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We present an experimental study into the impact of surface texturing on crank shaft bearing friction. This was achieved by applying a series of laser-etched patterns to the surface of shell components and comparing friction measurements - obtained using a journal bearing test rig - to those of a non-texture reference. Tests were performed under a range of applied loads, speeds and lubricant viscosities in order to simulate the operating conditions encountered in an engine. Three texture configurations were assessed with varying dimple density and area coverage. The lubricant film temperature was also measured in order to remove shear heating effects and deduce accurate friction values. The bearing shell with a texture pattern applied to the whole of its circumference showed substantial reduction in friction (approximately 18%) when compared with the non-textured reference. This reduction in friction increases as the film thickness increases (i.e.: moving right on the Stribeck curve) and is most pronounced under full film hydrodynamic conditions. Friction reductions of up to 13% were observed using bearing shells that were only textured outside the loaded region. This is attributed to a reduction in shear rate due to an increased gap without alteration of the pressure generation and is important since this texture configuration maintains its effectiveness, even after the heavily loaded region becomes worn. These reported hydrodynamic friction reductions have practical implications for automotive bearings.

Surface topography serves as the fingerprint of a machine element. Most researchers understood topographies from a single-stratum perspective, while some have started to concern a bi-Gaussian stratified viewpoint. In this paper, a two-stage abrasive manufacturing process was employed to produce designed bi-Gaussian stratified topographies on hydrophilic steel discs, and a chemical vapor deposition of 1H,1H,2H,2H-perfluorooctyltrichlorosilane was conducted to form a hydrophobic self-assembled monolayer on each disc. By carefully measuring the static contact angles on these hydrophilic and hydrophobic surfaces, for the first time we were able to provide insights into the mechanism by which bi-Gaussian stratified topographies affect the wettability. Wetting behaviors on hydrophilic surfaces can be well explained by the Wenzel model from a bi-Gaussian stratified viewpoint. Rough components and a small proportion of the upper component can contribute to a small contact angle because of the increased surface-area extension. However, wetting behaviors on hydrophobic surfaces cannot be completely answered by the Cassie model, indicating another significant influence factor, i.e., autocorrelation lengths.

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.

It has recently been shown that rectangular surface pockets are effective in reducing friction in a piston–liner type contact, providing that they are oriented with their long axis transverse to th...

This paper presents an experimental study into the flow behaviour of lubricant in a reciprocating contact simulating a piston ring–cylinder liner pair. The aim was to understand the effects of cavitation, starvation and surface texture, as well as the interaction between these, in order to improve automotive engine performance. A custom-built test rig was used, in which a section of piston ring is loaded against a reciprocating, laser-textured, fused silica pad representing the liner. A fluorescence microscope focusses through the silica specimen onto the contact in order to image the distribution of dyed oil. Tests were performed using a range of texture geometries and orientations, under starved and fully-flooded lubrication conditions, with measurements being compared against those from a non-textured reference. Under limited oil supply conditions, the non-textured reciprocating contact sweeps oil towards the reversal points (TDC and BDC), leading to starvation and increased friction. This issue is alleviated by the presence of surface texturing, with each pocket transferring oil from the inlet to the outlet of the contact as it passes; the result being 33% lower friction and oil distributed evenly over the liner surface. Even under fully flooded conditions, starvation is shown to occur following each reversal, as the change in sliding direction causes the cavitated outlet to become the oil-deprived inlet. This proof of cavitation-reversal-starvation, which occurs for up to the first 5% of the stroke length, depending on the lubricant’s viscosity, corresponds to regions of high wear, measured in this study and on actual cylinder liners reported in the literature. This process is also counteracted by the presence of surface texture, with each pocket depositing oil into the cavitated region prior to reversal. Fluorescence data also provides insights into other mechanisms with which different textures geometries control friction. Grooves oriented parallel to sliding direction increase friction as they appear to connect the high pressure inlet with the low pressure outlet, leading to oil film collapse. Grooves oriented transverse to sliding direction produce localised cavitation inside each pocket, which supports the theory that texture draws lubricant into the contact through the ‘inlet suction’ mechanism. These findings can aid texture design by showing how pockets can be used in practice to simultaneously control oil consumption, and reduce friction and wear along the stroke. It should be noted that the lubricant transport mechanisms described above should also result from other types of depressions, such those produced by porous coatings (provided they are smaller than the contact area).

