Received February 19, 2022, accepted April 14, 2022, date of publication May 17, 2022, date of current version May 27, 2022.
Digital Object Identifier 10.1109/ACCESS.2022.3175875

Less Frictional Skin Feels Softer in a
Tribologically Paradoxical Manner
NAOMI ARAKAWA

1 , NAOKI SAITO

1 , AND SHOGO OKAMOTO

1 MIRAI Technology Institute, Shiseido Company Ltd., Takashima, Yokohama 220-0011, Japan

2 , (Member, IEEE)

2 Department of Computer Sciences, Tokyo Metropolitan University, Hino, Tokyo 191-0065, Japan

Corresponding author: Naomi Arakawa (naomi.arakawa1@shiseido.com)
This work involved human subjects or animals in its research. Approval of all ethical and experimental procedures and protocols was
granted by the Shiseido Research Ethics Committee under Application No. C01724 and C01931.

ABSTRACT In general, the friction coefficient of a soft object, which has a low elastic modulus, is greaterthan that of a hard object. Briefly, friction and hardness are physically correlated. Given this relationship,a question naturally arises: are they perceptually coupled? We hypothesized that the higher an object’s surface
friction coefficient, the softer it would feel in a physically consistent manner. To confirm this hypothesis,
we conducted two types of psychophysical experiments using skin-like materials made of polyurethaneand human cheeks as stimuli considering the potential applications in cosmetics. In experiment 1, skin-likeobjects with the same dimensions and stiffness were coated with powders so that they had different friction
coefficients. Participants actively explored and evaluated the softness of these surfaces using their fingers.
Their exploratory motions were restricted to either pressing or rubbing. When participants repeatedly pressed
the surfaces with no sliding motion, they judged the softness of all the surfaces to be equal. In contrast, whenparticipants rubbed the surfaces, they judged the surfaces with lower friction (i.e., more slippery) as softer
than the surfaces with higher friction. In experiment 2, the same results were obtained using human cheeks,
one side of which was lubricated to be more frictional. This psychophysical interference between hardness
and friction is paradoxical in terms of tribology and contact mechanics. We discuss the potential reasons that
led to these results.
INDEX TERMS Cheek, friction, hardness, lubrication, polyurethane, softness perception.

I. INTRODUCTION

The tactile sensations of daily products are designed to satisfy the needs of consumers. It is important to know thepsychophysical relationship between the physical parameters of the products or materials and the tactile perception
to achieve consumer satisfaction. Various studies have been
conducted for such purposes [1]–[5]. We focus on the softness perception of human skin and objects as soft as humanskin.
Although haptic softness can be defined in terms of various aspects including furry, granulous, viscoelastic, anddeformable softness [6], most previous studies in this field
have conducted psychophysical experiments in which elastic
or deformable materials were pressed without sliding motions
or pinched by fingers or tools [7]–[16]. For example, the rolesof cutaneous and proprioceptive cues have been discussed,
The associate editor coordinating the review of this manuscript andapproving it for publication was Lei Wei
VOLUME 10, 2022.

and the former is a dominant or satisfactory cue for theperception of deformable objects’ softness [7], [9], [10]. Furthermore, softness perception by pressing or pinching relies
on both the stiffness, i.e., spring constant, and elastic modulus, i.e., Young’s modulus, of specimens [10], [11], [17].
The deformation of fingertip and object collectively determined by multiple types of hardness quantities potentiallycontributes to softness judgment [18]. Recently, the meaning of repetitive explorative motion, i.e., pressing, to judge
softness has been discussed [16]. Nonetheless, humans alsofeel softness when rubbing surfaces [5], [6], [19]. Althoughfriction occurs through rubbing objects, previous studies onperceptual dimensions for tactile material perception haveshown that perceived softness and perceived friction aremostly independent [1], [20]–[24]. The perceptual dimension of softness and friction, i.e., stickiness/slipperiness andmoistness/dryness, are judged differently, indicating that frictional properties such as the friction coefficient are considered to have little or no effect on the softness perception

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N. Arakawa et al.: Less Frictional Skin Feels Softer in Tribologically Paradoxical Manner

