Relating rheology and tribology of commercial dairy colloids to sensory perception
This study aims to investigate the relationship between rheological and tribological properties of commercial full fat and fat-free/low fat versions of liquid and soft solid colloidal systems (milk, yoghurt, soft cream cheese) with their sensory properties. Oscillatory measurements (strain, frequency), flow curves and tribological measurements (lubrication behaviour using Stribeck analysis) were conducted. Oral condition was mimicked using artificial saliva at 37 ○C. Discrimination test was conducted by 63 untrained consumers, followed by a qualitative questionnaire. Consumers significantly discriminated the fat-free/low fat from the full fat versions (p < 0.01) in all product classes, with most common verbatim used being “creamy”, “sweet” for the full fat versus “watery”, “sour” for the fat-free samples. Flow behaviour of both versions of milk showed overlapping trends with no significant differences identified both in absence and presence of saliva (p > 0.05). Full fat and fat free yoghurts had similar yielding behaviour and elastic modulus (G′), even in simulated oral conditions. However, in case of soft cream cheese, the full fat version had a moderately higher G′ than the low fat counterpart. Even in presence of artificial saliva, there was slight but significant difference in viscoelasticity between the cream cheese variants depending on fat content (p < 0.05). Stribeck curve analyses showed that at lower entrainment velocities (1–100 mm s−1), both full fat yoghurt and soft cream cheese exhibited a significantly lower traction coefficient when compared to fat-free/low fat versions (p < 0.05), which might be attributed to the lubricating effect of the coalesced fat droplets. Surprisingly, whole and skim milks showed no significant difference in traction coefficients irrespective of the entrainment speeds (p > 0.05). Results suggest that sensory distinction between fat-free and full fat versions, particularly in semi-solid systems could be better predicted by lubrication data as compared to bulk rheology.
Influence of ionic strength on the tribological properties of pre-adsorbed salivary films
We have studied tribological properties of pre-adsorbed salivary films formed in vitro on compliant hydrophobic surfaces. The adsorbed salivary film significantly decreases boundary friction under physiological ionic strength, which is related to a hydrophilic character of the adsorbed film and its structure. Decrease in the ionic strength below physiological conditions affects film’s structure, but it does not significantly affect boundary lubrication at low loads. Applications of high loads led to a gradual loss of lubrication due to shear-induced wear of the films. The wear became more extensive as the ionic strength of the solvent was lowered below physiological conditions.
Lubrication Properties of Protein Aggregate Dispersions in a Soft Contact
The lubrication, rheological, and molecular properties of two different protein aggregate dispersions were compared: globular aggregates of whey protein isolate (WPI) and fibrillar aggregates of ovalbumin from egg white. These dispersions are models for the lubricating fluid that is present between the tongue and the palate when consuming liquid or gelled products. To simulate oral conditions, a commercial tribometer was modified so that soft rubber surfaces could be used. This allowed us to measure friction at low contact pressures similar to those present between the tongue and palate. Clear correlations were observed between the measured friction coefficients and specific properties of the lubricating fluid such as protein concentration and aggregate size and shape. Furthermore, surface properties like elasticity, surface−surface interactions, and surface roughness had a significant effect on the friction under conditions that are relevant for texture perception. We conclude that in vitro measurements at low contact pressure provide valuable information for understanding and controlling food properties that modulate oral friction.
Investigation of the Thermostability of Bovine Submaxillary Mucin (BSM) and its Impact on Lubrication
Bovine Submaxillary Mucin (BSM) generates thin film layers via spontaneous adsorption onto hydrophobic surfaces such as Poly(dimethylsiloxane) (PDMS) and High Density Polyethylene (HDPE). A characteristic feature of mucin is its tribological- or lubricating properties. Circular dichroismspectroscopyrevealed that BSM is thermally stable over a wide range of temperatures (5–85°C) in its conformation, and Pin-on-Disk tribometry at low speeds showed negligible influence on lubricating properties. Employing the Mini Traction Machine, BSM was found to retain comparable lubricating properties after heating to 80°C and subsequent cooling. Random coiled secondary- and lack of tertiary structure in BSM is believed to contribute to the heat tolerance observed with regards to its conformational and lubrication properties.
Tribological Properties of Neutral Polysaccharide Solutions under Simulated Oral Conditions
Predictability of the perception of foods thickened by polysaccharides is only poor. Therefore, the effect of saliva on the lubrication properties of 2 types of neutral polysaccharides, cross-linked starch and locust bean gum, was studied. Despite the similar bulk rheological behavior of the 2 polysaccharides, the starch solution exhibited a significantly lower friction coefficient. Although starch viscosity was strongly decreased upon 10 s incubation with human saliva, a low friction coefficient was retained. The presence of remaining granules is held partly responsible for this. Addition of starch granules to locust bean gum also resulted in a decrease in the friction coefficient, but the effect was smaller compared to starch solutions digested by saliva. Smaller contact angles were measured for (digested) starch compared to locust bean gum solutions. This points to other parameters that assist in lubrication, such as the interaction of starch solution constituents with the rubbing surfaces. In addition, the importance of bulk viscosity for spreadability on surfaces was demonstrated. This study illustrates that the type of starch will determine not only the viscosity change but also the presence of intact granules upon digestion by saliva in the oral cavity; the combination of these 2 properties is regarded to be responsible for the poor predictability of sensory responses of starch containing foods.
