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How do we recognize water and oil through our tactile sense?
Colloids and Surfaces B: Biointerfaces 73 (2009) 80–83
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
How do we recognize water and oil through our tactile sense? Yoshimune Nonomura a,∗ , Yuichiro Arashi a , Takashi Maeno b a b
Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering, Yamagata University, Yonezawa 992-8510, Japan Human System Design Laboratory, Graduate School of System Design and Management, Keio University, Yokohama 223-8526, Japan
a r t i c l e
i n f o
Article history: Received 26 February 2009 Received in revised form 25 April 2009 Accepted 1 May 2009 Available online 9 May 2009 Keywords: Tactile impression Friction Wettability Water Silicone oil Glass
a b s t r a c t We can distinguish water and oil through our sense of feel, because the frictional properties of a water film are characteristic. However, we found that it was difficult to distinguish water and silicone oil on a glass substrate only by its feel. Friction evaluation showed that we recognize the liquid as water when the frictional resistance is large and changeable. On a glass substrate, water and silicone oil were not distinguishable, since both had a large frictional resistance. Statistical analysis suggested the contribution of the wettability of solid substrates to their tactile feel. This finding shows that we distinguish water from oil based on the significant friction properties; it is applicable to virtual reality systems, as well as cosmetics, food, and textiles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction We can easily distinguish water from oils thorough our tactile sense, because of their characteristic tactile feel. Many cosmetic chemists believe that water is the most effective textural control agent [1]. If the physical origin of tactile feel is clarified, it will be useful in designing virtual reality systems as well as food, cosmetics, and textiles. We previously showed that water has a stick–slip feel when a human fingertip rubs a small amount [2]. Friction analysis suggests that this stick–slip feel is caused by a drastic change in the frictional resistance. This is a well-known phenomenon, for example, the acoustic emission that arises when a wet finger is slid on the rim of a wine glass [3]. The stick–slip phenomenon occurs when dynamic friction is less than static friction [4]. Although humans can recognize various substances through their tactile sense, we often make mistakes. A tactile sense is an uncertain sensation, which changes with conditions or the environment. Extreme examples are tactile illusions, e.g., the fishbone tactile illusion, in which a concave surface is recognized as convex and the rubber hands illusion, in which a dummy hand is confused with the subject’s real hand [5,6]. In the present study, we evaluated the tactile feel when water and silicone oil were applied to various solid substrates. It is expected that the feel of water and oil will change with different kinds of substrates, because the frictional properties of a liquid film
depend on the surface properties of the solid substrate. In the tactile evaluations, we focused on the ability to identify water and silicone oil. Moreover, we evaluated the frictional force added to the solid substrates to clarify the factors governing the tactile feel. 2. Experiment 2.1. Material Water was purified by a water deionizing unit (DX-15 from Kurita Water Industries Ltd.). Silicone oil, KF-96-L 1CS (dimethyl polysiloxane, with a molecular weight of about 300, kinematic viscosity of 1.0 mm2 s−1 , and surface tension of 16.9 mN m−1 ) was obtained from Shin-Etsu Chemical Co. Ltd. These liquids were applied to four substrates: glass (Matsunami glass incorporated company), aluminum (Yasutoyo trading incorporated company), polyethylene (PE, As One Corp.), and polytetrafluoroethylene (PTFE, Nippon Valqua Industries, Ltd.). These substrates were washed by a commercial cleanser containing anionic surfactants and nonionic surfactants, and were immersed in running water for 30 min to rinse off the surfactants. The contact angle of the liquids on these solid substrates and the surface roughness were measured by a CA-X contact angle meter (Kyowa Interface Science Co. Ltd.) and a VK-8500 laser microscope (Keyence Co.), respectively. 2.2. Tactile evaluations
∗ Corresponding author. Tel.: +81 238 26 3164; fax: +81 238 26 3414. E-mail address: [email protected] (Y. Nonomura). 0927-7765/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2009.05.001
Tactile evaluations were achieved as follows: silicone oil’s similarity in feel with a standard water sample was evaluated. Subjects
Y. Nonomura et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 80–83
applied them to four different solid substrates installed on the tactile evaluation system. The tactile evaluations were rated on a seven-point scale, where a score of 7 meant “exactly the same texture as water,” while a score of 1 meant “exactly the opposite texture from water.” After filling in the questionnaire, the subjects were asked orally how they made their evaluations. Blind evaluations were carried out; the subjects did not know the composition of the liquids and the solid substrates. The subjects included 14 male students and 14 female students ranging in age from 21 to 25 years. The evaluations were carried out in a quiet room at 298 K after subjects washed both hands with a commercial liquid hand soap. Six milliliters of liquid hand soap was applied to each subject’s hands, and rinsed off with running water. Before the evaluations, their hands were wiped by paper towels. Subjects used their forefingers to rub first, 0.1 ml of water, and then 0.1 ml of silicone oil on the solid substrates installed on the tactile evaluation system. After filling in the questionnaire, the subjects washed their hands with water again. The evaluation items were selected based on a preliminary test by three professional panelists. One was a researcher, who has engaged in tactile evaluations for more than 10 years, while the other two were students who had studied the tactile sense. The unit process, i.e., applying the liquid samples, filling in the questionnaire, and washing the hands, was repeated for all substrates. The order of the substrates was random to eliminate order effects. The subjects touched the artificial skin through a blackout curtain. The procedure of the test was announced beforehand. The subjects volunteered to participate in the evaluation. The responsible party at Yamagata University confirmed that the ethical standards were followed and the test was reliable. The statistical analysis was carried out using SPSS 16.0 Base System and Amos.
