Physical origin of a complicated tactile sensation: ‘shittori feel'

Shittori feel is defined as a texture that is moderately moisturized; however, many people experience ‘shittori feel’ when they touch a dry solid material containing little liquid. Here, shittori feel was evaluated for 12 materials. We found that the highest score of shittori feel was achieved by powders. Multiple regression analysis showed that shittori feel is a complex sense of moist and smooth feels. We analysed the relationship between the physical properties and the moist/smooth feels to show how subjects felt certain feels simultaneously. The moist and smooth feels are related to the surface roughness and friction characteristics of the materials. The moist and smooth feels can be perceived when the finger starts to move on the material surface and when the finger moves and rubs the material surface, respectively.

properties relate to each other and to the overall assessment of the shittori effet.
Specific comments: -The variations in responses amongst participants are not shown nor discussed. It seems the score and values are the means across all participants but that is not clearly stated. I would really like to see the distribution of results across participants and a detailed explanation on how exactly the scores were computed. -The images, the equipment used for measurements and the sample preparation are very detailed, however, I don't think the raw data of individual responses to all the questions asked in the experiment is accessible and I think the journal requires it.
-It is unclear if the responses to Q2, Q4 and follow-up questions about when the effect was felt were analyzed or discussed in the paper. It would be good to add that or make it more explicit. -I would suggest adding a bit more detail on the importance of the "Shittori" feel. -Could you please specify the type/model of the load cells used to measure the frictions forces? -I would remove the variable "delay time". It's not super common and it doesn't add to your analysis.
-I would probably remove the capillary force equation since you just state it without going further. It would also make the text flow more instead of breaking it up as it is currently.
-Did the participants all know the shittori effect? Were they provided with a definition at the beginning of the experiments to ensure consistency in reporting across all participants?
Decision letter (RSOS-190039.R0) 24-Apr-2019 Dear Dr Nonomura, The editors assigned to your paper ("Physical origin of a complicated tactile sensation: "Shittori feel"") have now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Associate Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. Comments to the Author(s) The authors describe the tactile perception of a range of materials and rationalise the data mainly on the basis of corresponding measurements of their friction using a urethane resin probe. In particular, they argue that the difference between the static and dynamic values of the friction are an important contributory factor. Some of the differences are negative so that the meaning is not clear. The concept of a static friction is nebulous and is often only associated with extended contact times prior to sliding. To be convincing, the effect of this delay time should have been examined. In any case, the frictional characteristics of a finger pad are complex and cannot be satisfactorily mimicked by a probe. This is due to the extent of moisture accumulation in the contact region that is associated with an increase in the friction [e.g. Dzidek et al., 2017. Why pens have rubbery grips. Proc. Nat. Acad. Sci., 114, pp.10864-10869.] This so called occlusion effect is absent on rough surfaces and the friction reduces with sliding time for absorbent surfaces such as paper and probably some fabrics [Adams et al., 2013. Finger pad friction and its role in grip and touch. J. Roy. Soc. Interf., 10, p.20120467.] It will also be seen in this latter reference that the adhesion, due for example to capillary forces, is negligibly small compared to the applied normal forces. Consequently, an explanation based on capillary adhesion (eq. 4) is not correct. I suggest that the authors review the extensive literature on finger pad friction in order to provide a more convincing explanation of their data.

Reviewer: 2
Comments to the Author(s) "Shittori feel" is an interesting topic. The authors conducted the multiple regression analyses of different materials. The authors analyzed the relationship between the physical properties and the moist/smooth feels to show how subjects felt certain feels simultaneously. However, the following problems must be paid attention to. 1. Suggest that you add these diagrams and tables (figure S1, figure S2, figure S3, table S1…) to the proper place of the text so that the reader can understand them better. 2. For similar lists, recommend using tables. 3. The references in the past five years account for less than one sixth. Hope to refer to more recent articles.

Reviewer: 3
Comments to the Author(s) The paper is clearly written, the figures are easy to read and the information flows nicely. The authors followed a systematic and comprehensive approach to understanding what contributes to the shittori feel, by comparing how perceptual evaluations of material properties and materials properties relate to each other and to the overall assessment of the shittori effet.
Specific comments: -The variations in responses amongst participants are not shown nor discussed. It seems the score and values are the means across all participants but that is not clearly stated. I would really like to see the distribution of results across participants and a detailed explanation on how exactly the scores were computed. -The images, the equipment used for measurements and the sample preparation are very detailed, however, I don't think the raw data of individual responses to all the questions asked in the experiment is accessible and I think the journal requires it.
-It is unclear if the responses to Q2, Q4 and follow-up questions about when the effect was felt were analyzed or discussed in the paper. It would be good to add that or make it more explicit. -I would suggest adding a bit more detail on the importance of the "Shittori" feel.
-Could you please specify the type/model of the load cells used to measure the frictions forces? -I would remove the variable "delay time". It's not super common and it doesn't add to your analysis.
-I would probably remove the capillary force equation since you just state it without going further. It would also make the text flow more instead of breaking it up as it is currently.
-Did the participants all know the shittori effect? Were they provided with a definition at the beginning of the experiments to ensure consistency in reporting across all participants?