TABLE 1. Coefficient of friction of powdery surfaces.

or that their effect is relatively small compared with that ofhardness.
Physically, the contact theory indicates that the friction
coefficient depends on the elastic modulus of an object: thelower the elastic modulus, the greater the friction [25]–[27].
Briefly, a softer object yields a greater contact area, resulting
in greater adhesion friction. As a result, the friction coefficient is approximately proportional to −2/3 the power ofYoung’s modulus of a soft object. Further, the softer object
deforms more and leads to greater internal friction or damping effects, referred to as hysteresis friction, which is approximately proportional to −1/3 the power of Young’s modulus
of a soft object. Thus, the friction coefficient and elastic
modulus are negatively correlated (see section IV about thefurther connection between Young’s modulus, friction, andsoftness perception). Our question is whether this relationshipis also true between the friction and the softness perception ofan object. More specifically, do surfaces with higher friction
feel softer even when the elastic moduli of the objects are
identical?
Several studies indirectly suggested this possibility.
For example, Egawa et al. [28] investigated the correlation
between physical friction on the skin of the contralateralvolar forearm after applying emulsions and sensory evaluations during the application of emulsions on facial skinand found a weak correlation between physical friction andperceived softness. Takahashi et al. [29] investigated the relationship between several physical parameters, including frictional parameters and perceived softness, using five soft
sponges composed of polyester urethane foam. Their results
demonstrated that the sponges with higher friction were
judged as softer. Nonetheless, they did not control either
the friction or hardness of the objects being manipulated.
Horiuchi et al. [30] investigated the relationship between thefriction of powders on silicone rubber surfaces and the perceived softness of the powders on human skin; this relationship exhibited a positive correlation. However, the assessors,
who were cosmetic professionals, might have judged thesoftness of powders on the skin, considering that cosmetics
do not immediately change the skin stiffness. Hence, earlier studies have not investigated the relationship between
physical friction and softness perception on the skin or on
skin-like materials in which the lubrication conditions weremanipulated, and the hardness was controlled.
To investigate whether physical friction influences softness perception, in this study, we conducted two types ofpsychophysical experiments. In the first experiment, participants either rubbed or pressed soft polyurethane surfaces,
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i.e., artificial skins, made using the same polyurethane formulation and the same molding process. However, these surfaces were covered with different lubricant powders so thatthey exhibited different friction coefficients. Participants discriminated perceived softness; we then investigated whether
the friction would affect their perception of softness. In thesecond experiment, a protocol similar to the first experiment
was adopted for human cheek skin. Note that the potential
applications of the present study include cosmetic products,
and then we target skin-like materials and human cheeks.
II. MATERIALS AND METHODS

The following experimental protocol was approved by theinstitutional review board of Shiseido Co. Ltd. (approval
C01724 and C01931).
A. EXPERIMENT 1: SKIN MODEL
1) STIMULI

We used cylindrically artificial polyurethane skin (tailoredbased on existing products, Beaulax Co., Ltd, Saitama, Japan)
with a diameter and thickness of 50 mm and 15 mm, respectively, and the surface roughness transferred from a Japanese
female cheek. The hardness was designed to be similar tothat of human facial flesh, with a hardness value of AO 4.5(30.7 kPa in Young’s modulus) measured using a durometer(GS-721, Teclock, Nagano, Japan) in accordance with ISO
7619-1.
We adopted five types of lubrication conditions using powders. In each of the four conditions, boron nitride, mica,titanium oxide (particle diameter: 0.25 µm), and titaniumoxide (particle diameter: 0.03 µm, hydrophobic powder).
These are popular cosmetic ingredients. In the fifth condition,
no lubrication was applied. The amount of each type of
powder was adjusted such that the surface was uniformly
covered using a cosmetic brush. It should be noted that the
lubrication powders do not influence the physical hardnessof the artificial skins. The kinetic friction coefficient between
the fingers and the surface was measured using an instrument
described in [31] for each lubrication condition. Table 1 lists
the measured values.
2) PARTICIPANTS

We recruited thirty-nine Japanese females (age: 20–68 years,
native Japanese speakers), eleven American females(age: 28–62 years, native English speakers), and eleven Italian females (age: 26–44 years, native Italian speakers). All
participants had lived in their native countries for more thanten years until the age of twenty years and were living in
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N. Arakawa et al.: Less Frictional Skin Feels Softer in Tribologically Paradoxical Manner

FIGURE 1. Experiment 1: Setup using five different lubrication methods in polyurethane samples.

