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  • 1. Bennett, A. I.
    et al.
    Harris, Kathryn L
    University of Florida, USA.
    Schulze, K. D.
    Urueña, J. M.
    McGhee, A. J.
    Pitenis, A. A.
    Müser, M. H.
    Angelini, T. E.
    Sawyer, W. G.
    Contact Measurements of Randomly Rough Surfaces2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 4, article id 134Article in journal (Refereed)
    Abstract [en]

    This manuscript presents an experimental effort to directly measure contact areas and the details behind these scaled experiments on a randomly rough model surface used in the “Contact Mechanics Challenge” (2017). For these experiments, the randomly rough surface model was scaled up by a factor of 1000× to give a 100 mm square sample that was 3D printed from opaque polymethylmethacrylate (PMMA). This sample was loaded against various optically smooth and transparent samples of PDMS that were approximately 15 mm thick and had a range in elastic modulus from 14 kPa to 2.1 MPa. During loading, a digital camera recorded contact locations by imaging the scattering of light that occurs off of the PMMA rough surface when it was in contact with the PDMS substrate. This method of illuminating contact areas is called frustrated total internal reflection and is performed by creating a condition of total internal reflection within the unperturbed PDMS samples. Contact or deformation of the surface results in light being diffusely transmitted from the PDMS and detected by the camera. For these experiments, a range of reduced pressure (nominal pressure/elastic modulus) from below 0.001 to over 1.0 was examined, and the resulting relative contact area (real area of contact/apparent area of contact) was found to increase from below 0.1% to over 60% at the highest pressures. The experimental uncertainties associated with experiments are discussed, and the results are compared to the numerical results from the simulation solution to the “Contact Mechanics Challenge.” The simulation results and experimental results of the relative contact areas as a function of reduced pressure are in agreement (within experimental uncertainties).

  • 2. Bennett, A. I.
    et al.
    Rohde, S.
    Harris, Kathryn L
    University of Florida, USA.
    Schulze, K. D.
    Urueña, J. M.
    Pitenis, A. A.
    Ifju, P. G.
    Angelini, T. E.
    Müser, M. H.
    Sawyer, W. G.
    Deformation Measurements of Randomly Rough Surfaces2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 4, article id 123Article in journal (Refereed)
    Abstract [en]

    Measurements of surface deformations as part of the “Contact Mechanics Challenge” were collected using digital image correlation (DIC). For these experiments, a scaled version (1000×) of the periodic and random roughness surface provided for the “Contact Mechanics Challenge” was used. A 100 mm × 100 mm scale replica of the surface, approximately 10 mm thick, was 3D-printed using an opaque polymethylmethacrylate and pressed into contact against flat, transparent polydimethylsiloxane (PDMS) sheets with dead weight loads. Four different formulations of PDMS were used, and the resulting elastic moduli ranged from 64 kPa to 2.1 MPa. The DIC technique was used in situ to measure the deformation of the PDMS surface at each load increment from 22.5 to 450 N. Surface deformations in and out of contact were measured across the entire apparent area of contact and overlaid with the measurements of contact area to provide a complete description of the surface profile during loading. A direct comparison between these experiments and the simulations regarding the gap within the contact at a reduced pressure of 0.164 agrees to within ±10% when normalized to the maximum gap. 

  • 3. Chalifour, B.
    et al.
    Hoogveld, J. R. H.
    Derksen-Hooijberg, M.
    Harris, Kathryn L
    University of Florida, USA.
    Urueña, J. M.
    Sawyer, W. G.
    van der Heide, T.
    Angelini, C.
    Drought alters the spatial distribution, grazing patterns, and radula morphology of a fungal-farming salt marsh snail2019In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 620, p. 1-13Article in journal (Refereed)
    Abstract [en]

    Climate change is altering consumer− plant interactions in ecosystems worldwide. How consumers alter their spatial distribution, grazing activities, and functional morphology in response to climate stress can determine whether their effects on plants intensify or relax. Few studies have considered multiple consumer response metrics to elucidate the mechanisms underpinning the resulting changes in consumer− plant interactions. Here, we tested how drought stress influences the interaction between the dominant consumer, the fungal-farming periwinkle snail Littoraria irrorata, and a foundational plant, cordgrass Spartina alterniflora, in a southeastern US salt marsh. In a 4 mo field experiment, we maintained moderate snail densities in mesh control chambers and clear plastic climate chambers that simulated drought by elevating temperatures and drying soils. Monitoring revealed that snails more often congregated on cordgrass stems than leaves in climate chambers than in controls. Image analyses indicated that this behavioral shift corresponded to snails inflicting shorter, but more numerous, fungal-infested scars on cordgrass leaves, and causing less plant damage in climate chambers than controls. Coincident with their net reduction in grazing, snails maintained longer radulae, whose central teeth were blunter and lateral teeth were sharper, in climate chambers compared to controls. These results suggest that under drought, snail radulae may experience less frictional wear and that, at intermediate densities, snail−cordgrass interactions relax. Together with prior research showing that at high densities, snails can denude cordgrass during drought, we conclude that con-Saltmarsh snails (top left) stressed by drought conditions show reduced radula wear and shifts in tooth morphology (e.g. sharper lateral teeth; bottom right), suggesting that fewer resources are invested in maintaining the grazing apparatus. Climate change is expected to increase drought stress. © The authors 2019.

