ORIGINAL PAPER

Atomic Force Microscopy and Thermo-RheologicalCharacterisation of Lubricating Greases

M. C. Sanchez • J. M. Franco • C. Valencia •

C. Gallegos • F. Urquiola • R. Urchegui

Received: 10 February 2010 / Accepted: 3 December 2010 / Published online: 14 December 2010

� Springer Science+Business Media, LLC 2010

Abstract In this work, non-perturbed microstructures of

several commercial and model lubricating greases, differing

in nature and concentration of the thickener agent, were

examined using the atomic force microscopy (AFM) tech-

nique. Grease microstructure mainly depends on the nature

of the thickener employed and, also, on thickener concen-

tration and viscosity of the base oil. Thermal-induced

changes in the viscoelastic response of lubricating greases

have been investigated by using different rheological tech-

niques in a temperature range of 0–175 �C. Small-amplitude

oscillatory shear (SAOS) measurements were carried out to

determine the mechanical spectra of the different samples

studied. Lubricating grease rheological thermal suscepti-

bility was analysed by following the evolution of the plateau

modulus with temperature. SAOS functions dramatically

decrease, in most cases, above a characteristic temperature,

which depends on nature and/or concentration of the thick-

ener used and therefore on grease microstructure. The

thermo-mechanical reversibility of grease microstructure

has been studied by carrying out triple-step-shear stress tests

(shear stresses inside and outside of the linear viscoelasticity

range), at different temperatures. The degree of lubricating

grease non-reversible structural breakdown, which increases

with temperature, depends on the shear stress applied above

the linear viscoelasticity limits.

Keywords Atomic force microscopy � Lubricating

grease � Microstructure � Rheology � Temperature

1 Introduction

Lubricating greases are, in general, highly structured col-

loidal dispersions of a thickening agent, usually a metal

soap, in lubricant oil [1–5]. Fatty acid soaps of lithium,

calcium, sodium, aluminium and barium are most com-

monly used as thickeners. This component is added to

confer the consistency of greases, preventing loss of

lubricant under operating conditions and avoiding the

penetration of contaminants, such as solid particles and

water, without a significant reduction of the lubricating

properties. Greases are the preferred lubricant in hard-

to-reach places, for mechanically rubbing or dynamic

systems. Many important functional properties such as

their ability to flow under external forces, mechanical

stability under shearing, thermal susceptibility, dripping

and spattering, etc…, depend on the nature of its compo-

nents and the presence of some performance additives

[2, 5]. The thickener forms an entanglement network,

which traps the oil and confers the appropriate rheological

behaviour to the grease [4]. Consequently, suitable struc-

tural and physical properties may be reached from a proper

selection of the ingredients but, also, from process opti-

misation, as was previously reported [6]. In this sense, it is

relevant to understand how the development of grease

microstructure contributes to several functional properties

and, particularly, to the rheological characteristics of

lubricating greases [7].

The thermo-rheological behaviour of lubricating greases

under working conditions may be responsible for softening

or bleeding causing grease to flow away from contact areas

M. C. Sanchez � J. M. Franco (&) � C. Valencia � C. Gallegos

Departamento de Ingenierıa Quımica, Facultad de Ciencias

Experimentales, Campus de ‘‘El Carmen’’, Universidad de

Huelva, 21071 Huelva, Spain

e-mail: franco@uhu.es

F. Urquiola � R. Urchegui

Verkol Lubricantes, S.A., Barrio Zalain, 42, 31780 Bera,

Navarra, Spain

123

Tribol Lett (2011) 41:463–470

DOI 10.1007/s11249-010-9734-x

[8] or an improvement in the replenishment of the grease

into the contact zone decreasing starvation [9], depending

on the lubrication mechanism. On the other hand, the

rheology of lubricating greases is important not only under

working operation, being responsible for preventing loss of

lubricating under static conditions or acting as an effective

sealing agent. These rheological properties in quasi-

unperturbed conditions are determined by the microstruc-

ture induced by the thickener dispersed in the base oil.

Moreover, the rheological response of greases under high

shear rate situations, as for instance those achieved in

bearing lubrications, may be modelled from the charac-

terisation of the linear viscoelastic behaviour by applying a

factorable non-linear viscoelasticity model as previously

shown by Madiedo et al. [10].