Fluid gel lubrication as a function of solvent quality
Soft tribology is of increasing interest in biotribology applications to approximate skin/skin and oral contact environments. As such a number of studies explore the tribological behaviour of food and consumer product/model systems have been reported. This work builds on this understanding by examining soft surface lubrication of Agar fluid gel as a function of solvent quality. Firstly with alcohols at varying concentrations and secondly with salts, using the well-known lyotropic effect, which from previous studies is known to affect the properties of gels. Pure solvent systems showed patterns in friction reduction. Fluid gel lubrication behaviour was different when altering solvent quality and is suggested to be primarily an effect of changing gel structure, which is illustrated by approximate particle size distributions obtained for each sample. These effects can potentially be used to control food properties for enhanced behaviour in the mouth, or to better understand the structures that would form when using a fluid gel in more complex solvent containing products.
Tribological Study of Suspensions of Cysteine-Rich Protein Stabilized Microbubbles and Subsequent Triphasic a/O/W Emulsions
When developing low-fat foods, one of the main concerns is the sensory performance which is generally inferior to that of the full-fat version. Micron-sized air cells coated and stabilised by proteins (0.5–10 µm) have been produced sonochemically using different cysteine-rich proteins (hydrophobin, bovine serum albumin [BSA] and egg white proteins [EWPs]). These suspensions of air cells have been termed “air filled emulsions” (AFEs) and suggested for the production of low-fat emulsion-based products. This study explores the oral (tribological) behavior of AFEs as ingredients and within a triphasic A/O/W emulsion-based prototype salad dressing. Tribological measurement of AFEs yielded different results for BSA-AFE and EWP-AFE, indicating that the very nature of the protein may play a crucial role. However, the triphasic A/O/W emulsion showed similar, if not better, lubrication properties than the standard O/W version, indicating that AFEs may contribute to the perceived fat-related attributes. PRACTICAL APPLICATIONS The tribological behavior of a material has been postulated as relevant for its in-mouth perception, especially when considering fat-related attributes. The present study deals with tribological study of novel air-based ingredients. Knowing the tribological/lubrication behavior of these ingredients and their subsequent products could be useful for better understanding of their possible behavior in the mouth. Also, this could assist in redesigning ingredients and formulations with better sensory performance.
Sensory perception and lubrication properties of milk: Influence of fat content
The sensory perception of homogenized milk with a fat content between 0.06 and 8% was correlated with its friction coefficient and viscosity. Above a threshold of 1% fat, there was a strong decrease in friction coefficient at low speeds, which is associated with shear-induced coalescence. Creamy perception was perceived only for products with the friction coefficient below 0.25 for silicone rubber at entrainment speeds lower than 200 mm s−1. Under those conditions, a linear correlation between perceived creaminess and friction was obtained at a fat content above 1%. The increased creaminess and thus decreased friction was attributed to the coalescence of fat globules on the surface of the tongue and rubber disc, respectively. At higher speeds, fused fat droplets were broken into smaller droplets (reversing coalescence) due to the high shear, thereby eliminating the correlation.
Oral behaviour of food hydrocolloids and emulsions. Part 1. Lubrication and deposition considerations
Biopolymers (polysaccharides and proteins) and their mixtures are used in the Food Industry to impart stability, texture and appearance to fabricated foods. This paper reviews the physical measures conducted to elucidate textural properties such as creaminess, smoothness, sliminess and thickness of food products and discusses oral processing mechanisms in relation to the behaviour of hydrocolloids and emulsions in the oral cavity during eating. In particular, this article covers the use of Tribology and Evanescent Wave Spectroscopy techniques that enable the study of the lubrication and deposition behaviour of food components. Comparison of the physical measurements with sensory properties indicate that thin film rheology and surface deposition phenomena make an important contribution to sensory properties such as fattiness, smoothness and astringency.
Influence of load and elastic properties on the rolling and sliding friction of lubricated compliant contacts
Lubricated “soft” contacts, where one or both contacting solids have a low elastic modulus, are present in many practical engineering and biological applications including windscreen wipers, wet tyres, elastomeric seals, contact lenses and the tongue/palate system. In such contacts, the prevailing lubrication mode is “isoviscous EHL” (elastohydrodynamic lubrication). Unlike in steel–steel contacts, rolling friction can be considerable and this originates in part from the viscoelastic properties of the compliant surfaces. In this paper the influence on friction of both applied load and the elastic properties of the solids is studied using a mini traction machine. In this machine, the rolling and sliding friction can be separately determined. The viscoelastic properties of the polymers employed are measured using a dynamic mechanical analysis apparatus. The measured friction is compared to theoretical models for soft EHL and the viscoelastic energy losses arising from the contact deformation. Consideration of the frequency dependence of the substrate viscoelasticity enables reasonably accurate predictions of the rolling friction coefficient, especially within the mixed and boundary lubrication regimes.
Soft-tribology: Lubrication in a compliant PDMS–PDMS contact
We investigate the influence of surface roughness and hydrophobicity on the lubrication of a soft contact, consisting of a poly(dimethylsiloxane) (PDMS) sphere and a flat PDMS disk. The full Stribeck curves, showing boundary, mixed and elasto-hydrodynamic (EHL) lubrication, are presented for varying surface roughness and hydrophobicity. It is found that neither surface roughness nor hydrophobicity influence the friction coefficient (μ) within the EHL regime. However, increasing surface roughness decreases μ in the boundary regime, while extending the limits of the boundary and mixed lubrication regimes to larger values of the product of velocity and lubricant viscosity (Uη). The transition from the mixed lubrication to EHL regime is found to take place at lower values of the film thickness parameter Λ for increasingly rough surfaces. We found Λ=0.7 in the case of a root mean square (r.m.s.) surface roughness of 3.6 μm, suggesting that the effective surface roughness in a compliant compressed tribological contact is lower than that at ambient pressures. Rendering the PDMS surface hydrophilic promotes full-film lubrication and dramatically lowers μ in the boundary regime by more than an order of magnitude. This influence of surface wetting is also displayed when examining a range of lubricants using hydrophobic tribopairs, where the boundary μ decreases with decreasing lubricant–substrate contact angle. Implications of these measurements are discussed in terms of the creation of model surfaces for biotribological applications.