  • 4.
    Collier, Elizabeth S
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. Linköping University, Sweden.
    Costa, Elena
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. Gothenburg University, Sweden.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Bendtsen, Marcus
    Linköping University, Sweden.
    Niimi, Jun
    RISE Research Institutes of Sweden, Bioeconomy and Health, Agriculture and Food.
    Still just a matter of taste?: Sensorial appreciation of seafood is associated with more frequent and diverse consumption2024In: Appetite, ISSN 0195-6663, E-ISSN 1095-8304, Vol. 198, article id 107369Article in journal (Refereed)
    Abstract [en]

    Improving health and sustainability outcomes in WEIRD (Western, Educated, Industrial, Rich, Democratic) nations necessitates a reduction in red meat consumption. Seafood is often overlooked in achieving this goal. However, simply consuming more of familiar fish species places high stress on production of these species. For this reason, diversification of seafood consumption is also critical. Here the motives for seafood consumption (frequency and diversity) are investigated across two studies by adapting the 4Ns survey to the seafood category. This 16-item survey measures four factors underpinning meat consumption: namely that it is ‘Natural’, ‘Necessary’, ‘Normal’ and ‘Nice’. Swedish consumers’ hedonic and sensory expectations of two herring concepts (traditional pickled contra novel minced and presented as a burger) are also evaluated in relation to the 4Ns. Study 1 (N = 304) revealed that the seafood 4Ns scale had a similar underlying structure to that of meat and had good test-retest reliability. Study 2 (N = 514) showed that consumers expected to like the pickled herring (associated with being ‘seasoned’, ‘salty’, ‘sweet’, ‘firm’, ‘juicy’, ‘chewy’, and ‘slimy’) more than the minced herring (associated with being ‘mushy’, ‘fishy’, ‘grainy’, ‘dry’ and having ‘small bones’), and that ‘Nice’ scores affected expectations of both herring concepts. Food neophobia correlated inversely with seafood consumption frequency, expected liking, the ‘Nice’ subscale, and food agency. Critically, in both studies, enjoyment of seafood (higher ‘Nice’ scores) predicted more frequent and diverse seafood consumption, whilst agreeing that seafood is ‘Necessary’ for health predicted only consumption frequency, not diversity. Communicating the positive sensory attributes of seafood and developing novel product concepts in ways that disconfirm sceptical consumers’ negative sensory expectations may increase acceptance of both familiar and unfamiliar seafood concepts. © 2024 The Authors

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  • 5.
    Collier, Elizabeth S
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. Linköping University, Sweden.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Bendtsen, Marcus
    Linköping University, Sweden.
    Norman, Cecilia
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Niimi, Jun
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Just a matter of taste?: Understanding rationalizations for dairy consumption and their associations with sensory expectations of plant-based milk alternatives2023In: Food Quality and Preference, ISSN 0950-3293, E-ISSN 1873-6343, Vol. 104, article id 104745Article in journal (Refereed)
    Abstract [en]

    Although knowledge surrounding the obstacles omnivorous consumers face when substituting meat products with plant-based alternatives has increased dramatically, less is known about their perspectives on plant-based alternatives to dairy products. Here, these perspectives are assessed in two survey-based studies. Study 1 (N = 175) adapts an existing scale (the 4Ns of meat consumption) to dairy products in an effort to identify similarities and differences between rationalizations for meat and dairy consumption. This 16-item scale quantifies four factors (Natural, Necessary, Normal, and Nice) describing common rationalizations for meat consumption. The results revealed that the 4Ns transfer well to the dairy category, and that endorsement of dairy products as Nice was the strongest predictor of dairy consumption, relative to the other 3Ns. This is further supported by evaluation of consumers’ own qualitative descriptions of why they do or do not consume meat/dairy products, where “taste” was the most frequently used word in both categories. Study 2 replicates the relationships between dairy 4Ns scores and reported dairy consumption found in Study 1 and builds upon these results by showing that 4N score could accurately categorize consumers as frequent (N = 192) or infrequent (N = 210) consumers of plant-based milk alternatives (PBMAs). Differences in consumers’ expectations for the sensory characteristics of cow’s milk and PBMAs are identified, and the impact of rationalization (total 4N score) on the likelihood of expected sensory attribute associations is described. The role of rationalization in shaping sensory expectations and impacting dietary choices, in particular resistance to adopting PBMAs, is discussed.

  • 6.
    Collier, Elizabeth S
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. Linköping University, Sweden.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Jecks, Michael
    Independent Scholar, UK.
    Bendtsen, Marcus
    Linköping University, Sweden.
    Don’t throw the individual perspective out while waiting for systemic change2023In: Behavioral and Brain Sciences, ISSN 0140-525X, E-ISSN 1469-1825, Vol. 46, article id e154Article in journal (Refereed)
    Abstract [en]

    Although it is clear that i-frame approaches cannot stand alone, the impact of s-frame changes can plateau. Combinations of these approaches may best reflect what we know about behavior and how to support behavioral change. Interactions between i-frame and s-frame thinking are explored here using two examples: alcohol consumption and meat consumption.

  • 7.
    Collier, Elizabeth S
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. Linköping University, Sweden.
    Normann, Anne
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Oberrauter, Lisa-Maria
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Bergman, Penny
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Making More Sustainable Food Choices One Meal at a Time: Psychological and Practical Aspects of Meat Reduction and Substitution2022In: Foods, E-ISSN 2304-8158, Vol. 11, no 9, article id 1182Article in journal (Refereed)
    Abstract [en]

    Switching out meat in favour of plant-based alternatives such as meat substitutes is an important step towards eating more sustainably. Here, the aim was to identify and explore the specific barriers experienced by Swedish consumers when replacing meat with more sustainable alternatives. All meat-eating participants in this study reported some interest in reducing their meat consumption. Aspects of home-use and central-location test methods were combined by using a digital conferencing system to host cooking sessions and focus group discussions online, which was shown to be a viable setup even in this hands-on setting. The discussions targeted participants’ experience preparing meals using meat substitutes as well as their perceived motivators and barriers to reducing meat consumption. Four themes identified through thematic analysis indicated that meat-eating participants, despite their desire or intent to reduce their meat consumption, experienced barriers relating to the following: internal conflict due to holding multiple positive and negative beliefs about meat simultaneously (ambivalence), justification of eating meat (rationalisation), a desire for variety in and control over their food choices (agency), and sensitivity to the views and expectations of other people and the situational context regarding meat (social and structural factors). Possible strategies to support ambivalent individuals in aligning their behaviour with their beliefs instead of vice versa are discussed in the context of the meat paradox. Agency and practical skills, including increasing knowledge in preparing meals with plant-based proteins, likely play a role in bridging this intention–behaviour gap. © 2022 by the authors. 

