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ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 17, 2009A Flexible Platform for Tribological Measurements on a RheometerPatrick Heyer1 and Jörg Läuger11Anton Paar Germany GmbHHelmuth-Hirth-Str. 6, D-73760 Ostfildern / GermanyPhone: 49-711-72091-0, info.de@anton-paar.com, www.anton-paar.comABSTRACTA newly designed tribological cell for aRheometer and its application on varioussamplesisdescribed.Tribologicalmeasurements on dry, oil and greaselubricated systems have been conducted.Tests on some food products reveal a goodcorrelation between the friction propertiesand the fat content of the samples.INTRODUCTIONCompared to oils greases have a numberof advantages with respect to theconstruction and service of lubricatedcomponents. However, due to the viscoelastic behavior of greases there are certainconstraints to consider. Therefore having aninstrument and methods to investigate thevisco-elastic and frictional behavior ofgreases over an extended temperature rangeis highly desirable. Oscillatory amplitudesweeps are very well suited to investigatethe visco-elastic behavior and consistency oflubrication greases. Valuable information onthe visco-elastic behavior, i.e. the storageand the loss moduli as well as on the stressvalues at the flow point, i.e. the yield stress,are obtained1.In shear rheology the surface of thefixtures do not have any influence on therheological data as long as the conditions oflaminar flow are met. In some cases thesurfaces are treated or roughened in order toprevent slip and to assure a laminar flowfield. Therefore rheology uses the testfixtures to apply deformation onto thesample, whereas they are part of the testspecimen in tribological testings. However,in tribological tests forces, movements andnormal loads need to be applied or measuredas in the case of rheology. Modern rotationalrheometers are equipped with excellentspeed and torque control as well as anaccurate normal force detector and a precisecontrol of the temperature in a wide range.All these features can also be used fortribology as well, which led to the idea todesign an accessory enabling tribologicalmeasurements on a conventional rotationalrheometer.Recently, the lubricating properties ofcertain liquid and semi-solid foods, asdetermined with a variety of experimentaltribology instruments, was shown tocorrelate well with mouthfeel attributes2.However no standard instruments arecurrently available. The lubrication behaviorof foods has been measured using tribologyequipment, e.g. pin-on-disk or a ball-on-diskset up originally designed for industrialapplications such as the evaluation ofmachine wear2,3. Ideally, food lubricationwould involve the measurements ofphenomenon more similar to the actions thatoccur in the mouth during foodconsumption. These entail rubbing andsqueezing actions of the food item betweenthe tongue and palate, which are notmeasured by rheology. These rubbing andsqueezing motions generate a frictional
force in which the food-saliva mixture actsas a lubricant. The oral friction can bedescribed with the help of Stribeck curves,in which the boundary and mixed regimesdescribe the friction of a food material,which is at least in partial contact withsurfaces. As friction is the force that resistsmotion, it is highly system related and not amaterial property4. Therefore the load,surface and sliding speed at which theexperiments are performed are equallyimportant and several investigators havebegun studies to find the appropriateconditions. Values obtained from oralmeasurements show sliding speeds up to 200mm/s and loading regimes between 0.01 and90 N5. Instruments intended to measure thelubricity of food products must be capableof precise control of test parameters toaccurately capture the complex actions thatoccur in the mouth.An important aspect in the determinationof lubricity is the use of appropriatesurfaces. Attempts have been made usingtetrafluorethylene (PTFE) and zirconia6 orsteel against silicon rubber2. These surfaces,however, are smooth, and do not reallymimic the oral tissue of the mouth3. Morerecently, friction coefficients of foods weredetermined using additional materials thatincluded pig tongues and silicone surfacesand materials with well-defined surfaceIt was found that surfaceprofiles7,8.structure critically affects the frictionalbehavior of the investigated system.Furthermore the importance of the salivaryfilm’s