81
Fig. 1. Photograph of the tactile evaluation system. This device measures the friction forces and vertical stresses using strain gauges on two plate springs. Subjects rub water and silicone oil on solid substrates to evaluate tactile feels.
physical properties considered were the Young’s modulus E [9–11], surface roughness Ra , and the contact angle of substrates with water w and silicone oil s . When silicone oil was dropped on PE or PTFE, the similarity score was about 2; many subjects judged that they did not feel similar. When silicone oil was dropped on aluminum,
2.3. Friction evaluations In the present study, we used a system that simultaneously evaluated tactile sensations and frictional properties [7,8]. This device measured the frictional force and vertical stress using strain gauges on two plate springs, as shown in Fig. 1. The time resolution was 1 ms. The four solid substrates mentioned above were attached to this device. The strain gauges on two plate springs showed the linearity and reproducibility with an error less than 0.20 or 0.08 N for the frictional force and vertical stress, respectively. The maximum measurable load of the device was 5 N. 3. Results 3.1. Tactile evaluations The similarity score for silicone oil with water on each substrate, and the physical properties of each substrate are shown in Fig. 2. The
Fig. 2. Similarity scores of silicone oil on four substrates. Score 7 means “exactly the same texture as water”, while 1 means “exactly the opposite texture from water”. The physical properties of the substrates, i.e., Young’s modulus E, surface roughness Ra , contact angle of water w and contact angle of silicone oil s are also shown. Asterisks mean the results of t-tests; ** P < 0.01, *** P < 0.001.
Table 1 The counts of the substrates who mentioned the specific tactile feels for water nw and oil no . The symbol n means the difference between the counts; no − nw . Substrate
Liquid
Stick–slip
Glass
Silicone oil Water nGl
6 13 −7
Scratchy 9 9 0
Silky 6 7 −1
Smooth 0 0 0
Slimy 2 0 2
Slippery 2 2 0
Sticky 1 0 1
Aluminium
Silicone oil Water nAl
2 5 −3
1 4 −3
3 8 −5
12 4 8
10 1 9
0 2 −2
1 2 −1
PE
Silicone oil Water nPE
1 6 −5
1 9 −8
2 5 −3
12 0 12
11 0 11
2 1 1
2 0 2
PTFE
Silicone oil Water nPTFE
0 5 −5
0 6 −6
4 7 −3
10 1 9
16 2 14
3 1 2
0 0 0
82
Y. Nonomura et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 80–83
Fig. 4. A path diagram of tactile factors and physical factors; s : static friction coefficient, : difference between contact angles of water and silicone oil.
