Evaluation of Adhesion Forces in Alginate–Filler System Using an AFM Colloidal Probe Technique

Journal of Adhesion Science and Technology 25 (2011) 1159–1173brill.nl/jast

Evaluation of Adhesion Forces in Alginate–Filler SystemUsing an AFM Colloidal Probe Technique

B. Benli a,∗, J. Nalaskowski b, S. Assemi b, M. S. Celik c and J. D. Miller b

a Department of Chemical Engineering, Istanbul Technical University, Istanbul 34469, Turkeyb Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112, USA

c Department of Mineral Processing Engineering, Istanbul Technical University,Istanbul 34469, Turkey

AbstractInteraction and adhesion forces between polymer and mineral fillers are key factors in controlling theiraffinity during the preparation of high-quality nanobiocomposites. In this work, the compatibility betweenpolymer (alginate) and mineral filler (mica, borax or bentonite) surfaces was characterized by direct forcemeasurements using atomic force microscopy (AFM). Direct force measurements, using a single sphericalalginate particle prepared by the spinning disc atomization method and attached to a cantilever, have beenemployed to characterize adhesion forces between alginate and several mineral filler surfaces in air. The freeenergies of adhesion for alginate–mica, alginate–bentonite and alginate–borax systems were compared. Theexperimental adhesion forces for these systems were compared with those predicted using the Johnson–Kendall–Roberts (JKR) approximation and the Liftshitz–van der Waals and Lewis acid–base interactiontheory. The discrepancies between the experimental values and theoretical predictions were discussed onthe basis of the surface roughness effect on the measured adhesion forces.© Koninklijke Brill NV, Leiden, 2011

KeywordsAlginate, filler, bentonite, borax, mica, colloidal probe, AFM

1. Introduction

Adhesion and interaction forces between biopolymer and inorganic mineral fillerare of significant importance in the preparation of high-quality nanobiocomposites[1, 2]. Such forces provide compatibility between these surfaces and play a crucialrole in the stability [3, 4], rheology [5] and mechanical properties [6] of polymer–filler blends [1]. Besides compatibility, adhesion is influenced by additional factorsincluding the rate of applied force, viscous energy dissipation, mechanical proper-ties of both contacting materials, and also roughness [7]. Therefore, the selection ofthe ideal filler is very important in the preparation of high-quality nanocomposites.

* To whom correspondence should be addressed. E-mail: [email protected]

© Koninklijke Brill NV, Leiden, 2011 DOI:10.1163/016942410X12627768497437

1160 B. Benli et al. / J. Adhesion Sci. Technol. 25 (2011) 1159–1173

The ideal filler should at least have good surface wetting and bonding properties,high strength, good chemical resistance, as well as being readily available at a rea-sonably low cost [8].

A knowledge of the interaction forces is important and necessary for a betterunderstanding of the compatibility and the adhesion mechanisms in nanocompositeinterfaces [9, 10] and in the calculation of the free energy of adhesion. Althoughdirect force measurements have been made in the past, using a surface force appa-ratus (SFA), only with the development of the atomic force microscope (AFM) andthe colloidal probe technique has it been possible to quantify interaction forces andmeasurements of adhesion between a colloidal probe with a single particle and aselected flat surface in air [11–13].

Sodium alginate is a particularly attractive biopolymer that has a wide range ofapplications as a result of it being biocompatible, hydrophilic, and biodegradableunder normal environmental conditions. After cross-linking with Ca2+ ions, Ca-alginate is widely used as a gelling agent in many industries, including the food,pharmaceutical, cosmetics, paper, and textile industries. Some recent applicationsinclude biomedical applications, cell transplantation in tissue engineering, biosen-sors, and encapsulation of cell, protein and drug delivery systems [14–16]. Anotherinnovative use of alginate is its application as a packaging material as an alternativeto petroleum-based traditional plastics. It is biodegradable and a non-toxic biopoly-mer and can be used as a natural edible film [15]. Thus, biodegradable plastics suchas alginate provide one plausible approach to solving the ecological problem ofplastic waste disposal.