interaction with the surface and itsinfluence on the friction measurements arepointed out. The relationship of thesesurfaces in the prediction of human sensoryattributes has still not been fully understood.A better understanding of the materialproperties that influence the mouthfeel ofdairy products would be of great benefit tothe food and beverage industries, especiallyfor low fat and calorie-reduced products.Developing further instrumental techniquesto elucidate the physical origins of sensoryattributes such as “creaminess” “smooth-ness” and “thickness” would largely benefitproduct developers and allow the moreconsistently delivery of key mouthfeelattributes in their products9.TRIBOLOGY DEVICEFor testing tribological properties a newdevice has been designed, which can bemounted as an accessory onto a standardrotational rheometer10. It makes use of thelarge measurement ranges as well as themotor control mechanism of the rheometer,thus transforming the rheometer into ahighly sophisticated tribometer. The setup isbased on the ball-on-three-plates-principle(or ball-on-pyramid) consisting of ageometry in which a sphere is held, an insetwhere three small plates can be placed, anda bottom stage movable in all directions onwhich the inset can be fixed. Figs.1 and 2depict the new tribometer setup. Theflexibility of the bottom plate is required toget the same normal load acting evenly onall the three contact points of the upper ball.The rotating sphere is adjusted automaticallyand the forces are evenly distributed on thethree friction contacts. An overload of onecontact point would result in wrong frictionvalues.The ball as well as the plates for the insetcan be exchanged so that the system can beadapted to desired material combinations.For tribological measurements on foodsamples elastomers are used as bottomplates. Due to the elasticity of the bottomcontact the ball will press into the elastomerand the rotation of the ball will lead to abuild up of a lubrication film. This processis called soft elasto hydrodymaniclubrication (Soft-EHL).The rotational speed applied to the shaftis producing a sliding speed of the ball withrespect to the plates at the contact points.The resulting torque can be correlated withthe friction force by employing simplegeometric calculations. The normal force ofthe rheometer is transferred into a normalload acting perpendicular to the bottomplates at the contact points. The tribology
setup is temperature controlled by Peltierelements from -40 C up to 200 C. APeltier hood ensures the same temperature atthe bottom plates and at the upper ball.solution and sunflower oil at sliding speedsin the range of 0.4 and 20 mm/s 11.MEASUREMENTS ON LUBRICATIONGREASESIn Fig. 3 rheological strain sweeps ofthree different grease samples at –40 C and25 C are shown. The storage modulus G’representing the elastic part, and the lossmodulus G’’ representing the viscousportion of the samples response to theoscillation as a function of the shear stressare shown.106Pa104G'3G'' 1010102110Figure 1. Schematic setup Tribologyaccessory in side and top view.Figure 2. The Tribology accessory without(left) and with (right) additional Peltierhood.The described device was mounted on aMCR301 rheometer from Anton Paar. Testswith various thermoplastic elastomersrevealed that the elastomers suited fortribological testing on food samples having adifference in friction factor of more than 0.6between an aqueous 10% (w/w) sucrose-210-101101010Shear Stress τ2103Pa105Figure 3. Strain sweeps from γ 0.001 % upto 100 % at 25 C (full symbols) and -40 C(open symbols). Data plotted versus theshear stress. The big dots mark therespective flow points of the three greases atthe two temperatures.With the use of the tribology device andbyincreasingtheslidingspeedlogarithmically, starting from small valuesfriction factor measurements have beenobtained while applying a normal load of14 N. In Fig. 4 the friction coefficient as afunction of the sliding speed for two greasesmeasured at 25 C and -40 C are shown. Forboth greases, at low sliding speeds up to 5mm/s the friction values are lower at -40 C(squares) compared to 25 C (circles),whereas at higher speed up to 100 mm/s thefriction coefficient is lower at 25 Ccompared to -40 C. A possible explanationof these effects might be given byconsidering the rheology data from Fig. 3,which show that at -40 C the structure of the