In general, the microstructure of lubricating greases

depends on the type and concentration of the thickener

soap, as well as on the nature and viscosity of the base oil

employed. Thus, for instance, lithium lubricating greases

typically consist of a fine dispersion of the crystallized

lithium soap in the mineral or synthetic oil, usually in the

form of long, twisted and well-entangled fibres [3, 11–13].

These fibres arrange themselves to form a characteristic

microstructure, resulting from physical interactions

between colloidal particles and oil medium [12]. Other

types of thickener agents crystallize as rods, spheres or

platelet configurations [3, 11]. Traditionally, the analysis of

grease microstructure has been characterised through TEM

and SEM techniques [11, 14]. The most common criticism

associated to SEM or TEM observations is that both

techniques are vacuum based, and grease sample has to be

submitted to either a freezing treatment or oil removing,

thus distorting the original microstructure. Up to now, little

work has been carried out on the characterisation of grease

microstructure by means of atomic force microscopy

(AFM) [15–17]. In spite of some experimental difficulties,

this technique provides a great advantage. Thus, grease

samples do not need to be perturbed to carry out suitable

microstructural observations [16, 17], since the experi-

ments can be performed under atmospheric pressure. In this

paper, different microstructures presented by a set of

commercial and model lubricating greases were examined

using the AFM technique. This microstructure was related

to both some compositional variables and the resulting

thermo-rheological behaviour.

2 Experimental

2.1 Materials

Six lubricating grease samples differing in composition

were analysed from a rheological and microstructural point

of view. Lubricating greases C1–C4 were commercial

products manufactured by Verkol S.A. (Spain). Samples

M1 and M2 were model lubricating greases manufactured

in a 25 kg size pilot plant, following the Verkol S.A.

property method. Table 1 shows some compositional and

technical data for all the samples studied.

2.2 Atomic Force Microscopy

Microstructural characterisation of lubricating greases was

carried out by means of AFM, using a multimode AFM

connected to a Nanoscope IV scanning probe microscope

controller (Digital Instruments, Veeco Metrology Group

Inc., Santa Barbara, CA). All the observations were per-

formed in the tapping mode, using Veeco NanoprobeTM

Table 1 Main compositional and technical data for the lubricating greases studied

Lubricating grease C1 C2 C3 C4 M1 M2

Thickener type Lithium 12-

hydroxystearate

Lithium 12-

hydroxystearate

Lithium

complex

Calcium sulphonate

complex

Lithium

complex

Lithium

complex

% Thickener (w/w) 14.2 7.6 13.6 32.1 16.1 16.5

Base fluid Mineral Mineral Mineral/

synthetic

Mineral Synthetic Synthetic

Polymeric additive No No No No No Yes

Lubricating oil viscosity at 40 �C,

ASTM D-445 (mm2/s)

100 150 26 400 32 32

Dropping point, ASTM D-566 (�C) 190 200 263 249 294 274

Consistency, NLGI Grade 3 2 2 2 2 2

Worked penetration, ASTM D-217

(dmm)

227 271 275 287 270 268

464 Tribol Lett (2011) 41:463–470

123

tips, in Phase Imaging, where the cantilever oscillates at its

resonant frequency. The amplitude was used as a feedback

signal.

2.3 Rheological Tests

The rheological characterisation was performed in both a

controlled-stress (RS150 from Haake, Germany) and a

controlled-strain (ARES, Rheometric Scientific, UK) rhe-

ometer, in a temperature range between 0 and 175 �C.

Small amplitude oscillatory shear tests, inside the linear

viscoelasticity range, were carried out in a frequency range

between 0.01 and 100 rad/s, using a plate–plate geometry

(35 mm diameter, 1 mm gap). Strain or stress sweep tests,

at the frequency of 1 Hz, were previously performed on

each sample to determine the linear viscoelasticity range.

Rheo-destruction measurements were also performed in

oscillatory mode, following the evolution of the complex

modulus when a step-shear stress outside the linear vis-

coelasticity range was applied on the lubricating grease

sample, at different temperatures (25, 70 and 110 �C), and

the subsequent recovery when a step-shear stress inside the

linear region was again restored. Electric and forced con-

vection ovens, with environmental chambers that enclose

the sample, were used to control the temperature in the

controlled-stress and controlled-strain rheometers, respec-

tively. At least two replicates of each test were carried out

on fresh samples.