Despite abundant research on alginate biopolymers, AFM study for identifyingalginate and filler interactions, have not previously been reported in the literature.The main aim of this study was to measure the pull-off forces of alginate/alginateand alginate/mineral filler systems by AFM direct force measurements. For thispurpose, an alginate colloidal probe was first prepared from a single alginate parti-cle using a modified spinning disc atomization (SDA) technique [7]. In this study,the normalized adhesion forces (F/R) and free energies of adhesion (�G) betweenthe colloidal probe and alginate film, bentonite pellet, borax pellet or mica waferwere determined. The results of the spherical alginate colloidal probe were alsocompared with three different kinds of colloidal probes, i.e., glass, silica and Si3N4AFM tip in order to characterize surface irregularities. Furthermore, measured inter-action forces were compared with the values calculated theoretically, using both theJohnson–Kendall–Roberts (JKR) [17] and the Liftshitz–van der Waals and Lewisacid–base interaction theories [18].

2. Experimental

2.1. Materials

Natural sodium bentonite was obtained from Resadiye (Tokat, Turkey). The rawbentonite sample was dispersed in tap water at room temperature for 48 h and then

B. Benli et al. / J. Adhesion Sci. Technol. 25 (2011) 1159–1173 1161

subjected to a multistage purification process using a multi-hydrocyclone separator[19, 20]. Under optimum conditions, a high purity concentrate with a cation ex-change capacity of 98 meq/100 g and average particle size of 2.5 µm, was obtained.The sample was dried and gently ground for further use.

Borax decahydrate was crystallized from industrial grade borax pentahydratefrom Kirka (Eskisehir, Turkey). A saturated solution of borax pentahydrate wasprepared according to the solubility data of Mullin [21]. The borax pentahydratewas dissolved in a crystallizer for purification and production of the borax decahy-drate. The solution was then heated above the saturation temperature (∼69◦C) andthe required quantity of Na2CO3 was added to the solution in order to precipitateprobertite. The precipitated probertite was removed from the hot solution using afilter press. Finally, the purified hot solution was cooled to room temperature. Thecrystals obtained were dried at room temperature. In the second step of borax pu-rification, the crystals were purified by recrystallization without adding Na2CO3.

Sodium alginate purchased from Sigma-Aldrich as alginic acid of sodium saltfrom brown algae was used. Its grade is specifically recommended for immobiliza-tion applications. Muscovite mica was obtained from Wards Inc. (Rochester, NY,USA) and was freshly cleaved before each experiment.

2.2. Mineral Pellet Substrates Preparation

Bentonite and borax mineral samples used in the AFM studies were prepared asmineral pellets. These pellets containing approximately 1 g of mineral (with par-ticles smaller than 65 µm in diameter) were pressed on a hydraulic press at 9 barcompression pressure for one hour.

2.3. Preparation of Alginate Biocomposite Substrates

Biocomposite films were prepared by a casting and solvent evaporation technique atroom temperature. The desired quantity of sodium alginate was hydrated overnightin deionized water (18 M� cm, obtained using a Milli-Q System, Millipore) tofacilitate the dissolution of alginate and to obtain solutions of 1–3 wt%. The dis-solved polymer solution was cast onto a clean glass plate of 10 × 20 cm with theaid of a casting knife at room temperature. Then, all the samples were soaked in abath of 20 wt% CaCl2 solution for half an hour. After the cross-linking reaction,the films were taken out of the reaction solution, rinsed twice with ethyl alcohol tostrengthen the film, and washed with demineralized water to eliminate any possibleresidues and remaining traces of solvent. The film thus formed was dried at roomtemperature in a box for 3 days. Dried films of thickness ranging from 45–50 µmwere obtained.