Japan at the time of the experiments. None of the participants
reported any sensory, cutaneous, or kinetic impairments.
We recruited participants irrespective of their handedness.
Written informed consent was obtained from all participants.

TABLE 2. Descriptions that mean soft or friction.

3) RANKING TASKSFirstly, each participant conducted ranking tasks of fivespecimens for each evaluation descriptor, i.e., softness andfriction during rubbing. The tasks for judging softness andfriction were conducted separately in randomized order. After
that, they did another ranking task of five specimens forsoftness during pressing. Here, pressing is an indentation
perpendicular to the surface without sliding motion and intentional exertion of shearing forces. For individuals, three tasks,
i.e., softness and friction judgment during rubbing and softness judgment during pressing, were conducted only once.
All participants completed each task within 4 min.
Fig. 1 (a) shows the experimental setup for thepsychophysical experiments. A lace cloth was placed
between the participant and the five types of stimuli to prevent
them from judging the stimulus conditions based on thevisual assessment of the surface, because one type of powder
looked slightly white on the artificial skin. Nonetheless, theparticipant was made aware of the location of the stimuli.
The experimenter randomly placed the five types of stimuli
in front of participants in each trial. As shown in Fig. 1(b), the participant touched the five types of stimuli using
different fingers in a way that the lubrication powders were
not mixed. Zoeller et al. [13] showed that the sensitivity isdifferent among fingers during pressing objects. Therefore,
we checked whether the fingers used in Experiment 1 exhibited biases of ranks during the rubbing task, and the Friedman
test did not reveal differences among the fingers during
rubbing in perceived softness (N = 61, p = 0.112, χ 2 = 7.5,
df = 4) and in perceived friction (N = 61, p = 0.165,
χ 2 = 6.5, df = 4). There was no bias introduced by the
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specific fingers used in the study, i.e., no finger effect on thesoftness ranks in the following tasks.
The participants were presented with all types of stimuli
simultaneously and asked to rub each surface. They thenranked the stimuli based on each of the two perceptual
descriptions: softness and friction, presented in their own
native languages, as shown in Table 2. ‘‘Yawarakai,’’ whichmeans soft in Japanese, does not have the meaning of smoothness [32]. Inversely, ‘‘soft’’ in English and ‘‘morbida’’, whichmeans soft in Italian, include the meaning of smooth [33],[34]. We just focused on the softness perception and thenpresented the definition of the description to the participants
highlighting that the definition of ‘‘soft’’ and ‘‘morbida’’ did
not have the meaning of smoothness. Specifically, softness
was defined as follows: not hard, firm, or stiff, but easy topress and deform; soft was therefore regarded as the oppositeof hard. Here, softness largely corresponds to physical elasticity but does not include the nuance of surface uniformity,
unlike in a previous study [35]. Hence, the softness defined
here corresponds to the deformability among multiple typesof softness [6]. Friction was defined as the feeling of a force
that prevents one surface from sliding easily over anothersurface. High friction is the opposite of low friction. The
participants did not have any difficulty understanding thesedefinitions. The experimenter instructed participants as follows: ‘‘rank the five specimens in order of ‘softness/friction’feeling when you rub your finger on the surface.’’
The order of the two descriptors used for ranking was
randomized for each individual. The participants ranked theone of the descriptors after rubbing all five stimuli. They55281

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FIGURE 2. Friction coefficient, vibration, and stiffness of the stimuli (cheeks with powders) of experiment 2 measured bythree original sensors, which were customized to measure the physical conditions of human cheek skin. Bars indicate thearithmetic mean of the friction coefficient, mean amplitude spectrum (0.56–1000Hz), and stiffness, respectively. Error bars
indicate the standard error. n.s. and *** denote p ≥ 0.05 and p < 0.001, respectively.

