use of nanometric powders as solid lubricants. Full article
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18 pages, 4720 KiB  
Article
A Mesoscopic Simulation Approach Based on Metal Fibre Characterization Data to Evaluate Brake Friction Performance
by Francesco Varriale, Gabriele Riva, Jens Wahlström and Yezhe Lyu
Lubricants 2022, 10(3), 34; https://doi.org/10.3390/lubricants10030034 - 25 Feb 2022
Cited by 2 | Viewed by 2224
Abstract
The coefficient of friction (COF) is an important parameter when evaluating brake system performance. It is complex to predict friction due to its dependence on parameters, such as sliding velocity, contact pressure, temperature, and friction material mixtures. The aim of this work is [...] Read more.
The coefficient of friction (COF) is an important parameter when evaluating brake system performance. It is complex to predict friction due to its dependence on parameters, such as sliding velocity, contact pressure, temperature, and friction material mixtures. The aim of this work is to evaluate the macroscopic COF of a disc brake system under specific braking conditions by a meso-scale approach, using a cellular automaton simulation where the friction material mixture is modelled starting from its basic components. The influence of the local components in contact is taken into account. Simulated COF values are in line with the experimental values. Full article
(This article belongs to the Special Issue Automotive Tribology II)
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14 pages, 10875 KiB  
Article
Tribo-Surface Variation Caused by Ti2AlC and Ti3AlC2 Particles-Containing Lubricant under Cyclic Impact Loading
by Yan Shen, Zhixiang Liu, Chuanfei Xing, Qi Li and Junjing Fan
Lubricants 2022, 10(3), 33; https://doi.org/10.3390/lubricants10030033 - 23 Feb 2022
Cited by 2 | Viewed by 2140
Abstract
Mn+1AXn (MAX) phase materials present an attractive potential for friction reduction and wear resistance applications due to the ternary layered structure. This work was done to investigate how the combination of Ti2AlC and Ti3AlC2 MAX [...] Read more.
Mn+1AXn (MAX) phase materials present an attractive potential for friction reduction and wear resistance applications due to the ternary layered structure. This work was done to investigate how the combination of Ti2AlC and Ti3AlC2 MAX phase particles with zinc dialkyl dithiophosp (ZDDP) additives in the lubricant affected the tribo-surface by means of a reciprocating test rig with cyclic impact loading. The results indicated that the friction and wear properties of Ti3AlC2-containing lubricant were better than those of Ti2AlC-containing lubricant. The distinctive microstructure of worn surface caused by the Ti3AlC2 particles was characterized by the uniform distribution of many fine scratches, while the other was distributed with more peeling pits by the Ti2AlC particles. The tribo-chemical reaction of ZDDP involving Ti3AlC2 particles promoted a larger regional distribution of the tribofilm and the generation of short-chain phosphates. The synergistic effect of Ti3AlC2 particles and ZDDP additives led to excellent tribological properties. Full article
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18 pages, 2207 KiB  
Article
Modelling of Lubricated Electrical Contacts
by Robert L. Jackson and Santosh Angadi
Lubricants 2022, 10(3), 32; https://doi.org/10.3390/lubricants10030032 - 22 Feb 2022
Cited by 12 | Viewed by 4777
Abstract
Electrical contacts, although critically important for a wide range of applications, are susceptible to degradation due to fretting corrosion, especially when sliding and vibrations occur. To overcome fretting corrosion and sliding wear, lubricants are often used. However, the use of lubricants can cause [...] Read more.
Electrical contacts, although critically important for a wide range of applications, are susceptible to degradation due to fretting corrosion, especially when sliding and vibrations occur. To overcome fretting corrosion and sliding wear, lubricants are often used. However, the use of lubricants can cause other detrimental issues. Lubricants usually consist of non-conductive fluids such as hydrocarbons and fluorocarbons. Due to fluid dynamics, when sliding, vibration or other excitation occurs, these fluids can cause prolonged gaps between the conducting metal surfaces. Practically, this has been observed in data centers where vibrations due to technician maintenance or even earthquakes can occur. Depending on the viscosity and roughness of the surfaces, the time it takes these connector surfaces to return to solid conductive contact can be many seconds or longer. This work uses a novel theoretical model of the coupled fluid and solid mechanics between the rough metallic surfaces to evaluate these intermittent breaks in contact due to sliding. The influence of variation in lubricant properties, roughness, contact radius and contact force are considered by the model. Full article
(This article belongs to the Special Issue Special Issue in Elastohydrodynamics: Remembering Ramsey Gohar)
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