characterization_of_yield_stress_and_slip_behaviour_of_skin_and_hair_care_gels

Characterization of yield stress and slip behaviour of skin/hair care

gels using steady flow and LAOS measurements and their

correlation with sensorial attributes

S. Ozkan, T. W. Gillece, L. Senak and D. J. Moore*

Materials Science Group, Global R&D, Ashland Specialty Ingredients, 1361 Alps Road, Wayne, NJ 07470, U.S.A.

Received 29 June 2011, Accepted 19 December 2011

Keywords: gels, LAOS, rheology, sensory correlation, slip, yield stress

Synopsis

Gels made with three different polymers widely used as rheology

modifiers in cosmetic formulations (cross-linked poly(acrylic acid),

cross-linked poly(maleic acid-alt-methyl vinyl ether) copolymer and

cross-linked poly(acrylic acid-co-vinyl pyrrolidone) copolymer) were

characterized by rheological and sensory evaluation methods to deter-

mine the relationship between sensorial perception and corresponding

rheological parameters. Both conventional rheological characteriza-

tion methods and a more recent method, Fourier Transform Rheology

with Large Amplitude Oscillatory Flow data (LAOS), were utilized to

characterize the material with and without wall slip. Sensorial analy-

ses were implemented in vivo to evaluate the perceived ease of initial

and rub-out spreadability, cushion, pick-up and slipperiness attributes

of the gels. Results were statistically analysed by both variance (ANO-

VA) and principle component analysis (PCA). Sensorial panel testing

characteristics discriminated the three materials, and PCA analyses

revealed that sensory attributes could be well predicted by rheological

methods. Rheological experiments, without wall slip, revealed that gel

strength in the linear viscoelastic region (LVR) and yield stress of these

materials are similar, but exhibit significantly different wall slip and

thixotropy behaviour in the low shear rate region under wall slip con-

ditions. Above the critical shear rate, which corresponds to the yield

stress, all tested materials did not slip and behaved as conventional,

shear thinning polymeric fluids. In particular, the rheological parame-

ters and sensorial perception of the 1% cross-linked vinyl pyrrolidone/

acrylic acid copolymer were significantly affected by wall slip and/or

thixotropy-related shear banding phenomena.

Introduction

Sensory properties of personal care products contribute substantially

to overall consumer acceptance. Therefore, costly time-consuming

sensory evaluation techniques are applied to guide the formulator in

identifying and defining the sensory profile of a product. Further, by

correlating quantitative instrumental parameters with critical, yet

subjective, sensorial ratings, consumer perception may be better

understood.

Conventional rheological and mechanical testing methods, such

as dynamic oscillatory flow and steady torsional flow measure-

ments, measuring Young’s modulus or maximum normal force,

etc., have been widely utilized to characterize hydrogels in the

food industry for the purpose of correlating structural properties

with sensorial perception [1–6]. Even though a significant

amount of recent literature is available for this purpose, certain

aspects of hydrogel rheology, such as thixotropy, wall slip and

shear banding phenomena, were not addressed in these studies.

In a recent review [7], Fisher et al. mention that a description of

the rheology of gels and concentrated food systems, such as gel-

like glassy matter exhibiting ageing behaviour, is considered a

new approach and is not embraced as a method of choice by

food scientists [7].

From a rheologist’s point of view, characterization of hydrogels

and gel-like percolated suspensions/emulsions, and the determina-

tion of their accurate yield stress present special challenges. These

materials are associated with thixotropy, viscoplasticity and wall

slip. Ideally, the true yield stress should be determined directly as

the minimum values of the shear stress (1-D) or stress magnitude

(3-D) at which deformation is observed, giving special consideration

to the wall slip effect. Typically, the yield stress is determined from

shear stress vs. shear rate data by fitting the Bingham or Herschel–

Bulkley equations to the data. These methods are, however, prone

to experimental errors and wall slip effects, and not all materials

comply with these common methods. Consequently, measuring the

rheological parameters without paying attention to thixotropy and

wall slip, as well as correlating these parameters with sensorial

attributes, may not be optimal.

In this study, we conducted an extensive rheological study

addressing wall slip and thixotropy to investigate the correlations

between the rheological properties (conventional and Fourier trans-

form rheology with large amplitude oscillatory flow data (LAOS))

and the sensorial attributes of three commercially available cosmetic

rheological modifiers. The purpose of the study was to discern which

rheological methods and parameters are more appropriate for corre-

lation to specific sensorial attributes. To the extent of our knowl-

edge, a sensorial correlation study that combines conventional and

LAOS rheological parameters with and without wall slip has not

been published as it relates to cosmetics.

