Imperial College London

Dr Tom Reddyhoff

Faculty of EngineeringDepartment of Mechanical Engineering

Reader in Tribology
 
 
 
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Contact

 

+44 (0)20 7594 3840t.reddyhoff Website

 
 
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Location

 

670City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

132 results found

Gutierrez R, Fang T, Mainwaring R, Reddyhoff Tet al., 2024, Predicting the coefficient of friction in a sliding contact by applying machine learning to acoustic emission data, Friction, ISSN: 2223-7690

It is increasingly important to monitor sliding interfaces within machines, since this is where both energy is lost, and failures occur. Acoustic emission (AE) techniques offer a way to monitor contacts remotely without requiring transparent or electrically conductive materials. However, acoustic data from sliding contacts is notoriously complex and difficult to interpret. Herein, we simultaneously measure coefficient of friction (with a conventional force transducer) and acoustic emission (with a piezoelectric sensor and high acquisition rate digitizer) produced by a steel–steel rubbing contact. Acquired data is then used to train machine learning (ML) algorithms (e.g., Gaussian process regression (GPR) and support vector machine (SVM)) to correlated acoustic emission with friction. ML training requires the dense AE data to first be reduced in size and a range of processing techniques are assessed for this (e.g., down-sampling, averaging, fast Fourier transforms (FFTs), histograms). Next, fresh, unseen AE data is given to the trained model and the resulting friction predictions are compared with the directly measured friction. There is excellent agreement between the measured and predicted friction when the GPR model is used on AE histogram data, with root mean square (RMS) errors as low as 0.03 and Pearson correlation coefficients reaching 0.8. Moreover, predictions remain accurate despite changes in test conditions such as normal load, reciprocating frequency, and stroke length. This paves the way for remote, acoustic measurements of friction in inaccessible locations within machinery to increase mechanical efficiency and avoid costly failure/needless maintenance.[Figure not available: see fulltext.]

Journal article

Massocchi D, Chatterton S, Lattuada M, Reddyhoff T, Dini D, Pennacchi Pet al., 2023, Effect of Friction Reducers with Unreinforced PEEK and Steel Counterparts in Oil Lubrication, Lubricants, Vol: 11

The increasing adoption of PEEK (polyetheretherketone) in many industrial applications has promoted intense research to optimize its lubrication along with the development of friction reducers (FRs), additives that help in reducing fuel consumption and, consequently, CO2 emissions. In this study, the effect of FRs in improving the lubrication of PEEK–steel couplings was evaluated and their mechanism studied using the Mini Traction Machine (MTM) tribometer. Different types of FRs (such as Molybdenum dithiocarbamate, glycerol monooleate, amine and polymeric derivatives) and coupling combinations (steel/steel, steel/PEEK and PEEK/steel) were considered. The oil samples were evaluated as fresh and after a rubbing time considering different operative conditions (from high to low T, fixed load and type of contact motion), and a measurement of the tribofilm was acquired. The experimental campaign showed a ranking among FRs friction-reducing behavior and, in some cases, a synergistic effect was noted between the tribofilm containing the friction modifier and the PEEK surface. Comparing the top performing FRs with reference oil showed a reduction in friction of 22%, 21% and 37%, respectively, in steel–steel, PEEK–steel and steel–PEEK couplings, while in the standard steel–steel coupling, two out of four FRs did not reduce the friction. After conditioning in the presence of PEEK, all friction-modifier additives reduced the friction effectively. This demonstrates the promising performance of PEEK, its compatibility with friction-reducing additives, and its applicability to sliding machine parts in order to improve efficiency and thus reduce CO2 emissions.