rubbed them until they could assess. After assigning ranks
to the stimuli in terms of the first descriptor, the participants’fingers were wiped using wet papers and air-dried for fiveminutes. The five types of stimuli were then randomly repositioned, and the participants assessed the stimuli using the next
perceptual descriptor. For each participant, the ranking tasks
were conducted only once for each of the two descriptors.
After these tasks, the fingers were cleaned and dried again.
The participants pressed the five types of rearranged stimuli
without sliding the fingers on the surface and ranked thembased on perceived softness, during which the rubbing motion
was prohibited.
During the experiments, the finger motions of the participants were monitored by the experimenter, who gave instructions to the participants if needed. Before the experiments,
participants washed their hands using warm water and soap
and waited for ten minutes in a room with stable humidity(45–55%) and temperature (22–23o C). The participants’ fingers were then wiped using wet paper cloths and air-dried forfive minutes before each task.
4) STATISTICAL ANALYSIS

We applied the Freidman test to the ranks of five skin models
for each descriptor as an omnibus test to investigate potential
differences among the stimuli. Pair-wise comparisons werethen conducted as a post-hoc test to identify the surfaces
responsible for the differences with a Bonferroni correction.
We also calculated the Spearman’s rank correlation coefficient between the ranks of friction and softness provided byparticipants in the rubbing trials. Statistical analyses wereconducted using SPSS Statistics (version 23, IBM Corp.,
Chicago, IL).
B. EXPERIMENT 2: HUMAN CHEEK
1) STIMULI

We used two types of loose powder formulations designatedas formulae P and Q, as shown in Table 3. The formulations
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TABLE 3. Formulations of the two loose powders in percentage.

were adjusted so that they exhibited different lubrication
properties and complied with the internal safety regulations.
Formula P or Q was applied on each cheek of participants. It isnoted that no dry condition was tested as a control condition.
The vibration and friction caused when stroking the cheek by
a contactor were measured using the tactile sensor developed
as in [36], [37]. The stiffness of the cheek was measured
using a force-displacement sensor [38]. Fig. 2 summarizes themeasurement results. The friction coefficient was lower for
formula P than for formula Q. On the other hand, there wereno significant differences between formulae P and Q in termsof vibratory amplitude and stiffness. Thus, formulae P and Q
differed in terms of friction but not in terms of stiffness andsurface roughness on the cheek. As mentioned below, these
two types of powder were placed on the cheeks of participants
and they examined their own cheeks. In order to control thestimuli, the same person could have provided his/her own
cheeks to be examined by participants. However, in this case,this provider has to wash the face many times, leading to skindamages. Hence, we adopted a method where participants
touched their own cheeks.
2) APPLICATION OF LOOSE POWDERS ON CHEEKS

Participants removed their makeup using a makeup removerand washed their face and hands using warm water and soap.
They then waited for 10 min in a room with stable humidity(45–55%) and temperature (22–23o C). The participants’ fingers were wiped using wet paper cloths and air-dried for five
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N. Arakawa et al.: Less Frictional Skin Feels Softer in Tribologically Paradoxical Manner

min before each task. As aforementioned, participants thenhad the moisturizer (Vital Perfection Uplifting and Firming
Cream Enriched, Shiseido Co., Ltd., Tokyo, Japan, 200 µL)
applied on their whole face by an experimenter and waitedfor 15 min. The moisturizer was used to adjust the individual
skin conditions and apply the loose powders equally acrossthe participants. After that, an experimenter applied each typeof loose powder (30 mg) using a sponge puff (114, ShiseidoCo., Ltd., Tokyo, Japan) randomly on either of the cheeks.
For example, if formulation P was applied on the right cheek,
then formulation Q was applied on the left cheek.

TABLE 4. Test results of between-stimuli softness and friction
comparison in experiment 1.

TABLE 5. Results of softness and friction comparisons between
lubrication conditions in experiment 1.

