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Tactile Perception: Role of Friction and Texture
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tactile perception is considered an important contributor to the overall consumer experience of a product. However, what physical properties that create the specifics of tactile perception, are still not completely understood. This thesis has researched how many dimensions that are required to differentiate the surfaces perceptually, and then tried to explain these dimensions in terms of physical properties, by interconnecting human perception measurements with various physical measurements. The tactile perception was assessed by multidimensional scaling or magnitude estimation, in which methods human participants assign numbers to how similar pairs of surfaces are perceived or to the relative quantity of a specified perceptual attribute, such as softness, smoothness, coarseness and coolness. The role of friction and surface texture in tactile perception was investigated in particular detail, because typically tactile exploration involves moving (at least) one finger over a textured surface. A tactile approach for measuring friction was developed by means of moving a finger over the surfaces, mounted on a force sensor. The contribution of finger friction to tactile perception was investigated for surfaces of printing papers and tissue papers, as well as for model surfaces with controlled topography. The overarching research goal of this thesis was to study, systematically, the role of texture in tactile perception of surfaces.

The model surfaces displayed a sinusoidal texture with a characteristic wavelength and amplitude, fabricated by surface wrinkling and replica molding techniques. A library of surfaces was manufactured, ranging in wavelengths from 270 nm up to 100 µm and in amplitudes from 7 nm up to 6 µm. These surfaces were rigid and cleanable and could therefore be reused among the participants. To my knowledge, this is the first time in a psychophysical experiment, that the surface texture has been controlled over several orders of magnitude in length scale, without simultaneously changing other material properties of the stimuli.

The finger friction coefficient was found to decrease with increasing aspect ratio (amplitude/wavelength) of the model surfaces and also with increasing average surface roughness of the printing papers. Analytical modeling of the finger’s interaction with the model surfaces shows how the friction coefficient increases with the real contact area, and that the friction mechanism is the same on both the nanoscale and microscale. The same interaction mechanism also explains the friction characteristics of tissue paper. Furthermore, it was found that the perceptions of smoothness, coarseness, coolness and dryness are satisfactorily related to the real contact area at the finger-surface interface. 

It is shown that it is possible to discern perceptually among both printing papers and tissue papers, and this differentiation is based on either two or three underlying dimensions. Rough/smooth and thin/thick were the two main dimensions of surface feel found for the printing papers, whereas friction and wavelength were strongly related to the perceptual cues employed in scaling the model surfaces. These experimental results support the duplex theory of texture perception, which holds that both a “spatial sense”; used to discriminate the roughest textures from the others, and a “vibration sense”; used to discriminate among the smoother textures, are involved. The perception of what is considered rough and smooth depends on the experimental stimulus context. It is concluded that friction is important for human differentiation of surface textures below about 10 µm in surface roughness, and for larger surface textures, friction is less important or can even be neglected.

The finger friction experiments also allowed the following conclusions to be drawn: (i) The interindividual variation in friction coefficients is too large to allow direct comparison; however, the trends in relative friction coefficients for a group of participants are the same. (ii) Lipids are transferred to the test surface of study, and this lowers the friction. (iii) Many of the studies point to a characteristic frequency during sliding of about 30 Hz, which is both characteristic of the resonance frequency of skin and the expected frequency associated with the fingerprints. (iv) The applied load in surface interrogation is in fact regulated in response to the friction force.

The limits in tactile perception were indirectly researched by similarity scaling experiments on the model surfaces. Wrinkle wavelengths of 760 nm and 870 nm could be discriminated from untextured reference surfaces, whereas 270 nm could not. The amplitude of the wrinkles so discriminated was approximately 10 nm, suggesting that nanotechnology may well have a role to play in haptics and tactile perception.