Astringency of tea catechins: More than an oral lubrication tactile percept
Oral astringency is the dry sensation experienced in the mouth on consumption of plant-based polyphenols (catechins) found in wine and tea as well as certain fruits and vegetables. It is commonly explained as arising from the loss of lubricity owing to the precipitation of proteins from the salivary film that coats and lubricates the oral cavity. Here, we investigate this hypothesis directly by probing the impact of astringent compounds on the lubricating properties of saliva. By preadsorbing saliva onto an elastic hydrophobic substrate to form a highly lubricating and robust film under conditions designed to mimic the low pressure rubbing contacts experienced in the oral cavity (Bongaerts, Rossetti, & Stokes, 2007), we probe the interaction of this film on exposure to solutions containing tea catechins. We examine the response of the adsorbed salivary film to polyphenol structure, concentration and temperature, as well as the influence of astringency modulating solutions consisting of a thickener (maltodextrin) and milk. We find that a significant increase in friction coefficient occurs upon exposure with epigallocatechin gallate (EGCG) solutions due to a depletion of the lubricating proteins from the elastic substrates. The friction coefficient increases more rapidly with increasing EGCG concentration, this is in line with a corresponding increase in astringency perception. In addition, the inclusion of a hydrocolloid thickener in EGCG solutions caused a decrease in astringency perception probably due to specific EGCG–maltodextrin interaction and to an increase in viscosity, which lowers the friction coefficient between the elastic substrates. These findings show that the physical interaction of saliva proteins with EGCG molecules, which we probe through the loss of saliva lubricity, can be advantageously used to predict the astringent acuity of EGCG using our simple oral mimetic technique, supporting the hypothesis that astringency is related to a loss of lubrication. However, epicatechin (EC) did not alter the lubricating properties of the salivary film, although the EC solutions were perceived to be astringent, an observation which seems to question a simple causal dependence on oral lubrication. In addition to that, milk mitigated the astringency perception of EGCG solutions although considerably reducing saliva lubricity. We conclude that the depletion of saliva lubricating proteins is not necessary to obtain an astringent perception, and that astringency is unlikely to be a purely tactile percept.
Applications of tribology in studying food oral processing and texture perception
Oral texture is one of the most important quality attributes that contribute to consumers’ acceptance and preference of a food product. Oral processing of food is intricate and involves a series of processes—ingestion, mastication and finally swallowing. The mechanical and rheological properties have been widely applied to understand and describe in-mouth flow properties of a food and associated sensory perception. However, as the oral processing continues and food particle size reduces, rheology alone is no longer effective in explaining the textural and mouthfeel properties of food, but the lubrication behaviour between oral surfaces becomes a dominating mechanism in relation to food texture and mouthfeel. For this reason, tribology is emerging as a new discipline for food texture studies, where lubricating properties of food are measured by using an equipment that operates on the same principle used in mechanical engineering for determining the frictional properties of lubricants. While there are many instruments and equipments available for engineering lubrication studies, choice is limited for food applications. This article will review various tribological systems currently utilised for various food systems, whether commercially purchased, custom-made, or by use of attachment tools for rheometers. The operation principles, advantages and disadvantages of these devices are discussed. Examples of food application are also given in each case, where measured frictional properties are used to interpret sensory mouthfeel properties.
Oral tribology: bridging the gap between physical measurements and sensory experience
Soft-tribology is emerging as an important field of research for
quantifying the physics occurring during oral tribological
processes. In a food oral processing context, recent research
indicates that tribology measurements are providing insights
into several texture-related sensory percepts, but obtaining
quantitative empirical relationships between the two is
challenging. Choosing a physiologically relevant tribological
‘system’ is paramount to the successful application of tribology
as an indicator for texture perception; the choice of instrument,
surfaces and model food system, as well as the role of saliva,
should be carefully considered. Tribology and sensory
perception are affected by multiple physico-chemical
properties, and therefore integrative approaches that combine
tribology with other characterisation techniques are necessary
for mechanistic understanding on their inter-relationship.
Friction measurements with yoghurt in a simulated tongue-palate contact
The perception of many food attributes is related to mechanical stimulation and friction experienced in the tongue-palate contact duringmastication. Friction in the tongue-palate is determined by the changing film properties (composition, component distribution, thickness) in the conjunction. We suggest this evolution is essentially determined by tongue-palate film loss rather than shear flow entrainment which predominates in conventional bearing lubrication. The paper reports friction measurements in a simulated tongue-palate contact for a range of high and lowfat dairy foods. A reciprocating, sliding contactwith restricted stroke length (bcontact width) was used; under these conditions there is negligible shear-entrainment of fluid from outside the contact area. The tongue-palate contact was simulated by a PDMS ball and glass surface. The effect of hydrophobic and hydrophilic surfaces on friction was investigated for different fat contents (0, 4.2, 9.5% wt fat). Friction was measured over 60 s of rubbing. Significant differenceswere observed in the friction changewith time for different fat contents (μ 9.5 b μ 4.2 b μ 0 wt%) and for different surface energy conditions (μ hydrophilic b μ hydrophobic). Post-test visualisation of the rubbed films showed that lowfriction coefficientwas associatedwith the formation of a thin oil film on deposited particulate solids.