3) PARTICIPANTS

We recruited twenty-five participants, nine Japanese females(age: 23–66 years, native Japanese speakers), nine American females (age: 23–62 years, native English speakers),
and seven Italian females (age: 26–53 years, native Italian
speakers). All participants had lived in their native countries
for more than ten years until the age of twenty years old
and were living in Japan as of the date of the experiments.
None of them reported any sensory, cutaneous, or kinetic
impairments. We recruited them irrespective of their handedness. Written informed consent was obtained from each participant. Two American participants were excluded from theanalysis because of cosmetic pilling of the skin while rubbing
their cheek in each trial. We investigated all participants as asingle group.
4) TASKS

The participants were asked to rub the surfaces of their rightand left cheeks using both index fingers simultaneously. The
right index finger rubbed the right cheek, and the left index
finger rubbed the left cheek. They were then asked whichside felt softer and which more frictional. The order of thesetwo questions, that is, softness and friction, was randomized.
The two perceptual descriptions (softness and friction) were
the same as in Experiment 1. A mirror was not used; theparticipants explored their cheeks without looking at theirfaces and behaviors. After these tasks, participants pressedboth sides of their cheeks without sliding their fingers andanswered which side felt softer. During the experiments, thefinger motions of the participants were monitored by theexperimenter, similarly, to Experiment 1. All participants
completed the task within 1 min.
5) STATISTICAL ANALYSIS

We applied a binomial test to the results of each descriptor
to investigate potential differences between stimuli. Statistical analysis was conducted using SPSS Statistics, as inExperiment 1.
III. RESULTS
A. EXPERIMENT 1: SKIN MODEL

Fig. 3 shows the results of the experiments using artificial
skin as stimuli. The horizontal axis indicates the lubrication
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condition (A: boron nitride, B: mica, C: no lubricant,D: titanium oxide (particle diameter: 0.25 µm), and E: titanium oxide (particle diameter: 0.03 µm)). The vertical axis
is the rank of softness or friction perceived by rubbing orsoftness perceived by pressing.
Significant differences (p < 0.05) in the ranks were foundamong the lubrication conditions for the rubbing tasks, but
not for the pressing tasks (Table 4). Hence, we performed
the Wilcoxon signed-rank test as a post-hoc pair-wise test for
all the pairs of rubbing task conditions (Table 5). Condition
A was judged softer than conditions B, C, D, and E, andconditions B and C were also judged softer than conditions D
and E. Condition D and E were judged to have higher friction
than conditions A, B, and C.
To examine the link between perceived friction and softness during rubbing, we calculated Spearman’s rank correlation coefficient between the ranks provided for friction andsoftness by all participants. The correlation coefficient wasr = −0.35 (p < 0.001).
Although the conditions physically differed only in termsof surface friction, the perceived softness differed among
the stimulus conditions during rubbing. The less frictional
conditions were judged as softer than the more frictional conditions. These results opposed our hypothesis that surfaces
with higher friction coefficients could feel softer, based on
the law of contact mechanics.
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N. Arakawa et al.: Less Frictional Skin Feels Softer in Tribologically Paradoxical Manner

FIGURE 3. Result of experiment 1 using artificial skin models. *, **, and*** denote p < 0.05, p < 0.01, and p < 0.001 in the Wilcoxon signed-rank testwith Bonferroni adjustment, respectively. A: boron nitride, B: mica, C: no lubricant, D: titanium oxide (particle diameter: 0.25 µm), and E: titanium oxide(particle diameter: 0.03 µm).

FIGURE 4. Results of experiment 2 during rubbing the cheeks with two
types of loose powders. The numbers of responders for each formula are
indicated on the bars. n.s. and * denote p ≥ 0.05 and p < 0.05 with abinomial test, respectively.