Correspondence: Seher Ozkan, Material Science Group, Global R&D, Ash-

land Specialty Ingredients, 1361 Alps Road, Wayne, NJ 07470, U.S.A.

Tel.: 9736283971; Fax: 9736283886; e-mail: [email protected]

This work has been presented at the 82nd Society of Rheology Annual

Meeting, Santa Fe, New Mexico, October 27, 2010 and MRS Fall meeting,

Boston, MA, December 4, 2009.

*Current address: TRI-Princeton, 601 Prospect Avenue, Princeton, NJ

08540, U.S.A.

International Journal of Cosmetic Science, 2012, 34, 193–201 doi: 10.1111/j.1468-2494.2012.00702.x

ª 2012 ISP Investments Inc

ICS ª 2012 Society of Cosmetic Scientists and the Societe Francaise de Cosmetologie 193

The selected hydrogel rheology modifiers were as follows: cross-

linked poly(acrylic acid), cross-linked poly(maleic acid methyl vinyl

ether) copolymer and cross-linked poly(acrylic acid-co-vinyl pyrroli-

done) copolymer. The slip behaviour of these hydrogels was investi-

gated by compiling steady torsional flow data at different gap

openings and dynamic strain sweep data with smooth and rough-

ened surfaces (to allow or suppress wall slip). The yield stress, consis-

tency index and power law index of the hydrogels were obtained by

fitting slip-corrected shear stress vs. shear rate data to the Herschel–

Bulkley (H-B) model. In addition to conventional rheological charac-

terization methods, a recent method, Fourier transform rheology

with LAOS was utilized to characterize the material with and with-

out wall slip. Sensorial analyses were implemented in vivo to evalu-

ate the perceived ease of initial and rub-out spreadability, cushion,

pick-up and slipperiness attributes of the gels. The overall rheological

parameters were correlated with sensory panel test results.

Materials and methods

Three different rheology modifiers were studied: cross-linked poly

(acrylic acid) (Carbopol� 980; Lubrizol, Wickliffe, OH, U.S.A.),

cross-linked poly(maleic acid-alt-methyl vinyl ether) copolymer

(Stabileze� QM; ASI, Wayne, NJ, U.S.A.) and cross-linked poly

(acrylic acid/vinyl pyrrolidone) copolymer (Ultrathix� P100; ASI).

We will use the following acronyms in the text from this point on

to reference the thickeners: PAA-XL for cross-linked poly(acrylic

acid); PVM/MA-XL for cross-linked poly(maleic acid-alt-methyl

vinyl ether) copolymer; and PAA/VP-XL for the cross-linked poly

(acrylic acid/vinyl pyrrolidone) copolymer.

Samples were prepared as 1% (w/w) gels in de-ionized water

and were neutralized and preserved following the protocols given

by the manufacturers. Air bubbles were removed by centrifuging

the samples. Prior to use, each sample was left unperturbed for at

least 24 h to rebuild the structure lost during preparation and han-

dling. The sample concentrations were confirmed by thermogravi-

metric analysis measurements (Hi-Res TGA 2950 from TA

Instruments, New Castle, DE, U.S.A.). During the study, all samples

were stored in sealed containers at room temperature.

Rheological characterization

The rheological properties of 1% gels were investigated using a

stress-controlled AR-G2 rheometer and a strain-controlled ARES

rheometer (TA Instruments). All tests were carried out at

25 ± 0.1�C. As sample loading conditions influence testing, a

meticulous and consistent routine for sample loading was followed

to promote reproducible results. Prior to data collection, a 5-min

delay was applied to ensure rebuilding of the gel structure that was

compromised during sample loading. Four different types of mea-

surement results are reported:

Dynamic oscillatory measurements

The strain and frequency dependency of the materials functions,

such as the magnitude of complex viscosity (g*), storage modulus

(G¢(x)), loss modulus (G¢¢(x)) and oscillatory stress, were measured

using dynamic testing. Generally, smooth-surfaced plates were used

for dynamic testing; however, strain sweep experiments were also

repeated at x = 1 rps using plates, the surfaces of which were cov-

ered with 400 grit, adhesive-backed, waterproof sandpaper (ARC

Abrasives, Inc., Troy, OH, U.S.A.) to eliminate wall slip. Fourier

transform analysis was applied to the large amplitude oscillatory

shear (LAOS) flow data that had been collected with smooth and

sandpaper-covered plates. The sinusoidal stress response signal col-

lected from the sample was separated into elastic and viscous stress

contributions using symmetry arguments [8]. Chebyshev polynomi-

als (closely related to the Fourier deconvolution) were utilized as

orthonormal basis functions to further decompose these stresses

into odd and even harmonic components having physical interpre-

tations [8]. Multiple steady-state wave forms were used for data

analysis (typically three cycles of data were collected, and the last

two cycles, where the data had equilibrated, were used) at each

coordinate pair (x, c0).