Insights into the dynamics of oral lubrication and mouthfeel using soft tribology: Differentiating semi-fluid foods with similar rheology
In-mouth lubrication or ‘oral tribology’ is believed to be a major contributor to the perception of surface-related mouthfeel attributes such as roughness and astringency. In this work, commercial soft-food systems – custards, yogurts and thickened creams, each formulated at varying fat levels with the aim of maintaining consumer acceptability – are characterized using rheology and soft tribology to gain insight into the physical origins of mouthfeel and the dynamics of oral lubrication, including the role of saliva. It is shown that, despite generating similar bulk rheological profiles (oscillatory and steady shear flow properties), foods within each product series exhibit unique tribological properties, attributed to the preferential entrainment of different phases and components at narrowgap. Food-saliva interactions are also shown to play a crucial role in determining transient lubrication properties, which may reflect dynamic oral processes. We explore the use of tribology to differentiate rheologically similar products as a starting point to develop improved approaches for investigating tribological processes occurring in the mouth during food consumption. Challenges associated with the application of tribology in this context are also discussed.
Tribology of swollen starch granule suspensions from maize and potato
The tribological properties of suspensions of cooked swollen starch granules are characterised for sys-tems based on maize starch and potato starch. These systems are known as granule ‘ghosts’ due to therelease (and removal) of polymer from their structure during cooking. Maize starch ghosts are less swollenthan potato starch ghosts, resulting in a higher packing concentration and greater mechanical stability.In a soft-tribological contact, maize ghost suspensions reduce friction compared to the solvent (water),generate bell-shaped tribological profiles characteristic of particle entrainment and show a marked con-centration dependence, whereas potato ghost suspensions exhibit lubrication behaviour similar to water.Microscopy analysis of the samples following tribological testing suggests that this is due to the rapidbreak-up of potato ghosts under the shear and rolling conditions within the tribological contact. A reduc-tion in the small deformation moduli (associated with a weak gel structure) is also observed when thepotato ghost suspensions are subjected to steady shear using parallel plate rheometry; both microscopyand particle size analysis show that this is accompanied by the partial shear-induced breakage of ghostparticles. This interplay between particle microstructure and the resultant rheological and lubricationdynamics of starch ghost suspensions contributes to an enhanced mechanistic understanding of texturaland other functional properties of cooked starches in food and other applications.
Aqueous lubrication by fractionated salivary proteins: Synergistic interaction of mucin polymer brush with low molecular weight macromolecules
The remarkable lubrication provided by saliva in the oral cavity is vital to human health and wellbeing. Yet, molecular mechanisms for saliva lubrication remain unclear. In this work we report a possible mechanism of synergistic interaction between salivary proteins. By isolating a number of salivary protein fractions, we identify major protein candidates that contribute to saliva lubrication. We discover that a key driver for low friction is a hydrated brush-like layer formed by glycosylated species, with an essential synergistic contribution coming from the low molecular weight components that facilitate spreading, adsorption and strengthening of the salivary film on hydrophobic substrates. Lessons may be learned from saliva for understanding other natural bio-aqueous lubrication systems and for the development of saliva mimics.
The influence of co-solutes on tribology of agar fluid gels
The effects of glucose and glycerol on the lubrication properties of agar fluid gels have been studied using soft tribology. A novel approach using the sediment and supernatant of centrifuged fluid gels has allowed investigation of the distinct contributions of both the gelled particulate phase and the continuous phase on fluid gel tribology. The friction coefficient of both the particulate phase and fluid gels was significantly lower than that of the continuous phase across the three lubrication regimes. This indicates that particle entrainment occurs at all entrainment speeds, enhancing lubrication by prevention of surface contact.
Softer fluid gel particles produced with intermediate levels of glycerol (up to 30%) show increased friction as would be expected for an increased contact area between the tribological surfaces. At high levels of glycerol, the friction does not increase. It is proposed that soft particles are produced but the increasing friction is overcome with the increased lubrication from the more highly viscous continuous phase. In contrast, the presence of intermediate levels of glucose (up to 30%) increases the friction of the aqueous continuous phase but does not affect the particle properties. Texture analysis, rheology and light scattering techniques were used to elucidate the structural changes of the fluid gels induced by the addition of co-solutes and the influence this has upon lubrication.
Kappa carrageenan fluid gel material properties Part 2: Tribology
Semi-solid and liquid food thickeners typically take the form of either polymeric or particulate structures. These structures are known to control flow properties and mixing efficiency which can influence performance, texture and the perception of tastants and aromas. However, their structural influence on thin-film rheology (tribology), which is also relevant for texture perception, is not so well understood. In this investigation, the tribology in a boundary regime of lubrication is tested using kappa carrageenan lubricants formulated both in solution and as gelled particles in suspension (fluid gels) to provide new insights into the structural influence of thickener type on tribology. Polymeric lubricated systems were shown to be dominated by elastic deformation of the tribo surfaces and particulate suspensions were dominated by particles acting as contacting asperities of the mating surfaces. The tribology of gelled particles was shown to depend strongly on particle elasticity where less deformable (stiffer) particles reduce surface–surface contact and therefore reduce friction coefficients. The effect of particle volume fraction on friction coefficient is related to the number of particles entraining the contact and not particle–particle interactions or bulk rheological behaviour.
