Knowledge

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.

Keywords: Soft-EHL, Isoviscous-elastic, Friction, Couette friction

Polymer powders are commonly added to cosmetic formulations to improve product performance and skin feel. This study investigates the effect of particle concentration and size on the lubricating properties of powder suspensions. Results are reported for various particle sizes and concentrations.

When the tribological contact was fully immersed the addition of particles had no effect. However different behaviour was observed when the contact was only partially lubricated. In this case, a three-stage friction coefficient curve was observed. By varying the particle size and concentration it was shown that the duration and magnitude of each stage can be controlled.

Keywords: Sliding friction, Elastomer, Particulate, Real contact area

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.

Keywords: Tribology, Lubrication, Soft contacts, Polymer solutions, Isoviscous-elastic, Soft EHD

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.

Keywords: Tribology, Lubrication, Soft contacts, Isoviscous-elastic, Carbopol

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.

Keywords: Oral processing, Texture, Mouthfeel, Tribology, Lubrication, Rheology, Thin film, Gap dependent, Saliva, Fat, Oil, Emulsions, Hydrocolloid, Polysaccharide, Microstructure, Health, Obesity

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.

Keywords: Milk chocolate, Chocolate, Friction, Milk, Mouth, Hydrodynamics, Mouth, Particle Size

In this work the lubrication behaviour of emulsions, gels, and emulsion-filled gels was studied in relation to their composition and structure. It was found that emulsions had much lower friction coefficients than their continuous phases. Emulsions with 40 wt% oil had the same friction coefficient as the pure oil. The lubrication properties of the gels, sheared by pressing them through a syringe, strongly depended on the molecular properties of the gelling agent and on the breakdown behaviour of the gel matrix. For each type of emulsion-filled gel, the lubrication behaviour was affected by the interactions between oil droplets and matrix. For gels containing oil droplets bound to the matrix, the friction coefficient gradually decreased with increasing oil concentration. For gels containing oil droplets non-bound to the matrix, the friction coefficient of the filled gels was lower than that of the same gel matrix without oil.

However, no effect of the oil concentration on friction was observed. The different effects of the oil concentration on the lubrication behaviour of the various gels were explained by the relation between droplet–matrix interactions and the ‘apparent viscosity’ of the sheared gels. For gels with bound droplets, increasing the oil concentration resulted in an increase of the ‘apparent viscosity’ of the sheared gel. For gels with unbound droplets, the oil concentration did not affect the ‘apparent viscosity’. Confocal laser scanning microscopy (CLSM) observations of both emulsions and filled gels did not reveal coalescence of the oil droplets as a result of the shear treatment inherent to friction measurements.

Keywords: Lubrication, Friction coefficient, Protein gels, Polymer gels, Carrageenan, Gelatine, WPI, Emulsion

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.

Keywords: Tribology, Lubrication, Soft contacts, Elastomer, Viscoelasticity, Isoviscous-elastic

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.