Abstract [sv]

Taktil perception bidrar starkt till den sammantagna upplevelsen av en produkt, men hur materials olika ytegenskaper påverkar och styr perceptionen är ännu inte helt klart. Den här avhandlingen undersöker hur många och vilka egenskaper som är viktiga när känslan mellan två ytor jämförs. Tillvägagångssättet är tvärvetenskapligt där fysikaliska mätningar kopplas ihop med perceptions mätningar där människor används som instrument. Två typer av perceptionsförsök har utförts, multidimensionell skalning där försökspersoner sätter siffror på hur lika två ytor känns, samt magnitud estimation där i stället intensiteten på specifika perceptuella storheter som t.ex. upplevt lenhet, upplevd mjukhet och upplevd strävhet bedömdes. Eftersom taktil perception innebär kontakt samt relativ rörelse mellan hud och ytor, har fokus i avhandlingen varit att undersöka hur friktion och ytans struktur (ytråhet) påverkar och bidrar till den taktila perceptionen. Förutom fysikaliska mätningar på friktion och ytstruktur har värmekonduktivitet, mjukhet samt olika standard mätningar inom pappersindustrin mätts. En metod för att mäta friktion mellan ett finger och olika ytor har utvecklats för att i möjligaste mån återspegla friktionskomponenten i upplevt taktil perception. Friktionskoefficienter beräknades och jämfördes mellan alla ytor. De stimuli som har studerats är tryckpapper och mjukpapper samt modellytor, gjorda för att systematiskt undersöka hur ytstruktur påverkar perceptionen.

Tillverkningsmetoden för modellytorna valdes så att ytorna var tåliga och kunde tvättas och därmed återanvändas. Strukturen på ytorna bestod av ett vågformat mönster där våglängden varierade mellan 270 nm och 100 µm och amplituden mellan 7 nm och 6 µm. Enligt vår vetskap är det första gången som strukturer i de här skalorna har gjorts utan att samtidigt ändra andra material egenskaper.

Friktionskoefficienten minskade med ökad kvot mellan amplituden och våglängden på modellytorna samt med ytråheten på tryckpappren. En analytisk modell tillämpades på kontakten mellan ett finger och ytorna som visade att friktionskoefficienten beror av den verkliga kontaktarean. För de mycket grövre mjukpappren uppmättes inga stora skillnader i friktion förmodligen för att kontakarean mellan de olika mjukpapprena var lika. Den faktiska kontakarean visade sig också vara viktig för perceptionen av lenhet, strävhet, torrhet och svalhet.

Det visade sig vara en stor perceptuell skillnad mellan olika typer av tryckpapper och mjukpapper utifrån hur stimuli placerade sig på en taktil karta. För de tre materialen användes enbart två alternativt tre egenskaper hos materialet för att särskilja mellan alla olika par. För tryckpapper verkade en viktig dimension kunna beskrivas av alla de perceptuella och fysikaliska egenskaper som har med kontaktarean att göra, d.v.s. lenhet, svalhet, torrhet, ytråhet, värmekonduktivitet samt friktion. För att taktilt särskilja mellan olika ytor där bara strukturen är varierade, kunde friktion och våglängden relateras till spridningen i kartan. Båda studierna stödjer duplex theory of texture perception, där ett spatialt sinne används för att särskilja en av de grövre ytorna från en slät, och ett vibrationssinne för att särskilja mellan olika släta strukturer. Friktionen visade sig alltså vara en viktig fysikalisk egenskap för strukturer under åtminstone 10 µm i ytråhet.

Från fingerfriktions mätningar kunde även följande slutsatser dras: (i) Stora skillnader i friktionskoefficient mellan olika personer uppmättes, men trenderna mellan olika individer var samma, vilket gör att relativa skillnader i friktion från en individ är representativa. (ii) Lipider (fingerfett) som överförs från fingret till ytan vid kontakt sänker friktionen. (iii) Frekvensinnehållet i friktionskraften varierar mellan olika ytor och den frekvenstopp som ses vid 30 Hz kan möjligtvis bero på fingrets struktur eller resonansfrekvensen på huden. (iv) Den pålagda kraften under en friktionsmätning visar sig omedvetet regleras av den friktionskraft som fingret möter under rörelse. 