Journal article

Ardah S, Profito FJ, Reddyhoff T, Dini Det al., 2023, Advanced modelling of lubricated interfaces in general curvilinear grids, TRIBOLOGY INTERNATIONAL, Vol: 188, ISSN: 0301-679X

Journal article

Kirkby T, Smith JJ, Berryman J, Fowell M, Reddyhoff Tet al., 2023, Soot wear mechanisms in heavy-duty diesel engine contacts, WEAR, Vol: 524, ISSN: 0043-1648

Journal article

Weston A, Vladescu S-C, Reddyhoff T, Griffiths A, Crouzier T, Fielden M, Garnett JA, Carpenter GHet al., 2023, The influence of ions on the lubricative abilities of mucin and the role of sialic acids, COLLOIDS AND SURFACES B-BIOINTERFACES, Vol: 227, ISSN: 0927-7765

Journal article

Hu S, Huang W, Li J, Reddyhoff T, Cao X, Shi X, Peng Z, Demello A, Dini Det al., 2023, Rigid-flexible hybrid surfaces for water-repelling and abrasion-resisting, FRICTION, Vol: 11, Pages: 635-646, ISSN: 2223-7690

Journal article

Haimov E, Chapman A, Bresme F, Holmes AS, Reddyhoff T, Urbakh M, Kornyshev AAet al., 2023, Addendum: Theoretical demonstration of a capacitive rotor for generation of alternating current from mechanical motion., Nature Communications, Vol: 14, Pages: 483-483, ISSN: 2041-1723

Journal article

Vladescu S-C, Agurto MG, Myant C, Boehm MW, Baier SK, Yakubov GE, Carpenter G, Reddyhoff Tet al., 2023, Protein-induced delubrication: How plant-based and dairy proteins affect mouthfeel, FOOD HYDROCOLLOIDS, Vol: 134, ISSN: 0268-005X

Journal article

Zhang J, Yu M, Joedicke A, Reddyhoff Tet al., 2022, Characterising the effects of simultaneous water and gasolinedilution on lubricant performance, Tribology International, ISSN: 0301-679X

Journal article

Jia Y, Dou P, Zheng P, Wu T, Yang P, Yu M, Reddyhoff Tet al., 2022, High-accuracy ultrasonic method for in-situ monitoring of oil film thickness in a thrust bearing, Mechanical Systems and Signal Processing, Vol: 180, Pages: 109453-109453, ISSN: 0888-3270

Journal article

Dou P, Zou L, Wu T, Yu M, Reddyhoff T, Peng Zet al., 2022, Simultaneous measurement of thickness and sound velocity of porous coatings based on the ultrasonic complex reflection coefficient, NDT & E International, Vol: 131, Pages: 102683-102683, ISSN: 0963-8695

Journal article

Malik S, O'Sullivan C, Reddyhoff T, Dini D, Holmes Aet al., 2022, An acoustic 3D positioning system for robots operating underground, IEEE Sensors Letters, Vol: 6, Pages: 1-4, ISSN: 2475-1472

Underground robots are potentially helpful in many application domains, including geotechnical engineering, agriculture, and archaeology. One of the critical challenges in developing underground robotics is the accurate estimation of the positions of the robots. Acoustic-based positioning systems have been explored for developing an underground 3D positioning system. However, the positioning range is limited due to attenuation in the medium. This letter proposes an underground positioning system that utilizes a novel and easy-to-implement electronic approach for measuringthe acoustic propagation times between multiple transmitters and a receiver. We demonstrate a prototype using four transmitters at the surface and a single buried acoustic sensor as a proof-of-concept. The times of arrival for signals emitted by the different sources are measured by correlating the transmitted and received signals. The distances between the multiple transmitters and a receiver are estimated, and a tri-linearization algorithm is used to estimate the position of the buried sensor in 3D with respect to reference coordinates. The system is tested in a soil tank. The experimental results show that the proposed system is able to estimate the 3D position of buried sensors with an error of less than ±2.5 cm within a measurement field of size 50 cm × 50 cm × 35 cm in X, Y, and Z (width × length × depth). The proposed electronic synchronization approach allows increasing the positioning range of the system by increasing the number of transmittersat the surface. This paves the way for the development of a positioning system for robots operating underground.