Soft tribology of oil-continuous emulsions
Lubrication behaviour of foodstuff is related to mouthfeel perception and consumer appreciation. Soft tribology of food related products has mainly been investigated with semi-solid food, polymer solutions and water continuous emulsions, and this is the first study aimed at investigating soft tribolocigal behaviour of oil continuous emulsions. All the emulsions considered here exhibit the same trends in terms of lubrication behaviour, where little boundary lubrication is observed at the entrainment speed considered. The volume of dispersed aqueous phase affects overall tribology of oil continuous emulsions via an increase in their dynamic viscosity. Increasing the phase volume leads to an increase in friction in the elastohydrodynamic regime whereas the lubrication in the boundary regime is improved. Elastohydrodynamic lubrication is independent of the aqueous phase composition and the type of emulsifier present at the water–oil interface. These parameters affect boundary lubrication of emulsion systems exhibiting droplet size bigger than the elastohydrodynamic oil film thickness. This is expected to have a significant impact on the design of low fat emulsions that match the lubrication properties of their full fat versio
Laser-induced fluorescence for film thickness mapping in pure sliding lubricated, compliant contacts
A laser-induced fluorescence (LIF) technique has been used to measure fluid film thickness in a compliant, sliding contact under low-load/low-pressure conditions. The soft contact between an elastomer hemisphere and a glass disc is lubricated by a liquid containing fluorescent dye. The contact is then illuminated with 532 nm laser light through the glass disc, and viewed with a fluorescence microscope. From the intensity of emitted radiation, film thickness maps of the contact are determined. Previous calibration procedures have used a separate calibration piece and test specimen with possible errors due to differences in reflectivity between the calibration and test specimens. In the work reported in this paper a new calibration process is employed using the actual test sample, thereby avoiding such errors.
Results are reported for a sliding contact between PDMS and glass, lubricated with glycerol and water solutions under fully flooded and starved conditions. It was found that, for glycerol, the measured film thickness is somewhat lower than numerical predictions for both lubrication conditions. It is suggested that a combination of thermal effects and the hygroscopic nature of glycerol may cause the lubricant viscosity to drop resulting in thinner films than those predicted for fully flooded contacts. Starvation occurs above a critical entrainment speed and results in considerably thinner films than predicted by fully flooded I-EHL theory. A numerical study has been carried out to determine the effect of the observed starvation on film thickness. Predicted, starved film thickness values agree well with those obtained experimentally.
Inlet protein aggregation: a new mechanism for lubricating film formation with model synovial fluids
This paper reports a fundamental study of lubricant film formation with model synovial fluid components (proteins) and bovine serum (BS). The objective was to investigate the role of proteins in the lubrication process. Film thickness was measured by optical interferometry in a ball-on-disc device (mean speed range of 2-60 mm/s). A commercial cobalt-chromium (CoCrMo) metal femoral head was used as the stationary component. The results for BS showed complex time-dependent behaviour, which was not representative of a simple fluid. After a few minutes sliding BS formed a thin adherent film of 10-20 nm, which was attributed to protein absorbance at the surface. This layer was augmented by a hydrodynamic film, which often increased at slow speeds. At the end of the test deposited surface layers of 20-50 nm were measured. Imaging of the contact showed that at slow speeds an apparent ‘phase boundary’ formed in the inlet just in front of the Hertzian zone. This was associated with the formation of a reservoir of high-viscosity material that periodically moved through the contact forming a much thicker film. The study shows that proteins play an important role in the film-forming process and current lubrication models do not capture these mechanism
The frictional properties of Newtonian fluids in rolling–sliding soft-EHL contact
A combined experimental and numerical study has been carried out to explore friction in rolling–sliding, soft-EHL contact. Experimental work has employed corn syrup solutions of different concentrations in water to provide a range of lubricant viscosities and has measured Couette friction in mixed rolling–sliding conditions over a wide range of entrainment speeds. A Stribeck curve has been generated, ranging from the boundary to full film, isoviscous-elastic lubrication regime. In the latter regime, friction coefficient is approximately proportional to the product of (entrainment speed × viscosity) raised to the power 0.55. Numerical solution of the isoviscous-elastic lubrication regime has been used to derive predictive equations for both Couette and Poiseuille friction in circular, soft-EHL contacts. This shows that in soft-EHL the Poiseuille or “rolling” friction can have magnitude comparable to the Couette friction. The calculated Poiseuille friction coefficient can be predicted from non-dimensional load and speed using a simple power law expression similar to that used for film thickness. However accurate prediction of calculated Couette friction coefficient requires a two-term power law expression. Comparison of experimental and numerical Couette friction coefficients shows quite good agreement between the two, with a similar non-dimensional speed dependence, but slightly lower predicted than measured values
Rolling and sliding friction in compliant, lubricated contact
Friction is investigated in a rolling–sliding, lubricated, steel ball on elastomer flat contact. Two different types of friction are identified: rolling friction, which results from the movement of the surfaces relative to the contact, and sliding or interfacial friction, which arises from relative motion of the two contacting surfaces. A novel experimental technique is
described to measure these two types of friction simultaneously in a single test. This enables separate rolling and interfacial ‘Stribeck-type’ friction curves to be produced for Newtonian lubricants. These curves are compared with theoretical predictions of friction. The results show that rolling friction originates primarily from two sources: Poiseuille flow of lubricant in the contact and elastic hysteresis. There are also two main types of interfacial friction; due to Couette flow and solid surface adhesion. For compliant elastomer-on-steel contacts, rolling friction forms a significant proportion of the total friction even at quite high slide–roll ratios.