greases is stronger compared to 25 C. Itseems that due the stronger structure andhigher yield stresses a thicker film is presentat lower temperatures, resulting in a smallerfriction at low speeds. At higher speeds partof the grease in the contact area might bemoved out and the higher yield behaviorrestricts a back flow of the grease. Thereforethe film thickness is reduced and the frictionincreases.0.3NLGI-0 25 C0.25μNLGI-2 25 C0.20.15NLGI-2 -40 CNLGI-0 -40 C0.100.35100.3NLGI-2 -40 C0.20.15NLGI-2 25 C0.1NLGI-0 -40 CNLGI-0 25 C0.05020-510-4-3-2-1101010Sliding Speed vs100101mm/s 103Figure 5. Static friction measurement for 2lubrication greases at 25 C and -40 C.0.2500.40.350.050.4μsmaller at -40 C compared to 25 C for bothgreases.4060Sliding Speed vs80 mm/s 100Figure 4. Friction factor vs. sliding speed fortwo different greases at 25 C and -40 C.In order to investigate the static frictionbehavior measurements in which the frictionforce was logarithmically increased wereperformed for the same two greases. Again anormal load of 14 N was used. Before theactual measurement a run-in at a speed of 10mm/s was performed for 10 min. followedby a rest period for another 10 min. In Fig. 5the results of these tests are shown.At small forces there is practically nomovement and the data scatter around zerospeed, represented by small values in alogarithmic scale. If the force is largeenough to overcome the static friction thespeed jumps from zero (or very smallvalues) to larger values. At 25 C (circles)the friction coefficient stays more or lessconstant up to the maximum plotted speed.However at -40 C (squares) a step in speedcan be seen as well, but at higher speeds thefriction increase again. The static friction,which can be taken as the friction value atwhich the big jump in the speed occurs, isBall bearings are often lubricated withgreases ensuring lowest possible frictionunder working conditions. Therefore aspecial holder for standard ball bearing hasbeen designed and is mounted onto thetribology cell. Starting friction, runningfriction and roll out time after an appliedspeed are the most interesting parameters.The settings for the starting torquemeasurement are the same as for the staticfriction determination. The torque islogarithmically increased and the occurringspeed is measured. Fig. 6 shows suchstarting torque measurements at -40 C, 25 C and 60 C.Due to the influence of the yield point ofthe lubricant on the starting torque the ballbreak-off requires more force at lowertemperatures. It has been shown before thatthe shear stress to overcome the yield pointis strongly temperature dependent. Thelower the temperature, the higher the yieldpoint and the more force is required toinduce flow.The rolling friction test presented in Fig.7 is showing a rotational speed ramp rangingfrom 0.1 up to 3000 rpm. The curvesmeasured at 25 C and 60 C look similareven though the measured values are lowerfor 60 C. For both curves the torque
0.5mNm-40 C0.40.35 25 C0.3M 60 C0.250.20.150.10.05010-710-610-510-4-3-21010Speed n10-11001011021/min104Figure 6. Starting torque measurement for aball bearing at -40 , 25 C, and 60 C.101mNm10-40 C0 25 C 60 CM1010-1-210-1100110Speed n1021031/min 104Figure 7. Speed ramp measurement for aball bearing at -40 C, 25 C, and 60 C.Torque versus rotational speed.4101/min102-40 C10n101010 60 C 25 C0-2-4-602040Interval Time tint6080s100Figure 8. Roll out measurement for a ballbearing at -40 C, 25 C, and 60 C.Rotational speed as a function of the time inthe roll-out interval.slightly in the speed range form 0.1 to 100rpm, whereas at higher speeds a steep slopecan be observed indicating a lubricationproblem. At -40 C the lubrication isinsufficient right from the beginning sincethe running torque increases continuously.At 200 rpm the lubricant’s viscosity dropsdue to friction heating resulting in animproved lubrication.A roll-out test consists of two intervals.In the first interval the rotational speed is setto 3000rpm and held at this speed for 10seconds, whereas in the second interval thedeceleration is measured. For clarity reasonin Fig. 8 only the second interval ispresented. The roll out time of the ballbearing increases in the order of thetemperature. At -40 C the bearing roll-out isthree times shorter compared to 60 C.MEASUREMENTS ON FOOD SAMPLESDifferentiation of Fluid Dairy ProductsVarious commercial fluid dairy productswith varying fat amount were studied usingthe described tribological device. Fig. 9shows the friction factor of