Hur små strukturer som kan diskrimineras har indirekt undersökts genom likhetsförsöket på modellytorna där försökspersoner skulle bedöma hur lika alla par av ytor kändes. Resultaten visade att ytorna med våglängder på 760 nm och 870 nm upplevdes olika jämfört med referens ytor utan något systematiskt mönster, medan ytan med 270 nm i våglängd inte kunde särskiljas. Amplituden på ytan som kunde diskrimineras var endast ca 10 nm, vilket indikerar att nanoteknologi mycket väl kan bidra inom haptiken och för att i framtiden kontrollera den taktila perceptionen.  

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xii, 78 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:48
Keyword [en]
human skin, tactile friction, finger friction, skin friction, skin tribology, biotribology, tactile perception, haptic perception, psychophysics, haptics, surface roughness, surface texture, contact area, nanostructure, model surfaces, surface wrinkling, printing paper, tissue paper, magnitude estimation, multidimensional scaling, tactile threshold, psychophysical relations, smoothness, coolness, coarseness, softness, force sensor, skin lipids, topical formulations, skin creams
Keyword [sv]
taktil friktion, fingerfriktion, hudfriktion, hudtribologi, biotribologi, taktil perception, psykofysik, haptik, ytråhet, ytstruktur, ytveckning, nanostruktur, kontaktarea, modellytor, friktionskoefficient, kraftmätare, bestruket papper, obestruket papper, tryckpapper, mjukpapper, multidimensionell skalning, magnitud estimation, taktilt tröskelvärde, lenhet, mjukhet, svalhet, strävhet, hudlipider, topikala beredningar, hudkräm
National Category
Tribology Physical Chemistry Psychology (excluding Applied Psychology) Materials Chemistry Other Chemistry Topics
Identifiers
URN: urn:nbn:se:kth:diva-103916ISBN: 978-91-7501-499-9 (print)OAI: oai:DiVA.org:kth-103916DiVA: diva2:562422
Public defence
2012-11-16, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20121026

Available from: 2012-10-26 Created: 2012-10-24 Last updated: 2012-11-13Bibliographically approved
List of papers
1. Finger Friction Measurements on Coated and Uncoated Printing Papers
Open this publication in new window or tab >>Finger Friction Measurements on Coated and Uncoated Printing Papers
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2010 (English)In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 37, no 2, 389-399 p.Article in journal (Refereed) Published
Abstract [en]

A macroscopic finger friction device consisting of a piezoelectric force sensor was evaluated on 21 printing papers of different paper grades and grammage. Friction between a human finger and the 21 papers was measured and showed that measurements with the device can be used to discriminate a set of similar surfaces in terms of finger friction. When comparing the friction coefficients, the papers group according to paper grade and the emerging trend is that the rougher papers have a lower friction coefficient than smoother papers. This is interpreted in terms of a larger contact area in the latter case. Furthermore, a decrease in friction coefficient is noted for all papers on repeated stroking (15 cycles back and forth with the finger). Complementary experiments indicate that both mechanical and chemical modifications of the surface are responsible for this decrease: (1) X-ray photoelectron spectroscopy measurements show that lipid material is transferred from the finger to the paper surface, (2) repeated finger friction measurements on the same paper sample reveal that only partial recovery of the frictional behaviour occurs and (3) profilometry measurements before and after stroking indicate small topographical changes associated with repeated frictional contacts.