This paper presents a comparison between the results from numerical modelling and experiments to shed light on the mechanisms by which surface texture can reduce friction when applied to an automotive cylinder liner. In this configuration, textured features move relative to the piston-liner conjunction and to account for this our approach is to focus on the transient friction response to individual pockets as they pass through, and then leave, the sliding contact. The numerical approach is based on the averaged Reynolds’ equation with the Patir & Cheng's flow factors and the p-θ Elrod-Adams mass-conserving cavitation model. The contact pressures that arise from the asperity interactions are solved simultaneously to the fluid flow solution using the Greenwood and Tripp method. The experimental data is produced using a pin-on-disc set up, in which laser textured pockets have been applied to the disc specimen. Under certain conditions in the mixed and boundary lubrication regimes, both model and experimental results show i ) an increase in friction as the pocket enters the contact, followed by ii ) a sharp decrease as the pocket leaves the contact, and then iii ) a gradual decay back to the pre-entrainment value. From the evidence obtained for the first time from the proposed combined modelling and experimental investigation conducted under carefully controlled conditions, we suggest that these three stages occur due to the following mechanisms: i ) a reduction in fluid pressure due to the increased inlet gap, ii ) inlet suction as the cavitated fluid within the pocket draws lubricant into the contact, and iii ) film thickness decay as oil is squeezed out of the contact. The interplay of these three mechanisms is shown to control the response of micro-textured surfaces under all lubrication regimes.

Artificial liquid-repellent surfaces have attracted substantial scientific and industrial attention with a focus on creating functional topological features; however, the role of the underlying structures has been overlooked. Recent developments in micro-nanofabrication allow us now to construct a skin-muscle type system combining interfacial liquid repellence atop a mechanically functional structure. Specifically, we design surfaces comprising bioinspired, mushroom-like repelling heads and spring-like flexible supports, which are realized by three-dimensional direct laser lithography. The flexible supports elevate liquid repellency by resisting droplet impalement and reducing contact time. This, previously unknown, use of spring-like flexible supports to enhance liquid repellency provides an excellent level of control over droplet manipulation. Moreover, this extends repellent microstructure research from statics to dynamics and is envisioned to yield functionalities and possibilities by linking functional surfaces and mechanical metamaterials., Science Advances, 6 (32), ISSN:2375-2548

Artificial liquid-repellent surfaces have recently attracted vast scientific attention; however, achieving mechanical robustness remains a formidable challenge before industrialization can be realized. To this end, inspired by plateaus in geological landscapes, a self-compensating strategy is developed to pave the way for the synthesis of durable repellent surfaces. This self-compensating surface comprises tall hydrophobic structural elements, which can repel liquid droplets. When these elements are damaged, they expose shorter structural elements that also suspend the droplets and thus preserve interfacial repellency. An example of this plateau-inspired stratified surface was created by three-dimensional (3D) direct laser lithography micro-nano fabrication. Even after being subjected to serious frictional damage, it maintained static repellency to water with a contact angle above 147° and was simultaneously able to endure high pressures arising from droplet impacts. Extending the scope of nature-inspired functional surfaces from conventional biomimetics to geological landscapes, this work demonstrates that the plateau-inspired self-compensating strategy can provide an unprecedented level of robustness in terms of sustained liquid repellency.

Wetting mechanisms on rough surfaces were understood from either a monolayer or a multiscale perspective. However, it has recently been shown that the bi-Gaussian stratified nature of real surfaces should be accounted for when modeling mechanisms of lubrication, sealing, contact, friction, acoustic emission, and manufacture. In this work, a model combining Wenzel and Cassie theories was put forward to predict the static contact angle of a droplet on a bi-Gaussian stratified surface. The model was initially applied to numerically simulated surfaces and subsequently demonstrated on hydrophilic steel and hydrophobic self-assembled monolayer specimens with preset bi-Gaussian stratified topographies. In the Wenzel state, both the upper and the lower surface components are fully wetted. In the Cassie state, the upper component is still completely wetted, while the lower component serves as gas traps and reservoirs. By this model, wetting evolution was assessed, and the existence of different wetting states and potential state transitions was predicted.

The aim of this study is to gain insights into the interactions between laser-textured surface pockets and friction and wear behaviour of an automotive piston-liner pairing. To do this, a recently developed, reciprocating, test apparatus was used to conduct wear tests under highly loaded conditions. Fused silica specimens with a range of pocket geometries were rubbed against a convex steel pad and the resulting friction and wear data were compared with those from a non-textured specimen. Contact conditions were set to remove the influence of initial surface roughness on texture behaviour. These tests showed that, as the specimen become worn and surface roughness increases, the contact progresses further into the mixed and boundary regime. This leads to a significant improvement in the relative performance of the textured specimens, showing reductions in friction of up to 70%, compared with the non-texture case. This is consistent with previous results that have shown texture to have the effect of boosting film thickness in the mixed lubrication regime. Surface texture was also shown to reduce the volume of wear, by up to 69% (corresponding to a change in wear coefficient from 2.67e−4 to 0.81e−4 [mm3/N m]). Another, important, finding is that both friction and wear reduce monotonically as the sum of the pocket volumes along the stroke increases. This may aid texture design, since it means that individual pocket width and depth values can largely be ignored, so long as the overall volume is maximised. The only exception to this trend is when the pockets are larger than the contact area. In this case, friction increases due to a collapse of the lubricant film, while wear reduction remains unaffected – a discrepancy which may suggest that pockets reduce wear and friction via separate mechanisms.