these samples asa function of the sliding speed. Whenassessing the friction and lubricationproperties of the tested dairy products, aclear discrimination between samples couldbe seen at several sliding speeds. TheStribeck curves generated by fluid dairyproducts could be divided into 3 distinctregimes, which are the boundary, mixed andhydrodynamic regime. In the low speedboundary regime, where there is virtually nopressure build-up, the friction resultspredominantly from the interaction of theasperities of the interacting surfaces withvery small layer of lubricant. At highersliding speeds the contact area of the twointeracting surfaces are no longer in contact(hydrodynamic regime) and the friction isgenerated by the flow properties of thelubricant. In between these two regimes laysthe mixed regime, where the contact area arein partial contact. The Stribeck curves of thefluid dairy emulsion with less than 5% fat,the mixed regime is defined by the inflectionpoint of the Stribeck curves between asliding speed of 3-10 mm/s. For the fluiddairy emulsion with higher fat content, thereseems to be a more drawn out mixed regimebetween sliding speeds of 5-50 mm/s. This
suggests that at low sliding speeds thesample is excluded from the contactmeasuring surface between the ball and thethree thermoplastic elastomer plates anddoes not act as a lubricating agent. As thesliding speed increases the sample isadsorbed into the contact measuring surfaceand becomes a more effective lubricatingagent.1.62%ReducedFat MilkFriction Factor1.2Whole MilkHalf & Half0.8Heavy Cr eamFat Free0.4Skim Milk0.00.11101001000Sliding Speed (m m /sec)Figure 9. Friction factor as a function ofsliding speed for 5 different fluid dairysamples.The dairy samples with a fat contentbelow 2% also have a very distinctive stickand slide pattern, which is very reproducibleand seems to be a system property (asfunction of the sample and interactingsurfaces). For whole milk and dairy productswith higher fat contents we still observe thecharacteristic stick and slide pattern, but it islesspronounced,suggestingmorelubricating action of the fat globules, due tohigher fat globule population. The stick andslide pattern is visible well into thehydrodynamic regime, where the pressurebuild-up is enough to draw lubricantbetween the interacting surfaces.Improvement of Mouthfeel of MilkFat-in-water dairy emulsions, thickenedwith maltodextrin or xanthan gum, wereproduced with identical micelle sizedistribution, apparent viscosity (20mPa.sand 70 mPa.s at 50s-1) and fat volumefraction (5 vol.% and 20 vol.%) as describedby Akhtar et al.12,13.Fat-in-water emulsions (30 vol.%, 2.8wt.% sodium caseinate) were prepared at50 C using a Rannie homogenizer operatingat 350 bar. The hydrocolloid (maltodextrinor xanthan gum) was dissolved indemineralised water at room temperature toa solution of known concentration. Fat-inwateremulsions(30vol.%)andhydrocolloid solution were mixed by gentlestirring in order to adjust the fat content (20vol.% and 5 vol.%) and the viscosity. Theaverage droplet size of the 30 vol.%emulsion was 0.86 µm and there was nochange in the average droplet size whendiluted to achieve 20 vol.% and 5 vol.% fatin-wateremulsions.Thermoplasticelastomer strips were cleaned with dilutedsoap, rinced thoroughly with tap water anddried with tissue paper by blotting. Theupper ball-shaped element was made ofsteel. The tests were performed in duplicateor triplicate at random at a temperature of20 C and a normal force 3 N.The emulsions were assessed induplicate. Panel members were asked to ratecreaminess on a scale of 1 to 10, where 10corresponds to the intensity highest rating.PrincipalComponentAnalysiswasperformed with CAMO Unscramblersoftware. The same emulsions of 70 mPa.sand 20 mPa.s apparent viscosities werecharacterised by tribology (Figs. 10 and 11)and sensory analysis (Table 1).The sensory score increase when theapparent viscosity of the emulsion is raisedfrom 20 to 70 mPa.s. This is in agreementwith the results of Akhtar et al. 13. With abackground of identical droplet sizedistribution, fat content and apparentviscosity, the 1DE maltodextrin is far moreefficient than xanthan in reducing thefriction factor. Dickinson et al.13 also foundthat iso-viscous emulsions (of identical fatcontent and droplet size distribution) gives abetter score with maltodextrin than xanthan.They suggest that non-rheological factorsmay enhance the perception of creaminess.