Keyword
Biotribology, Coated paper, Finger friction, Friction test methods, Paper friction, Perception, Skin friction, Surface roughness, Tactile feel, Uncoated paper
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-11930 (URN)10.1007/s11249-009-9538-z (DOI)000274222300031 ()2-s2.0-77649237221 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20110126

Available from: 2010-01-27 Created: 2010-01-26 Last updated: 2017-12-12Bibliographically approved
2. Tactile perception: Finger friction, surface roughness and perceived coarseness
Open this publication in new window or tab >>Tactile perception: Finger friction, surface roughness and perceived coarseness
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2011 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 44, no 5, 505-512 p.Article in journal (Refereed) Published
Abstract [en]

Finger friction measurements performed on a series of printing papers are evaluated to determine representativeness of a single individual. Results show occasionally large variations in friction coefficients. Noteworthy though is that the trends in friction coefficients are the same, where coated (smoother) papers display higher friction coefficients than uncoated (rougher) papers. The present study also examined the relationship between the measured friction coefficients and surface roughness to the perceived coarseness of the papers. It was found that both roughness and finger friction can be related to perceived coarseness, where group data show that perceived coarseness increases with increasing roughness.

Keyword
Finger friction, Paper friction, Roughness, Perception
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-33206 (URN)10.1016/j.triboint.2010.04.010 (DOI)000289398900001 ()2-s2.0-79952443407 (Scopus ID)
Note
QC 20110519Available from: 2011-05-19 Created: 2011-05-02 Last updated: 2017-12-11Bibliographically approved
3. Haptic perception of fine surface texture: Psychophysical interpretation of the multidimensional space
Open this publication in new window or tab >>Haptic perception of fine surface texture: Psychophysical interpretation of the multidimensional space
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(English)Manuscript (preprint) (Other academic)
National Category
Psychology (excluding Applied Psychology) Other Chemistry Topics Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-103724 (URN)
Note

QS 2012

Available from: 2012-10-24 Created: 2012-10-19 Last updated: 2012-10-26Bibliographically approved
4. Feeling small: Exploring the Tactile Perception Limits
Open this publication in new window or tab >>Feeling small: Exploring the Tactile Perception Limits
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2013 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, 2617- p.Article in journal (Refereed) Published
Abstract [en]

The human finger is exquisitely sensitive in perceiving different materials, but the question remains as to what length scales are capable of being distinguished in active touch. We combine material science with psychophysics to manufacture and haptically explore a series of topographically patterned surfaces of controlled wavelength, but identical chemistry. Strain-induced surface wrinkling and subsequent templating produced 16 surfaces with wrinkle wavelengths ranging from 300 nm to 90 mu m and amplitudes between 7 nm and 4.5 mu m. Perceived similarities of these surfaces (and two blanks) were pairwise scaled by participants, and interdistances among all stimuli were determined by individual differences scaling (INDSCAL). The tactile space thus generated and its two perceptual dimensions were directly linked to surface physical properties - the finger friction coefficient and the wrinkle wavelength. Finally, the lowest amplitude of the wrinkles so distinguished was approximately 10 nm, demonstrating that human tactile discrimination extends to the nanoscale.

Keyword
Individual-Differences, Texture-Perception, Finger Friction, Surface-Texture, Thin-Films, Touch, Discrimination, Exploration, Mechanisms, Vibrations
National Category
Psychology (excluding Applied Psychology) Polymer Chemistry Other Physics Topics Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-103901 (URN)10.1038/srep02617 (DOI)000324228600001 ()2-s2.0-84884253667 (Scopus ID)
Funder
VinnovaKnowledge FoundationSwedish Research CouncilSwedish Foundation for Strategic Research
Note

QC 20131004. Updated from manuscript to article in journal.