B. EXPERIMENT 2: HUMAN CHEEK

Fig. 4 shows the proportion of answers to the question about
which side of the cheeks felt softer or more frictional. Eighteen among the 23 participants judged the cheek with formula
P to be softer (p = 0.011, z = 2.50, N = 23) and the cheekwith formula Q to be more frictional (p = 0.011, z = 2.50,
N = 23) while rubbing. In contrast, the proportions forformulae P and Q were almost tied in terms of the perceived
softness felt during pressing (p = 0.678, z = 0.42, N = 23).
These results are consistent with those in Experiment 1 but
disagree with our hypothesis introduced in Section I.
IV. DISCUSSION

Most previous studies of softness perception have focusedon the compliance of objects during pressing or pinching[7]–[16]. However, humans also feel softness while rubbing
surfaces [4], [6], [19]. As mentioned earlier, although friction occurs during rubbing objects, previous studies on the55284

perceptual dimensions have shown that perceived softness
and perceived friction are mostly independent [1], [20]–[24].
We investigated the hypothesis that surfaces with greater
friction coefficients would feel softer considering the law ofcontact mechanics.
In the two experiments, participants judged the hardness
perceived by rubbing several types of artificial skins or theirown cheeks only when their surface lubrication was manipulated. Interestingly, the effect of friction was the oppositeof that predicted by our hypothesis. The smaller the friction
coefficient of the surface, the softer it felt. To the best of ourknowledge, this is the first study to report that physical surface friction affects softness perception. As previously mentioned in Section I, earlier studies [28]–[30] did not directly
investigate the effect of physical friction on the perceptionof softness. They did not control either the friction or hardness of objects while manipulating one of them [29] or only
performed a sensory evaluation to characterize emulsions or
powders themselves [28], [30]. Their objectives differed fromthose of the present study, in which participants rubbed andjudged the softness under controlled conditions with the same
hardness in the basal layer and only performed rubbing orpressing.
For the softness perception, the features of the contact between the object and fingertip are recognized as animportant cue [7]–[11], [18], [39]–[43], based on which wehave a hypothetical explanation of the present results. First,
a hypothesis of softness perception is introduced. According
to Hertz theory, when an elastic sphere contacts a planewith the normal force fn , the contact area A is given as
follows [44]: 2321−νobj
1−νf2
+
)
f
(
n
Ef
Eobj 

(1)
A∝

1
1
Rf + Robj
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N. Arakawa et al.: Less Frictional Skin Feels Softer in Tribologically Paradoxical Mannerwhere Rf and Robj are the radii of the sphere and flat object(1/Robj = 0), respectively. The sphere corresponds to the fingertip and Rf is its size. Ef and Eobj are Young’s moduli of thesphere and flat object, respectively. Provided that the sphere isa fingertip, Ef and Rf are known values. The formula suggeststhat Eobj can be estimated by fn and A. The Poisson ratios νfand νobj are nearly constant, i.e., 0.5 [45], considering that theflat object is skin: νf ∼ νobj ∼ 0.5. Briefly, based on the ratioof A and fn , the hardness of the object is estimated as follows:2
3
1
1∝ Rf ( +
) .
2
Ef
Eobj
3


A

(2)

fn

Although (1) and (2) mention the contact area, a similar discussion can be held for the maximum pressure or maximum
normal deformation of the sphere.
When the sphere slides on a flat surface, a tangential force
ft is applied to the objects. If the components of the contact
force, that is, ft and fn (normal force), are not perceptually
precisely decoupled, which may be a strong assumption to betested in the future, and the hardness perception is based on
the ratio of the resultant contact force fr and A, (2) can be
replaced by:
A
fr

2
3

∝

1
1
Rf ( + 0 )
Ef
Eobj

!2

3

(3)

1

fr = (fn2 + ft2 ) 2 .

(4)

0 is the perceived Young’s modulus. By using AmontonsEobj
Coulomb’s law (ft = µfn ) and the coefficient of friction µ,(3) can be rewritten as

1
1∝ Rf ( + 0 )
2
1
Ef
Eobj
fn3 (1 + µ2 ) 3
A

!2

3

.