of the subjects who mentioned each tactile feel for silicone oil and water, respectively. When water was applied on a glass substrate, there were 13 subjects reported a stick–slip feel and 9 reported a scratchy texture among the 28 subjects. When silicone oil was applied to a glass substrate, there were 6 and 9 subjects who felt these textures, respectively. Conversely, when water was applied on PTFE, only 5 and 6 subjects, respectively, noted those them. When silicone oil was applied on PTFE, no one reported either the stick–slip feel or the scratchy feel. In this case, many subjects reported a smooth or a slimy feel. The correlation coefficients r between the similarity score and n are shown in Table S1 (see Supplementary data). In the seven tactile factors, the scratchy feel had the highest value, r = 0.328, while the slimy feel showed a strong negative correlation, r = − 0.314. These results show that the similarity of the feel of silicone oil with water depends on the solid substrate, and we distinguish water and oil by sensing a scratchy or slimy feel on our fingertips. 3.2. Friction evaluations
Fig. 3. Change in friction coefficient (red lines), friction force (blue lines) and vertical stress (green lines) with application of water on glass (a), silicone oil on glass (b), water on PE (c) and silicone oil on PTFE (d). Positive signals on the friction force and frictional coefficient demonstrate a frictional resistance to the right, and negative signals to the left. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
the score was 3.3 ± 1.6, and when silicone oil was dropped on a glass substrate, the similarity score was 5.3 ± 1.7. Many subjects judged that they felt similar. The t-test showed that the inequality of glass and aluminum, and that of aluminum and PE were significant at the critical rate of <0.1% and <1%, respectively. Table 1 shows the reasons the subjects gave the similarity scores. The numbers in the table are the counts of the subjects who mentioned the specific keywords. Moreover, n is the difference between the numbers n = no − nw . Here, no and nw are the counts
We evaluated the frictional resistance to determine the origin of the scratchy feel on our fingertips. Some profiles of the friction force, vertical stress, and friction coefficient are shown in Fig. 3. The static and dynamic friction coefficients of water and silicone oil on each substrate are shown in Table S2 (see Supplementary data). Fig. 3a and b are friction profiles of water and silicone oil applied on glass, respectively. The motion of a fingertip on the solid substrates induced positive or negative signals in the friction force and the friction coefficient. Positive signals demonstrated frictional resistance to the right and the negative ones to the left. In both cases, the dynamic friction coefficient was about 1, and a periodic change in the frictional resistance was observed. In the present study, we call this friction profile periodic pattern 1. When water was applied to the glass substrate, 17 of 18 subjects who felt the scratchy feel showed periodic pattern 1. Conversely, when silicone oil was applied to the glass substrate, 10 of 13 subjects showed this pattern. The friction profile generated when water was applied to PTFE is shown in Fig. 3c. In this case, the dynamic friction coefficient was about 1, and a change in the frictional resistance was observed occasionally. For this study, the profile is called periodic pattern 2. The friction profile when silicone oil was applied to PTFE is shown in Fig. 3d. The dynamic friction coefficient was about 0.25–0.30 of the
Y. Nonomura et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 80–83
coefficient when water was applied, and a change in the frictional resistance was not observed. We called this profile a broad pattern, and 16 of 20 subjects who showed the broad pattern reported the slimy feel. Periodic pattern 1 was observed for both water and silicone oil on the glass substrate. On the PTFE substrate, periodic pattern 2 was observed for water and the broad pattern for silicone oil. These results predict a high similarity score when a similar friction pattern is observed for water and silicone oil, and a low score when the friction patterns differ. 4. Discussion The present sensory evaluations show that the tactile feel of silicone oil changes with the nature of the solid substrate, and reflects friction profiles. In this section, we discuss the recognition mechanism of water and oil through our tactile sense. We analyze the relationship between tactile factors and physical factors with statistical methods, and find that we distinguish water from oil by the scratchy and slimy feel. Fig. 4 shows a path diagram between the similarity scores, tactile factors, and physical factors. The comparison fidelity index (CFI) of this model and root mean square error of approximation (RMSEA) showed statistical adequacy; CFI = 1.00 and RMSEA = 0. Statistical suitability requires CFI > 0.95 and RMSEA < 0.05 [12]. The contribution of two tactile factors, the scratchy feel and the slimy feel, to the similarity with water was ascertained. Each contribution was 0.18 and −0.17, respectively. This result suggests