Steady torsional flow experiments

The AR-G2 rheometer with parallel disc fixtures of 20 mm diame-

ter was used for rotational viscometry. At each shear rate, a fresh

sample was used to avoid pre-shearing of the gels. Steady torsional

flow experiments with parallel plates were carried out at two differ-

ent gap heights, 1.0 and 1.5 mm, for each shear rate. Each condi-

tion was repeated at least three times with fresh samples. As will

be discussed in the results section, the results from these experi-

ments are indicative of the presence of wall slip in torsional flow

[9, 10]. The wall slip behaviour of the gels, characterized in terms

of the slip velocity vs. the wall shear stress relationship, was used

in correcting the rheological data for wall slip and calculation of

Navier’s slip coefficients following Kalyon et al. [11]. Herschel–

Bulkley-derived parameters, such as the yield stress, consistency

index and power law index, were determined from fitting slip-cor-

rected steady torsional flow data to the Herschel–Bulkley model

defined by the following equation (1),

s ¼ s0 þm _cn ð1Þ

where s is the shear stress (Pa), s0 is the yield stress (Pa), m is the

consistency index (Pa s1/n), _c is the shear rate (1 s)1) and n is

the power law index.

Continuous shear rate ramp tests

The AR-G2 rheometer, equipped with 20-mm parallel disc fixtures,

was used for stress/shear rate ramp tests by ramping up to 500 s)1

in 1 min at two different gap openings (1.0 mm and 1.5 mm) to

determine the shear rate range where wall slip influenced the mea-

surements.

Extensional tests

The AR-G2 rheometer, with 20 mm stainless steel parallel plates,

was used to carry out extensional force measurement tests. The

same pre-test protocols used in rotational experiments were used

for all extensional testing. The initial gap was set to 400 lm, and

the top plate was raised to a 3-mm gap with a speed of 3 mm s)1.

Force vs. gap data were collected, and the maximum force reading

was reported. Each sample was tested at least six times, and the

results were reported as averages of maximum force values with

95% confidence intervals. One-factor analysis of variance (ANOVA)

test was applied to test the significance of differences.

Sensory evaluation

Primary skin-feel parameters, such as pick-up, cushion/body/firm-

ness, initial spreadability, and secondary skin-feel parameters, such

as rub-out spreadability, slipperiness/lubrication, were evaluated for

correlation against rheological parameters. We used the existing

lexicon to define these attributes and to determine the intensity

scales and reference values for each attribute [12].


ICS ª 2012 Society of Cosmetic Scientists and the Societe Francaise de Cosmetologie

International Journal of Cosmetic Science, 34, 193–201194

Sensory correlation with rheology S. Ozkan et al.

A randomized complete-block experimental design was carried

out for evaluation of the samples, where panellists are the ‘blocks’

and the samples are the ‘treatments’. Each panellist evaluated all

three samples to form a ‘complete block’. This design type is effec-

tive when panellists are consistent in rating the samples, but may

be using different ranges of the scale to express their perceptions.

Primary and secondary skin-feel parameters were evaluated by six

selected untrained panellists. Each attribute was tested on separate

days. The panellists rated the pick-up, cushion/body/firmness, ini-

tial spreadability, rub-out spreadability and slipperiness/lubrication

character of the gels on a scale of 0–10. Reference standards were

available as given in Meilgaard et al. [12]. The same sample

batches were used for both sensory evaluation and rheological

characterization of 1% PAA-XL, 1% PVM/MA-XL and 1% PAA/VP-

XL. Samples were stored in sealed ointment jars at 25�C and equili-

brated at least 24 h prior to evaluation to let the internal structure

completely rebuild. All tests were conducted in a temperature- and

humidity-controlled environment. Panel members were informed

that the samples were composed of three different thickeners, but

the identity of individual samples was not disclosed. Members

worked individually, and no discussions took place during the ses-

sions.

A rectangle of silicone release paper was taped to the bench top,

and 0.3 grams of each sample and reference materials were deli-

cately and simultaneously applied to the substrate surface using a

measuring spoon to ensure the minimal disruption of the internal

structure of the gel network. Sample jars were three-digit coded

and were presented to panellists in random order. Panellists were

instructed to rotate their finger at a defined rotation rate and dura-

tion for each test. They were asked to use the same finger to test

each sample and were subsequently instructed to clean fingers

between samples. Only one direction of rotation was permitted for

each panellist during the experiment. Prior to probing the perfor-

mance of the studied gels, the panellists were instructed to first cal-

ibrate the scale of the sensorial measurement by examining the

control(s).