  • 8.
    Harris, Kathryn L
    University of Florida, USA.
    TRIBOCHEMICAL INTERACTIONS OF A PTFE/ALPHA ALUMINA COMPOSITE AT THE SLIDING INTERFACE: A MECHANISM FOR ULTRA LOW WEAR2016Doctoral thesis, monograph (Other academic)
    Abstract [en]

    The wear and friction behavior of ultralow wear polytetrafluoroethylene (PTFE)/αalumina composites first described by Burris and Sawyer in 2006 has been studied intensively in the years hence. The mechanisms responsible for the remarkable improvement in wear over unfilled PTFE are not yet fully understood. The formation of tribofilms on the countersurface and the running face of the polymer is crucial to the ultra-low wear behavior of the composite on a metal countersurface. The complete chemical mechanism of transfer film formation and adhesion, and its role in the exceptional wear performance has yet to be elucidated. Some debate exists regarding the role of chemical interactions between the PTFE, the filler, and the metal countersurface. Some have concluded that chemical changes are not an important part of the ultralow wear mechanism in these materials at all. A “stripe” test allowed comprehensive spectroscopic studies of PTFE/α-alumina transfer films in various stages throughout development and led to a proposed mechanism which details the initiation and adhesion of the tribofilms formed on both surfaces of the wear pair. PTFE chains (carbon-carbon bonds) are broken mechanically during sliding and undergo a cascade of reactions to produce carboxylate chain ends that chelate to the metal surface and to the surface of e porous, friable alumina filler particles. This tribochemical process forms a robust polymeron-polymer system that protects the steel countersurface from abrasion, and the polymer surface from wear. The system is able to withstand hundreds of thousands, and possibly millions of cycles of sliding with almost no wear of the polymer composite after an initial period of high wear during run-in. A mathematical model in support of the hypothesis of mechanical scission of carboncarbon bonds in the backbone of PTFE in simple sliding contact is detailed, using the Hamaker model for van der Waals interactions between polymer fibrils and the countersurface (a cylinder and a flat surface). The proven necessity of ambient moisture and oxygen is explained in the mechanism, and model experiments using small molecules further support the assignment of reactions in the proposed mechanism to the processes at the sliding interface.

  • 9.
    Harris, Kathryn L
    et al.
    University of Florida, USA.
    Bennett, A. I.
    Rowe, K. G.
    Sawyer, W. G.
    Janus Blocks: A Binary System Wear Instability2016In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 63, no 1, article id 8Article in journal (Refereed)
    Abstract [en]

    In this manuscript, a simple binary model is devised that describes the wear behavior of two blocks coupled under a constant, dynamically partitioned normal load. In this simple system, the frictional force is reacted by two independent springs and the blocks are allowed to move and wear independently based on system dynamics and kinematics. The only coupling between the blocks occurs through the partitioning of the applied normal load, which uses a pair of springs in parallel to model elasticity. This system is found to preferentially wear one of the blocks until two disparately unique conditions of steady wear are reached in the system: (1) a condition in which the partitioning of the load between the blocks yields equal wear and thus steady partitioning of the load and (2) a condition in which the pair of blocks go to zero wear by having one block not sliding but carrying all of the load and the other block completely slipping but carrying none of the load. These “Janus blocks,” the simplest of binary spring–block systems, begin life in a nominally identical state and then their behavior bifurcates, producing runaway or irregular wear. The onset of this instability can initiate from any differences in load partitioning, spring constants, friction coefficient, or wear rates (no matter how small). 

  • 10.
    Harris, Kathryn L
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Collier, Elizabeth S
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Skedung, Lisa
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Rutland, Mark W.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. KTH Royal Institute of Technology, Sweden.
    A Sticky Situation or Rough Going?: Influencing Haptic Perception of Wood Coatings Through Frictional and Topographical Design2021In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 69, no 3, article id 113Article in journal (Refereed)
    Abstract [en]

    Improving the tactile aesthetics of products that can be described as touch intensive is an increasing priority within many sectors, including the furniture industry. Understanding which physical characteristics contribute to the haptic experience of a surface, and how, is therefore highly topical. It has earlier been shown that both friction and topography affect tactile perception. Thus, two series of stimuli have been produced using standard coating techniques, with systematic variation in (physical) friction and roughness properties. This was achieved through appropriate selection of matting agents and resins. The stimuli sets were then evaluated perceptually to determine the extent to which discrimination between pairs of surfaces followed the systematic materials variation. In addition to investigating the role of the physical properties in discrimination of the surfaces, their influence on perceived pleasantness and naturalness was also studied. The results indicate that changes in tactile perception can be understood in terms of friction and roughness, and that varying the matting agents (topography) and resins (material properties) in the coatings provide the controlling factors for furniture applications. Perceived pleasantness is associated with low friction and smoother topography, whilst perceived naturalness is found to be described by an interaction between tactile friction and the average maximum peak height of the surface features. Graphic Abstract: [Figure not available: see fulltext.] © 2021, The Author(s).

  • 11.
    Harris, Kathryn L
    et al.
    University of Florida, USA.
    Curry, J. F.
    Pitenis, A. A.
    Rowe, K. G.
    Sidebottom, M. A.
    Sawyer, W. G.
    Krick, B. A.
    Wear Debris Mobility, Aligned Surface Roughness, and the Low Wear Behavior of Filled Polytetrafluoroethylene2015In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 60, no 1, article id 2Article in journal (Refereed)
    Abstract [en]

    PTFE/α-alumina composites are well known to exhibit very low wear rates compared to unfilled PTFE and various other PTFE-matrix composites. The improved wear life of these composites is attributed in part to the formation of a uniform protective transfer film on the metal countersurface. It is postulated that the retention of transferred material and the recirculation of third bodies between the transfer film and running surface of the polymer composite are necessary for the maintenance of low wear within this tribological system. The accumulation of these third bodies was observed in reciprocating sliding tests on countersamples prescribed with aligned roughness. Wear performance of the polymer composite was tested as a function of the between the sliding direction and the aligned roughness of the countersample, ranging from parallel to perpendicular to the sliding direction. The wear rate of roughness oriented with the sliding direction was 300 times higher than roughness perpendicular to the sliding direction, revealing the importance of surface morphology and third body retention.