that we distinguished water and oil from these tactile characteristics—liquids with a higher scratchy feel and a lower slimy feel are very similar to water. What are the physical and tactile factors governing this similarity? Friction evaluations show that we distinguish water from oils based on the friction resistance and its periodic changes. The similarity score was large when the periodic pattern appeared for both water and oil, while the score was low when the broad pattern appeared for the oil. The contribution of frictional resistance to the similarity score is found in the path diagram in Fig. 4. The similarity score increases with a decrease in the absolute value of s w s ; s = ss − w s , where s and s are the static friction coefficients of silicone oil and water, respectively. The contribution of to the similarity score is also predicted in the path diagram. The similarity score increases as the absolute value of decreases. These predictions are important in understanding the texture of liquid materials. Here, we discuss the origin of frictional resistance and the periodic changes observed in the present study. There are some reports on the remarkable frictional resistance of water films between solid substrates. Adams et al. proposed that frictional resistance increases by the plasticizing effect of the skin when skin that is covered with water is rubbed by a glass tip. The periodic changes are caused by the formation and rupture of water film on the glass surface [13]. Periodic pattern 1 is explained by this mechanism, when water is applied to glass and aluminum. However, when water is applied to a hydrophobic substrate, a high friction resistance is always observed, because a stable water film is not formed on a hydrophobic surface. The hydrophobicity of PE and PFPE is reflected in the contact angle w in Fig. 2. Periodic pattern 2, when water is applied to PE and PTFE, and explains this mechanism. The conventional model cannot explain the periodic pattern observed when silicone oil was applied to a glass substrate, because
83
oils does not swell and increase the friction resistance of a skin practically [14]. We assume that the periodic pattern is caused by the hydrogen bonds between skin and a glass substrate. In general, hydrogen bonds form between one electronegative atom and a hydrogen covalently bonded to another electronegative atom, such as oxygen, nitrogen, fluorine, and chloride [15]. In low polarity oils, such as silicone oils, the keratin proteins on the human skin might be bound with hydroxyl groups on the glass substrate by hydrogen bonds. This specific affinity raises an ankylosis and a frictional resistance between skin and glass. The formation and rupture of the lubricating film induced the periodic pattern when silicone oil was applied to the glass substrate. 5. Conclusions We studied the tactile feel of water and oil on some solid substrates. The results of the tactile evaluations show that we distinguish water and oil by the scratchy or slimy feel. These tactile feels reflect the frictional resistance and are caused by the wettability of the liquids with the solid substrates. These findings are useful for controlling the tactile sensation of liquids, and developing cosmetics, food, and fibrous products. In addition, methods of expressing the texture of water and oil could be used in virtual reality technology. Acknowledgements This work was supported by a grant for basic science research, project no. 070023, from The Sumitomo Foundation. The authors thank Dr. Tadahiro Aida of Yamagata University for the contact angle measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfb.2009.05.001. References [1] N.F. Estrin, J.M. Akerson, Cosmetic Regulation in a Competitive Environment, Informa Health Care, New York, 2002 (Chapter 24). [2] Y. Nonomura, T. Fujii, Y. Arashi, T. Miura, T. Maeno, K. Tashiro, Y. Kamikawa, R. Monchi, Colloids Surf. B 69 (2009) 264. [3] G. Jundt, A. Radu, E. Fort, J. Duda, H. Vacha, N. Fletcher, J. Acoust. Soc. Am. 119 (2006) 3793. [4] F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, Oxford, 2001 (Chapter 5). [5] M. Nakatani, R.D. Howe, S. Tachi, The 2nd Joint Eurohaptics Conference and Symposium on Haptic Interface for Virtual Environment and Teleoperator Systems, 2006, p. 3. [6] M. Botvinick, J. Cohen, Nature 391 (1998) 756. [7] H. Shirado, T. Maeno, The First Joint Eurohaptics Conference and Symposium on Haptic Interface for Virtual Environment and Teleoperator Systems, 2005, p. 57. [8] K. Kamikawa, Y. Nonomura, T. Maeno, Trans. Jpn. Soc. Mech. Eng., Ser. C 73 (2007) 1827. [9] M. Yamane, et al., Handbook of Glass Engineering, Asakura Publishing, Tokyo, 1999, p. 83. [10] The Japan Society of Mechanical Engineers (Ed.), Engineering Materials: Data and Guide for Mechanical Engineers, Maruzen, 2006, p. 252. [11] K. Ito, Plastic Handbook, Kogyochyosakai, 1980, p. 137. [12] L. Hu, P.M. Bentler, Psychol. Methods 3 (1998) 424. [13] M.J. Adams, B.J. Briscoe, S.A. Johnson, Tribol. Lett. 26 (2007) 239. [14] S. Nacht, J. Close, D. Yeung, E.H. Gans, J. Soc. Cosmet. Chem. 32 (1) (1981) 55. [15] J.N. Israelachivili, Intermolecular and Surface Forces, Academic Press, London, 1992 (Chapter 8).