Journal article

Hu S, Cao X, Reddyhoff T, Ding X, Shi X, Dini D, deMello AJ, Peng Z, Wang Zet al., 2022, Pneumatic programmable superrepellent surfaces, Droplet, Vol: 1, Pages: 48-55, ISSN: 2769-2159

Morphological transformation of surface structures is widely manifested in nature and highly preferred for many applications such as wetting interaction; however, in situ tuning of artificial morphologies independent of smart responsive materials remains elusive. Here, with the aid of microfluidics, we develop a pneumatic programmable superrepellent surface by tailoring conventional wetting materials (e.g., polydimethylsiloxane) with embedded flexible chambers connecting a microfluidic system, thus realizing a morphological transformation for enhanced liquid repellency based on a nature-inspired rigid-flexible hybrid principle (i.e., triggering symmetry breaking and oscillator coupling mechanisms). The enhancement degree can be in situ tuned within around 300 ms owing to pneumatically controllable chamber morphologies. We also demonstrate that the surface can be freely programmed to achieve elaborated morphological pathways and gradients for preferred droplet manipulation such as directional rolling and bouncing. Our study highlights the potential of an in situ morphological transformation to realize tunable wettability and provides a programmable level of droplet control by intellectualizing conventional wetting materials.

Journal article

Dou P, Wu T, Luo Z, Yang P, Peng Z, Yu M, Reddyhoff Tet al., 2022, A finite-element-aided ultrasonic method for measuring central oil film thickness in a roller-raceway tribo-pair, Friction, Vol: 10, Pages: 944-962, ISSN: 2223-7690

Roller bearings support heavy loads by riding on an ultra-thin oil film (between the roller and theraceway), the thickness of which is critical as it reflects the lubrication performance. Ultrasonicinterfacial reflection, which facilitates a non-destructive measurement of oil film thickness, has beenwidely studied. However, insufficient spatial resolution around the rolling line contact zone remainsa barrier, despite the employment of miniature piezoelectric transducers. In this paper, a finiteelement-aided method is utilized to simulate wave propagation through a three-layered structureof roller-oil-raceway under elastohydrodynamic lubrication (EHL), with nonlinear characteristicsof i) the deformed curvature of the cylindrical roller and ii) the non-uniform distribution of fluid bulkmodulus along the circumference of the oil layer taken into account. A load- and speed-dependentlook-up table is then developed to establish an accurate relationship between the overall reflectioncoefficient (directly measured by an embedded ultrasonic transducer) and the objective variable ofcentral oil film thickness. Moreover, the proposed finite-element-aided method is verifiedexperimentally in a roller-raceway test rig, with the ultrasonic measured oil film thicknessessentially corresponding to the calculated values by EHL theory.

Journal article

Dou P, Zheng P, Jia Y, Wu T, Yu M, Reddyhoff T, Liao W-H, Peng Zet al., 2022, Ultrasonic measurement of oil film thickness in a four-layer structure for applications including sliding bearings with a thin coating, NDT & E International, Pages: 102684-102684, ISSN: 0963-8695

Journal article

Beamish S, Reddyhoff T, Hunter A, Dwyer-Joyce RSet al., 2022, A method to determine acoustic properties of solids and its application to measuring oil film thickness in bearing shells of unknown composition, MEASUREMENT, Vol: 195, ISSN: 0263-2241

Journal article

Yu M, Zhang J, Kirkby T, Joedicke A, Reddyhoff Tet al., 2022, Electrical impedance spectroscopy enabled in-depth lubrication condition monitoring, The 2022 STLE Annual Meeting & Exhibition

Electrical contact resistance or capacitance as measured between twointerfaces of a lubricated contact has been used in tribometers, partiallyreflecting the lubrication condition. In contrast, the electrical impedancespectroscopy (EIS) provides rich information of magnitude/phase spectrum,which is thoroughly investigated using a combination of electrical circuitmodels (equivalent to the lubricated contact) and in-situ measurements with aball-on-disc contact. Results indicate a promising potential of EIS inlubrication condition monitoring, including the variation of lubricant filmthickness as estimated using high-frequency magnitude response; thetransition between full-film, mixed, and boundary lubrication regimes, asdifferentiated using extracted electrical resistance together with phasespectrum; the forming of anti-wear boundary film, where extraresistor/capacitor are added; and the degradation of lubricant, such as fueldilution, oil oxidization, and water emulsifying.