2.4 Penetration and Mechanical Stability Tests

The lubricating greases were worked 60 strokes in a

Mechanical Stability Tester and penetration values were

determined using a Koehler Penetrometer model K 95500

following the ASTM D-217 standard. Classical consistency

NLGI grade was established according to these penetration

values [3].

3 Results and Discussion

3.1 AFM Observations

Atomic force microscopy (AFM) is not a simple technique,

particularly for studying the microstructure of lubricating

greases. Some experimental problems, which may distort

structural data, are derived from the textural and rheolog-

ical properties of these materials when using the ‘contact

mode’. Thus, Hurley and Cann [15] pointed out that they

were not able to obtain good observations of untreated

lubricating greases by using the AFM contact mode. They

used the non-contact mode to successfully examine non-

washed samples, although some discrepancies in both types

of observations were detected. However, suitable micro-

structural observations were previously obtained with

lithium greases by using the tapping mode, once the sample

was heated up to a temperature below the dropping point

and, then, cooled down to room temperature in order to

obtain a very smooth surface [16, 17]. This protocol was

followed in this study. All observations were carried out by

using tapping mode in phase imaging. The phase lag is

dependent on several parameters such as grease composi-

tion, adhesion, friction and viscoelastic properties.

Figure 1a–d shows AFM micrographs for different

commercial lubricating greases. Typical entangled fibrous

microstructures were observed in lithium greases. Samples

C1 and C2 are lithium lubricating greases differing in soap

content (see Table 1). As can be observed in the phase

images shown in Fig. 1a–b, sample C1, with higher soap

content, shows thicker and more entangled and densely

arranged fibres than grease C2. Moreover, fibres in C1

appear clearly more agglomerated, yielding large hollow

spaces among them, where the oil is trapped. A lower density

of fibres, and not so agglomerated, is noticed in the case of

grease C3 (Fig. 1c), which contains a lithium complex soap

concentration similar to grease C1, although manufactured

with a blend of mineral and synthetic oils of lower viscosity

(Table 1). On the contrary, as expected, a completely dif-

ferent microstructure was observed for grease C4, based on a

complex calcium sulphonate thickener, at high concentra-

tion (32.1% w/w). In this case, the structure consists of a fine

dispersion of spherical and agglomerated particles (Fig. 1d)

with average size around 0.23 lm.

The influence of temperature on the microstructure of

lubricating greases was analysed on a model lithium

complex grease manufactured with a low-viscosity syn-

thetic oil (M1). Thus, Fig. 2a, b shows AFM images of this

sample obtained at 25 �C and 75 �C, respectively. Similar

microstructural arrangements are clearly observed, due to

the fact that this temperature window is not particularly

wide for a typical lubricating grease. However, it can be

noticed that highly entangled and agglomerated fibres are

apparent at 25 �C, whilst lower density of fibres and less

agglomerated, yielding larger hollow spaces among them,

are apparent at 75 �C, which may be attributed to a higher

solvency of the base oil. Unfortunately, phase images

cannot be obtained at higher temperatures, due to an

excessive softening of the sample that induces sticking of

the tip to the sample.

3.2 Small-Amplitude Oscillatory Shear (SAOS) Tests

Figures 3 and 4 show the evolution of the storage (G0) and

loss (G00) moduli with frequency, within the linear visco-

elasticity range, for two selected lubricating grease for-

mulations, in a temperature window of 0–175 �C. As can

Tribol Lett (2011) 41:463–470 465

123

be observed, the values of the storage modulus, G0, are

always higher than those found for the loss modulus, G00, in

the whole frequency range studied. G0 slightly evolves with

frequency, with a slope that increases with temperature,

whilst G00 usually shows a minimum value, which tends to

vanish as temperature increases. These results mainly

correspond to the plateau region of the mechanical spec-

trum, typical of highly entangled systems [18], and support

the idea that lubricating greases are highly structured sys-

tems, as has been previously discussed. In addition, there is

a tendency to a crossover between both viscoelasticity

functions at low frequencies, more apparent at the highest

temperatures, evidencing the beginning of the terminal or

viscous region.