Data analysis

Data were analysed by a two-factor (assessor, sample) analysis of

variance (ANOVA) test using Excel� software. The mean rating

and Fisher’s least significant differences for each term were calcu-

lated by ANOVA. Principle component analysis (PCA) of the mean

rating for each sensory attribute was used to visualize the relation-

ship between variables and samples using XLSTAT (Addinsoft, New

York, NY, U.S.A.).

Results and discussion

Rheological characterization

Dynamic oscillatory measurements

The strain amplitude dependency of dynamic material properties

were investigated using smooth-surfaced fixtures at 1, 10 and 20

rps frequency, and rough-surfaced fixtures at 1 rps frequency.

Material functions, such as the magnitude of complex viscosity

(g*), storage modulus (G¢(x)), loss modulus (G¢¢(x)), shear stress

and elastic stress (product of storage modulus and magnitude of

strain amplitude) [13, 14], of the 1% hydrogels were measured

over a strain amplitude (c0) range of 0.1–500%. All three hydro-

gels exhibited linear behaviour for a range of strain amplitudes up

to 1% for both low and high frequencies. However, at higher strain

amplitudes of 1% (or at high shear stress), the dynamic response

becomes non-linear and G¢, G¢¢ and g* decrease dramatically. Strain

sweep results have indicated that at low and high frequencies, G¢ is

higher than G¢¢, showing typical physical gel behaviour.

Strain sweep experiments were repeated at x = 1 rps with

plates, of which surfaces were covered with 400 grit adhesive-

backed waterproof sandpaper to eliminate slip. Figure 1a shows

that the modulus values started to decrease at a lower critical

strain, when compared to those shown in Fig. 1b, because of the

onset of slip. This trend is exacerbated in the 1% PAA/VP-XL,

where the slip effect is more pronounced. The yield stress values of

the three gels for slip and no-slip conditions, determined from the

maximum value of the elastic stresses (product of storage modulus

and magnitude of strain amplitude) at different frequencies, are

given in Table I. Table I shows that the calculated maximum elas-

tic stress values are frequency dependent (shear rate dependent)

because of shear thinning and/or thixotropic behaviour of the gels.

These values are also surface roughness–dependent because of the

onset of slip. In the low shear rate region (x = 1 rps), there is a

significant difference between the maximum elastic stress values

measured using smooth surfaces. In contrast, no significant differ-

ence between the gels was noted when rough surfaces were used.

The maximum elastic stress values increased with increasing fre-

quency for all samples.

The storage modulus in the linear viscoelastic region (LVR), G¢,is another important parameter that reflects the strength and/or

ratio of the interactions among the polymer chains or swollen

cross-linked domains. The G¢ values of all three samples in the LVR

are given in Table II. Table II shows that, in the LVR, there was no

significant effect because of slip and that there was only a slight

increase in G¢ with increasing frequency in all samples.

(a) (b)

Figure 1 Strain amplitude dependency of the

storage modulus (G¢ (dyn cm)2)) at 1 rps using

smooth surface fixtures (a) and rough surface f-

ixtures (b).



International Journal of Cosmetic Science, 34, 193–201 195


Fourier transform analysis was also performed on the LAOS data

collected with rough and smooth surfaces at 1%, 200% and 400%

strain amplitude values. The sinusoidal stress response signal col-

lected from the sample was decomposed into elastic and viscous

stress contributions using symmetry arguments following the meth-

ods given by Ewoldt et al. (2008) [8]. Chebyshev polynomials were

calculated using MITlaos software, and the results are shown in

Tables III and IV [8]. One percent of strain amplitude was chosen

to represent the LVR, 200% strain amplitude to represent transition

region from linear to non-linear and 400% strain amplitude to rep-

resent the non-linear region of the material. Table III shows that

1% gels exhibit shear thickening / strain softening behaviour in

the LVR (1% strain amplitude). PAA/VP-XL gel exhibits shear thin-

ning / strain stiffening behaviour, whereas 1% PVM/MA-XL and

1% PAA-XL exhibit shear thickening / strain stiffening in the tran-

sition region (200% strain amplitude). All three gels exhibited

shear thinning / strain stiffening behaviour in the non-linear

region (400% strain amplitude). For smooth surfaces, all 1% gels

showed shear thickening / strain stiffening behaviour in the transi-

tion region (200% strain amplitude) and shear thinning / strain

stiffening behaviour in the non-linear region (400% strain ampli-

tude). The absolute values of all Chebyshev coefficients that were

measured with rough surfaces were different than the correspond-

ing coefficients measured with smooth surfaces, indicating the pres-

ence of wall slip effects on the measurements.

