ERC Project LubSat

LubSat is a European Research Council (Starting Grant) funded research project for 5 years (2017-22) with a value of €1.5 million to understand the role of oral lubricity on satiety. In particular, our quantitative multi-scale understanding of lubrication of the human salivary film when exposed to stimuli from food biomolecules, which in turn can have significant appetite suppression consequences, remains poorly understood. The key limitation to accurately measure oral lubrication is the lack of availability of tribo-contact surfaces that effectively emulate the oral surfaces (i.e. the soft, slippery mucous-coated human tongue and the upper palate). The project will apply classical theories from Physics and tools from Mechanical Engineering to design novel soft lubricious surfaces emulating our saliva-coated human tongue. This will be then used to create fundamental understanding of how food molecules lubricate the oral surfaces and the implications this has on the satisfaction of the food and perceived satiety.

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New Results on ‘Salivary lubrication (ex vivo) enhancement upon moderate exercise  published in Archives of Oral Biology

Saliva is a nature-engineered lubricant found in the oral cavity and is fundamental to eating, swallowing, speech and one’s daily functioning. Recently, there has been an escalation in research interest on salivary lubrication largely fueled by the increased incidence of dry mouth or xerostomia resulting in poor quality and quantity of saliva. In this study pulished in Archives of Oral Biology, Volume 116, Article No. 104743,, we examined changes in salivary lubricity after a bout of moderate intensity cycling for 45 min in healthy females as compared to a time-matched rest period. It was hypothesized that exercise would induce enhancement in salivary lubrication performance due to its effects on increases in protein and MUC5B content. Tribology results revealed that moderate intensity exercise resulted in enhanced lubricity of saliva with an order-of-magnitude lower friction coefficients in the boundary regime 45 minutes post exercise, with frictional forces being significantly lower as compared to the Control procedure. Total protein and α-amylase secretion also increased in the Exercise procedure. Inisghts from this proof-of-concept study reveals that moderate intensity exercise leads to an increase in α-amylase and total protein secretion resulting in enhanced lubrication performance of the saliva. However, the lubrication performance was not related to MUC5B content, suggesting the role of other proteinaceous species acting as lubricants. These preliminary results serve to design future exercise interventions in populations with dry mouth conditions.

New Results on ‘Tribology, Rheology and Sensory Perception of Bead-layered Hydrogels published in Food Hydrocolloids

Foods in general are heterogeneous composite structures with particles of various sizes, shapes and viscoelastic moduli embedded in complex polysaccharide and protein networks. Examples of such composite foods may range from the conventional use of freeze-dried fruit pieces in porridge and yoghurt, and starch granules in custards to the more recent usage of flavoured gelatine pearls in confectionery, pieces of cookies in ice creams and seeds/nuts inclusion in cheese, etc. Indeed such interesting inclusions of particles are increasingly enabling novel texture creations and triggering hedonic escalation of these palatable foods. In addition to creating new hedonic textural experiences, there is an increasing body of evidence showing texturally complex foods containing inclusions can influence oral processing behaviour in human subjects. Nevertheless, it remains elusive in literature whether consumers can distinguish particle sizes for soft particles or not? It is also important to understand how the instrumental and sensorial response of  hydrogels with polymeric soft gel particle inclusions might alter if the soft gel particles were present as “layers” rather than being incorporated homogeneously in the matrix.