Regarding the influence of temperature on the linear

viscoelastic functions, both G0 and G00 decrease as tem-

perature increases, although this decay depends on the

nature of the thickener used. Thus, for instance, the SAOS

functions of the lithium grease C1 (Fig. 3) are almost

unaffected by temperature below 50 �C, and then pro-

gressively decrease. On the contrary, the calcium sulpho-

nate grease C4 shows a decrease in the values of both linear

viscoelasticity functions up to 100 �C, which dramatically

drop at 125 �C (Fig. 4).

The characteristic parameter of the plateau region of the

mechanical spectrum previously mentioned is the plateau

modulus, GNo , defined, for polymeric materials, as the

extrapolation of the contribution of the entanglements to G0

at high frequencies [19], which may be considered as a

measure of the aggregation number among the dispersed

structural units and, consequently, related to the strength of

the microstructural network. The temperature dependence

on the plateau modulus, GNo , with temperature, for the

commercial and model lubricating greases studied, is

shown in Figs. 5 and 6. As can be observed, the above-

mentioned dependence dramatically changes at a charac-

teristic temperature, which depends on the nature of the

lubricating grease studied. Table 2 shows the values of this

characteristic temperature (Tc) and the slopes (S1 and S2) of

the two regions below and above this temperature, which

are related to the thermal susceptibility of the grease in the

respective temperature windows. Commercial lithium

grease C1 shows the lowest characteristic temperature and,

also, the lowest value of the slope in the first region, S1. As

was previously mentioned, SAOS functions for this sample

were almost constant at low temperature. On the contrary,

lithium grease C2, having lower soap concentration and,

also, lower values of SAOS functions than C1, shows a

Fig. 1 AFM micrographs

(window size 20 lm) for

selected commercial lubricating

greases: a C1; b C2; c C3; d C4

466 Tribol Lett (2011) 41:463–470

123

continuous decrease in GNo , being the slope in the whole

temperature range studied similar to that found for lithium

grease C1 in the second temperature region, S2. On the

other hand, lubricating grease C3, which contains a lithium

complex soap, displays a very similar behaviour to that

found with sample C2 in a wide temperature window,

although, in this case, a dramatic increase in the slope of

the plateau modulus versus temperature is found at the

highest temperatures studied (150–175 �C). Grease C3 also

exhibits intermediate GNo values than those found for C1

and C2 samples, respectively, which can be attributed to an

intermediate degree of fibres agglomeration. Finally, as

expected attending to its microstructure, calcium sulpho-

nate lubricating grease C4 shows a quite different

Fig. 2 AFM micrographs (window size 20 lm) for a model lubri-

cating grease (M1), at different temperatures: a 25 �C; b 75 �C

10-2 10-1 100 101 102103

104

105

106

ω (rad/s)

G' (

Pa)

ω (rad/s)

0 °C 25 °C 50 °C 75 °C 100 °C 125 °C 150 °C 175 °C

10-2 10-1 100 101 102103

103

104

105

106

G"

(Pa)

0 °C 25 °C 50 °C 75 °C 100 °C 125 °C 150 °C 175 °C

Fig. 3 Frequency dependence of the storage and loss moduli for

lubricating grease C1, at different temperatures

10-2 10-1 100 101 102102

103

104

105

106

10-2 10-1 100 101 102103

102

103

104

105

106

G"

(P

a)

0°C 25°C 50°C 75°C 100°C 125°C 150°C 175°C

G' (

Pa)

ω (rad/s)

0°C 25°C 50°C 75°C 100°C 125°C 150°C 175°C

ω (rad/s)

Fig. 4 Frequency dependence of the storage and loss moduli for

lubricating grease C4, at different temperatures

-25 0 25 50 75 100 125 150 175 200103

104

105

106

GN

o (P

a)

Temperature (°C)

sample C1 sample C2 sample C3 sample C4 linear fittings

Fig. 5 Temperature dependence of the plateau modulus for the

commercial lubricating greases studied (samples C1–C4)

Tribol Lett (2011) 41:463–470 467

123

behaviour. Thus, GNo steadily decreases up to a temperature

of around 100 �C, and then a dramatic drop (almost one

decade) in the plateau modulus is noticed. After Tc, this

lubricating grease shows the lowest value of the slope S2.

Figure 6 shows the evolution of GNo with temperature for

two model greases thickened with a complex lithium soap.