Available from: 2012-10-24 Created: 2012-10-24 Last updated: 2017-12-07Bibliographically approved
5. Tactile friction of controlled fine surface textures: Role of real contact area and adhesion
Open this publication in new window or tab >>Tactile friction of controlled fine surface textures: Role of real contact area and adhesion
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(English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics Other Chemistry Topics Polymer Chemistry Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-103903 (URN)
Note

QS 2012

Available from: 2012-10-24 Created: 2012-10-24 Last updated: 2012-10-26Bibliographically approved
6. Robust Hydrophobic Surfaces Displaying Different Surface Roughness Scales While Maintaining the Same Wettability
Open this publication in new window or tab >>Robust Hydrophobic Surfaces Displaying Different Surface Roughness Scales While Maintaining the Same Wettability
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2011 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 27, no 13, 8153-8159 p.Article in journal (Refereed) Published
Abstract [en]

A range of surfaces coated with spherical silica particles, covering the size range from nanometer to micrometer, have been produced using Langmuir-Blodgett (LB) deposition. The particles were characterized both in suspension and in the Langmuir trough to optimize the surface preparation procedure. By limiting the particle aggregation and surface layer failures during the preparation steps, well-defined monolayers with a close-packed structure have been obtained for all particle sizes. Thus, this procedure led to structured surfaces with a characteristic variation in the amplitude and spatial roughness parameters. In order to obtain robust surfaces, a sintering protocol and an AFM-based wear test to determine the stability of the deposited surface layer were employed. Hydrophobization of the LB films followed by water contact angle measurements showed, for all tested particle sizes, the same increase in contact angle compared to the contact angle of a flat hydrophobic surface. This indicates nearly hexagonal packing and gives evidence for nearly, complete surface wetting of the surface features.

Keyword
LANGMUIR-BLODGETT-FILMS, AIR/WATER INTERFACE, SILICA PARTICLES, WENZEL, CASSIE, NANOPARTICLES, DEPOSITION, MONOLAYERS, CRYSTALS, WATER
National Category
Other Basic Medicine
Identifiers
urn:nbn:se:kth:diva-36230 (URN)10.1021/la201121p (DOI)000292124000025 ()2-s2.0-79959788273 (Scopus ID)
Note

QC 20110711

Available from: 2011-07-11 Created: 2011-07-11 Last updated: 2017-12-11Bibliographically approved
7. Tactile friction of topical formulations
Open this publication in new window or tab >>Tactile friction of topical formulations
2016 (English)In: Skin research and technology, ISSN 0909-752X, E-ISSN 1600-0846, Vol. 22, no 1, 46-54 p.Article in journal (Refereed) Published
Abstract [en]

Background: The tactile perception is essential for all types of topical formulations (cosmetic, pharmaceutical, medical device) and the possibility to predict the sensorial response by using instrumental methods instead of sensory testing would save time and cost at an early stage product development. Here, we report on an instrumental evaluation method using tactile friction measurements to estimate perceptual attributes of topical formulations. Methods: Friction was measured between an index finger and an artificial skin substrate after application of formulations using a force sensor. Both model formulations of liquid crystalline phase structures with significantly different tactile properties, as well as commercial pharmaceutical moisturizing creams being more tactile-similar, were investigated. Friction coefficients were calculated as the ratio of the friction force to the applied load. The structures of the model formulations and phase transitions as a result of water evaporation were identified using optical microscopy. Results: The friction device could distinguish friction coefficients between the phase structures, as well as the commercial creams after spreading and absorption into the substrate. In addition, phase transitions resulting in alterations in the feel of the formulations could be detected. A correlation was established between skin hydration and friction coefficient, where hydrated skin gave rise to higher friction. Also a link between skin smoothening and finger friction was established for the commercial moisturizing creams, although further investigations are needed to analyse this and correlations with other sensorial attributes in more detail. Conclusion: The present investigation shows that tactile friction measurements have potential as an alternative or complement in the evaluation of perception of topical formulations.

Place, publisher, year, edition, pages
John Wiley & Sons, 2016
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-103723 (URN)10.1111/srt.12227 (DOI)000370302100007 ()25783057 (PubMedID)2-s2.0-84952838395 (Scopus ID)
Note

Updated from Manuscript to Article.

QC 20160204. QC 20160319

Available from: 2012-10-19 Created: 2012-10-19 Last updated: 2017-12-07Bibliographically approved

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