New work by Ecaterina Stribiţcaia (PhD Student, ERC LubSat Project) on Tribology and rheology of bead-layered hydrogels: Influence of bead size on sensory perception” published in Food Hydrocolloids, Volume 104, Art. No. 105692 has answers to some of these questions. Layered hydrogels were designed in thsi study using a monolayer of calcium alginate (CaA) beads of small, medium and large size (diameter of 805, 1413 or 1725 μm, respectively) sandwiched in between layers of kappa-carrageenan (κC) gel matrix, with controls created using pure κC hydrogels and κC + sodium alginate (NaA) mixed gels. Large deformation rheology of the hydrogels followed by apparent viscosity as well as tribological properties of the hydrogel boli (after homogenising with simulated saliva) were analysed. Sensory discrimination tests (n = 113) and intensity ratings (n = 60) were conducted with untrained panellists. Bead size did not have an influence on the rheological properties of the layered hydrogels and hydrogel boli, respectively (p > 0.05). However, the lubrication behaviour of the layered hydrogel boli was influenced by bead size, with gels containing large-sized beads showing highest lubrication in both boundary and mixed regimes (p < 0.05). Untrained panellists were able to discriminate non-layered gels from bead-layered counterparts based on textural attributes, such as “hard”, “chewy” and “pasty”, but surprisingly, they could not distinguish between small and large-sized bead-layered gels in contrast to the oral tribology results. In summary, modulus of beads matters! In other words, the low modulus of the beads appeared to be the limiting factor to detect differences based on soft bead sizes in this study. Check out the open access paper at to know more about some of the exciting results on bead layered hydrogels containing soft beads of different sizes.

A Review on ‘Probing the frictional properties of soft materials at the nanoscale’ published in Nanoscale

Friction becomes particularly relevant at the nanoscale in a wide range of systems operating under mild to extreme load conditions. Applications of nanoscale friction spanning from device miniaturization i.e. micro- and nano-electromechanical devices (MEMS/NEMS) to complex frictional phenomena occurring in biological applications including protein motors on microtubules to prokaryotic/eukaryotic cells under naturally-occurring sliding motions and biomimetics have placed it at the cutting edge of nanotechnological research. At a fundamental level, friction force microscopy (FFM) plays a crucial analytical role in understanding nanoscale interactions at biological interfaces, in addition to the increasing demands of traditional soft material industries (e.g. food, healthcare, biomedical, agrochemical, cosmetics) among many others. In this Review published in Nanoscale (2020), Volume 12, Pages 2292-2308, we specifically provide a critical analysis of FFM focusing on the booming area of nanotribology in soft surfaces, describing the rapid evolution of FFM, the gradual transition from the use of sharp tips to well-defined colloidal probes  and more recently performing friction measurements incorporating flexibility into the material physics and chemistry of the contact surfaces (see figure in the top for the key miletones).

We extensively review studies where FFM has been used on soft surfaces to measure these tiny lateral forces, and discuss the impact that (i) surface interactions, (ii) surface roughness, (iii) intrinsic material properties, and (iv) experimental conditions have on the frictional properties of hard-on-soft and soft-on-soft contact systems. We examine aspects of molecular dynamics (MD) simulations that allow the prediction of frictional behaviour at the nanoscale for hard and soft contacts. Finally, we outline the systems where FFM using soft surfaces can be applied, before looking into the future opportunities, including the fabrication of precisely tailored soft probes for FFM along with the growing need for new mathematical models to overcome the current limitations of FFM-based approaches for soft tribology. It is noteworthy that still most of the published studies on friction using FFM are utilizing conventional polymeric probes with an elastic modulus higher than 1 GPa (see Figure below) and there are relatively rare instances where a system of soft probe/soft surface has been used to measure lateral forces (see the green region in the Figure below). Designing colloidal probes that exploit the parallel developments in material chemistry can help to address many fundamental research challenges. The investigation of this knowledge gap, regarding the capability of performing nanotribology experiments with elastic moduli ranging from tens of kPa to few MPa at the nanoscale, will be of great importance, and will find use in a wide range of future biological and technological applications, where soft materials with desired frictional properties are in demand. Nanoscale friction in hydrogel-based and biomaterial-based colloidal probes that emulate biological surfaces with relevant modulus and roughness offers excellent opportunities for future interdisciplinary research involving material scientists, biomaterial engineers, mechanobiologists, nanotribologists, and physicists. Such fundamental knowledge is key to eventually design a new generation of soft materials with the desired frictional properties that will tackle a variety of global challenges, from reduction in energy consumption to biological tissue repair. To read this comprehensive review, click here for the full text:


New results on ‘Multi-scale salivary lubrication mechanism‘ published in Advanced Materials Interfaces

Salivary pellicle, the absorbed layer of saliva within the oral cavity, is an excellent aqueous lubricant that maintains low‐friction movement in the mouth that is of first‐order importance to human life for feeding and speech. Saliva is unique as compared to all other bodily lubricants, as it bathes the hardest (enamel) to one of the softest tissues in the mouth. Mucin, and more recently small molecular salivary proteins have been separately mooted as biological lubricants responsible for reducing friction in saliva. However, none of these by themselves have managed to replicate the remarkable aqueous lubrication in boundary and fluid film regimes as well as load‐bearing abilities of adsorbed real human saliva in direct in vitro experiments. This study titled “A self‐assembled binary protein model explains high‐performance salivary lubrication from macro to nanoscale”published in in Advanced Materials Interfaces sheds light on the molecular mechanism of salivary pellicle lubrication by involving multilayer fabrication using salivary proteins and multi-scale lubrication experiments. We have uncovered that an electrostatically driven multilayer architecture of binary salivary proteins, i.e., negatively charged salivary mucin (bovine submaxillary mucin, shown in branched bluish structure in the 1st Figure)  and positively charged salivary protein (lactoferrin, shown in red globules in the 1st Figure) is key to facilitate the lubrication of soft sliding interfaces. Despite the simplicity of such a binary protein model compared to the complexity of multiple proteins in real saliva, the tribological properties at macro to nanoscale of this architecture closely resemble those of real human saliva. Experimental techniques (see 2nd Figure below) ranged from multi-scale tribological analysis covering 9 orders of magnitude of normal forces to dynamic film formation monitoring of the multilayered architecture using a quartz crystal microbalance with dissipation (QCM‐D) on substrates with varying surface chemistries, which were supported by self-consistent field theory calculations. With these experimental and theoretical approaches, we isolated the specific role of each protein component, and found that the positively-charged proteins (lactoferrin) in saliva acts as a “molecular glue” in the mucin network that synergistically promotes mucin–mucin assembly to trap water molecules. Such a network facilitates macromolecule‐mediated viscous lubrication, while the lactoferrin also glues salivary mucin strongly to the oral surfaces, thus enabling effective boundary lubrication as shown in the 1st Figure.  Also, the multilayered (mucin-lactofferin-mucin)n architecture replicates the behavior of saliva at a certain ion concentration highlighting the importance of electrostatic interaction in the salivary lubrication phenomena. These unprecedented results present an optimal approach to study multi-scale aqueous lubrication, and will open the doors to fabrication of nature‐mimetic aqueous lubricants for a range of biological applications.

To read the full article, click here:

A Review on ‘Ageing related changes in quantity and quality of saliva published in Journal of Texture Studies

Saliva is crucial to oral processing of food and consequently is also related to the sensory and textural experience. It is often assumed that the secretion and properties of saliva change with age, which can result in dry mouth conditions, taste aberrations. Such changes may result in reduced nutrient intake and malnutrition besides adversely affecting the quality of life. Based on some recent research findings, this paper reviews our current understandings on age-dependent changes on quantity (bulk salivary flow rate) as well as quality of saliva (e.g. composition, viscosity, lubrication) in healthy elderly individuals published in Journal of Texture Studies (2020), Volume 50, 1, Pages 27-35 As shown in the summary figure below, it is clear that age-related changes in saliva are multifactorial. On one hand, a reduced salivary flow rate has an influence on increasing the ionic concentration of saliva, inadequate levels of fluid to coat the oral cavity and reduced ability to cope with the rate of pellicle replenishment. On the other hand, the decrease of salivary mucins and calcium might render loss of the lubricating properties of bio-lubricant saliva, which can result in reduced wettability of oral surfaces eventually leading to xerostomia, associated oral symptoms and potential aberrations in sensorial and/or textural perception. With carefully planned procedures, such as use of appropriate age groups, healthy versus older adults with health conditions and/or type of medication used, the knowledgebase on agre-dependent change in quantity and quality of saliva will be expanded offering potential applications to designing optimized food and oral therapies for maintaining oral health and nutritional status in elderly population.