Concerning complex lithium grease M1, the slope below

the characteristic temperature (130 �C) is quite similar to

that obtained for the commercial lubricating grease C3,

having the same thickener. However, M1 thermal suscep-

tibility, S2, is much lower above Tc, probably due to its

higher soap concentration (16.1 vs. 13.6%). Including a

polymeric additive in a grease formulation seems to affect,

in a qualitative manner, the influence of temperature on

GNo . Thus, GN

o slightly increases with temperature for

grease M2 below Tc, temperature significantly lower than

that obtained for the polymer-free formulation. Above Tc,

GNo values remain almost constant.

The overall results presented are in general agreement

with those reported in the literature. Thus, Couronne et al.

[20] studied the influence of thermal degradation on the

rheological and physico-chemical characteristics of lithium

lubricating greases, indicating that thermally aged greases

showed lower values of their rheological functions, due to

the degradation of the microstructural network into isolated

fibres. Similarly, Hurley and Cann [15], using scanning

electron and atomic force microscopes, found that thermal

ageing of a lithium lubricating grease, at 120 �C, caused

clumping and agglomeration of the thickener particles and,

eventually, the fibre network was fully destroyed. Although

the influence of temperature on grease microstructure, in a

limited temperature range, can be deduced from the

micrographs shown in Fig. 2, unfortunately, the AFM

technique used in this work does not allow direct suitable

morphological observations above the characteristic tem-

perature previously discussed.

3.3 Rheo-Destruction and Recovery Tests

When an external stress beyond the limits of the linear

viscoelasticity range is applied on a lubricating grease, a

microstructural modification, which may be partially irre-

versible, is induced [21]. Consequently, grease non-linear

viscoelastic and viscous functions, at constant shear strain/

rate, will decrease with shear time. The degree of irre-

versible structural breakdown is governed, among other

factors, by temperature, elapsed time and shear strain/stress

applied on the sample [21, 22]. Figure 7 shows results

obtained from triple-step-shear stress tests in oscillatory

shear carried out on selected lubricating grease samples. As

can be observed, the complex modulus decreases when a

-25 0 25 50 75 100 125 150 175 200103

104

105

106G

No (

Pa)

Temperature (ºC)

sample M1 sample M2 linear fittings

Fig. 6 Temperature dependence of the plateau modulus for the

model lubricating greases studied (samples M1 and M2)

Table 2 Lubricating grease characteristic temperatures and plateau modulus versus temperature slopes of the two regions below and above the

corresponding characteristic temperature

Lubricating grease C1 C2 C3 C4 M1 M2

Characteristic temperature (�C) 50 – 149 100 130 75

Thermal susceptibility, S1 (1/�C) -1.6 9 10-4 -3.2 9 10-3 -3.8 9 10-3 -2.4 9 10-2 -5.3 9 10-3 1.5 9 10-3

Thermal susceptibility, S2 (1/�C) -4.4 9 10-3 – -1.8 9 10-2 -9.3 9 10-4 9.6 9 10-4 -9.8 9 10-4

0 1000 2000 3000 4000

100

101

102

103

104

105

G*

(Pa)

stress programme 5-500-5 Pa 5-1000-5 Pa 5-2000-5 Pa

time (s)

Fig. 7 Evolution of the complex modulus with time, during triple-

step-shear stress tests in oscillatory shear, for lubricating grease M1,

at 70 �C. (Step 1: shear stress inside the linear viscoelasticity region;

Step 2: shear stress outside the linear viscoelasticity region; Step 1:

shear stress inside the linear viscoelasticity region)

468 Tribol Lett (2011) 41:463–470

123

stress outside the linear viscoelasticity range is applied, and

subsequently recovers when a shear stress inside the linear

region is, once again, restored. As expected, the complex

modulus values are lower as the magnitude of the stress

applied increases, as a consequence of a much larger

structural breakdown. The percentage of lubricating grease

rheo-destruction and reversible structural breakdown can

be quantified as follows:

%Rheo-destruction ¼ G�o � G�1G�o

� 100 ð1Þ

%Recovery ¼ G�2 � G�1G�o � G�1

� 100 ð2Þ

being G�o;G�1 and G�2 the values of the complex modulus in

the linear viscoelastic region (first step–stress), after the

application of a stress value outside the linear viscoelastic

range (second step–stress), and after restoring the initial

stress value (third step–stress), respectively.