Lubrication properties of non-adsorbing polymer solutions in soft elastohydrodynamic (EHD) contacts
The in-use performance and processing of many consumer products in the food, home and personal care industries are dependent on their tribological properties. A major component of these products is often a high molecular weight polymer, which is typically used to thicken aqueous systems. Polymer solutions tend to be non-Newtonian, and in particular their viscosity varies with shear rate, such that it is difficult to predict their friction or hydrodynamic film-forming behaviour. The present work relates the tribology of aqueous polymer solutions to their rheological properties in thin films in ‘soft’ contacts at high shear rates. The friction properties of three types of polymers in aqueous solution, polyethylene oxide, PEO; xanthan gum, XG; and guar gum, GG, have been studied as a function of polymer concentration over a wide range of entrainment speeds in a point contact formed between silicone rubber and steel. This has enabled the boundary lubrication and isoviscous-elastic lubrication properties of the solutions to be investigated using both hydrophilic and hydrophobic silicone surfaces.
It is found that the friction vs. entrainment speed dependence follows the shape of a classical Stribeck curve. In general, a lower friction is observed with increasing polymer concentration in the mixed-regime. Using scaling factors for the entrainment speed, we have shown that this decrease in friction is likely to be due to viscous effects and that the scaling factors represent effective high shear rate viscosities. In the case of PEO and XG, and GG at low concentrations, a good correlation is found between this effective viscosity and the apparent viscosity measured at the highest shear rates attainable with the available rheometer. However, for GG at concentrations above 0.2%, the effective viscosity decreases with increasing polymer content.
The three polymers do not significantly reduce friction in the boundary regime and in general give essentially the same response as water when an effective viscosity is taken into account. However, a slight increase in friction in comparison to pure water has been observed for XG and GG on hydrophobic surfaces. It is suspected that this may be due to a blocking of fluid entrainment, or possibly exclusion of polymer from the contact, due to the large hydrodynamic volume and rigid nature of the two biopolymers. Finally, for PEO solutions with full-film elastohydrodynamic conditions were reached, the measured friction coefficient of the film correlated quite well with the value calculated from the effective viscosity
Soft lubrication of model hydrocolloids
Many food products are highly structured, complex hydrocolloids that are characterized by either a gel-like or shear thinning behaviour and are thus strongly non-Newtonian systems. Their sensory perception during use is dependent on their behaviour in very thin film conditions at high shear rates. In order to better understand their behaviour in thin films, we have formulated two kinds of solution which are typically present in real systems. We examine the lubricating properties (i.e. tribology) of thin films of structured and non-structured fluids between a hard and soft surface (‘soft-EHL’ lubrication), when the film thickness is of a similar size to the microstructural elements. The present work involves relating the tribology of both shear thinning polymer solutions and swollen microgel suspensions to their rheological and microstructural properties in thin films and at high shear rates in the boundary, mixed and fluid film lubrication regimes.
We show that the polymer solutions are typically entrained into rubbing contacts to form mixed fluid/boundary lubricating films while model yield stress fluids generate only boundary lubricating films. Soft-EHL theory satisfactorily predicts friction measurements at high entrainment speeds in full film lubrication. The microgels are found to form a confined film which minimizes contact between the surfaces, causing a lowering of the friction coefficient in the boundary-regime, while the friction in the mixed-regime is also dependent on the high-shear viscosity.
Oral processing, texture and mouthfeel: From rheology to tribology and beyond
Texture and mouthfeel arising from the consumption of food and beverages are critical to consumer choice and acceptability. While the food structure design rules for many existing products have been well established, although not necessarily understood, the current drive to produce healthy consumer acceptable food and beverages is pushing products into a formulation space whereby these design rules no longer apply. Both subtle and large scale alterations to formulations can result in significant changes in texture and mouthfeel, even when measurable texture-related quantities such as rheology are the same. However, we are only able to predict sensations at the initial stages of consumption from knowledge of material properties of intact food.
Research is now on going to develop strategies to capture the dynamic aspects of oral processing, including: from a sensory perspective, the recent development of Temporal Dominance Sensation; from a material science perspective, development of new in vitro techniques in thin film rheology and tribology as well as consideration of the multifaceted effect of saliva. While in vivo, ex vivo, imitative and empirical approaches to studying oral processing are very insightful, they either do not lend themselves to routine use or are too complex to be able to ascertain the mechanism for an observed behaviour or correlation with sensory. For these reasons, we consider that fundamental in vitro techniques are vital for rational design of food, provided they are designed appropriately to capture the important physics taking place during oral processing. We map the oral breakdown trajectory through 6 stages and suggest a dynamic multi-scale approach to capture underlying physics. The ultimate goal is to use fundamental insights and techniques to design new food and beverages that are healthy yet acceptable to consumers.
Oral processing of two milk chocolate samples
Oral processing of two milk chocolates, identical in composition and viscosity, was investigated to understand the textural behaviour. Previous studies had shown differences in mouthcoating and related attributes such as time of clearance from the oral cavity to be most discriminating between the samples. Properties of panellists’ saliva, with regard to protein concentration and profile before and after eating the two chocolates, were included in the analysis but did not reveal any correlation with texture perception. The microstructure of the chocolate samples following oral processing, which resembled an emulsion as the chocolate phase inverts in-mouth, was clearly different and the sample that was found to be more mouthcoating appeared less flocculated after 20 chews. The differences in flocculation behaviour were mirrored in the volume based particle size distributions acquired with a laser diffraction particle size analyser. The less mouthcoating and more flocculated sample showed a clear bimodal size distribution with peaks at around 40 and 500 μm, for 10 and 20 chews, compared to a smaller and then diminishing second peak for the other sample following 10 and 20 chews, respectively. The corresponding mean particle diameters after 20 chews were 184 ± 23 and 141 ± 10 μm for the less and more mouthcoating samples, respectively. Also, more of the mouthcoating sample had melted after both 10 and 20 chews (80 ± 8% compared to 72 ± 10% for 20 chews). Finally, the friction behaviour between a soft and hard surface (elastopolymer/steel) and at in-mouth temperature was investigated using a commercial tribology attachment on a rotational rheometer. Complex material behaviour was revealed. Observations included an unusual increase in friction coefficient at very low sliding speeds, initially overlapping for both samples, to a threefold higher value for the more mouthcoating sample. This was followed by a commonly observed decrease in friction coefficient with increasing sliding speed (mixed and elasto-hydrodynamic regime), steeper in the case of the more mouthcoating sample until the differences between the two samples became negligible at a sliding speed of ≈0.2 mm s−1. The stark differences in the tribological behaviour in these regimes begin to allow correlation of data from sensory and physical measurements based on insight into the underlying material behaviour. The complex picture also included comparable behaviour of both samples in the late stages of the elasto-hydrodynamic regime and the early stages of the hydrodynamic regime, until a change of slope was observed and then, at higher sliding speeds, the less mouthcoating sample showed higher friction coefficients. In conclusion, this research uncovered novel correlations of a complex food composite between the sensory behaviour and the physical material properties relating to melting and friction behaviour.