New results on ‘Microgels acting as viscosity modifiers and lubricants’ published in Soft Matter

Microgels are soft colloids made of cross-linked polymers that are capable of entrapping significant proportions of solvent or water. In this work published in Soft Matter, Volume 15, Pages 9599-9599, the mechanical performance of colloidal whey protein microgels (hydrodynamic diameter ∼100 nm measured using dynamic light scattering and atomic force microscopy (AFM) (see artwork (top) and experimental AFM image, (bottom)) of different rigidity (soft microgels (G′ ∼ 100.0 Pa) and hard microgels (G′ ∼ 10.0 kPa) ) dispersed in Newtonian or complex non-Newtonian fluids was investigated for the first time via rheology and soft tribology complemented with theoretical considerations. Dispersions of both soft microgels (G′ ∼ 100.0 Pa) and hard microgels (G′ ∼ 10.0 kPa) were observed to act as thickeners in low viscosity Newtonian fluids and correspondingly reduced the friction, latter decreased as a function of the increased rigidity of the microgels. Differently, in high viscosity continuum, the microgels acted as thinning agents and increased the friction. Variation of mechanical characteristics of microgels and continuum allowed establishing a relationship between the high shear rheology of the dispersions and their lubrication properties, which to the best of our knowledge has never been reported for this kind of complex continuum before. Thus, in order to obtain a benefit from microgels on the tribology performance of a dispersion, simplistically, it is necessary to have harder particles when the continuum viscosity is increased until the microgels become too hard that they can increase the abrasiveness in biological contacts. To read the full article, click here:

An Invited Review on ‘Human and Model Saliva’ published in Advances in Colloid and Interface Science

Human saliva, a seemingly simple aqueous fluid, is, in fact, an extraordinarily complex biocolloid that is not fully understood, despite many decades of study. In the last few decades, colloid scientists have attempted designing model (i.e. ‘saliva mimicking fluid’) salivary formulations to understand saliva-food colloid interactions in an in vitro set up and its contribution on microstructural aspects, lubrication properties and sensory perception. We present the first review published in  Advances in Colloid and Interface Science 27 Article No. 102034 on the the current state of knowledge on bulk and interfacial properties of model saliva in comparison to real human saliva and highlight how far such model salivary formulations can match the properties of real human saliva. Real human saliva has an extremely complex architecture with multiple proteins and consequently specialized properties, understanding of which is far from complete. Hence, designing one standardized formulation of real human saliva is not straightforward. Indeed, model saliva and real human saliva show similarities in food colloid-saliva interaction outcomes when the model saliva emulates the biochemical composition (e.g. ions, negatively charged commercially available mucin) and the electrostatic charge, to a certain extent. Such understanding has helped to decipher the physico-chemical mechanisms behind sensory perceptions. However, considering the growing research interests on oral lubrication, model saliva studied mainly using less expensive and crude pig gastric mucin (PGM) is particularly problematic in replicating the lubrication and adsorption properties of saliva. Based on surface adsorption and lubrication studies from a wide pool of studies, we recommend that bovine submaxillary mucin (BSM) as the most optimal choice for the commercially available mucin source for model saliva preparation in terms of lubrication properties. Despite that mucin is recognized as the chief contributor to the lubricating properties of real saliva, it is evident that aqueous solution of mucin alone cannot fully represent various physicochemical and biophysical properties of saliva. Systematic studies on designing mucin-multilayers with targeted tribological properties have to be investigated in future, particularly with respect to examining its potential to replicate human salivary pellicle. Tribological measurements in the presence of mucins and mucin-polycationic additive systems (multilayers) in comparison to ex vivo human salivary conditioning films are needed in first place to warrant its use in food tribology experiments. To read this comprehensive review paper, please click in the open access link:

A Systematic Review on ‘Oral Tribology-Sensory Relationship’ published in Current Opinion in Food Science

Oral tribology is rapidly entering into the food scientists’ toolbox because of its promises to predict surface-related mouthfeel perception. We present the first systematic review published in  Current Opinion in Colloid Food Science 27 Pages 64-73 on the relationships between instrumental tribology measurements and sensory perception. In recent years, an impressive suite of commercially available and bespoke tribometers have surfaced to quantify the friction in presence of model and real food systems that allow the plotting of Stribeck curve. We discussed the advantages and disadvantages of these tribometers that are currently used across various food material science laboratories. It is exciting times that food scientists have already moved on from using steel-steel to steel-elastomers (such as polydemthylsiloxane (PDMS)) or PDMS–PDMS surfaces to mimic tongue-palate contacts, which have enabled developing empirical relationships with some sensory attributes that are either fat-related or non-fat related. We narrowed down the recent studies involving model and real food systems from 2016 onwards from 4857 to only 16 articles that found correlations between friction coefficients (μ) and specific sensory attributes, such as ‘smoothness’, ‘slipperiness’, ‘pastiness’. However, considering lubrication is a system property and not an intrinsic material property of the lubricant, generalizability of such relationship can be questioned. It is highly likely that such existing relationship is only valid within the remits of those specific experimental conditions, and such tribology–sensory relationship might not hold well with other equipment or experimental conditions. Hence, it is crucial to build mechanistic hypotheses before trying to examine tribology–sensory relationships. Finally, to marry oral tribology to sensory, we have listed challenges and opportunities on the road to identify generalized relationships between instrument and sensory perception. To read the full paper, please click in the open access link:

New results on ‘Rheology and Tribology of Polysaccharides’ published in Biotribology

Combination of rheology and tribology can be a promising tool to characterize polysaccharides and understand their potential for acting as promising thickeners for dysphagia (swallowing disorder) patients. In this study, which has been recently published in Biotribology 33 Pages 14699-14708, gellan gum (GG) (0.075–0.3 wt%), a, bacterial exopolysaccharide from Sphingomonas elodea was compared against commercial starch-based thickeners (modified starch with or without gums, 5 wt%) and xanthan gum (XG, 0.5–1.0 wt%) for rheological and oral tribological properties. Gellan gum is a linear tetrasaccharide, composed of 4)-L-rhamnopyranosyl-(α-1 to 3)-d-glucopyranosyl-(β-1 to 4)-D-glucuronopyranosyl-(β-1 to 4)-d-glucopyranosyl-(β-1 to 4) containing acetyl substituents on the 3-linked glucose. Gelation occurs upon cooling of a heat-treated gellan gum solutions under quiescent conditions through the aggregation of its double helices.  We compared the polysaccharides using apparent viscosity, oral tribology using polydimethylsiloxane (PDMS) ball-on-disc set up and ζ-potential measurements. The measurements were conducted in presence of artificial saliva containing mucin with or without α-amylase at 37 °C to mimic oral conditions. Viscosity results suggested that the commercial starch-based thickeners behaved like water in orally relevant shear, largely associated with the hydrolysis of modified starch by α-amylase, whereas, XG and GG showed no responsiveness to α-amylase. In the case of oral tribology, artificial saliva containing mucin adsorbed to the PDMS surfaces reducing friction as compared to water. From a tribological perspective, the lubrication profile of all thickeners was found to be controlled mainly by their viscous component. Hence, samples with higher viscosities at orally relevant shear rates (>0.1 Pa s at 100 s−1 shear rates i.e. two orders of magnitude higher viscosity than water) showed promising lubrication performances (i.e. μ ≤ 0.07 in the mixed regimes) as compared to that of water (μ ~ 1.0 in the mixed regimes) due to their larger drag force to allow entrainment as compared to water. The increase in boundary friction coefficients in commercial starch-based thickeners was likely associated with α-amylase-induced hydrolysis, increasing the PDMS-PDMS asperity contacts. In simulated oral conditions, the increase in friction coefficients in presence of XG and GG was influenced by the exclusion of mucin-rich artificial saliva from the PDMS surfaces due to electrostatic interaction between the gums and mucin. A combination of rheological and tribological techniques in orally relevant conditions appears as a reliable approach to understand the potential of GG (0.3 wt%) to act as a dysphagia thickener that offers similar mechanical properties as XG (1.0 wt%) at a lower concentration. Further studies on extensional viscosity measurement of gellan gum in simulated oral processing conditions and relationship of instrumental data with sensory measurements (e.g. thickness, cohesiveness, stickiness) are needed to warrant its applications in dysphagia management. You can read the paper at

