micro/nano-particle manipulation and adhesion studies

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Micro/Nano-particle Manipulation andAdhesion StudiesWeiqiang Ding aa Department of Mechanical and Aeronautical Engineering, ClarksonUniversity, Potsdam, NY 13699-5725, USAPublished online: 02 Apr 2012.

To cite this article: Weiqiang Ding (2008): Micro/Nano-particle Manipulation and Adhesion Studies, Journalof Adhesion Science and Technology, 22:5-6, 457-480

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Journal of Adhesion Science and Technology 22 (2008) 457–480www.brill.nl/jast

Micro/Nano-particle Manipulation and Adhesion Studies

Weiqiang Ding ∗

Department of Mechanical and Aeronautical Engineering, Clarkson University,Potsdam, NY 13699-5725, USA

Received in final form 9 November 2007

AbstractIn this paper recent progress in micro/nano-particle manipulation and particle–substrate adhesion studies isreviewed. Various particle manipulation and particle–substrate adhesion measurement techniques are sum-marized. An atomic force microscope (AFM) is the most commonly used tool for particle manipulation,and there are also some custom-made devices that employ different interactions such as van der Waals,electrostatic and capillary forces to manipulate micro/nano-particles. In particle–substrate adhesion study,the contact-based measurement techniques such as pull-off test and lateral push test explore the adhesionbetween a single particle and a substrate, while non-contact adhesion measurement techniques such as elec-tric field detachment and centrifugal detachment methods simultaneously explore the adhesion properties ofa group of particles. Also reviewed are recent studies on factors that affect the particle–substrate adhesion,including surface roughness, electrostatic charge and relative humidity. Koninklijke Brill NV, Leiden, 2008

KeywordsAtomic force microscopy, detachment force, nanomanipulation, nanoparticles, particle adhesion, particledetachment, roughness

1. Introduction

A nanoparticle is one of the major fundamental building blocks for nanotechnology.In recent years nanoparticles have attracted considerable research interest becauseof their unique electronic, optical, mechanical and chemical properties. Nanopar-ticles have found applications in a variety of areas such as advanced materials,electronics, catalysis, biomedicine and pharmaceuticals. For some of the potentialapplications such as single-electron transistor or nanoparticle memory, it is desir-able to precisely position an individual nanoparticle, or assemble nanoparticlesinto two-dimensional (2-D) or three-dimensional (3-D) structures. Over the pasttwo decades there have been numerous experimental investigations on micro/nano-

* Tel.: 315-268-2205; Fax: 315-268-6695; e-mail: [email protected]

Koninklijke Brill NV, Leiden, 2008 DOI:10.1163/156856108X295563

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458 W. Ding / Journal of Adhesion Science and Technology 22 (2008) 457–480

particle manipulation. A number of techniques have been developed to manipulateparticles on a variety of substrates and under different environmental conditions.

The adhesion behavior of particles has been a subject of study for a longtime [1–8]. Understanding the particle–substrate adhesion is of both scientific andtechnological interest. For instance, such knowledge is essential for a number ofindustrial processes involving micro/nano-particles such as coating, printing, pol-ishing and cleaning. The particle–substrate interactions have been theoreticallyevaluated using analytical or numerical methods and also experimentally studiedthrough adhesion measurements.

The interaction between a sphere and a flat surface has been theoretically inves-tigated for several decades. Many adhesion models have been proposed, such asthe JKR (Johnson–Kendall–Roberts) [9], DMT (Derjaguin–Muller–Toporov) [10],MYD (Muller–Yushchenko–Derjaguin) [11] and M–D (Maugis–Dugdale) [12]models. The van der Waals interaction was considered in these models, and theadhesion force was found to depend on the material properties and the geome-try of both the particle and the substrate. In the past two decades, a variety oftechniques have been developed to experimentally explore the particle–substrateadhesion properties, and such techniques can be classified into two categories:(1) contact techniques and (2) non-contact techniques. The contact-based particleadhesion measurements are typically performed with an atomic force microscope(AFM) or other custom-made manipulation devices, and a probe is used to explorethe particle–substrate adhesion. In non-contact measurements the adhesion prop-erties of particles are explored by applying external forces such as electrostatic,centrifugal or inertia forces without direct contact.

Other than the material properties and dimension, the adhesion between a particleand a substrate is influenced by several other parameters such as the surface rough-ness, the electrostatic charge and the charge distribution, and the relative humidity.Researchers have observed significant discrepancy between the experimental re-sults and the values predicted with contact mechanics models. In the past decadeconsiderable research efforts have been devoted to develop models that incorporateadditional parameters such as surface roughness, electrostatic charge and relativehumidity in the analysis. These models were reported to better predict the experi-mental results.

2. Micro/Nano-particle Manipulation Techniques

AFM is a technique for imaging surface topography with nanometer resolution. Theinvention of the AFM in the 1980’s provides opportunities for direct probing of in-dividual nanoscale particles [13]. The principles of the AFM are well documentedin the literature [13–17]. AFM offers high positioning precision, and thus becomesan ideal tool for controlled manipulation of micro/nano-particles on a substrate. Nu-merous micro/nano-particle manipulation studies have been carried out with AFMin the past two decades.