  • 12.
    Harris, Kathryn L
    et al.
    University of Florida, USA.
    Pitenis, A. A.
    Sawyer, W. G.
    Krick, B. A.
    Blackman, G. S.
    Kasprzak, D. J.
    Junk, C. P.
    PTFE Tribology and the Role of Mechanochemistry in the Development of Protective Surface Films2015In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 48, no 11, p. 3739-3745Article in journal (Refereed)
    Abstract [en]

    The wear and friction behavior of ultralow wear polytetrafluoroethylene (PTFE)/α-alumina composites first described by Burris and Sawyer in 2006 has been heavily studied, but the mechanisms responsible for the 4 orders of magnitude improvement in wear over unfilled PTFE are still not fully understood. It has been shown that the formation of a polymeric transfer film is crucial to achieving ultralow wear on a metal countersurface. However, the detailed chemical mechanism of transfer film formation and its role in the exceptional wear performance has yet to be described. There has been much debate about the role of chemical interactions between the PTFE, the filler, and the metal countersurface, and some researchers have even concluded that chemical changes are not an important part of the ultralow wear mechanism in these materials. Here, a "stripe" test allowed detailed spectroscopic studies of PTFE/α-alumina transfer films in various stages of development, which led to a proposed mechanism which accounts for the creation of chemically distinct films formed on both surfaces of the wear couple. PTFE chains are broken during sliding and undergo a series of reactions to produce carboxylate chain ends, which have been shown to chelate to both the metal surface and to the surface of the alumina filler particles. These tribochemical reactions form a robust polymer-on-polymer system that protects the steel countersurface and is able to withstand hundreds of thousands of cycles of sliding with almost no wear of the polymer composite after the initial run-in period. The mechanical scission of carbon-carbon bonds in the backbone of PTFE under conditions of sliding contact is supported mathematically using the Hamaker model for van der Waals interactions between polymer fibrils and the countersurface. The necessity for ambient moisture and oxygen is explained, and model experiments using small molecules confirm the reactions in the proposed mechanism. .

  • 13.
    Karlsson, Simon
    et al.
    RISE Research Institutes of Sweden, Safety and Transport, Measurement Technology.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Melin, Jeanette
    RISE Research Institutes of Sweden, Safety and Transport, Measurement Technology.
    Lahne, Jacob
    Virginia Polytechnic Institute and State University, USA.
    Wolfson, Julia
    John Hopkins University, USA.
    Collier, Elizabeth S
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. Linköping University, Sweden.
    An evaluation and shortening of the Cooking and Food Provisioning Action Scale (CAFPAS) using item response theory2023In: Food Quality and Preference, ISSN 0950-3293, E-ISSN 1873-6343, Vol. 108, article id 104880Article in journal (Refereed)
    Abstract [en]

    The Cooking and Food Provisioning Action Scale (CAFPAS) is a 28-item validated tool for measuring food agency, a latent construct representing an individual's ability to make and achieve food-preparation and -provisioning goals. Here, key measurement parameters (targeting, threshold ordering, item fit, unidimensionality, differential item functioning, local dependency, and person reliability) of the CAFPAS are evaluated using a specific case of item response theory, Rasch analysis, on data from a development sample (N = 1853; 910 from Sweden; 943 from the US). Winsteps (v.5.1.7) is used for this analysis. The similarity of the Swedish version of the CAFPAS to the original is also assessed. Based on an iterative assessment of the measurement properties with different combinations of items in the development sample, ways to shorten the CAFPAS without jeopardizing construct validity or person reliability are examined. After removing items that do not fit the Rasch model, or that appear redundant in relation to other items, an 11-item version (CAFPAS-short) is suggested and tested using further Rasch analysis on both the development sample and an additional US-based validation sample (N = 1457). Scores of cooking confidence and attitudes are then modelled with measures from the CAFPAS and CAFPAS-short using frequentist and Bayesian analysis. Results suggest that the CAFPAS-short performs similarly to the full-length version, and potential future improvements to the CAFPAS are discussed. This study represents a successful application of item response theory to investigate and shorten a psychometric scale, reducing cognitive load on participants in studies using the CAFPAS whilst minimizing loss of data reliability. © 2023 The Author(s)

  • 14. Krick, B. A.
    et al.
    Marchman, K. R.
    Harris, Kathryn L
    University of Florida, USA.
    Sawyer, W. G.
    Atomic origins of wear in ionic solids2013In: 5th World Tribology Congress, WTC 2013, Politecnico di Torino (DIMEAS) , 2013, p. 1479-1480Conference paper (Refereed)
  • 15. Krick, B. A.
    et al.
    Pitenis, A. A.
    Harris, Kathryn L
    University of Florida, USA.
    Junk, C. P.
    Sawyer, W. G.
    Brown, S. C.
    Rosenfeld, H. D.
    Kasprzak, D. J.
    Johnson, R. S.
    Chan, C. D.
    Blackman, G. S.
    Ultralow wear fluoropolymer composites: Nanoscale functionality from microscale fillers2016In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 95, p. 245-255Article in journal (Refereed)
    Abstract [en]

    Polytetrafluoroethylene (PTFE) filled with certain alumina additives has wear rates over four orders of magnitude lower than unfilled PTFE. The mechanisms for this wear reduction have remained a mystery. In this work, we use a combination of techniques to show that porous, nanostructured alumina microfillers (not nanofillers) are critical for this wear reduction. The microscale alumina particles break during sliding into nanoscale fragments. X-ray microtomography, transmission electron microscopy and infrared spectroscopy reveal nanoscale alumina fragments accumulated in the tribofilms. Tribochemically generated carboxylate endgroups bond to metal species in the transfer film and to alumina fragments in the surface of the polymer composite. These mechanically reinforced tribofilms create robust sliding surfaces and lead to a dramatic reduction in wear. © 2015 The Authors. 