(5)

This indicates that the left side rate decreases as µ increases,
0 . Hence, the change in µ may
leading to an increase in Eobj
interfere with the judgment of object hardness while rubbing
it. The above discussions focused on the contact force andskin-object contact area; however, the confusion or incorrect
separation of skin deformations caused by the normal and tangential forces may also explain the results of our experiment.
In Experiment 1, some participants reported that higher
frictional conditions D and E were felt softer than lower frictional conditions A and B. This tendency was in contradiction
to the average overall results. It is not clear whether theyfelt conditions D and E as softer than the others, or whether
instead, they failed to distinguish perceived softness among
the five types of stimuli and randomly made up their answers.
The titanium oxide powder used in condition E is hydrophobic, while the others are not. Therefore, the skin water content
or sweat of individual fingers might have affected the lubrication and their judgment of friction and softness. Further
studies on the interaction between, for example, finger-sweat
and powder characteristics are necessary.
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Some questions remain unanswered about the perceptionof softness caused by the difference in friction coefficients.
In this study, participants evaluated the feeling of softness
when rubbing the surfaces of artificial skin models or theircheeks. It is not clear whether the same phenomenon isobserved when rubbing objects with stiffness markedly different from human skin. Moreover, the stiffness of the stimuli
in the experiments was the same, and we did not compare
the difference in physical stiffness with the difference inperceived softness caused by friction. We did not conduct
this comparison mainly because the surface friction is covaried with the object stiffness, and it is difficult to preparea stimulus set with different stiffnesses but the same friction. Additionally, although the ranking task was adopted
in Experiment 1, the psychophysical method of constantstimuli provides more solid results. However, the methodof constant stimuli requires long task periods, which were
not allowed under the pandemic situation. Previous studies have shown differences in attitudes and sensory perception toward cosmetic products among cultures [46]–[49].
For example, American people tend to rub their skins tojudge its softness, whereas Japanese people opt to press [48].
We recruited Japanese, Italian, and American participants
to our experiments and investigated cross-cultural differences. However, we could not conclude potential similarities
or differences among different cultures mainly because of
the limited number of participants from different cultures.
Finally, we introduced a hypothetical principle of frictiondriven softness perception; however, this principle needs to be
pursued in the future. The present study merely demonstrated
a relationship between the surface friction and softness perception when rubbing the skin and skin-like objects.
V. CONCLUSION

This work aimed to test the hypothesis that surface friction
could influence the softness perceived when a finger rubs thesurface of a soft material. We created controlled stimuli withequivalent stiffness and different friction coefficients on thesurface using artificial skin and human cheeks. The artificial
skin was covered with different kinds of lubricant powders,
and the human cheeks were covered with loose powders
made of cosmetic powder ingredients. We conducted two
psychophysical experiments using artificial skin and humancheeks, where participants discriminated the softness of thestimuli during rubbing. The first experiment, using artificial
skin, found that friction affected the perceived softness. Thisfinding was replicated in the second experiment using humanskin.
We conclude that surface friction affects the perception ofsoftness when rubbing soft objects. This influence appears
to be observed only when the surfaces were rubbed, but notwhen they were pressed. Interestingly, the effect of friction
was the opposite to that predicted by our hypothesis. It wasdemonstrated that the smaller the surface friction coefficient,
the softer the surface felt. We suspect that human’s incorrect
separation between the normal and tangential forces or skin55285

N. Arakawa et al.: Less Frictional Skin Feels Softer in Tribologically Paradoxical Manner

deformations at the finger pad explains this phenomenon;
however, the root cause of this phenomenon remains to beelucidated in the future.
ACKNOWLEDGMENT

The authors are grateful to Tomomi Shimizu for her help
during this work and to Taiki Kazama for his special sensor.
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NAOKI SAITO received the master’s degree fromthe Graduate School of Engineering, Tohoku University, in 2009. He is currently a Researcher with
the Shiseido Global Innovation Center, Yokohama,Japan. His research interests include tactile perception and affective engineering.

NAOMI ARAKAWA received the master’s degreefrom the Faculty of Engineering, Osaka University, in 2004. She is currently a Researcher with
the Shiseido Global Innovation Center, Yokohama,Japan. Her research interests include tactile perception and affective engineering.

SHOGO OKAMOTO (Member, IEEE) received
the Ph.D. degree in information sciences fromTohoku University, in 2010. He was an Associate
Professor at the Department of Mechanical Systems Engineering, Nagoya Unviersity. Currently,
he is an Associate Professor with the Department of Computer Sciences, Tokyo Metropolitan
University. His research interests include haptics,
assistive robotics, and affective engineering.

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