2.4. The Production of Spherical Alginate Particles for the Colloidal Probe

Spherical particles of alginate polymer were produced using the modified spin-ning disc atomization (SDA) procedure shown in Fig. 1 [22]. In this procedure,the spinning disk represents the critical step that controls the atomization process.


Figure 1. The preparation of a spherical alginate particle using the spinning disc atomizationmethod [22].

Therefore, the speed of rotating disc was controlled electronically. The alginatepolymer solution (3 wt%) was dropped onto the disc rotating at a rate of 20 ml/h.Each drop was accelerated by the rotating disc due to the adhesion of the wettinglayer and its viscosity. Under proper conditions, the liquid left the edge of the discas isolated droplets. In addition, floating drops were dissolved in the continuouslystirred CaCl2 bath (20 wt% of CaCl2). As the alginate drop entered the bath aself arrangement of the di-block copolymers occurred at the interface with the hy-drophilic blocks pointing towards the outside and the hydrophobic ones towards theinside. The alginate drop was covered by the hydrophobic cross-linked copolymer.As a result of immediate and complete wetting, the spherical alginate beads wereeasily formed.

2.5. Colloidal Probe Preparation

Micron-sized spherical alginate particles prepared by the modified SDA method,silica particles (Bangs Laboratories Inc., Fishers, IN, USA) and glass spheres (Poly-sciences Inc. Warrington, PA, USA) of diameter ranging from 8 to 33 µm wereprecisely glued to the AFM cantilever using the following procedure [23]. The can-tilever was placed under the charge-coupled device (CCD) camera fitted with a longdistance lens of 500× magnification. A small amount of epoxy resin was transferredon the tip of the cantilever, using a 50 µm diameter tungsten wire attached to a mi-cromanipulator. Subsequently, a selected sphere was picked up using a new tungstenwire and carefully placed on the cantilever using the micromanipulator. Scanningelectron microscope (SEM) images confirmed the absence of contamination of thespherical particles with epoxy resin during gluing. The prepared colloidal probeswere used after at least 24 h of drying. An example of the colloidal probe with thealginate particle is shown in Fig. 2.


Figure 2. (a) A CCD camera image of an alginate particle glued on the AFM cantilever; (b) SEMimage of the same alginate particle after force measurements.

2.6. Adhesion Force Measurement

The adhesion force (pull-off force) between the polymer and the filler surface wasmeasured using a Nanoscope V Multimode AFM (Veeco Metrology, Santa Bar-bara, CA, USA) with the colloidal probe technique. During the measurement, thelaser beam was focused on the back of the cantilever in order to detect its deflectionas it interacted with the surface beneath it. The reflected light was directed onto asplit photodiode detector, which produces a voltage signal proportional to the can-tilever deflection. As the spring constant was known, the deflection of the springcould be converted to a force using Hooke’s law. The samples were imaged using agold-coated Si3N4 cantilever, with a nominal spring constant of 0.12 N/m (DigitalInstrument, Santa Barbara, CA). Our experience indicated that there was no sig-nificant difference in the spring constants of these triangular cantilevers declaredby the manufacturers and that measured using the thermal tune method. The maxi-mum applied load during contact was 2.82 nN and the scan rate was kept constantat 1 Hz (speed 1 µm/s). Under a given set of conditions, the measurements weremade with flat surface samples, each at four different locations. At each location atleast 10 representative force profiles were captured for analysis. Then, the value ofthe cantilever deflection was converted to an adhesion force, FA using equation (1):

FA = Dk, (1)

where D is the distance from where the retract portion of the curve intersects the setpoint to the tip pull-off (rebound) and k is the spring-constant. The calculation ofadhesion forces from the 40 profiles resulted in a maximum relative standard devia-tion of about 10%. This magnitude of relative standard deviation is not uncommonas shown by Chirstendat et al. [24]. Subsequently, the measured interaction forcewas normalized using the radius R of the particle. The normalized adhesion forceswere analyzed by the JKR theory [17] using polymer/filler composite films similarto those reported for polymer particles [23, 25] and elastic solids [26]. The relation


between the normalized adhesion forces (F/R) and the free energies of adhesion(�G) is given by equation (2) [17]:

FA = −3

2πR�GPF. (2)

From this equation the free energies of adhesion of surfaces using the AFM forcemeasurements can be obtained. After the force measurements, SEM photographs ofthe cantilevers were taken in order to measure the diameter of the alginate particleand the dimensions of the cantilevers. No permanent deformation of the particleswas found (Fig. 2(b)).