Conference paper

Vladescu S-C, Tadokoro C, Miyazaki M, Reddyhoff T, Nagamine T, Nakano K, Sasaki S, Tsujii Yet al., 2022, Exploiting the Synergy between Concentrated Polymer Brushes and Laser Surface Texturing to Achieve Durable Superlubricity, ACS APPLIED MATERIALS & INTERFACES, Vol: 14, Pages: 15818-15829, ISSN: 1944-8244

Journal article

Yu M, Reddyhoff T, Dini D, Holmes A, O'Sullivan Cet al., 2022, Acoustic emission enabled particle size estimation via low stress-varied axial interface shearing, IEEE Transactions on Instrumentation and Measurement, Vol: 71, ISSN: 0018-9456

Acoustic emission (AE) refers to a rapid release of localized stress energy that propagates as a transient elastic wave and is typically used in geotechnical applications to study stick-slip during shearing, and breakage and fracture of particles. This article develops a novel method of estimating the particle size, an important characteristic of granular materials, using axial interface shearing-induced AE signals. Specifically, a test setup that enables axial interface shearing between a one-dimensional compression granular deposit and a smooth shaft surface is developed. The interface sliding speed (up to 3mm/s), the compression stress (0-135kPa), and the particle size (150μm-5mm) are varied to test the acoustic response. The start and end moments of a shearing motion, between which a burst of AE data is produced, are identified through the variation of the AE count rates, before key parameters can be extracted from the bursts of interests. Linear regression models are then built to correlate the AE parameters with particle size, where a comprehensive evaluation and comparison in terms of estimation errors is performed. For granular samples with a single size, it is found that both the AE energy related parameters and AE counts, obtained using an appropriate threshold voltage, are effective in differentiating the particle size, exhibiting low fitting errors. The value of this technique lies in its potential application to field testing, for example as an add-on to cone penetration test systems and to enable in-situ characterization of geological deposits.

Journal article

Bahshwan M, Gee M, Nunn J, Myant CW, Reddyhoff Tet al., 2022, In situ observation of anisotropic tribological contact evolution in 316L steel formed by selective laser melting, Wear, Vol: 490-491, Pages: 1-12, ISSN: 0043-1648

A consensus on the tribological performance of components by additive-versus conventional manufacturing has not been achieved; mainly because the tribological test set-ups thus far were not suited for investigating the underlying microstructure's influence on the tribological properties. As a result, utilization of additive manufacturing techniques, such as selective laser melting (SLM), for tribological applications remains questionable. Here, we investigate the anisotropic tribological response of SLM 316L stainless steel via in situ SEM reciprocating micro-scratch testing to highlight the microstructure's role. As-built 316L SLM specimens were compared against annealed wire-drawn 316L. We found that: (i) microgeometric conformity was the main driver for achieving steady-state friction, (ii) the anisotropic friction of the additively manufactured components is limited to the break-in and is caused by the lack of conformity, (iii) the cohesive bonds, whose strength is proportional to frictional forces, are stronger in the additively manufactured specimens likely due to the dislocation-dense, cellular structures, (iv) low Taylor-factor grains with large dimension stimulate microcutting in the form of long, thin sheets with serrated edges. These findings uncover some microstructurally driven tribological complexities when comparing additive to conventional manufacturing.