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
How do we recognize water and oil through our tactile sense? Yoshimune Nonomura a,∗ , Yuichiro Arashi a , Takashi Maeno b a b
Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering, Yamagata University, Yonezawa 992-8510, Japan Human System Design Laboratory, Graduate School of System Design and Management, Keio University, Yokohama 223-8526, Japan
a r t i c l e
i n f o
Article history: Received 26 February 2009 Received in revised form 25 April 2009 Accepted 1 May 2009 Available online 9 May 2009 Keywords: Tactile impression Friction Wettability Water Silicone oil Glass
a b s t r a c t We can distinguish water and oil through our sense of feel, because the frictional properties of a water film are characteristic. However, we found that it was difficult to distinguish water and silicone oil on a glass substrate only by its feel. Friction evaluation showed that we recognize the liquid as water when the frictional resistance is large and changeable. On a glass substrate, water and silicone oil were not distinguishable, since both had a large frictional resistance. Statistical analysis suggested the contribution of the wettability of solid substrates to their tactile feel. This finding shows that we distinguish water from oil based on the significant friction properties; it is applicable to virtual reality systems, as well as cosmetics, food, and textiles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction We can easily distinguish water from oils thorough our tactile sense, because of their characteristic tactile feel. Many cosmetic chemists believe that water is the most effective textural control agent [1]. If the physical origin of tactile feel is clarified, it will be useful in designing virtual reality systems as well as food, cosmetics, and textiles. We previously showed that water has a stick–slip feel when a human fingertip rubs a small amount [2]. Friction analysis suggests that this stick–slip feel is caused by a drastic change in the frictional resistance. This is a well-known phenomenon, for example, the acoustic emission that arises when a wet finger is slid on the rim of a wine glass [3]. The stick–slip phenomenon occurs when dynamic friction is less than static friction [4]. Although humans can recognize various substances through their tactile sense, we often make mistakes. A tactile sense is an uncertain sensation, which changes with conditions or the environment. Extreme examples are tactile illusions, e.g., the fishbone tactile illusion, in which a concave surface is recognized as convex and the rubber hands illusion, in which a dummy hand is confused with the subject’s real hand [5,6]. In the present study, we evaluated the tactile feel when water and silicone oil were applied to various solid substrates. It is expected that the feel of water and oil will change with different kinds of substrates, because the frictional properties of a liquid film
depend on the surface properties of the solid substrate. In the tactile evaluations, we focused on the ability to identify water and silicone oil. Moreover, we evaluated the frictional force added to the solid substrates to clarify the factors governing the tactile feel. 2. Experiment 2.1. Material Water was purified by a water deionizing unit (DX-15 from Kurita Water Industries Ltd.). Silicone oil, KF-96-L 1CS (dimethyl polysiloxane, with a molecular weight of about 300, kinematic viscosity of 1.0 mm2 s−1 , and surface tension of 16.9 mN m−1 ) was obtained from Shin-Etsu Chemical Co. Ltd. These liquids were applied to four substrates: glass (Matsunami glass incorporated company), aluminum (Yasutoyo trading incorporated company), polyethylene (PE, As One Corp.), and polytetrafluoroethylene (PTFE, Nippon Valqua Industries, Ltd.). These substrates were washed by a commercial cleanser containing anionic surfactants and nonionic surfactants, and were immersed in running water for 30 min to rinse off the surfactants. The contact angle of the liquids on these solid substrates and the surface roughness were measured by a CA-X contact angle meter (Kyowa Interface Science Co. Ltd.) and a VK-8500 laser microscope (Keyence Co.), respectively. 2.2. Tactile evaluations
∗ Corresponding author. Tel.: +81 238 26 3164; fax: +81 238 26 3414. E-mail address: [email protected] (Y. Nonomura). 0927-7765/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2009.05.001
Tactile evaluations were achieved as follows: silicone oil’s similarity in feel with a standard water sample was evaluated. Subjects
Y. Nonomura et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 80–83
applied them to four different solid substrates installed on the tactile evaluation system. The tactile evaluations were rated on a seven-point scale, where a score of 7 meant “exactly the same texture as water,” while a score of 1 meant “exactly the opposite texture from water.” After filling in the questionnaire, the subjects were asked orally how they made their evaluations. Blind evaluations were carried out; the subjects did not know the composition of the liquids and the solid substrates. The subjects included 14 male students and 14 female students ranging in age from 21 to 25 years. The evaluations were carried out in a quiet room at 298 K after subjects washed both hands with a commercial liquid hand soap. Six milliliters of liquid hand soap was applied to each subject’s hands, and rinsed off with running water. Before the evaluations, their hands were wiped by paper towels. Subjects used their forefingers to rub first, 0.1 ml of water, and then 0.1 ml of silicone oil on the solid substrates installed on the tactile evaluation system. After filling in the questionnaire, the subjects washed their hands with water again. The evaluation items were selected based on a preliminary test by three professional panelists. One was a researcher, who has engaged in tactile evaluations for more than 10 years, while the other two were students who had studied the tactile sense. The unit process, i.e., applying the liquid samples, filling in the questionnaire, and washing the hands, was repeated for all substrates. The order of the substrates was random to eliminate order effects. The subjects touched the artificial skin through a blackout curtain. The procedure of the test was announced beforehand. The subjects volunteered to participate in the evaluation. The responsible party at Yamagata University confirmed that the ethical standards were followed and the test was reliable. The statistical analysis was carried out using SPSS 16.0 Base System and Amos.