3. Results and Discussion

3.1. Adhesion Forces between Colloidal Probes and Alginate Film Surfaces

Applying the JKR theory to the direct force measurements gives information on thecompatibility of the surfaces. The restriction of this theory is the use of a smoothand spherical colloidal probe [27]. For this reason, the results of alginate colloidalprobe (17 µm) were compared in Table 1 to various colloidal probes viz. glass(13.7 µm), silica (25 µm) and Si3N4 AFM tip (40 nm). The Si3N4 AFM tip, used asa colloidal probe, can be envisaged as a small particle with a radius equal to the tipradius, as the apex of the pyramidal tip can be approximated by a spherical shape[7]. According to Table 1, the highest adhesion value was obtained for alginatesphere — namely 21.1 ± 0.9 mN/m. Silica sphere had the second highest adhesionvalue of 19.4 ± 0.4 mN/m for the same alginate film surface.

In order to make a comparison of the substrates of alginate surfaces, 1 and 3 wt%bulk concentration of alginate solutions were prepared before film casting. FromTable 1, it is clear that by increasing the alginate concentration three-fold, the ad-hesion force increased nearly three-fold from 77 to 253 nN, in response to the moredense surface structure. Similarly, confirming this increase, the adhesion measure-

Table 1.Measured adhesion force, free energy of adhesion and normalized adhesion force (F/R) for differentbulk alginate concentrations and tip materials in air

Material Adhesion Free energy of F/R (mN/m)force (nN) adhesion (mJ/m2) (measured)

Alginate sphere/alginate surface (1%∗) 77 −1.4 6.4 ± 0.6Alginate sphere/alginate surface (3%) 253 −4.5 21.1 ± 0.9Si3N4 tip/alginate (1%) surface 10 −0.05 0.25 ± 0.03Si3N4 tip/alginate (3%) surface 25 −0.14 0.59 ± 0.08Glass sphere/alginate (3%) surface 91 −1.4 6.6 ± 0.3Silica sphere/alginate (3%) surface 503 −4.1 19.4 ± 0.4

* Alginate surface prepared from 1 wt% alginate aqueous solution.


ments between the silicon nitride tip and alginate surface also increased by nearlythree-fold, from 10 to 25 nN.

The JKR model should appropriately describe the adhesion for large sphereswith high surface energies and low Young’s moduli [28]. The improvements in theJKR theory do not directly take viscoelastic phenomena into account. However, theJKR model and another contact mechanics model — the Derjagin, Muller, Toropov(DMT) — are frequently used to interpret the pull-off forces measured by the AFMtechnique [25]. The DMT model has been most successfully applied to systemswith submicroscopic particles of high Young’s modulus that interact with othersurfaces through relatively weak surface forces compared to that applied to the JKRmodel. The selection of an appropriate system requires the calculation of either theTabor number or the Maugis number [29–31]. For example, for the alginate system(1 wt%), the Maugis number (λ) calculated from equation (3) is approximately 197:

λ = 2.06

z0

√RW 2

πK2, (3)

where z0 is the equilibrium separation distance between the probe and substrate(∼0.2 nm for many systems [31]), R is the radius of the probe (17 µm for alginatesingle particle), W is the work of adhesion (W = 2γ and γ is 0.064 N/m for 1 wt%of alginate solution measured by Hershko and Nussinovitch [32]), and K is thereduced elastic modulus, K = 2E/3(1 − ν2) ≈ 14.75 kPa. In this equation E is theYoung’s modulus (20 kPa measured by Draget et al. [33]) and ν is the Poisson’sratio (0.31 suggested by Nguyen et al. [34]). Such a large Maugis number (λ > 5)indicates that the JKR model should be used in the analysis of elastic deformationsand pull-off forces for the alginate solution–alginate system.