Applying surface texture to piston liners may provide an effective means of controlling friction and hence improving engine efficiency. However, little is understood about the mechanisms by which pockets affect friction, primarily because of a lack of reliable experimental measurements. To address this, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing, under conditions that closely replicate an automotive piston ring-liner conjunction. Film thicknesses were measured using a modified version of the ultra-thin film optical interferometry approach, enabling film thicknesses

Due to ethical issues and the difficulty in obtaining biological tissues, it is important to find synthetic elastomers that can be used as replacement test media for research purposes. An important example of this is friction testing to understand the mechanisms behind mouthfeel attributes during food consumption (e.g. syrupy, body and clean finish), which requires an oral mimic. In order to assess the suitability of possible materials to mimic oral surfaces, a sliding contact is produced by loading and sliding a hemispherical silica pin against either a polydimethyl siloxane (PDMS), agarose, or porcine tongue sample. Friction is measured and elastohydrodynamic film thickness is calculated based on the elastic modulus of the samples, which is measured using an indentation method. Tests were performed with both saliva and pure water as the lubricating fluid and results compared to unlubricated conditions. PDMS mimics the tongue well in terms of protein adhesion, with both samples showing significant reductions in friction when lubricated with saliva versus water, whereas agarose showed no difference between saliva and water lubricated conditions. This is attributed to PDMS's O Si(CH3)2- group which provides excellent adhesion for the saliva protein molecules, in contrast with the hydrated agarose surface. The measured modulus of the PDMS (2.2 MPa) is however significantly greater than that of tongue (3.5 kPa) and agarose (66–174 kPa). This affects both the surface (boundary) friction, at low sliding speeds, and the entrained elastohydrodynamic film thickness, at high speeds. Utilising the transparent PDMS sample, we also use fluorescence microscopy to monitor the build-up and flow of dyed-tagged saliva proteins within the contact during sliding. Results confirm the lubricous boundary film forming nature of saliva proteins by showing a strong correlation between friction and average protein intensity signals (cross correlation coefficient = 0.87). This demonstrates a powerful method to study mouthfeel mechanisms.

The friction properties of a range of viscosity modifier-containing oils in an engine bearing have been studied in the hydrodynamic regime using a combined experimental and modelling approach. The viscometric properties of these oils were previously measured and single equations derived to describe how their viscosities vary with temperature and shear rate (Marx et al. Tribol Lett 66:92, 2018). A journal bearing machine has been used to measure the friction properties of the test oils at various oil supply temperatures, while simultaneously measuring bearing temperature using an embedded thermocouple. This shows the importance of taking account of thermal response in journal bearings since the operating oil film temperature is often considerably higher than the oil supply temperature. For Newtonian oils, friction coefficient measurements made over a wide range of speeds, loads and oil supply temperatures collapse onto a single Stribeck curve when the viscosity used in determining the Stribeck number is based on an effective oil film temperature. Journal bearing machine measurements on VM-containing oils show that these give lower friction than a Newtonian reference oil. A thermo-hydrodynamic model incorporating shear thinning has been used to explore further the frictional properties of the VM-containing oils. These confirm the findings of the journal bearing experiments and show that two key factors determine the friction of the engine bearing; (i) the low shear rate viscosity of the oil at the effective bearing temperature and (ii) the extent to which the blend shear thins at the high shear rate present in the bearing.

The effect of surface texture on friction has been investigated experimentally for a convergent-divergent bearing, operating under different lubrication regimes. This was achieved using recently developed test apparatus that closely simulates an automotive piston ring-liner contact, by sliding patterned plane fused silica surfaces against a convex steel pad. Textured patterns, consisting of pockets of varying shapes and orientations, were assessed on their ability to reduce frictional losses, and results compared with those from a reference smooth surface. These tests were performed under a range of lubricant viscosities and applied normal loads in order to vary lambda values (i.e. the ratio of minimum film thickness to composite roughness) and hence highlight the beneficial or detrimental effects of texture in boundary, mixed and full film regimes. In the boundary and mixed regimes – the regimes where surface texture was beneficial – grooves normal to the sliding direction were the most effective patterns among those investigated, reducing friction by up to 62%. The results suggest that pockets act to increase fluid entrainment and hence reduce any asperity contact that is present. However, pockets at reversal were shown to increase friction dramatically. In the full film regime, when a liquid film fully separates sliding surfaces, texture was shown to increase friction compared to the smooth case. In addition to this, the experimental setup allowed cavitation in the ring-liner pairing to be imaged. These results may suggest that the increase in full film friction due to surface texture may be due to the interaction of pockets with the cavitated region. Furthermore, imaging results confirm previous research in that the number of cavitation streamers increase as the film thickness decreases.