  • 16. Leblanc, K. J.
    et al.
    Niemi, S. R.
    Bennett, A. I.
    Harris, Kathryn L
    University of Florida, USA.
    Schulze, K. D.
    Sawyer, W. G.
    Taylor, C.
    Angelini, T. E.
    Stability of High Speed 3D Printing in Liquid-Like Solids2016In: ACS Biomaterials Science & Engineering, E-ISSN 2373-9878, Vol. 2, no 10, p. 1796-1799Article in journal (Refereed)
    Abstract [en]

    Fluid instabilities limit the ability of features to hold their shape in many types of 3D printing as liquid inks solidify into written structures. By 3D printing directly into a continuum of jammed granular microgels, these instabilities are circumvented by eliminating surface tension and body forces. However, this type of 3D printing process is potentially limited by inertial instabilities if performed at high speeds where turbulence may destroy features as they are written. Here, we design and test a high-speed 3D printing experimental system to identify the instabilities that arise when an injection nozzle translates at 1 m/s. We find that the viscosity of the injected material can control the Reynold's instability, and we discover an additional, unanticipated instability near the top surface of the granular microgel medium.

  • 17. McGhee, A. J.
    et al.
    Pitenis, A. A.
    Bennett, A. I.
    Harris, Kathryn L
    University of Florida, USA.
    Schulze, K. D.
    Urueña, J. M.
    Ifju, P. G.
    Angelini, T. E.
    Müser, M. H.
    Sawyer, W. G.
    Contact and Deformation of Randomly Rough Surfaces with Varying Root-Mean-Square Gradient2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 4, article id 157Article in journal (Refereed)
    Abstract [en]

    The “Contact Mechanics Challenge” posed to the tribology community by Müser and Dapp in 2015 detailed a 100 µm × 100 µm randomly rough surface with a root-mean-square gradient of unity, g ¯ = 1. Many surfaces, both natural and synthetic, can be described as randomly rough, but rarely with a root-mean-square gradient as steep as g ¯ = 1. The selection of such a challenging surface parameter was intentional, but potentially limiting for broad comparisons across existing models and theories which may be limited by small-slope approximations. In this manuscript, the root-mean-square gradients (g ¯) of the “Contact Mechanics Challenge” surface were produced on 1000 × scaled models such that there were three different surfaces for study with g¯=0.2,0.5, and 1. In situ measurements of the real area of contact and contact area distributions were performed using frustrated total internal reflectance along with surface deformation measurements performed using digital image correlation. These optical in situ experiments used the scaled 3D-printed rough surfaces that were loaded into contact with smooth, flat, and elastic samples that were made from unfilled PDMS: (10:1) E* = 2.1 MPa Δγ = 4 mJ/m2; (20:1) E* = 0.75 MPa Δγ = 3 mJ/m2; (30:1) E* = 0.24 MPa Δγ = 2 mJ/m2. All of the loading was performed using a uniaxial load frame under force control. A Green’s function molecular dynamics simulation assuming the small-slope approximation was compared to all experimental data. These measurements reveal that decreasing root-mean-square gradient noticeably increases real area of contact area under conditions of “equal” applied load, but variations in the root-mean-square gradient did not significantly alter the contact patch geometry under conditions of nearly equal real area of contact. Including g ¯ in the reduced pressure (p= P/ (E∗ g ¯)) reduced the root-mean-square error between the simulation (g ¯ = 1) and all experimental data for the relative area of contact as a function of reduced pressure over the entire range of surfaces, materials, and loads tested.

  • 18. Müser, M. H.
    et al.
    Dapp, W. B.
    Bugnicourt, R.
    Sainsot, P.
    Lesaffre, N.
    Lubrecht, T. A.
    Persson, B. N. J.
    Harris, Kathryn L
    University of Florida, USA.
    Bennett, A.
    Schulze, K.
    Rohde, S.
    Ifju, P.
    Sawyer, W. G.
    Angelini, T.
    Ashtari Esfahani, H.
    Kadkhodaei, M.
    Akbarzadeh, S.
    Wu, J. -J
    Vorlaufer, G.
    Vernes, A.
    Solhjoo, S.
    Vakis, A. I.
    Jackson, R. L.
    Xu, Y.
    Streator, J.
    Rostami, A.
    Dini, D.
    Medina, S.
    Carbone, G.
    Bottiglione, F.
    Afferrante, L.
    Monti, J.
    Pastewka, L.
    Robbins, M. O.
    Greenwood, J. A.
    Meeting the Contact-Mechanics Challenge2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 4, article id 118Article in journal (Refereed)
    Abstract [en]

    This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one..

  • 19.
    Pilkington, Georgia A.
    et al.
    KTH Royal Institute of Technology, Sweden.
    Harris, Kathryn L
    KTH Royal Institute of Technology, Sweden.
    Bergendal, Erik
    KTH Royal Institute of Technology, Sweden.
    Reddy, Akepati Bhaskar
    KTH Royal Institute of Technology, Sweden.
    Palsson, Gunnar K.
    Uppsala University, Sweden.
    Vorobiev, Alexei
    Uppsala University, Sweden.
    Antzutkin, Oleg N.
    Luleå University of Technology, Sweden.
    Glavatskih, Sergei
    KTH Royal Institute of Technology, Sweden ; Ghent University, Belgium.
    Rutland, Mark W.
    RISE - Research Institutes of Sweden (2017-2019), Bioscience and Materials, Chemistry and Materials. KTH Royal Institute of Technology, Sweden.
    Electro-responsivity of ionic liquid boundary layers in a polar solvent revealed by neutron reflectance2018In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 148, no 19, article id 193806Article in journal (Refereed)
    Abstract [en]