3.2. Adhesion Forces between Colloidal Probes and Mineral Pellet Surfaces

The pull-off force measurements performed between the alginate colloidal probeand the mineral fillers are given in Table 2. The results obtained for mineral fillersurfaces were interpreted using the diameter of alginate colloidal probe (17 µmand 33.4 µm) and compared with the commercial Si3N4 tip (40 nm). It is clearfrom Table 2 that the adhesion force between mica and alginate has the highestvalue, conversely, the free energy of adhesion between mica and alginate displaysthe lowest negative value, indicating a high compatibility of polymer–mica sur-face. Bentonite and borax exhibit smaller absolute values of adhesion force whichmay indicate low compatibility between these surfaces and alginate. The high freeenergy of adhesion values for alginate–bentonite system may be ascribed to theweaker adhesion between the hydrophobic alginate and the low hydrophilicity ofbentonite, compared to mica [35]. Structurally, these minerals can be classified asdisoctahedral/trisoctahedral in the case of mica and tetrahedral for bentonite.

However, the measurements on the filler surfaces using the silicon nitride tip,as opposed to alginate, showed very small adhesion values due to the low surfacearea and chemistry of the probe. The problem, however, is that the adhesion force


Table 2.Effect of colloidal probe size of alginate particle on the adhesion force of mineral filler surfaces duringdirect force measurements

Material Adhesion Free energy of F/R (mN/m)force (nN) adhesion (mJ/m2) (measured)

Alginate probe (17 µm)/bentonite surface 38 −0.47 1.82 ± 0.34Alginate probe (17 µm)/borax surface 48 −0.60 2.81 ± 0.39Alginate probe (33.4 µm)/bentonite surface 44 −0.23 1.09 ± 0.09Alginate probe (33.4 µm)/borax surface 50 −0.32 1.50 ± 0.3Alginate probe (33.4 µm)/mica surface 501 −3.19 15.01 ± 1.19Si3N4 tip (r = ave. 40 nm)/bentonite surface 13 −0.05 0.32 ± 0.03Si3N4 tip (r = ave. 40 nm)/borax surface 21 −0.11 0.53 ± 0.02Si3N4 tip (r = ave. 40 nm)/mica surface 33 −0.18 0.82 ± 0.01

particularly on mica is very small (33 nN in air) (Table 2). In the literature, thetheoretical value of the adhesion force on freshly cleaved mica is given as 31±5 nNand the measured data are given as 23 ± 3 nN and 24 ± 4 nN, respectively [36].These demonstrate that our experimental data are consistent with the calculatedvalues.

Similar results were reported by Sokolov et al. [36]. Silicon nitride may form anoxide layer on the surface. Silicon oxide has hydroxyl groups on its surface whichwould give rise to a negative charge and consequently increase the free energy ofthe system and result in a repulsive force [37]. Thus, the measured values of ad-hesion force on the AFM tip can be smaller than that on the colloidal probe. Inthe literature, the effect of probe radius on adhesion forces for different pairs ofparticles differs by a factor of 5 to 10 [38]. Similarly, the absolute values of themeasured pull-off forces were approximately fifty times smaller than that predictedby the JKR approximation [39]. This relatively big difference between measuredand calculated values could be explained by surface roughness [40]. Also, adhesionis dramatically reduced due to the roughness of component surfaces, because thereal contact area is reduced [41]. In this study, the variation in colloidal probe di-ameter of different materials seems to induce little change in adhesion force. Forinstance, the adhesion force of bentonite using the silicon nitride tip gave smallervalues (13 nN) than that of mica (33 nN). Conversely, adhesion forces between thesame surfaces and alginate colloidal probe increased from 44 nN to 501 nN usingthe colloidal probe diameter of 33.4 nm.