Influence of load and elastic properties on the rolling and sliding friction of lubricated compliant contacts
Lubricated “soft” contacts, where one or both contacting solids have a low elastic modulus, are present in many practical engineering and biological applications including windscreen wipers, wet tyres, elastomeric seals, contact lenses and the tongue/palate system. In such contacts, the prevailing lubrication mode is “isoviscous EHL” (elastohydrodynamic lubrication). Unlike in steel–steel contacts, rolling friction can be considerable and this originates in part from the viscoelastic properties of the compliant surfaces.
In this paper the influence on friction of both applied load and the elastic properties of the solids is studied using a mini traction machine. In this machine, the rolling and sliding friction can be separately determined. The viscoelastic properties of the polymers employed are measured using a dynamic mechanical analysis apparatus. The measured friction is compared to theoretical models for soft EHL and the viscoelastic energy losses arising from the contact deformation. Consideration of the frequency dependence of the substrate viscoelasticity enables reasonably accurate predictions of the rolling friction coefficient, especially within the mixed and boundary lubrication regimes.
A conceptual model for fluid gel lubrication
Although fluid gels have found a large number of applications (in every day food products and cosmetics), their lubrication behaviour is still not fully understood. In this work the lubrication behaviour of agarose fluid gels has been investigated and the mechanism describing the process is proposed. A wide range of agarose concentrations was studied (1% to 4%) and it gave fluid gel particles with different levels of elasticity E, which are shown to affect both their rheological and tribological behaviour. The critical velocity required to induce entrainment of the particles was found to also depend on the particles’ elasticity E but in addition to the applied normal load W. Fluid gel systems of the same elasticity E but of different particle sizes were also investigated and an overall reduction in friction with decreasing size was obtained. The present study is of interest to all processes involving soft contact between particles and surfaces; a clear example being the understanding of astringency during mastication.
Influence of Temperature on the Frictional Properties of Water-Lubricated Surfaces
The influence of temperature on the lubricating properties of neat water for tribopairs with varying bulk elasticity moduli and surface hydrophilicity, namely hard-hydrophobic interface (h–HB), hard-hydrophilic interface (h–HL), soft-hydrophobic interface (s–HB), and soft-hydrophilic interface (s–HL), has been investigated. With increasing temperature, the coefficients of friction generally increased due to the decreasing viscosity of water. This change was more clearly manifested from soft interfaces for more feasible formation of lubricating films. Nevertheless, dominant lubrication mechanism appears to be boundary and mixed lubrication even for soft interfaces at all speeds (up to 1200 mm/s) and temperatures (1 to 90 °C) investigated. The results from this study are expected to provide a reference to explore the temperature-dependent tribological behavior of more complex aqueous lubricants, e.g., those involving various additives, for a variety of tribosystems.
End-grafted Sugar Chains as Aqueous Lubricant Additives: Synthesis and Macrotribological Tests of Poly(l-lysine)-graft-Dextran (PLL-g-dex) Copolymers
Comb-like graft copolymers with carbohydrate side chains have been developed as aqueous lubricant additives for oxide-based tribosystems, in an attempt to mimic biological lubrication systems, whose surfaces are known to be covered with sugar-rich layers. As adopted in the previous studies of the graft copolymer poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), which showed both excellent lubricating and antifouling properties, a similar approach was chosen to graft dextran chains onto the same backbone, thus generating PLL-g-dex. PLL-g-dex copolymers readily adsorb from aqueous solution onto negatively charged oxide surfaces. Tribological characterization at the macroscopic scale, either under pure sliding conditions or a mixed sliding/rolling contact regime, shows that PLL-g-dex is very effective for the lubrication of oxide-based tribosystems. The relative lubricating capabilities of PLL-g-dex copolymers compared with PLL-g-PEG copolymers were observed to be highly dependent on the molecular structure of the copolymers (in particular, side-chain density along the backbone) and the measurement conditions (in particular, time between tribocontacts); the PLL-g-dex copolymers with a low degree of grafted side chains (≤20% grafting of available protonated primary amine groups along the backbone) showed better lubricating performance than their PLL-g-PEG counterparts at high tribocontact frequency (≥ca. 0.32 H
Methodological study on the removal of solid oil and fat stains from cotton fabrics using abrasion
Solid fats are one of the most difficult stains to remove at low temperatures. Mechanical action is beneficial for stain removal, but the potential and limitations of such an essential part of washing are not known. Fabric abrasion has been studied for the first time in a systematic and controlled manner using a tribometer. The efficiency of cotton–cotton abrasion, to simulate the rubbing of clothes, was studied in the absence of detergents using models of liquid and solid oils (hexadecane, octadecane, and undecanoic acid) and real fats (lard and buttermilk fat). In model oils, abrasion is not very effective at any temperature, whereas in typical fats abrasion significantly improves cleaning in a wide range of temperatures. The different behavior is caused by the temperature-dependent solid fat content of lard and butter. Fluorescence microscopy is introduced as a novel methodology for the quantification of the fat content in soiled fabrics
Tribology and sensory attributes of food dispersions