Steady torsional flow experiments

Steady torsional flow experiments with 20-mm parallel plates were

carried out at two different gap heights (1.0 and 1.5 mm) for each

different shear rate value. Each condition was repeated at least three

times with fresh samples. The results show that the shear stress val-

Table I Yield stress values determined from maximum elastic stress calcula-

tions for different frequency and surface conditions

1% PAA/

VP-XL

1% PVM/

MA-XL 1% PAA-XL

Maximum elastic stress,

Pa (x = 1 rps), smooth surface

29 139 136



159 206 209



191 246 259


Pa (x = 1 rps), rough surface

169 175 164

Table II Storage modulus, G¢, values in the linear viscoelastic region for dif-

ferent frequency and surface conditions

1% PAA/VP-XL 1% PVM/MA-XL 1% PAA-XL

G¢, Pa (x = 1 rps),

smooth surface

859 788 565

G¢, Pa (x = 10 rps),

smooth surface

932 830 626

G¢, Pa (x = 20 rps),

smooth surface

923 884 641

G¢, Pa (x = 1 rps),

rough surface

851 765 550

Table III Chebyshev coefficients, which are calculated from large amplitude oscillatory flow (LAOS) data using MITlaos software. Experiments were conducted

at 1 rps frequency using smooth surface fixtures at 1%, 200% and 400% strain amplitudes

Strain amplitude % G¢, Pa G¢L/G¢M e3, Pa tand gL/gM v3, Pa s Physical attribute

1% PAA/VP-XL 400 29.51 3.79 10.38 2.68 0.83 )5.55 Shear thinning, strain stiffening

1% PVM/MA-XL 400 34.27 4.46 12.48 2.59 1.03 )4.21 Shear thinning, strain stiffening

1% PAA-XL 400 41.10 1.86 8.48 1.80 1.04 )2.56 Shear thinning, strain stiffening


1% PVM/MA-XL 200 81.00 1.93 14.87 1.59 1.24 1.25 Shear thickening, strain stiffening

1% PAA-XL 200 84.50 1.40 9.54 1.23 1.25 1.70 Shear thickening, strain stiffening

1% PAA/VP-XL 1 843.2 0.91 )4.5 0.07 2.07 3.44 Shear thickening, strain softening

1% PVM/MA-XL 1 704.97 0.94 )1.79 0.08 1.89 1.70 Shear thickening, strain softening

1% PAA-XL 1 529.09 0.94 )1.58 0.08 1.87 1.56 Shear thickening, strain softening

Table IV Chebyshev coefficients, which are calculated from large amplitude oscillatory flow (LAOS) data using MITlaos software. Experiments were conducted

at 1 rps frequency using rough surface fixtures at 200% and 400% strain amplitudes

Strain amplitude, % G¢, Pa G¢L/G¢M e3, Pa tand gL/gM v3, Pa s Physical attribute


1% PVM/MA-XL 400 37.06 4.08 12.57 2.35 1.10 )3.15 Shear thinning, strain stiffening

1% PAA-XL 400 43.64 1.79 8.48 1.65 1.12 )1.61 Shear thinning, strain stiffening

1% PAA/VP-XL 200 89.22 1.78 15.16 1.32 1.43 3.06 Shear thickening, strain stiffening

1% PVM/MA-XL 200 86.96 2.07 14.79 1.44 1.31 2.63 Shear thickening, strain stiffening

1% PAA-XL 200 90.01 1.28 8.29 1.05 1.32 3.32 Shear thickening, strain stiffening





ues increase with increasing gap separation at a constant apparent

shear rate. The dependence of the data on the gap separation sug-

gests sensitivity to the surface-to-volume ratio of the geometry being

utilized. This behaviour indicates the presence of wall slip in tor-

sional flow. A number of recent studies, which could be considered

benchmarks for the characterization of these types of materials,

showed that the steady-state simple shear behaviour can be very

well represented by the Herschel–Bulkley model. The yield stress of

these viscoelastic materials can be determined by the extrapolation

of the shear stress vs. shear rate data to zero shear rate by fitting

the data to the model after Rabinowitsch and slip corrections

[15–18]. Apparent torque and shear rate data were corrected for

non-linearity using the Rabinowitsch correction. Apparent shear

rate data were also corrected for wall slip in the low shear rate

region using the modified Mooney method according to the existing

literature [9–11]. H-B parameters, such as s0 (yield stress (Pa)), m

(consistency index (Pa s1/n)), _c(corrected shear rate (1 s)1)) and

n (power law index), are determined from fitting the

Rabinowitsch and slip-corrected steady torsional flow data

to the equation (1). The results are given in Table V.Results show that the yield stress values determined from the H-B

fit are in the same range and order with the yield stress values

obtained from the maximum elastic stress calculations using

dynamic strain sweep data collected with rough surfaces (see

Tables I and V). The maximum elastic stress values are, however,

slightly higher than the yield stress values determined by the model.