Journal article

Dou P, Jia Y, Zheng P, Wu T, Yu M, Reddyhoff T, Peng Zet al., 2022, Review of ultrasonic-based technology for oil film thickness measurement in lubrication, Tribology International, Vol: 165, ISSN: 0301-679X

Lubricant film thickness is the most informative variable that reflects lubrication conditions and transmission efficiency in the mechanical equipment, therefore its measurement is highly important. Despite a large number of theoretical models that have been developed to describe the lubricant film, complexities and uncertainties in a real tribo-pair contact still hinder the implementation of accurate and robust methods of in-situ film thickness measurements. Recently, ultrasonic-based measurement has been widely studied, showing promising potential owing to its non-destructive characteristics, high sensitivity, and limited physical modifications. This paper comprehensively reviews basic principles of ultrasonic-based oil film measurement; summarizes progress on calculation models and associated signal processing methods; exhibits in-lab demonstrations and in-situ applications; and discusses key technical issues and possible solutions.

Journal article

Yu M, Reddyhoff T, Dini D, Holmes A, O'Sullivan Cet al., 2021, Using ultrasonic reflection resonance to probe stress wave velocity in assemblies of spherical particles, IEEE Sensors Journal, Vol: 21, Pages: 22489-22498, ISSN: 1530-437X

A high-sensitivity method to measure acousticwave speed in soils by analyzing the reflected ultrasonic signalfrom a resonating layered interface is proposed here.Specifically, an ultrasonic transducer which can be used to bothtransmit and receive signals is installed on a low-high acousticimpedance layered structure of hard PVC and steel, which in turnis placed in contact with the soil deposit of interest. The acousticimpedance of the soil (the product of density and wave velocity)is deduced from analysis of the waves reflected back to thetransducer. A system configuration design is enabled bydeveloping an analytical model that correlates the objectivewave speed with the measurable reflection coefficient spectrum.The physical viability of this testing approach is demonstratedby means of a one-dimensional compression device that probesthe stress-dependence of compression wave velocity of differentsizes of glass ballotini particles. Provided the ratio of thewavelength of the generated wave to the soil particle size issufficiently large the data generated are in agreement with dataobtained using conventional time-of-flight measurements. Inprinciple, this high-sensitivity approach avoids the need for thewave to travel a long distance between multiple transmitterreceiver sensors as is typically the case in geophysical testingof soil. Therefore it is particularly suited to in-situ observation ofsoil properties in a highly compact setup, where only a single transducer is required. Furthermore, high spatialresolution of local measurements can be achieved, and the data are unaffected by wave attenuation as transmitted insoil.

Journal article

Miyazaki M, Nakano K, Tadokoro C, Vladescu S-C, Reddyhoff T, Sasaki S, Tsujii Yet al., 2021, Enhancing durability of concentrated polymer brushes using microgrooved substrates, WEAR, Vol: 482, ISSN: 0043-1648

Journal article

Hu S, Reddyhoff T, Li J, Cao X, Shi X, Peng Z, deMello AJ, Dini Det al., 2021, Biomimetic water-repelling surfaces with robustly flexible structures, ACS Applied Materials and Interfaces, Vol: 13, Pages: 31310-31319, ISSN: 1944-8244

Biomimetic liquid-repelling surfaces have been the subject of considerable scientific research and technological application. To design such surfaces, a flexibility-based oscillation strategy has been shown to resolve the problem of liquid-surface positioning encountered by the previous, rigidity-based asymmetry strategy; however, its usage is limited by weak mechanical robustness and confined repellency enhancement. Here, we design a flexible surface comprising mesoscale heads and microscale spring sets, in analogy to the mushroomlike geometry discovered on springtail cuticles, and then realize this through three-dimensional projection microstereolithography. Such a surface exhibits strong mechanical robustness against ubiquitous normal and shear compression and even endures tribological friction. Simultaneously, the surface elevates water repellency for impacting droplets by enhancing impalement resistance and reducing contact time, partially reaching an improvement of ∼80% via structural tilting movements. This is the first demonstration of flexible interfacial structures to robustly endure tribological friction as well as to promote water repellency, approaching real-world applications of water repelling. Also, a flexibility gradient is created on the surface to directionally manipulate droplets, paving the way for droplet transport.