    Using neutron reflectivity, the electro-responsive structuring of the non-halogenated ionic liquid (IL) trihexyl(tetradecyl)phosphonium-bis(mandelato)borate, [P6,6,6,14][BMB], has been studied at a gold electrode surface in a polar solvent. For a 20% w/w IL mixture, contrast matched to the gold surface, distinct Kiessig fringes were observed for all potentials studied, indicative of a boundary layer of different composition to that of the bulk IL-solvent mixture. With applied potential, the amplitudes of the fringes from the gold-boundary layer interface varied systematically. These changes are attributable to the differing ratios of cations and anions in the boundary layer, leading to a greater or diminished contrast with the gold electrode, depending on the individual ion scattering length densities. Such electro-responsive changes were also evident in the reflectivities measured for the pure IL and a less concentrated (5% w/w) IL-solvent mixture at the same applied potentials, but gave rise to less pronounced changes. These measurements, therefore, demonstrate the enhanced sensitivity achieved by contrast matching the bulk solution and that the structure of the IL boundary layers formed in mixtures is strongly influenced by the bulk concentration. Together these results represent an important step in characterising IL boundary layers in IL-solvent mixtures and provide clear evidence of electro-responsive structuring of IL ions in their solutions with applied potential.

  • 20. Pitenis, A. A.
    et al.
    Ewin, J. J.
    Harris, Kathryn L
    University of Florida, USA.
    Sawyer, W. G.
    Krick, B. A.
    In vacuo tribological behavior of polytetrafluoroethylene (ptfe) and alumina nanocomposites: The importance of water for ultralow wear2014In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 53, no 1, p. 189-197Article in journal (Refereed)
    Abstract [en]

    Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many practical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of magnitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 9 10-6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1-10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 9 10-7 mm3/(Nm)

  • 21. Pitenis, A. A.
    et al.
    Harris, Kathryn L
    University of Florida, USA.
    Junk, C. P.
    Blackman, G. S.
    Sawyer, W. G.
    Krick, B. A.
    Ultralow wear PTFE and alumina composites: It is all about tribochemistry2015In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 57, no 2, article id 4Article in journal (Refereed)
    Abstract [en]

    Over the last decade, researchers have explored an intriguing polymer composite composed of granular polytetrafluoroethylene (PTFE) 7C and alumina particles. This material is extraordinary because a very small amount of alumina additive (<5 wt%) decreased the wear rate of the PTFE composite by over four orders of magnitude. Previous studies have shown that this wear resistance was initiated and maintained by the formation of a stable, robust, and uniform polymeric transfer film on the surface of the countersample. Although its importance to this tribological system is clear, the transfer film itself has not been well understood. Careful spectroscopic analysis throughout the stages of transfer film development revealed that tribochemistry plays a major role in the significant wear rate reductions achieved in PTFE and alumina composites. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy reveal that PTFE chains break due to the mechanical stresses at the wear surface and, in the presence of oxygen and water in the ambient environment, produce carboxylic acid end groups. These carboxylic acid end groups can chelate to the exposed metal on the steel surface and nucleate the formation of the transfer film. The resulting thin and robust fluoropolymer transfer film protects the surface of the steel and changes the sliding interface from polymer on steel to polymer on polymer transfer film. These effects keep friction coefficients and wear rates low and stable. Ultimately, the real mechanisms responsible for the exceptional wear performance of these materials are all about the tribochemistry. 

  • 22. Rohde, S. E.
    et al.
    Bennett, A. I.
    Harris, Kathryn L
    University of Florida, USA.
    Ifju, P. G.
    Angelini, T. E.
    Sawyer, W. G.
    Measuring Contact Mechanics Deformations Using DIC through a Transparent Medium2017In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 57, no 9, p. 1445-1455Article in journal (Refereed)
    Abstract [en]

    This paper describes the experimental methodology used to study the contact mechanics of a rigid, rough surface and a compliant, nominally flat surface using digital image correlation (DIC). The rough surface was produced by 3-D printing PMMA and the flat surface was produced with transparent PDMS (silicone rubber). The deformation of the speckled top surface (contact) of the PDMS was measured via DIC viewed through the transparent media. Four different PDMS formulations with moduli ranging from 64 to 2120 kPa were used in the experiment program to cover a wide range of modulus normalized loads. The deformation of the contact surface and depth of penetration versus normalized load were measured. The results were overlaid with previous measurements of contact area and complemented them extremely well. Additionally, it was shown that scaling laws associated with such contact mechanics problems extend many length scales. 

  • 23. Rowe, K. G.
    et al.
    Harris, Kathryn L
    University of Florida, USA.
    Schulze, K. D.
    Marshall, S. L.
    Pitenis, A. A.
    Urueña, J. M.
    Niemi, S. R.
    Bennett, A. I.
    Dunn, A. C.
    Angelini, T. E.
    Sawyer, W. G.
    Lessons from the lollipop: Biotribology, tribocorrosion, and irregular surfaces2014In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 56, no 2, p. 273-280Article in journal (Refereed)
    Abstract [en]

    Biotribology and tribocorrosion are often not included in numerical or computational modeling efforts to predict wear because of the apparent complexity in the geometry, the variability in removal rates, and the challenge associated with mixing time-dependent removal processes such as corrosion with cyclic material removal from wear. The lollipop is an accessible bio-tribocorrosion problem that is well known but underexplored scientifically as a tribocorrosion process. Stress-assisted dissolution was found to be the dominant tribocorrosion process driving material removal in this system. A model of material removal was described and approached by lumping the intrinsically time-dependent process with a mechanically driven process into a single cyclic volumetric material removal rate. This required the collection of self-reported wear data from 58 participants that were used in conjunction with statistical analysis of actual lollipop cross-sectional information. Thousands of repeated numerical simulations of material removal and shape evolution were conducted using a simple Monte Carlo process that varied the input parameters and geometries to match the measured variability. The resulting computations were analyzed to calculate both the average number of licks required to reach the Tootsie Roll® center of a Tootsie Roll® pop, as well as the expected variation thereof.