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W. Ding / Journal of Adhesion Science and Technology 22 (2008) 457–480 459

Other than the AFM, several custom-made manipulation tools have also beenused for contact manipulation of micro/nano-particles in vacuum as well as ambi-ent environments. Different interaction forces such as van der Waals interaction,electrostatic force and capillary force are used for particle manipulation in suchdevices.

2.1. Particle Manipulation with AFM

Reifenberger and co-workers [18–26] performed early work on nanoparticle ma-nipulation and adhesion with AFM. For example, Schaefer et al. [18] demonstratedthat nanoscale clusters deposited on flat substrates could be imaged with AFM.Mahoney et al. [19] studied the interactions of gold nano-clusters with a num-ber of atomically flat substrates with AFM. Later, they demonstrated the assemblyof gold nano-clusters into complex 2-D patterns through nanomanipulation [21].Early work on nanoparticle positioning was also reported by Junno et al. [27],who demonstrated controlled positioning of 30 nm diameter GaAs nanoparticleson a substrate with AFM.

A series of works have also been reported by Requicha and co-workers [28–33].They reported using two methods to manipulate gold nanoparticles on mica sur-face with non-contact AFM [28], namely “feedback off” protocol and “set-pointchange” protocol. With “feedback off” protocol the nanoparticle on a substrate wasfirst imaged in non-contact mode, the feedback was then switched off as the tipapproached the nanoparticle. The nanoparticle was thus pushed laterally by thetip. The “set-point change” protocol involved a change in the non-contact imag-ing set-point to achieve manipulation. According to the authors, the “feedbackoff” technique cannot be used to manipulate very small nanoparticles because theseparation between the tip and substrate is around 4–5 nm, while the “set-pointchange” technique can manipulate particles of a few nanometers in size. Withthese methods, they demonstrated that gold nanoparticles of 5–15 nm in diametercould be assembled into predetermined patterns on the surface of mica. Construc-tion of 3-D structures through individual nanoparticle manipulation has also beendemonstrated [30]. In a further study, they investigated the feasibility of push-ing nanoparticles on uneven substrates [33], and demonstrated that a nanoparticlecould be pushed up a step whose height was of the same order as the particle size.With a similar technique, they later demonstrated nanoparticle manipulation in liq-uids [32].

Martin et al. [34] performed manipulation of silver nanoparticles on silicon diox-ide substrate with AFM operated in non-contact mode. They developed a schemethat allowed real-time monitoring of the position of the moving particle by mon-itoring the vibration amplitude of the cantilever during continuous line scanning.Hansen et al. [35] reported positioning of iron nano-clusters with AFM, and theyspecifically discussed the choices of cantilever and shape of the cantilever tip thatenabled successful particle manipulation. Decossas et al. [36] presented three ma-nipulation methods for positioning silicon nano-crystals on silicon substrates. In the

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first method, nanoparticles are picked up with an AFM tip and deposited elsewhere.The second method allows the simultaneous manipulation of a set of nano-crystalsto draw well-defined lines, accomplished by sweeping the AFM tip on the surfacein contact imaging mode. The third method manipulates an individual nano-crystalwith high positioning precision through mechanical pushing with the AFM tip op-erated in contact mode. Ma et al. [37] performed push tests on La0.7Sr0.3MnO3(LSMO) nanoparticles deposited on a silicon dioxide surface with an AFM oper-ated in tapping mode. They reported that only 15% of nanoparticles were pushedaway, and they concluded that pushing of LSMO nanoparticles in tapping mode isdifficult.

Sitti and Hashimoto [38] proposed an AFM-based force-controlled pushing sys-tem for manipulation and assembly of nanoparticles. Interaction forces among theAFM probe tip, the nanoparticle and the substrate, including van der Waals force,capillary force, electrostatic force, repulsive contact force and frictional force, wereanalyzed in detail. Pushing experiments on gold-coated latex particles on a siliconsubstrate were performed with a custom-made AFM using a piezoelectric probe,and precise positioning of sub-micrometer particles with ∼30 nm accuracy wasachieved. Later, Sitti [39] introduced an autonomous nanorobotic manipulation sys-tem for nanoscale frictional study. With this system, pushing experiments on latexparticles of around 500 nm radius were performed, and several modes of particlemotion including sliding, rolling and rotation were observed.

2.2. Particle Manipulation in a Scanning Electron Microscope (SEM) withvan der Waals Force

In the past two decades a large number of nanomanipulation studies have beencarried out in the vacuum chamber of SEM. Readers are referred to the paper byFahlusch et al. [40] for a detailed review of such activities. Some of these activi-ties involve contact manipulation of micro/nano-particles relying on the mechanicalinteraction between the particle and probe.