We compared the results for 1% PAA-XL with the existing litera-

ture. Roberts and Barnes reported a study with Carbopol dispersions

taking slip phenomenon into account [19]. They used vane geome-

try with a slender gauze basket inserted inside the outer cylinder to

suppress wall slip and reported that the yield stress of 1% PAA-XL

as 115 Pa by fitting the data to the Herschel–Bulkley model. In this

study, we used a 20-mm stainless steel parallel plate working

against a Teflon�-coated surface and fitted the slip-corrected data

to the same model. This laboratory predicted the yield stress value

of 1% PAA-XL as 124 Pa, indicating consistency with the existing

literature. It should be emphasized that water quality, differences in

neutralization and mixing processes, measurement protocols and so

on will affect the quality and precision of the results. As per Piau

et al., ‘The accuracy of quantification with complex fluids is much

poorer than with polymer solutions, and an overall accuracy of

10% can usually be considered as very good indeed’ [16].

Continuous stress or shear rate ramp measurements to establish the

shear rate dependency of shear stress and viscosity at different gap

openings

Continuous shear rate ramp tests were performed by ramping from

0–500 s)1 in 1 min at two different gap openings (1.0 and

1.5 mm) to determine the impact of the sample gap on the mea-

sured shear stress and viscosity. The results are given in Fig. 2. Fig-

ure 2a shows the impact of wall slip effects on the measurements

in the shear rate range between 0.06 s)1 and 2 s)1. Slip velocity

increases up to 0.5 s)1 and then decreases with increasing shear

rate until it reaches 2 s)1, the critical shear rate, where the mate-

rial becomes fluid. These findings are also in agreement with the

results given in previous literature [19 and 16]. Figure 2b clearly

shows the onset of wall slip effects on the measurements for 1%

PVM/MA-XL in the shear rate range between 0.05 s)1 and 5 s)1.

Slip velocity increases to 0.1 s)1 and then begins decreasing with

increasing shear rate until it reaches 5 s)1, the critical shear rate,

where the material becomes fluid. Figure 2c indicates that 1%

PAA/VP-XL measurements are affected by wall slip over a much

wider shear rate range than the other samples. The material slips

in the shear rate range between 0.01 s)1 and 10 s)1, and the slip

velocity increases with increasing shear rate up to 0.02 s)1, but

then subsequently tapers.

Yield stress values, or the critical stress values corrected for wall

slip (Table I and V), show that the critical stresses necessary to dis-

rupt the entire internal structure for all three materials are similar.

In contrast, under wall slip conditions, the gel samples exhibit sig-

nificantly different behaviour below the critical shear rate. The

Table V Herschel–Bulkley model parameters and Navier’s slip coefficients

1% PAA/VP-XL 1% PVM/MA-XL 1% PAA-XL

s0, Pa 161.5 168.5 123.7

m, Pa s1/n 12 22.8 41.5

n 0.54 0.52 0.43

b, m (Pa s1/nb)nb 0.0033 0.141 0.024

sb 1.07 0.35 0.43

(a)

(b)

(c)

Figure 2 Continuous shear rate ramp results at 1.5-mm and 1.0-mm gap

openings for (a) PAA-XL, (b) PVM/MA-XL and (c) PAA/VP-XL.





results indicate that the PAA/VP-XL possesses the highest slip

velocity and wall slip onsets at much lower shear stresses and

shear rate as compared to PVM/MA-XL and PAA-XL (Table I,

Fig. 2).

Extensional tests

The maximum force data collected during extensional testing were

used to glean information about each material’s cohesiveness. As

the intent is to mimic the ‘pick-up’ sensory evaluation test, the ini-

tial gap was kept as low as 400 microns, and the top plate was

raised with a relatively high speed (3 mm s)1). After sample load-

ing, the material was equilibrated for 5 min prior to testing in

order to rebuild the internal structure. Each sample was tested at

least six times, and the results are given in Fig. 3 as averages of

maximum force values with 95% confidence intervals for each

sample. One-way ANOVA results indicate that 1% PAA/VP-XL gen-

erated significantly lower maximum forces than 1% PAA-XL,

whereas the 1% PVM/MA-XL forces were only directionally >1%

PAA/VP-XL. The lower extensional force of 1% PAA/VP-XL may

be attributed to increases in the surface-to-volume ratio, possibly

due to wall slip effects, shear banding and/or thixotropy.