Journal article

Hu S, Cao X, Reddyhoff T, Shi X, Peng Z, deMello AJ, Dini Det al., 2021, Flexibility-patterned liquid-repelling surfaces, ACS Applied Materials and Interfaces, Vol: 13, Pages: 29092-29100, ISSN: 1944-8244

Droplets impacting solid surfaces is ubiquitous in nature and of practical importance in numerous industrial applications. For liquid-repelling applications, rigidity-based asymmetric redistribution and flexibility-based structural oscillation strategies have been proven on artificial surfaces; however, these are limited by strict impacting positioning. Here, we show that the gap between these two strategies can be bridged by a flexibility-patterned design similar to a trampoline park. Such a flexibility-patterned design is realized by three-dimensional projection micro-stereolithography and is shown to enhance liquid repellency in terms of droplet impalement resistance and contact time reduction. This is the first demonstration of the synergistic effect obtained by a hybrid solution that exploits asymmetric redistribution and structural oscillation in liquid-repelling applications, paving the rigidity-flexibility cooperative way of wettability tuning. Also, the flexibility-patterned surface is applied to accelerate liquid evaporation.

Journal article

Haimov E, Chapman A, Bresme F, Holmes A, Reddyhoff T, Urbakh M, Kornyshev Aet al., 2021, Theoretical demonstration of a capacitive rotor for generation of alternating current from mechanical motion, Nature Communications, Vol: 12, Pages: 3678-3678, ISSN: 2041-1723

Innovative concepts and materials are enabling energy harvesters for slower motion, particularly for personal wearables or portable small-scale applications, hence contributing to a future sustainable economy. Here we propose a principle for a capacitive rotor device and analyze its operation. This device is based on a rotor containing many capacitors in parallel. The rotation of the rotor causes periodic capacitance changes and, when connected to a reservoir-of-charge capacitor, induces alternating current. The properties of this device depend on the lubricating liquid situated between the capacitor’s electrodes, be it a highly polar liquid, organic electrolyte, or ionic liquid – we consider all these scenarios. An advantage of the capacitive rotor is its scalability. Such a lightweight device, weighing tens of grams, can be implemented in a shoe sole, generating a significant power output of the order of Watts. Scaled up, such systems can be used in portable wind or water turbines.

Journal article

Zimmer M, Vladescu S-C, Mattsson L, Fowell M, Reddyhoff Tet al., 2021, Shear-Area Variation: A Mechanism that Reduces Hydrodynamic Friction in Macro-Textured Piston Ring Liner Contacts, Tribology International, ISSN: 0301-679X

Journal article

Yu M, Zhang J, Joedicke A, Reddyhoff Tet al., 2021, Experimental investigation into the effects of diesel dilution on engine lubrication, Tribology International, Vol: 156, Pages: 1-9, ISSN: 0301-679X

The dilution of lubricant due to contamination with diesel fuel is an increasingly prevalent, potentially importantand poorly understood issue. Thisstudy addressestwo fundamental questions: 1) How doesthe change in lubricantrheology due to diesel dilution affect engine lubrication? 2) How is the chemical performance of lubricantcomponents (base oil and performance additives) impacted by diesel dilution under different lubrication regimes(boundary/full film, hydrodynamic/elastohydrodynamic). This is achieved by testing three lubricant samples: 1)neat fully formulated 0W-30 engine oil, 2) fully formulated 0W-30 oil diluted with diesel at a concentration of15%, denoted “0W-30D”, and 3) neat, fully-formulated 0W-16, with the same base oil components andperformance additives as the 0W-30, but blended to give a viscosity equal to that of the diluted an equivalent“0W-30D”. Tribometer tests, including 1) low pressure, low shear viscosity, 2) Ultra-high Shear Viscosity (USV),3) elastohydrodynamic film thickness, 4) Stribeck friction and 5) boundary friction and wear, are then conducted.To further emulate engine lubrication conditions, Stribeck curve measurements are performed on the threelubricants using a journal bearing test rig, fitted with a connecting-rod and commercial diesel engine shells.Results suggest that diesel dilution only slightly affects chemical additive performance (with friction modifiersbeing more inhibited than anti-wear additives) but does reduce both viscosity and film thickness. However, caremust be taken in using viscometrics to predict dilution behaviour because 1) the pressure viscosity coefficient isalso affected by diesel dilution which has implications for elastohydrodynamically lubrication contacts, 2) shearthinning means that viscosity modifier additives effects lose their functions at high shear rates; whereas dieselcontamination affects viscosity behaviour throughout the whole shear rate range.