81
Fig. 1. Photograph of the tactile evaluation system. This device measures the friction forces and vertical stresses using strain gauges on two plate springs. Subjects rub water and silicone oil on solid substrates to evaluate tactile feels.
physical properties considered were the Young’s modulus E [9–11], surface roughness Ra , and the contact angle of substrates with water w and silicone oil s . When silicone oil was dropped on PE or PTFE, the similarity score was about 2; many subjects judged that they did not feel similar. When silicone oil was dropped on aluminum,
2.3. Friction evaluations In the present study, we used a system that simultaneously evaluated tactile sensations and frictional properties [7,8]. This device measured the frictional force and vertical stress using strain gauges on two plate springs, as shown in Fig. 1. The time resolution was 1 ms. The four solid substrates mentioned above were attached to this device. The strain gauges on two plate springs showed the linearity and reproducibility with an error less than 0.20 or 0.08 N for the frictional force and vertical stress, respectively. The maximum measurable load of the device was 5 N. 3. Results 3.1. Tactile evaluations The similarity score for silicone oil with water on each substrate, and the physical properties of each substrate are shown in Fig. 2. The
Fig. 2. Similarity scores of silicone oil on four substrates. Score 7 means “exactly the same texture as water”, while 1 means “exactly the opposite texture from water”. The physical properties of the substrates, i.e., Young’s modulus E, surface roughness Ra , contact angle of water w and contact angle of silicone oil s are also shown. Asterisks mean the results of t-tests; ** P < 0.01, *** P < 0.001.
Table 1 The counts of the substrates who mentioned the specific tactile feels for water nw and oil no . The symbol n means the difference between the counts; no − nw . Substrate
Liquid
Stick–slip
Glass
Silicone oil Water nGl
6 13 −7
Scratchy 9 9 0
Silky 6 7 −1
Smooth 0 0 0
Slimy 2 0 2
Slippery 2 2 0
Sticky 1 0 1
Aluminium
Silicone oil Water nAl
2 5 −3
1 4 −3
3 8 −5
12 4 8
10 1 9
0 2 −2
1 2 −1
PE
Silicone oil Water nPE
1 6 −5
1 9 −8
2 5 −3
12 0 12
11 0 11
2 1 1
2 0 2
PTFE
Silicone oil Water nPTFE
0 5 −5
0 6 −6
4 7 −3
10 1 9
16 2 14
3 1 2
0 0 0
82
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Fig. 4. A path diagram of tactile factors and physical factors; s : static friction coefficient, : difference between contact angles of water and silicone oil.