New apparatus is described to simulate a compliant seal interface, allowing the percolation of liquid to be viewed by a fluorescence microscope. A model, based on the boundary element (BE) methodology, is used to provide a theoretical explanation of the observed behavior. The impact of contact pressure, roughness, and surface energy on percolation rates are characterized. For hydrophilic surfaces, percolation will always occur provided a sufficient number of roughness length scales are considered. However, for hydrophobic surfaces, the inlet pressure must overcome the capillary pressure exerted at the minimum channel section before flow can occur.

The aim of this paper is to study the transient phenomena in hydrodynamic textured bearings. Both convergent and convergent–divergent reciprocating textured bearings are considered. A mass-conserving formulation of the Reynolds equation recently proposed by some of the authors and used to capture cavitation in steady-state lubricated contacts has been implemented to study transient effects in lubricated textured bearings. It is shown that the proposed solver is capable of capturing the frictional response of bearings characterised by various geometries and loading conditions in both steady-state and transient configurations. Depending on the boundary conditions governing the problems under investigation, changes in load support or film thickness variations are correctly predicted, demonstrating that the methodology developed in this paper is suitable to provide an efficient tool for the design and optimisation of textured bearings. A qualitative comparison with preliminary experimental data obtained using an apparatus developed to study reciprocating textured surfaces is performed, showing that the characteristic transient behaviour of such surfaces in different loading regimes can be correctly captured using the proposed numerical implementation.

Recent experiments have shown that automotive piston-liner friction may be reduced by up to 50 % if the surface of the liner is laser textured with certain configurations of micro-pockets. It is important to model this behavior to understand and optimize the friction reduction mechanisms that are occurring. However, until now, very few models that predict the lubrication performance of textured surfaces have been successfully validated against experimental data. This is because of the requirement for them to: (1) reproduce experimental configurations with a certain degree of fidelity, (2) conserve mass properly, and (3) account for transient, boundary lubrication conditions. To address this, the current paper presents a comparison between the results from a numerical model, which fulfils these criteria, and an experimental test rig operating under the same conditions. The mathematical modeling is based on the averaged Reynolds’ equation with Patir and Cheng’s flow factors and the p − θ Elrod–Adams mass-conserving cavitation model. Simultaneously to the fluid flow solution, the contact pressures that arise from the asperity interactions are also included into the calculations through the well-known stochastic Greenwood and Tripp model for rough contacts. The experimental data is produced using a reciprocating tribometer, whose contact conditions are closely controlled and accurately mimic those found in an automotive piston–liner conjunction. Data is presented in terms of friction force versus stroke angle, and the similarities and differences between the model and experiment are discussed.

To shed light on the mechanisms with which surface texture improves the tribological performance of piston–liner contacts, we have measured the transient friction response as individual pockets pass through a reciprocating sliding contact. Tests were performed at different sliding speeds and results compared to those from a non-textured, reference specimen under different lubrication regimes. At low speed when the contact is in the boundary regime, friction force falls abruptly as each pocket leaves the contact zone, before gradually returning to an approximately steady-state value. This suggests that each pocket acts to temporarily increase the film thickness, which then decays to its non-textured value as oil is squeezed out. At higher speeds, friction is seen to reduce in a stepwise fashion, since the period between pockets being entrained is less than the time taken for the film to decay. In addition, friction results obtained when the contact is operating in the middle of the mixed regime point to a temporary film thickness collapse as the pocket enters the contact, and this agrees with recent modelling predictions. At higher speeds, the compound effect of successive pockets is to shift the contact to the right on Stribeck curve. These results imply that each pocket gives rise to an increase in film thickness that is both short-lived and small in magnitude (we estimate a few tens of nm). However, the resulting effect on friction can be significant (up to 82 % in this study) for two reasons: (1) provided the pocket frequency is sufficiently high, each successive pocket entrainment builds the film up without there being time for it to reduce back to its steady-state value; (2) when the contact is in the mixed regime, the Stribeck curve is at its steepest and friction is therefore most sensitive to film thickness changes. This has important practical implications in that pocket spacing on piston liners should be varied as a function of reciprocating sliding speed.