  • 24. Shu, Ju
    et al.
    Harris, Kathryn L
    KTH Royal Institute of Technology, Sweden.
    Munavirov, Bulat
    Westbroek, Rene
    Leckner, Johan
    Glavatskih, Sergei
    Tribology of polypropylene and Li-complex greases with ZDDP and MoDTC additives2018In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 118, p. 189-195Article in journal (Refereed)
    Abstract [en]

    The influence of thickener and additive interactions on grease lubricating performance is examined. Polypropylene and lithium complex thickened (Li-complex) greases were tested both as neat greases and with a 2 wt% addition of ZDDP and/or MoDTC. A combination of ZDDP and MoDTC in the polypropylene grease provided the lowest friction with greater longevity compared to the Li-complex grease with the same additives, independent of sliding speed, contact pressure, temperature or type of sliding: continuous vs. reciprocating. The additive combination of ZDDP and MoDTC provided the best antiwear performance in both greases. Depending on the grease sample type, EDS revealed the presence of iron, zinc, phosphorous, sulfur, and molybdenum within the tribofilms.

  • 25. Sidebottom, M. A.
    et al.
    Pitenis, A. A.
    Junk, C. P.
    Kasprzak, D. J.
    Blackman, G. S.
    Burch, H. E.
    Harris, Kathryn L
    University of Florida, USA.
    Sawyer, W. G.
    Krick, B. A.
    Ultralow wear Perfluoroalkoxy (PFA) and alumina composites2016In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 362-363, p. 179-185Article in journal (Refereed)
    Abstract [en]

    Fluoropolymers have unique mechanical, chemical, and tribological properties (low friction coefficients) but their use as solid lubricants is inhibited by high wear rates (1-5×10-4 mm3/Nm). The addition of certain types of α-alumina has been shown to reduce the wear rate of PTFE by over three orders of magnitude, but due to its extremely high molecular weight PTFE cannot be screw injection molded. However, PFA, a perfluorinated copolymer of tetrafluoroethylene (TFE) and a perfluorinated alkylvinyl ether (PAVE), can be. Teflon® PFA 340 samples with various weight fractions of α-alumina (0%, 5%, 7.5%, 10%) were injection molded, and samples from each mold were wear tested against stainless steel (P=6.3 MPa, v=50.8 mm/s). Experiments showed that the friction behavior of the PFA 340-α alumina composite was very close to that of both unfilled PFA 340 and PTFE-α alumina composites. The wear rate of unfilled PFA 340 was 1.4×10-4 mm3/Nm, and dropped to 4.0×10-8 mm3/Nm for the PFA-α alumina composites. Just as in the case of PTFE-α alumina composites, these PFA composites generated brown-colored tribofilms on both the polymer and metal surfaces, which were indicative of tribochemical changes. ATR-IR and FTIR spectra of each surface showed evidence for the generation of perfluorinated carboxylate salts and waters of hydration. This spectral similarity between PTFE and PFA 340 samples shows that the same tribological mechanism found in PTFE-α alumina composites is responsible for ultralow wear in PFA-α alumina composites as well. 

  • 26.
    Skedung, Lisa
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Collier, Elizabeth S
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Wallqvist, Viveca
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Nyhus, Anne Kari
    Microbeads, Norway.
    Björndal, Lene
    Microbeads, Norway.
    FINE-TUNING THE TACTILE PERCEPTION OF COATINGS2021In: European Coatings Journal, ISSN 0930-3847, Vol. 6, p. 32-37Article in journal (Other academic)
    Abstract [en]

    Human tactile evaluations were combined with tactile friction measurements to quantify the perceptual experience of touching coated panels. Monosized beads of nine different polymer compositions were added to a soft-touch waterborne two-component PUR coating. Introducing beads of different composition affected tactile perception.

  • 27.
    Skedung, Lisa
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Collier, Elizabeth S
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Arvidsson, Martin
    RISE - Research Institutes of Sweden (2017-2019).
    Wäckerlin, Aneliia
    Glas Trösch AG, Switzerland.
    Haag, Walter
    Glas Trösch AG, Switzerland.
    Bieri, Marco
    Glas Trösch AG, Switzerland.
    Romanyuk, Andriy
    Glas Trösch AG, Switzerland.
    Rutland, Mark W.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. KTH Royal Institute of Technology, Sweden.
    Feeling smooth: Psychotribological probing of molecular composition2018In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 66, no 4, p. 1-10, article id 138Article in journal (Refereed)
    Abstract [en]

    The aim of this study was to evaluate whether smooth surfaces varying in surface chemistry could be perceptually distinguished with the sense of touch. A set of ten glass surfaces was prepared which varied systematically in terms of the molecular composition of a thin coating of low topography. The contact angle, contact angle hysteresis, and surface energy were evaluated as objective physical parameters characterizing each coating. Additionally, the interaction forces between a human finger and the different coatings were quantified and compared in terms of tactile friction coefficients. The surfaces were evaluated psychophysically in terms of perceived similarities and were then ranked according to pleasantness. The participants could perceptually distinguish between surfaces varying in surface chemistry and a primary and secondary perceptual dimension were identified as sufficient to distinguish them. The primary dimension correlates with surface free energy, but both tactile friction and surface energy contribute to this dimension depending on whether the coatings are organic or inorganic. The secondary dimension could not be identified explicitly in terms of a physical quantity but is discussed in terms of recent developments in the literature. Coated glass is characterized by high friction coefficient upon interaction with a human finger as well as significant hysteresis in the stroking directions (lower applied load and higher friction in the backward stroke). Despite the complexity of the tribology, pleasantness can be clearly linked to it, where low friction (high contact angle) materials receive a higher ranking. © The Author(s) 2018.