of the subjects who mentioned each tactile feel for silicone oil and water, respectively. When water was applied on a glass substrate, there were 13 subjects reported a stick–slip feel and 9 reported a scratchy texture among the 28 subjects. When silicone oil was applied to a glass substrate, there were 6 and 9 subjects who felt these textures, respectively. Conversely, when water was applied on PTFE, only 5 and 6 subjects, respectively, noted those them. When silicone oil was applied on PTFE, no one reported either the stick–slip feel or the scratchy feel. In this case, many subjects reported a smooth or a slimy feel. The correlation coefficients r between the similarity score and n are shown in Table S1 (see Supplementary data). In the seven tactile factors, the scratchy feel had the highest value, r = 0.328, while the slimy feel showed a strong negative correlation, r = − 0.314. These results show that the similarity of the feel of silicone oil with water depends on the solid substrate, and we distinguish water and oil by sensing a scratchy or slimy feel on our fingertips. 3.2. Friction evaluations
Fig. 3. Change in friction coefficient (red lines), friction force (blue lines) and vertical stress (green lines) with application of water on glass (a), silicone oil on glass (b), water on PE (c) and silicone oil on PTFE (d). Positive signals on the friction force and frictional coefficient demonstrate a frictional resistance to the right, and negative signals to the left. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
the score was 3.3 ± 1.6, and when silicone oil was dropped on a glass substrate, the similarity score was 5.3 ± 1.7. Many subjects judged that they felt similar. The t-test showed that the inequality of glass and aluminum, and that of aluminum and PE were significant at the critical rate of <0.1% and <1%, respectively. Table 1 shows the reasons the subjects gave the similarity scores. The numbers in the table are the counts of the subjects who mentioned the specific keywords. Moreover, n is the difference between the numbers n = no − nw . Here, no and nw are the counts
We evaluated the frictional resistance to determine the origin of the scratchy feel on our fingertips. Some profiles of the friction force, vertical stress, and friction coefficient are shown in Fig. 3. The static and dynamic friction coefficients of water and silicone oil on each substrate are shown in Table S2 (see Supplementary data). Fig. 3a and b are friction profiles of water and silicone oil applied on glass, respectively. The motion of a fingertip on the solid substrates induced positive or negative signals in the friction force and the friction coefficient. Positive signals demonstrated frictional resistance to the right and the negative ones to the left. In both cases, the dynamic friction coefficient was about 1, and a periodic change in the frictional resistance was observed. In the present study, we call this friction profile periodic pattern 1. When water was applied to the glass substrate, 17 of 18 subjects who felt the scratchy feel showed periodic pattern 1. Conversely, when silicone oil was applied to the glass substrate, 10 of 13 subjects showed this pattern. The friction profile generated when water was applied to PTFE is shown in Fig. 3c. In this case, the dynamic friction coefficient was about 1, and a change in the frictional resistance was observed occasionally. For this study, the profile is called periodic pattern 2. The friction profile when silicone oil was applied to PTFE is shown in Fig. 3d. The dynamic friction coefficient was about 0.25–0.30 of the
Y. Nonomura et al. / Colloids and Surfaces B: Biointerfaces 73 (2009) 80–83
coefficient when water was applied, and a change in the frictional resistance was not observed. We called this profile a broad pattern, and 16 of 20 subjects who showed the broad pattern reported the slimy feel. Periodic pattern 1 was observed for both water and silicone oil on the glass substrate. On the PTFE substrate, periodic pattern 2 was observed for water and the broad pattern for silicone oil. These results predict a high similarity score when a similar friction pattern is observed for water and silicone oil, and a low score when the friction patterns differ. 4. Discussion The present sensory evaluations show that the tactile feel of silicone oil changes with the nature of the solid substrate, and reflects friction profiles. In this section, we discuss the recognition mechanism of water and oil through our tactile sense. We analyze the relationship between tactile factors and physical factors with statistical methods, and find that we distinguish water from oil by the scratchy and slimy feel. Fig. 4 shows a path diagram between the similarity scores, tactile factors, and physical factors. The comparison fidelity index (CFI) of this model and root mean square error of approximation (RMSEA) showed statistical adequacy; CFI = 1.00 and RMSEA = 0. Statistical suitability requires CFI > 0.95 and RMSEA < 0.05 [12]. The contribution of two tactile factors, the scratchy feel and the slimy feel, to the similarity with water was ascertained. Each contribution was 0.18 and −0.17, respectively. This result suggests that we distinguished