Journal article

Vlădescu S-C, Bozorgi S, Hu S, Baier SK, Myant C, Carpenter G, Reddyhoff Tet al., 2021, Effects of beverage carbonation on lubrication mechanisms and mouthfeel, Journal of Colloid and Interface Science, Vol: 586, Pages: 142-151, ISSN: 0021-9797

The perception of carbonation is an important factor in beverage consumption which must be understood in order to develop healthier products. Herein, we study the effects of carbonated water on oral lubrication mechanisms involved in beverage mouthfeel and hence taste perception. Friction was measured in a compliant PDMS-glass contact simulating the tongue-palate interface (under representative speeds and loads), while fluorescence microscopy was used to visualise both the flow of liquid and oral mucosal pellicle coverage.When carbonated water is entrained into the contact, CO2 cavities form at the inlet, which limit flow and thus reduce the hydrodynamic pressure. Under mixed lubrication conditions, when the fluid film thickness is comparable to the surface roughness, this pressure reduction results in significant increases in friction (>300% greater than under non-carbonated water conditions). Carbonated water is also shown to be more effective than non-carbonated water at debonding the highly lubricious, oral mucosal pellicle, which again results in a significant increase in friction. Both these transient mechanisms of starvation and salivary pellicle removal will modulate the flow of tastants to taste buds and are suggested to be important in the experience of taste and refreshment. For example this may be one reason why flat colas taste sweeter.

Journal article

Reddyhoff T, Ewen J, Deshpande P, Frogley M, Welch M, Montgomery Wet al., 2021, Macroscale superlubricity and polymorphism of long-chain n-alcohols, ACS Applied Materials and Interfaces, Vol: 13, Pages: 9239-9251, ISSN: 1944-8244

Simple n-alcohols, such as 1-dodecanol, show anomalous film-forming and friction behaviors under elastohydrodynamic lubrication (EHL) conditions, as found inside bearings and gears. Using tribometer, diamond anvil cell (DAC), and differential scanning calorimetry (DSC) experiments, we show that liquid 1-dodecanol undergoes a pressure-induced solidification when entrained into EHL contacts. Different solid polymorphs are formed inside the contact depending on the temperature and pressure conditions. Surprisingly, at a moderate temperature and pressure, 1-dodecanol forms a polymorph that exhibits robust macroscale superlubricity. The DAC and DSC experiments show that superlubricity is facilitated by the formation of lamellar, hydrogen-bonded structures of hexagonally close-packed molecules, which promote interlayer sliding. This novel superlubricity mechanism is similar to that proposed for the two-dimensional materials commonly employed as solid lubricants, but it also enables the practical advantages of liquid lubricants to be maintained. When the pressure is increased, 1-dodecanol undergoes a polymorphic transformation into a phase that gives a higher friction. The DAC and DSC experiments indicate that the high-friction polymorph is an orthorhombic crystal. The polymorphic transformation pressure coincides with the onset of a dimple formation in the EHL films, revealing that the anomalous film shapes are caused by the formation of rigid orthorhombic crystals inside the contact. This is the first demonstration of a macroscale superlubricity in an EHL contact lubricated by a nonaqueous liquid that arises from bulk effects rather than tribochemical transformations at the surfaces. Since the superlubricity observed here results from phase transformations, it is continuously self-replenishing and is insensitive to surface chemistry and topology. This discovery creates the possibility of implementing superlubricity in a wide range of machine components, which would resul

Journal article

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