Rheo-destruction and recovery percentages are listed in

Tables 3 and 4, as a function of temperature, for both

model lubricating grease formulations, M1 and M2,

respectively. As can be observed, in most cases, rheo-

destruction percentage is close to 100%, and increases with

both stress magnitude and temperature. Moreover, the final

value of the complex modulus after the third step-stress

(stress inside the linear viscoelasticity region) is always

inferior to that shown by the original sample (complex

modulus values after the first step-stress), which indicates a

certain degree of irreversible structural modification in the

samples, enhanced, as expected, by increasing stress and

temperature. Figure 8 shows the AFM image of sample M1

submitted to a stress programme of 10–1000–10 Pa in a

rheo-destruction test, which illustrates the effect of a rel-

atively high shear stress on grease microstructure. As can

be clearly observed, the new shear-induced microstructure

consists of more agglomerated shorter fibres and much

larger hollow spaces among them, than those found in the

non-perturbed microstructure (see Fig. 2a). Finally, the

addition of a polymeric thickener to a model grease

formulation does not generally modify rheo-destruction

percentages, excepting for the lowest stress applied in the

non-linear viscoelastic range (i.e. 500 Pa). On the contrary,

this polymeric thickener usually improves recovery per-

centages (decreased irreversible rheo-destruction).

4 Conclusions

The non-perturbed microstructure of different lubricating

grease samples was analysed by means of the AFM tech-

nique. Grease microstructure mainly depends on the nature

Table 3 Rheological destruction and recovery percentages, during

triple-step-shear-stress tests in oscillatory shear, for grease sample M1

Stress-step

test (Pa)

Temperature

(�C)

Rheo-destruction

(%)

Recovery

(%)

10–500–10 25 89.5 73.1

70 99.6 31.1

110 99.8 66.0

10–1000–10 25 99.5 53.4

70 100 43.7

110 99.9 27.3

10–2000–10 25 100 22.8

70 100 19.1

110 100 7.6

Table 4 Rheological destruction and recovery percentages, during

triple-step-shear-stress tests in oscillatory shear, for grease sample M2

Stress-step

test (Pa)

Temperature

(�C)

Rheo-destruction

(%)

Recovery

(%)

10–500–10 25 78.8 91.9

70 93.0 76.7

110 99.2 70.5

10–1000–10 25 98.1 66.1

70 99.4 56.1

110 99.9 37.2

10–2000–10 25 99.3 57.3

70 99.9 14.1

110 99.9 8.5

Fig. 8 AFM micrograph (window size 20 lm) for lubricating grease

sample M1 submitted to stress programme of 10–1000–10 Pa in a

rheo-destruction test

Tribol Lett (2011) 41:463–470 469

123

of the thickener agent employed, and, also, on thickener

concentration and viscosity of the base oil.

In the case of lithium soap-based greases, the higher the

soap content more entangled and agglomerated fibres

appear in grease microstructure. Lower density of fibres

and less agglomerated, yielding larger hollow spaces

among them, are noticed as temperature increases. On the

other hand, the microstructure of the complex calcium

sulphonate soap-based grease consists of a fine dispersion

of spherical, and agglomerated, particles. The values of the

linear viscoelasticity moduli of the lubricating grease

samples studied dramatically decrease, in most cases,

above a characteristic temperature, which depends on the

nature and/or concentration of the thickener used and

therefore on the resulting microstructure. Higher values of

the linear viscoelastic functions were found for lithium

greases showing more agglomerated fibres. Lubricating

grease rheological thermal susceptibility was analysed by

following the evolution of the plateau modulus with tem-

perature. The complex calcium sulphonate soap-based

grease displays a thermo-rheological behaviour quite dif-

ferent to those found for lithium greases, as a result of a

very different microstructure. The thermo-mechanical

reversibility of lubricating grease microstructure was

studied by carrying out triple-step-shear stress tests in

oscillatory shear (shear stresses inside and outside the

linear viscoelasticity range), at different temperatures. The

degree of lubricating grease non-reversible structural

breakdown, which increases with temperature, depends on

the shear stress applied above the linear viscoelasticity

limits. Including a polymeric additive in a grease formu-

lation significantly dampens its non-reversible shear-

induced structural breakdown.

Acknowledgments This work is part of the research project AVI-

2015 (CENIT programme, sponsored by CDTI-MCI, Spain). The

authors gratefully acknowledge its financial support.

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