Sensory evaluation

Primary skin-feel parameters, such as pick-up, cushion/body/firm-

ness, initial spreadability and secondary skin-feel parameters, such

as rub-out spreadability and slipperiness / lubrication, were evalu-

ated by six selected naıve panellists. The purpose of the study was

to investigate the correlation of sensory attributes with the mea-

sured rheological properties of the three thickeners. At the initial

stage of the study, gels were characterized at 25�C by placing the

sample on a skin simulant surface, rather than on the forearm of

the panellist. This was performed to eliminate transient structural

changes of the materials during testing, because of temperature,

electrolyte effects and pH differences of the skin surface. Even

though trained panellists were not used, very explicit instructions

were prepared to make sure that each panellist would work at the

same shear rate range during the evaluation of each attribute.

ANOVA analysis conveyed that variations between assessors were

significant for cushion, initial spreadability and slipperiness ratings,

but were not significant for rub-out spreadability and pick-up

ratings. On the other hand, differences between samples were

significant for all attributes except rub-out spreadability. ANOVA

analysis showed that panellists used different parts of the scale to

express their perceptions, but were consistent in ranking and differ-

entiating their differences in general.

In PCA of the sensory evaluation data for the three different gel

samples, the first two principle components accounted for 81% and

19% of the variance, respectively. Sensory ratings of the three gel

samples plotted for the first two principle components are shown in

Fig. 4. One percent PAA/VP-XL scored to the left side of F1, show-

ing low values of cushion, slipperiness and pick-up ratings, but

high values of initial and rub-out spreadability ratings. One percent

PAA-XL scored on the right side of F1, contrasting with 1% PAA/

VP-XL, showing high values of cushion, slipperiness and pick-up

ratings, but low values of initial and rub-out spreadability ratings.

One percent PVM/MA-XL scored low in F2, showing low values of

all attribute ratings. The correlation between sensory attributes

showed that cushion, slipperiness and pick-up are related, whereas

initial and rub-out spreadability are related, but are in contrast

with cushion, slipperiness, and pick-up.

Correlation of sensory ratings with conventional rheological

parameters

Principle component analysis analysis was applied to the rheologi-

cal parameters and sensory evaluation ratings data together, and

the results are in Fig. 5. The first two principle components

accounted for 76.01% and 23.99% of the variance, respectively.

The correlation between the sensorial and rheological material

parameters indicates that pick-up, slipperiness and cushion are

related to each other and also to the consistency index (m) from

the H-B fit, as well as the maximum normal force (MNF) values

obtained from extensional experiments. However, pick-up, slipperi-

ness and cushion are in contrast with the parameter group, includ-

ing the power law index of H-B fit (n), gel strength values in the

LVR measured with smooth surfaces at 1, 10 and 20 rps frequency

values (G¢ S (x = 1, 10, 20 rps) and rough surfaces at 1 rps fre-

quency (G¢ R(x = 1 rps)). Another related parameter group formed

by initial spreadability, rub-out spreadability and slip power low

index (s) is in the negative side of both F1 and F2. This group of

parameters is in contrast with the elastic stress values measured

with smooth surfaces (ESS (x = 1, 10 and 20 rps)) and the shear

viscosity values measured at 10, 100 and 500 s)1 shear rate val-

ues (SV (@10, 100 and 500 s)1)).

Figure 3 Average maximum force values measured during extensional tests

for 1% PAA/VP-XL, 1% PVM/MA-XL and 1% PAA-XL (*P < 0.009). Figure 4 Sensory data: Principle component analysis.





Correlation of sensory ratings with large amplitude oscillatory

shear flow (LAOS) parameters (Chebyshev coefficients)

Principle component analysis was applied to the Chebyshev coeffi-

cients given in Tables III and IV and to the set of sensory evalua-

tion ratings data in Fig. 6. The first two principle components

accounted for 65.72% and 34.28% of the variance, respectively.

The correlation between the sensorial attribute ratings and Cheby-

shev coefficients demonstrates that pick-up, slipperiness and cush-

ion are related to the alternative modulus measured with rough

and smooth surfaces at 400% strain (G¢ CR4 and G¢ CS4) and to

the v3 coefficient calculated from the data measured with smooth

surfaces at 400% strain (v3 CS4). The parameter v3 is indicative of

the shear thickening or shear thinning behaviour of the material

(v3 > 0 shear thickening, v3 < 0 shear thinning) [8]. This parame-

ter group is in contrast with the related parameter group, including

the alternative modulus, G¢, measured in the LVR at 1% strain

with smooth surfaces (G¢ CS001) and the tand value measured in

the transition region with smooth surfaces at 200% strain (tand

CS2).