10-Jun-2019
Dear Dr Nonomura, I am pleased to inform you that your manuscript entitled "Physical origin of a complicated tactile sensation: "Shittori feel"" is now accepted for publication in Royal Society Open Science.
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Reviewer 1: I would like to appreciate your suggestive comments. In the present paper, we have revised these points.

･"Some of the differences are negative so that the meaning is not clear. The concept of a static friction is nebulous and is often only associated with extended contact times prior to sliding. To be convincing, the effect of this delay time should have been examined."
The contact probe was in contact with the material for 30 seconds prior to sliding. As the reviewer mentioned, the static friction coefficient depends on the conditions of sliding. In particular, the contact time between the contact probe and materials before sliding can bring a significant effect on the static friction. Therefore, we confirmed that the static and dynamic friction coefficients were almost the same even if the contact time was changed from 30-300 seconds (section 2.4 p. 10 line 22-p.11 line 3). However, it was difficult to show the effect of delay time, because the time was only 0.05-0.06 seconds and was too short to check the effect precisely. We will conduct systematic analysis to show the effect of the delay time after developing a more precise evaluation method. ･"In any case, the frictional characteristics of a finger pad are complex and cannot be satisfactorily mimicked by a probe. This is due to the extent of moisture accumulation in the contact region that is associated with an increase in the friction [e.g. Dzidek et al., 2017. Why pens have rubbery grips. Proc. Nat. Acad. Sci., 114, pp.10864-10869.] This so called occlusion effect is absent on rough surfaces and the friction reduces with sliding time for absorbent surfaces such as paper and probably some fabrics [Adams et al., 2013. Finger pad friction and its role in grip and touch. J. Roy. Soc. Interf., 10, p.20120467.] It will also be seen in this latter reference that the adhesion, due for example to capillary forces, is negligibly small compared to the applied normal forces. Consequently, an explanation based on capillary adhesion (eq. 4) is not correct. I suggest that the authors review the extensive literature on finger pad friction in order to provide a more convincing explanation of their data." The hypothesis based on the capillary adhesion force may be too simple to explain the phenomena that occur on the actual skin surface. In future, we will propose more suitable model of interfacial phenomena on human skin. For example, as the reviewer mentioned, the extent of moisture accumulation in the contact region is associated with an increase in the friction [38]. The morphology of the sliding surface in the powder bed is complex: During the sliding process the powder is packed into the groove of the fingerprint, which lowers the friction significantlly [39]. The uneven surface of the paper absorbs sweat and changes the friction characteristics drastically [40]. Dussaud et al. measured flows within the fine channels on the human skin surface and demonstrated that this network can act as a microfluidic device to allow the surface transport of liquids. [41]. The description was added in page 23 lines 4-12.
Reviewer 2: We thank you for appreciating our manuscript very much.
･"Suggest that you add these diagrams and tables (figure S1, figure S2, figure S3, table S1…) to the proper place of the text so that the reader can understand them better." In the revised paper, Figure S1, S2, and S6 are added in the main text (Figure 1, 2 ･"For similar lists, recommend using tables." The images and chemical composition of 12 materials, which were described in Figure  S1 and section 2.1. in the previous version, are shown in Table 1.
･"The references in the past five years account for less than one sixth. Hope to refer to more recent articles." Some recent papers on modeling of tactile texture and tribology of human skin were added as references I would like to appreciate your suggestive comments. In the present paper, we have revised these points. ･"The variations in responses amongst participants are not shown nor discussed. It seems the score and values are the means across all participants but that is not clearly stated. I would really like to see the distribution of results across participants and a detailed explanation on how exactly the scores were computed." In the revised paper, shittori score of all subjects, means and standard deviations are described in Figure 2 and Table S1 to show the distribution of results across participants. The means are arithmetic averages. The descriptions are added in page 14 lines 2-12.
･"The images, the equipment used for measurements and the sample preparation are very detailed, however, I don't think the raw data of individual responses to all the questions asked in the experiment is accessible and I think the journal requires it. " The raw data of individual responses to all the questions are added in Table S1-3. The descriptions were added in the section 3.2. In the comments of Q2 and 4, the words concerning sticky or moist feel are appeared with relatively high probability for powders A and B. On the other hand, the words related to dry feel were described for cloths K and L with the lowest score of shittori feel (Table S2,3 and Fig. S4).
･"It is unclear if the responses to Q2, Q4 and follow-up questions about when the effect was felt were analyzed or discussed in the paper. It would be good to add that or make it more explicit. " The comments and appearance rate of the words on each tactile dimension in the subject's comment were added in Table S2, S3 and Figure S4. In the comments of the subjects of Q2 and 4, the words concerning sticky or moist feel appeared with relatively high probability for powders A and B. On the other hand, the words related to dry feel were described for cloths K and L with the lowest score of shittori feel. These descriptions were added in the section 3.2 page 14 lines 9-12.
･"I would suggest adding a bit more detail on the importance of the "Shittori" feel. " In the field of cosmetics, Shittori feel is the most important evaluation item for many formulators, because the feeling is the characteristic texture of healthy skin with high water