3.3. Theoretical Calculations of Free Energy of Adhesion

The theoretical interfacial tension calculations based on the Lifshitz–van der Waalsand Lewis acid–base interaction theory [18] for the alginate/alginate and algi-nate/mineral fillers system, were carried out using equations (4) and (5). The polar


Table 3.Surface tension components for alginate, bentonite, borax andmica (mJ/m2)

Material γ LW γ + γ − Reference

Alginate 20.7 7.71 5.40 [16]Wyoming bentonite 41.2 1.50 33.30 [44]Muscovite (mica) 38.2 0.50 47.30 [44]

surface tension component of each material obtained from Table 3 and the corre-sponding equations were modified as follows:

γi = γ LWi + 2

√γ +i γ −

i (4)

and

γPF = (√γ LW

P −√

γ LWF

)2 + 2(√

γ +P γ −

P +√

γ +F γ −

F −√

γ +P γ −

F −√

γ −F γ +

P

),

(5)

where γPF is the interfacial tension between two different materials, γ LWi is the

Lifshitz–van der Waals component of the surface tension, γ + is the electron ac-ceptor and γ − is the electron donor parameter of the Lewis acid–base componentof the surface tension of the material. The subscripts P, F and PF refer to alginatepolymer, mineral filler and alginate/mineral filler surfaces, respectively.

In our case, since borax is known to be soluble and bentonite as a high swellingmaterial in water, the measurements were made using compressed pellets in air.Although a simple and easy method was provided to obtain a substrate for themeasurement of interaction forces between alginate and mineral fillers, the surfaceroughness of these pellets were relatively high. It is well known that the force ofadhesion increases with increasing contact area [7, 40], but it is important to recog-nize that when the two surfaces are in contact, the asperities on the surfaces are inreal contact [44]. These asperities in contact were analyzed using the AFM imagesshown in Fig. 3; the preparation method of pellet or cleavage surface influencesthe average roughness (Ra) and the root mean square (RMS) roughness (Rq) deter-mined by AFM Nanoscope 7.2 software. The roughness values of the surfaces usedare significantly different, as seen in Table 5.

It can be seen from the AFM imaging studies and Table 5 that bentonite and bo-rax surfaces are much rougher than either the alginate or mica surfaces. Accordingto the measured RMS roughness values, surfaces can be classified into two groups;firstly, rough surfaces like bentonite and borax with Rq values of 417 and 138 nmand secondly, smooth surfaces like alginate (3 wt%), alginate (1 wt%) and mica cal-culated as 4.35, 12.3, and 7.15 nm, respectively. Busscher et al. [45] measured theeffect of surface roughness on the wetting properties of at least 12 different com-mercial polymers and concluded that surface roughness values lower than 100 nm


(a)

(b)

(c)

Figure 3. The AFM images (contact mode) of the surfaces and their corresponding error images of:(a) alginate 1 wt%, (b) alginate 3 wt%, (c) bentonite, (d) mica and (e) borax. (Left) 3-D surface plotand (right) deflection error image.

do not influence the wetting of a substrate. However, Miller et al. [45] showedthat even extremely low surface roughness values (in the nanometer range) are im-portant and influence the wetting behavior of polytetrafluoroethylene (PTF). Also,the results from this work show that the difference in roughness parameters of all


(d)

(e)

Figure 3. (Continued.)