The impact of food industry on societal aspects grows dynamically, thereby posing new challenges for food developers to comply to consumer demands. Consumers expect new and more sophisticated products that are tuned to their very specific needs, like health and life-style. This means that food products must be fully programmable to meet these market demands. The taste, smell, and texture perception of food are the most important properties that influence the consumer’s experience. To provide control over these features the physics behind them during oral manipulation and structure breakdown must be understood. Most desirably, the physical properties can be tightly linked to attributes of oral perception. In this thesis the texture of semi-solid and liquid (model) food is studied using Quantitative Descriptive Analysis. Despite the quantification of sensory scores, it remains difficult to translate these into food physical characteristics in an absolute mode. For that reason, in parallel a physical study is performed using mainly tribology to provide quantitative description of the mechanical processes in the oral environment. Therefore, the experimental setup is adapted such to mimic oral conditions by employing soft surfaces. Next to more complex commercial products, like milk, the samples studied include aqueous dispersions of protein aggregates or polysaccharides, and emulsion filled gels. These components may contribute to fat-feel in low or no-fat dairy products and provide a major uncertainty in predicting the sensorial restraints of the product. In this thesis we investigate how various factors influence the friction of above-mentioned systems and how this friction can be understood at a more fundamental level. In addition, the surface effects are studied in detail to determine how sensitive the choice of the oral surface analog is on the friction and what the most important surface properties are that are relevant to better predict sensory attributes. In this thesis we present an overview of correlations obtained between different sensory attributes and frictional data. We show that oral perception can be translated into physical quantities using friction. The best correlations are obtained for physical conditions that resemble most the oral environment, i.e. speed below 50 mm/s, shear rate of about 50 s-1 and soft, rough surface. For instance a low friction coefficient correlates well with the creamy perception of the consumer. This finding may contribute to develop healthy foods with a full fat creamy taste. Moreover, samples with low friction coefficient can be perceived as soft and velvety, while high friction results in rough texture perception. Some attributes, like filmy or slimy, correlate better with viscosity, as their perception is determined by the bulk properties of a sample rather than interaction with oral surfaces. In a few cases the correlation with friction coefficient could not be established, e.g. for sticky or powdery attributes. The latter one, however, shows a good correlation with particles size if they are present in the solution. This thesis presents a comprehensive picture of the relation between physical quantities and sensory attributes and provides a step forward towards a better control over oral perception of food.
Structural modifications of the salivary conditioning film upon exposure to sodium bicarbonate: implications for oral lubrication and mouthfeel
The salivary conditioning film (SCF) that forms on all surfaces in the mouth plays a key role in lubricating the oral cavity. As this film acts as an interface between tongue, enamel and oral mucosa, it is likely that any perturbations to its structure could potentially lead to a change in mouthfeel perception. This is often experienced after exposure to oral hygiene products. For example, consumers that use dentifrice that contain a high concentration of sodium bicarbonate (SB) often report a clean mouth feel after use; an attribute that is clearly desirable for oral hygiene products. However, the mechanisms by which SB interacts with the SCF to alter lubrication in the mouth is unknown. Therefore, saliva and the SCF was exposed to high ionic strength and alkaline solutions to elucidate whether the interactions observed were a direct result of SB, its high alkalinity or its ionic strength. Characteristics including bulk viscosity of saliva and the viscoelasticity of the interfacial salivary films that form at both the air/saliva and hydroxyapatite/saliva interfaces were tested. It was hypothesised that SB interacts with the SCF in two ways. Firstly, the ionic strength of SB shields electrostatic charges of salivary proteins, thus preventing protein crosslinking within the film and secondly; the alkaline pH (≈8.3) of SB reduces the gel-like structure of mucins present in the pellicle by disrupting disulphide bridging of the mucins via the ionization of their cysteine’s thiol group, which has an isoelectric point of ≈8.3.
Lubricating properties of human whole saliva as affected by β-lactoglobulin
The effect of β-lactoglobulin (β-LG) at pH 3.5 and 7.0 on lubricating property of saliva as related to astringency perception was investigated using tribology. Saliva was adsorbed onto surfaces of a rotating poly dimethylsiloxane (PDMS) ball and disc to form a film under conditions that mimic the rubbing contacts in the oral cavity (Bongaerts, Rossetti, & Stokes, 2007) and the lubricity of saliva films upon exposure to astringent compounds was measured. While addition of non-astringent β-LG at pH 7.0 slowly increased friction of saliva film between tribopair surfaces, β-LG at pH 3.5 rapidly increased the friction coefficients of saliva, similar to other astringent compounds (epigallocatechin gallate and alum). This supports the hypothesis that astringency of β-LG arises from the loss of lubrication of saliva which is in agreement with the well-accepted astringency model of polyphenols. Increasing β-LG concentration at pH 3.5 (0.5–10% w/w) caused a rapid increase in friction coefficient; however, at the highest protein concentration, the friction coefficient, although higher than observed for water, was below the values observed for the lower protein concentrations. This suggests that static tribology testing is different from the dynamic in-mouth system such that a simple relationship between friction and sensory astringency cannot be found for all conditions.