  • 28.
    Skedung, Lisa
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Hörlin, Elizabeth
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Rutland, Mark W.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. KTH Royal Institute of Technology, Sweden.
    The finishing touches: the role of friction and roughness in haptic perception of surface coatings.2020In: Experimental Brain Research, ISSN 0014-4819, E-ISSN 1432-1106, Vol. 238, p. 1511-1524Article in journal (Refereed)
    Abstract [en]

    Humans are extraordinarily skilled in the tactile evaluation of, and differentiation between, surfaces. The chemical and mechanical properties of these surfaces are translated into tactile signals during haptic exploration by mechanoreceptors in our skin, which are specialized to respond to different types of temporal and mechanical stimulation. Describing the effects of measurable physical characteristics on the human response to tactile exploration of surfaces is of great interest to manufacturers of household materials so that the haptic experience can be considered during design, product development and quality control. In this study, methods from psychophysics and materials science are combined to advance current understanding of which physical properties affect tactile perception of a range of furniture surfaces, i.e., foils and coatings, thus creating a tactile map of the furniture product landscape. Participants' responses in a similarity scaling task were analyzed using INDSCAL from which three haptic dimensions were identified. Results show that specific roughness parameters, tactile friction and vibrational information, as characterized by a stylus profilometer, a Forceboard, and a biomimetic synthetic finger, are important for tactile differentiation and preferences of these surface treatments. The obtained dimensions are described as distinct combinations of the surface properties characterized, rather than as 'roughness' or 'friction' independently. Preferences by touch were related to the roughness, friction and thermal properties of the surfaces. The results both complement and advance current understanding of how roughness and friction relate to tactile perception of surfaces.

  • 29.
    Skedung, Lisa
    et al.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Hörlin, Elizabeth
    RISE Research Institutes of Sweden.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Rutland, Mark W.
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design. KTH Royal Institute of Technology, Sweden.
    Applebaum, Mara
    L’Oréal Research and Innovation, USA.
    Greaves, Andrew
    L’Oréal Research and Innovation, France.
    Luengo, Gustavo
    L’Oréal Research and Innovation, France.
    A Curly Q: Is Frizz a Matter of Friction?2021In: Perception, ISSN 0301-0066, E-ISSN 1468-4233, Vol. 50, no 8, p. 728-732Article in journal (Refereed)
    Abstract [en]

    The oft discussed and fretted over environmental influences on hair have led to a popular consensus which suggests that elevated temperature and humidity lead to frizzier, wilder hair. However, few attempts at actually quantifying these effects have been made. Although frizziness is usually perceived visually, here the influence of variations in temperature and humidity on the tactile perception and friction of curly and straight hair were investigated. It is shown that changes in humidity may disproportionately affect perceived frizziness of curly hair by touch due to concurrent changes in the tactile friction. © The Author(s) 2021.

  • 30. Urueã, J. M.
    et al.
    Pitenis, A. A.
    Harris, Kathryn L
    University of Florida, USA.
    Sawyer, W. G.
    Evolution and wear of fluoropolymer transfer films2015In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 57, no 2, article id 9Article in journal (Refereed)
    Abstract [en]

    Polytetrafluoroethylene (PTFE) is a solid lubricant known for its low friction coefficient and high wear rate. When filled with a low volume percent of alumina particles (5 wt%), its wear rate is decreased over four orders of magnitude. The development of a thin, uniform and well adhered transfer film during sliding is partially responsible for this decrease in wear rate by creating a low shear interface and forming a protective layer between the PTFE/alumina sample and metal countersample. In this work, a ''striped'' transfer film was generated by sliding up to one million reversals over a gradually decreasing stroke length. Wear and friction experiments were performed on a microtribometer to determine the robustness of the transfer film. Interferometry and profilometry were used to measure the height and wear of the film. Microscopy was used to investigate the morphology of the transfer film over sliding distance. 

  • 31.
    Van Meter, Kyie
    et al.
    Florida A&M University, USA.
    Pitenis, Angela
    University of California, USA.
    Harris, Kathryn L
    RISE Research Institutes of Sweden, Bioeconomy and Health, Material and Surface Design.
    Sawyer, W Gregory
    University of Florida, USA.
    Krick, Brandon
    Florida A&M University, USA.
    Contact pressure dependent mechanisms of ultralow wear PTFE composites2023In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 152, article id 204715Article in journal (Refereed)
    Abstract [en]

    One of the most dramatic reductions in the wear of PTFE has been achieved by compositing PTFE with as little as 1–5 wt% of alumina particles; this has been reported to produce wear rates K ∼10−7 mm3/Nm. The mechanisms associated with this reduction in wear are multifaceted, including 1) preventing crack propagation and delamination of the PTFE wear surface, 2) promoting tribochemistry and more recently 3) tribologically-induced breaking of the filler into nanoscale fragments to stabilize and reinforce tribofilms. However, in an effort to keep experiments comparable, many of the studies throughout the literature have focused on a narrow contact pressure range. In these experiments, we explored the effects of contact pressure on the tribological behavior of different PTFE and alumina composites, one of which is reported to achieve ultra-low wear (∼10−7 mm3/Nm) and another that is reported to only have mild reductions in wear (∼1 × 10−5 mm3/Nm) compared to unfilled PTFE (∼4 × 10−4 mm3/Nm). We found that with decreased contact pressures, the PTFE-alumina composite that was previously reported as high wear could achieve ultralow wear rates. The PTFE-alumina composite previously reported to achieve ultralow wear achieved ultralow wear at a range of low to high contact pressures, with a higher pressure limit corresponding to increases in wear. The friction behavior of PTFE-alumina composites was found to be highly dependent on contact pressure, with increasing pressures resulting in decreasing friction coefficients (∼0.5–0.17 over a 0.62–8.5 MPa range). This effect became more pronounced when the contact pressure was incrementally varied during testing resulting in up to a 70% decrease or increase in friction coefficient due to increasing or decreasing the pressure, respectively. IR spectra of the polymer wear surface showed that tribofilms rich in carboxylates and metal oxides form at the full range of contact pressures tested, even at the extremes. This formation of tribofilms at the sliding interface not only contributes to the ultralow wear of these materials, but plays a role in the friction behavior observed. From this, we gained new insight into the role, functionality and limitations of the alumina fillers. 

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