An Invited Review on ‘Oral Lubrication’ published in Current Opinion in Colloid & Interface Science

Oral lubrication deals with one of the most intricate examples of bio-lubrication, where surfaces under sliding conditions span from the hardest enamel to soft oral tissues in human physiology. The situation is more complex as these surfaces are covered by an endogenous biolubricant ‘saliva‘ before the exogenous food particles can wet, stick, or slip at the surfaces. In this Invited Review published in the Special Issue of “Outstanding Young Researchers in Colloid and Interface Science” in Current Opinion in Colloid and Interface Science 39 Pages 61-75, we covered the latest advances in tribology research in soft oral contacts and theoretical developments relevant for this field. We have briefly covered the material physics and chemistry (roughness, wettability and modulus) of human soft oral architectures under shear fields, which influence oral lubrication and highlight the transition from rheological to tribological limits. Talking about surface roughness of human tongue, the human filiform papillae can contain 6–12 protruding hairs, each of which are 34–50 μm wide contributing to overall 420–500 μm diameter in the root and height of 250 μm, whereas fungiform papillae almost doubling the diameter of the filiform ones. Overall, all these surface topographic analyses of tongue point out that engineering of these filiform papillae on tongue surface by nature provides it a more coarse texture than even a 100-grit sandpaper. However, the tongue does not feel as coarse as represented by its roughness largely due its reduced “stiffness” (Young’s modulus).

In terms of lubricants, we discussed one endogenous lubricant (saliva) and one exogenous lubricant (food-based microgels/hydrogel particles)  to capture the recent knowledge gathered in how they reduce friction in sliding soft contacts in an in vitro set up. We discussed the legacy of materials used for oral lubrication studies in the last decade covering latest experiments conducted mainly using polydimethylsiloxane (PDMS) tribopairs in in vitro and pig tongue surfaces in ex vivo experiments. For example, according to classical Hertz contact theory, in the case of the popular PDMS ball on disc tribological set-up (E* ∼ 2 MPa, R ∼ 0.01 m) with a typical load of 2 N, the maximum contact pressure is ∼200 kPa. This pressure is about one order of magnitude higher than that measured in oral-palate contact of healthy adults (30–50 kPa). For future, we highlight the window of opportunity offered by a range of polymeric surfaces together with the advent of 3D Printing technology that can be used to emulate oral surfaces with accurate roughness and modulus to enable colloid scientists to underpin principles that govern oral lubrication at colloidal scale. To read the full paper, please click in the link:

New results (with Slide Presentation) on ‘Lubrication of Emulsion Microgel Particles’ published in ACS Applied Materials & Interfaces