water and oil from these tactile characteristics—liquids with a higher scratchy feel and a lower slimy feel are very similar to water. What are the physical and tactile factors governing this similarity? Friction evaluations show that we distinguish water from oils based on the friction resistance and its periodic changes. The similarity score was large when the periodic pattern appeared for both water and oil, while the score was low when the broad pattern appeared for the oil. The contribution of frictional resistance to the similarity score is found in the path diagram in Fig. 4. The similarity score increases with a decrease in the absolute value of s w s ; s = ss − w s , where s and s are the static friction coefficients of silicone oil and water, respectively. The contribution of to the similarity score is also predicted in the path diagram. The similarity score increases as the absolute value of decreases. These predictions are important in understanding the texture of liquid materials. Here, we discuss the origin of frictional resistance and the periodic changes observed in the present study. There are some reports on the remarkable frictional resistance of water films between solid substrates. Adams et al. proposed that frictional resistance increases by the plasticizing effect of the skin when skin that is covered with water is rubbed by a glass tip. The periodic changes are caused by the formation and rupture of water film on the glass surface [13]. Periodic pattern 1 is explained by this mechanism, when water is applied to glass and aluminum. However, when water is applied to a hydrophobic substrate, a high friction resistance is always observed, because a stable water film is not formed on a hydrophobic surface. The hydrophobicity of PE and PFPE is reflected in the contact angle w in Fig. 2. Periodic pattern 2, when water is applied to PE and PTFE, and explains this mechanism. The conventional model cannot explain the periodic pattern observed when silicone oil was applied to a glass substrate, because
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oils does not swell and increase the friction resistance of a skin practically [14]. We assume that the periodic pattern is caused by the hydrogen bonds between skin and a glass substrate. In general, hydrogen bonds form between one electronegative atom and a hydrogen covalently bonded to another electronegative atom, such as oxygen, nitrogen, fluorine, and chloride [15]. In low polarity oils, such as silicone oils, the keratin proteins on the human skin might be bound with hydroxyl groups on the glass substrate by hydrogen bonds. This specific affinity raises an ankylosis and a frictional resistance between skin and glass. The formation and rupture of the lubricating film induced the periodic pattern when silicone oil was applied to the glass substrate. 5. Conclusions We studied the tactile feel of water and oil on some solid substrates. The results of the tactile evaluations show that we distinguish water and oil by the scratchy or slimy feel. These tactile feels reflect the frictional resistance and are caused by the wettability of the liquids with the solid substrates. These findings are useful for controlling the tactile sensation of liquids, and developing cosmetics, food, and fibrous products. In addition, methods of expressing the texture of water and oil could be used in virtual reality technology. Acknowledgements This work was supported by a grant for basic science research, project no. 070023, from The Sumitomo Foundation. The authors thank Dr. Tadahiro Aida of Yamagata University for the contact angle measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfb.2009.05.001. References [1] N.F. Estrin, J.M. Akerson, Cosmetic Regulation in a Competitive Environment, Informa Health Care, New York, 2002 (Chapter 24). [2] Y. Nonomura, T. Fujii, Y. Arashi, T. Miura, T. Maeno, K. Tashiro, Y. Kamikawa, R. Monchi, Colloids Surf. B 69 (2009) 264. [3] G. Jundt, A. Radu, E. Fort, J. Duda, H. Vacha, N. Fletcher, J. Acoust. Soc. Am. 119 (2006) 3793. [4] F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, Oxford, 2001 (Chapter 5). [5] M. Nakatani, R.D. Howe, S. Tachi, The 2nd Joint Eurohaptics Conference and Symposium on Haptic Interface for Virtual Environment and Teleoperator Systems, 2006, p. 3. [6] M. Botvinick, J. Cohen, Nature 391 (1998) 756. [7] H. Shirado, T. Maeno, The First Joint Eurohaptics Conference and Symposium on Haptic Interface for Virtual Environment and Teleoperator Systems, 2005, p. 57. [8] K. Kamikawa, Y. Nonomura, T. Maeno, Trans. Jpn. Soc. Mech. Eng., Ser. C 73 (2007) 1827. [9] M. Yamane, et al., Handbook of Glass Engineering, Asakura Publishing, Tokyo, 1999, p. 83. [10] The Japan Society of Mechanical Engineers (Ed.), Engineering Materials: Data and Guide for Mechanical Engineers, Maruzen, 2006, p. 252. [11] K. Ito, Plastic Handbook, Kogyochyosakai, 1980, p. 137. [12] L. Hu, P.M. Bentler, Psychol. Methods 3 (1998) 424. [13] M.J. Adams, B.J. Briscoe, S.A. Johnson, Tribol. Lett. 26 (2007) 239. [14] S. Nacht, J. Close, D. Yeung, E.H. Gans, J. Soc. Cosmet. Chem. 32 (1) (1981) 55. [15] J.N. Israelachivili, Intermolecular and Surface Forces, Academic Press, London, 1992 (Chapter 8).