Table 4.Comparison of pull-off forces obtained from AFM measurements

Material γP γF γPF �G12 F/R (calc.) F/R (meas.)(mJ/m2) (mN/m) (mN/m)

Alginate sphere/alginate 33.6 33.6 0.00 −67.20 317 25.3 ± 6.1surface (3 wt%)

Alginate sphere/bentonite pellet 33.6 55.3 −7.2 −96.15 453 3.8 ± 0.8Alginate sphere/mica pellet 33.6 47.9 −16.2 −97.72 460 15.0 ± 1.2

alginate and mineral surfaces used, have a significant impact on the measured adhe-sion force, resulting in very low values of adhesion for the relatively rough pellets.According to a previous study [7], adhesion reaches a maximum at Rq roughnessvalues of about 3 nm, for example, toner particles and silicon substrate where thedimensions of the asperities of particle and substrate are approximately equal. Inthis case, the substrate surface roughness is approximately equal to the roughnessof the particle at the point where the two roughnesses match.


Table 5.Roughness parameters of surfaces for the same surfacearea of 400 µm2

Material name Rq (nm) Ra (nm)

Alginate, 1% 4.35 3.15Alginate, 3% 12.3 9.33Bentonite 417 334Borax 138 109Mica 7.15 5.78

Figure 4 summarizes the results of the pull-off force measurements using differ-ent alginate probes with diameters of 12 µm and 33.4 µm, for the RMS roughnessof alginate films, bentonite and borax pellets, and mica cleaved surfaces. The de-pendence of the pull-off forces on the substrate roughness can be seen in Fig. 4.One can see that in the case of rough surfaces (approx. 420 nm) the probe diame-ter was found to have little effect on the measured force. A similar result has beenreported by Segeren [7], suggesting that there is a threshold RMS value for theprobe/substrate pair, at which pull-off forces become independent of roughness. Asexpected, from a multiple asperity contact point of view, the largest adhesion wasfound for the mica surface in contact with the probe [47, 48]. The difference inroughness between mica surface and bentonite/borax pellets has a significant im-pact on measured pull-off forces, resulting in very low values of adhesion for therelatively rough pellets. However, in the literature [49], adhesion of particles smallerthan or similar in size to the asperities depends mainly on the size and shape of theasperities and only weakly on the size of the particle. For large particles also theparticle size has a significant effect on the adhesion [49]. The plateau values ofpull-off forces in the case of bentonite and borax surfaces suggest that the thresholdRMS values may have been attained. In order to overcome the roughness effect,Tormoen and Drelich [50] suggested a very practical solution and a soft probe wasselected during the pull-off force measurements. Small asperities of viscoelasticand plastic polymers, such as alginate polymer, can be squeezed during the pull-offforce measurements by using larger loads in order to eliminate/reduce the effectof roughness on adhesion. The average pull-off force value increased with appliedload, indicating deformation of the probe and increased contact area. Therefore,cantilevers stiffer than those used in this study are needed for further investigation.

4. Conclusions

An AFM colloidal probe technique was used to measure the adhesion and interac-tion forces for alginate/alginate and alginate/mineral fillers of bentonite, borax andmica systems. The following conclusions were drawn from direct measurements ofadhesion forces:


Figure 4. Effect of the probe diameter and surface RMS roughness on adhesion force measurements.

• spherical particles of alginate polymer were successfully produced using thespinning disk method;

• for the first time atomic force microscopy was employed to characterize adhe-sion in alginate–filler surfaces;

• the free energy of adhesion between mica and alginate had the lowest value.Bentonite and borax exhibited low values of adhesion forces which may in-dicate a smaller compatibility between these surfaces and alginate; howeversurfaces roughness has to be taken into account;

• the bentonite–alginate system had the lowest adhesion force and the highestfree energy of adhesion which may be the result of weaker adhesion betweenhydrophobic alginate and less hydrophilic bentonite;


• the difference in roughness between mica surface and bentonite/borax pelletshad a significant impact on the measured adhesion force, resulting in very lowvalues of adhesion for the relatively rough pellets. It may be possible to over-come the roughness effect by selecting a soft probe, operating at adequate loadsto conform to the substrate irregularities [50]. The roughness effect does requirefurther investigation.

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Evaluation of Adhesion Forces in Alginate–Filler System Using an AFM Colloidal Probe Technique

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