1Friction factor (-)XG5v%20v%0.1Mdx5v%20v%0.010.11101001000Sliding speed (mm/sec)Figure 10. Stribeck curves of 20 mPa.semulsions.Friction factor ng speed (mm/sec)Figure 11. Stribeck curves of 70 mPa.semulsions.Table 1. Summary of analytical data.Fat(vol%)Hydrocolloid dextrin0.19%xanthan20Viscosity(mPa.s)at 50s-128Friction .340.027810.820.233.5In particular, when panelists were asked toscorestickiness(ormouthcoating),maltodextrin give a significantlybetter score than xanthan. This work showsthat beyond rheology, friction factorprovides insights to explain the differencesin the perception of creaminess. ThePrincipal Component Analysis in this workreveals that the perceived creaminess ismore sensitive to the friction factor than theapparent viscosity.CONCLUSIONSA state of the art rheometer is obviouslynot limited on the determination of flowbehavior only. Due to the excellent speedand torque characteristics and the capabilityof setting and reading normal forces, it canbe used for applications it was initially notdesigned for. This statement is underlinedby the presented accessories for tribologyand additional holder for ball bearings. Theresults obtained in the performance tests ofthe tribological accessory on dry and oillubricatedfrictionpartnerswerereproducible and in good agreement withdata from literature. It could be shown that asingle instrument can measure frictionfactors as function of sliding speed as wellas static friction values. The temperaturedependency of the starting friction, therunning friction and the roll-out behavior ofball bearings were studied on a speciallydesigned fixture mountable on thetribological accessory.Data, obtained on food samples, showedthat the friction and lubrications propertiesof different food samples can be assessed.Models, derived from the data appeared tobe capable of predicting human sensoryattributes. Tribology could be a valuabletool in assessing the mouthfeel propertiesassociated with food systems or theircomponents. This tool should allow fooddevelopers to better optimize the mouthfeelof fluid foods and beverages.
REFERENCES1. Mezger, Th. (2006) “The RheologyHandbook”, 2nd Edition, Vincentz Network,Hannover.(2005), “Relating the sensory sensation‘creamy mouthfeel’ in custards torheological measurements”, J. Chemom., 19,191-2000.2. Malone, M.E., Appleqvist, I.A.M., andNorton, I.T. (2003), “Oral behavior of foodhydrocolloids and emulsions. Part 1:Lubrication and deposition considerations”,Food Hydrocoll., 17, 763-773.10. Heyer, P. and Läuger, J. (2009),“Correlation between friction and flow oflubricanting greases in a new tribometerdevice”, TriboTest, to be published.3. Dresselhuis, D.M., de Hoog, E.H.A.,Cohen-Stuart, M.A., and van Aken, G.A.(2008), “Application of oral tissue intribological measurements in an emulsionperception context”, Food Hydrocoll., 22(2),323-335.4. Prinz, J.F., de Wijk, R.A., and Huntjens,L. (2007), “Load dependency of thecoefficient of friction of oral mucosa”, FoodHydrocoll., 21, 402-408.5. Miller, J.L. and Watkins, K.L. (1996)“The influence of bolus volume andviscosity on anterior lingual force during theoral stage of swallowing”, Dysphagia 11,117-124.6. Lee, S., Heuberger, M., Rousset, P., andSpencer, N.D. (2002), “Chocolate at asliding interface”, J. Food Sci., 67 (7), 27122717.7. Ranc, H., Elkhyat, A., Servais, C., MacMary, S., Launay, B., and Humbert, Ph.(2006), “Friction coefficient and wettabilityof oral mucosal tissue: Changes induced bya salivary layer”, Colloids Surf. A:Physicochem. Eng. Asp., 276, 151-1668. Ranc, H., Servais, C., Chauvey, P.-F.,Debaud, S., and Mischler, S. (2006), “Effectof surface structure on frictional behavior ofa tongue and plate tribological system”,Tribol. Int., 39, 1518-1526.9. Jellema, R.H., Janssen, A.M., Terpstra,M.E.J., de Wilk, R.A., and Smilde, A.K.11. Tribology Device for Accessingmouthfeel attributes of foods, InternationalPatent Application, WO 2008/148536 A1(2008).12. Akhtar, M., Stenzel, J., Murray, B.S.,and Dickinson, E. (2005), “Factors affectingthe perception of creaminess of oil-in-wateremulsions”, Food Hydrocoll., 19, 521-526.13. Akhtar, M., Murray B.S., and DickinsonE. (2006), “Perception of creaminess ofmodel oil-in-water dairy emulsions:Influence of the shear-thinning nature of aviscosity-controlling hydrocolloid”, FoodHydrocoll., 20, 839-847.14. Dickinson, E., Akhtar M., and MurrayB.S. (2006). “Perception of the creaminessof dairy emulsions of well-defined dropletsize and controlled rheology”, Proceeding ofthe 4th International Symposium on FoodRheology and Structure P. Fischer, P. Erni& E.J. Windhab, editors, 293-297.
values at the flow point, i.e. the yield stress, are obtained1. In shear rheology the surface of the fixtures do not have any influence on the rheological data as long as the conditions of laminar flow are met. In some cases the surfaces are treated or roughened in order to prevent slip and to assure a laminar flow field.