Biolubricants  reduce friction between soft biological contacting surfaces, such as the tear in eyes, saliva in the oral cavity, synovial fluid in the articular joints, to prevent discomfort and wear of epithelial tissues and cartilage. The use of submicron to micron sized particles (e.g., microgels) as biolubricant additives has attracted recent attention in biomaterial science research due to their ability to reduce friction and wear in the boundary regime of sliding contact in biological areas, such as tongue-oral palate contacts, cartilage, or eye blinking. In this paper published in ACS Applied Materials & Interfaces 10 (32), Pages 26893–26905, starch-based emulsion microgel particles with different starch (15 and 20 wt %) and oil contents (0–15 wt %) were synthesized, and their lubrication performance under physiologically relevant conditions was investigated. Emulsion microgels were subjected to skin mimicking or oral cavity mimicking conditions, i.e., smooth hydrophobic polydimethylsiloxane ball-on-disc tribological tests, in the absence or presence of salivary enzyme (α-amylase). In the absence of enzyme, emulsion microgel particles (30–60 vol % particle content) conserved the lubricating properties of emulsion droplets, providing considerably lower friction coefficients (μ ≤ 0.1) in the mixed lubrication regime compared to plain microgel particles (0 wt % oil). Upon addition of enzyme, the lubrication performance of emulsion microgel particles became strongly dependent on the particles’ oil content. Microgel particles encapsulating 5–10 wt % oil showed a double plateau mixed lubrication regime having a lowest friction coefficient μ ∼ 0.03 and highest μ ∼ 0.1, the latter higher than with plain microgel particles. An oil content of 15 wt % was necessary for the microgel particles to lubricate similarly to the emulsion droplets, where both systems showed a normal mixed lubrication regime with μ ≤ 0.03. The observed trends in tribology, theoretical considerations, and the combined results of rheology, light scattering, and confocal fluorescence microscopy suggested that the mechanism behind the low friction coefficients was a synergistic enzyme- and shear-triggered release of the emulsion droplets, improving lubrication. The present work thus demonstrates experimentally and theoretically emulsion microgel particle as a novel biolubricant additive with stimuli-responsive properties capable of providing efficient boundary lubrication between soft polymeric surfaces. At the same time, the additive should provide an effective delivery vehicle for oil soluble ingredients in aqueous media. These findings demonstrate that emulsion microgel particles can be developed into multifunctional biolubricant additives for future use in numerous soft matter applications where both lubrication and controlled release of bioactives are essential.. To read the full paper, please click in the link To hear an audio presentation on this paper by Ophelie Torres, please click below:

New results on Aqueous Lubrication using Microgel Particles published in Langmuir

Aqueous lubrication has emerged as an active research area in recent years due to its prevalence in nature in biotribological contacts and its enormous technological soft-matter applications. In this study, which has been recently published in Langmuir 33 Pages 14699-14708, we designed aqueous dispersions of biocompatible whey-protein microgel particles (WPM) (10–80 vol %) cross-linked via disulfide bonding and focused on understanding their rheological, structural and biotribological properties (smooth polydimethylsiloxane (PDMS) contacts, Ra < 50 nm, ball-on-disk set up). The WPM particles (Dh = 380 nm) displayed shear-thinning behavior and facilitated lubrication between bare hydrophobic  PDMS surfaces (HB) (static water contact angle 108°), leading to a 10-fold reduction in boundary friction force at increased volume fraction (ϕ ≥ 65%), largely attributed to the close packing-mediated layer of particles between the asperity contacts acting as “true surface-separators”. In addition, WPM particles employed a rolling mechanism analogous to “ball bearings”, the latter supported by negligible change in size and microstructure of the WPM particles after tribology. An ultralow boundary friction coefficient, μ ≤ 0.03 was achieved using WPM between O2 plasma-treated hydrophilic PDMS contacts coated with bovine submaxillary mucin (HL+BSM) (static water contact angle 47°), and electron micrographs revealed that the WPM particles spread effectively as a layer of particles even at low ϕ∼ 10%, forming a lubricating load-bearing film that prevented the two surfaces from true adhesive contact. However, above an optimum volume fraction, μ increased in HL+BSM surfaces due to the packing and interpenetration of particles that possibly impeded effective rolling, explaining the  increase in friction. The present work demonstrates a novel approach for providing ultralow friction in soft polymeric surfaces using proteinaceous microgel particles that satisfy both load bearing and kinematic requirements. To read the full paper, please click in the link