Another related parameter group includes initial spreadability,

rub-out spreadability and g¢L/g¢M values measured with rough sur-

faces in the transition and non-linear regions (g¢L/g¢M CR2 and g¢L/

g¢M CR4). These parameters are positioned on the positive side of

both F1 and F2. This group contrasts the g¢L/g¢M values measured

with smooth surfaces in the transition and non-linear region (g¢L/

g¢M CS2 and g¢L/g¢M CS4), G¢L/G¢M values measured in the LVR

using smooth surfaces at 1% strain amplitude (G¢L/G¢M CS001) and

e3 values measured in the LVR using smooth surfaces at 1%

strain amplitude (e3 CS001). The e3 values are indicative of strain

Figure 5 Sensory and conventional rheological

parameter data together: Principle component

analysis.





stiffening or softening behaviour of the material (e3 > 0 strain stiff-

ening, e3 < 0 strain softening) [8]. The strain stiffening or strain

softening behaviour of the material represents the increase or

decrease of the resistance of the material to the deformation in the

extensional direction and, for some cases, may be different than the

material’s response to shear deformation. For instance, random-

coiled high-molecular polymer chains may exhibit much higher

extensional viscosity than their shear viscosity with increasing

shear or strain rates. Therefore, a material can be shear thinning

(exerts less resistance to shear flow with increasing shear rate), but

at the same time strain stiffening (exerts higher resistance to exten-

sional flow with increasing strain rate) [20, 21].

The case can be made that the wall slip is expected to show its

effect in the even harmonics of the LAOS data, which can be

extracted from the Fourier amplitude spectrum. For this reason, we

have included the relative intensity of the second harmonic (I2/I1,

the ratio of the second harmonic to the principle harmonic) to

investigate the effect of slip on the even harmonics and their corre-

lation with sensory attributes. The results are given in Fig. 7. Fig-

ure 7a shows the strong correlation between the slip velocity and

the even harmonics of the material. Figure 7b,c shows the strong

correlation between the relative intensity of the even harmonics

and spreadability ratings, indicating the effect of slip on the percep-

tion of spreadability of the material. These results also confirm the

effect of slip on the rheological parameters and sensory perception

of the material.

Conclusions

Rheological methods can be successfully applied to objectively and

quantitatively describe sensory attributes of thickeners if necessary

attention is paid in choosing the right rheological methods. The

occurrence of wall slip and thixotropy may contribute to the sen-

sory perception of hydrogel-based personal care products and

should be characterized. We have determined that a significant

correlation can be made between slip velocity and the initial and

rub-out spreadability of the hydrogels. The applied shear rate

range may contribute to the material’s response to a given defor-

mation and to the sensorial perception of the product. The correla-

tion between sensory attributes showed that cushion, slipperiness

and pick-up are related, whereas initial and rub-out spreadability

are related, but are in contrast with cushion, slipperiness and

pick-up. These trends are in good agreement with existing litera-

ture.

Using FT analysis in LAOS can be effective in correlating sensory

rating results in skin/hair gels. Results indicate that the surface

Figure 6 Sensory and LAOS analysis data

together: Principle component analysis.





roughness of the plates, and choosing to test in the linear, transi-

tion or non-linear region, will determine which LAOS analysis

parameters will correlate best with which sensory parameters. This

indicates that wall slip and thixotropy have to be taken into

account when correlating LAOS analysis parameters. With these

precautions, rheological techniques, and LAOS in particular, can be

considered an exciting way to make inroads into sensorial percep-

tion analysis.

Acknowledgements

We thank Dr. Dilhan M. Kalyon of Stevens Institute of Technology for

his discussions regarding slip correction, Dr. Gareth H. McKinley of

MIT and Dr. Randy H. Ewoldt of University of Illinois for providing the

MITlaos software and for their guidance regarding LAOS analysis,

and Dr. Aloyse Franck of TA Instruments for his valuable comments

and help with analysing the LAOS data. We are grateful to the review-

ers for their time and comments that helped improve this study.

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(a) (b) (c)

Figure 7 Linear regression fit for relative intensity (I2/I1) vs. slip velocity coefficient, s, collected with smooth surfaces at 400% strain (a), and sensory ratings

vs. relative intensity (I2/I1) data collected with smooth surfaces at 400% strain (b and c).





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