Miyazaki et al. [41] studied the adhesion between polyvinyltoluene (PVT) mi-crospheres and a gold-coated glass probe with a custom-made adhesion measure-ment system in the SEM. They investigated the influence of electron beam exposureon the particle–probe adhesion, and reported a dependence of the adhesion forceon the electron beam flux. The authors proposed that the electron beam irradi-ation induced an electric double layer at the contact interface, which resulted instrong electrostatic interaction at the interface. Saito et al. [42] proposed protocolsto pick up and deposit microspheres with a needle-shaped tool in the SEM. A de-tailed analysis of the interaction forces between the microsphere, the substrate andthe manipulation probe was presented, and the existence of a maximum rolling re-sistance was suggested. With a micromanipulation system, micro-size polystyrenelatex (PSL) or silica spheres deposited on a gold-coated glass plate were pushedwith a gold-coated tapered glass needle. An increase of adhesion force with theincrease of electron beam irradiation time was observed. Based on their experimen-

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tal results, the authors proposed a kinematic model for picking up and depositingmicroparticles. Later, they constructed an automatic system that could arrange ran-domly deposited micro-size metal particles into arbitrary 2-D patterns [43]. Animage recognition system was developed that could automatically determine thepositions of the manipulation probe and particles. The van der Waals force was em-ployed to pick up and deposit particles, based on the observation that the particleadhered strongly to the center of the probe but weakly to the edge of the probe.Recently, Blideran et al. fabricated a mechanically actuated micro-gripper [44],and performed manipulation on micro-size glass spheres [45] in the SEM. Morerecently, Ding et al. [46] explored the micro-size PSL particle adhesion with acustom-made nanomanipulator in the vacuum chamber of an SEM.

2.3. Particle Manipulation with Electrostatic Force

Electrostatic force generated from an applied electric field has also been used tomanipulate micro/nano-particles. For example, Egashira et al. [47] demonstratedmanipulation of gold and nickel alloy powder particles with a needle throughelectrostatic interaction. Takahashi et al. [48] theoretically estimated the voltagerequired to pick up an adhered particle from a substrate based on the JKR model.Later, they performed electric field detachment tests in the ambient environmentwith a manual positioning stage [49, 50]. Solder balls of ∼30 µm in diameter settledon a stainless steel plate were picked up by a sharp stainless steel needle using elec-trostatic force. Two types of tests were performed with either the needle-particle gapor the voltage kept constant, and the experimental results agreed well with the the-oretical predictions. Later, they reported kinetic control of the particle detachmentthrough a sequence of voltages for non-impact electrostatic manipulation [51].

2.4. Particle Manipulation with Capillary Force

As will be discussed later, capillary force is commonly involved in particle adhesionstudy in the presence of water vapor. Recently it has been shown that capillary forcecan be used to manipulate microparticles. Obata et al. [52] first proposed a schemefor micro-manipulation of spherical objects with capillary force. The force neededto collapse a liquid bridge between a sphere and a plate at a given value of liquidvolume was numerically analyzed according to the theory of Orr et al. [53]. Later,the authors proposed a concave probe-tip design for particle manipulation [54],which was expected to generate much larger capillary force than a flat probe-tip.Probes with different concave dimensions were fabricated and the maximum cap-illary forces between these concave probes and spheres of varied diameters weremeasured. The experimental results were in good agreement with the theoreticalpredictions. It was found that a much larger capillary force could be generated asthe concave dimension of the probe approached the curvature of the particle.

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2.5. Comparison of Different Particle Manipulation Techniques

Table 1 summarizes the particle manipulation studies we have discussed and pro-vides a comparison in terms of the particle size and material, substrate material,operative environment and observation method.

Among the particle manipulation techniques presented, AFM has the highestpositioning and imaging resolution. It can be used to manipulate particles as smallas a few nanometers in diameter. Another advantage of using AFM is that it canalso measure the substrate surface morphology, which is an important parameter inthe adhesion analysis. The main disadvantage of AFM-based particle manipulationtechnique is that it does not provide real-time visual monitoring of the manipulationprocess, because the same AFM probe is used for both imaging and manipulationtasks.

Compared with AFM-based particle manipulation, van der Waals force-basedparticle manipulation in SEM allows in situ visual monitoring of the manipulationprocess. Since SEM has high lateral imaging resolution, high positioning precisioncan also be achieved. However, due to the inevitable electron beam charging ef-

Table 1.Comparison of particle manipulation techniques

Manipulation Particle Particle Substrate Operative Observation Referencetechnique material diameter environment method

Atomic Gold 9–20 nm HOPG Ambient AFM [21]force GaAs 30 nm GaAs Ambient AFM [27]microscopy Gold 15 and 30 nm Silicon Ambient AFM [30, 32]

5 nm Mica [28]15 and 28 nm Mica [33]

Silver 10–100 nm Silicon Ambient AFM [34]dioxide

Iron 5–50 nm GaAs Ambient AFM [35]Silicon 4–30 nm Silicon Ambient AFM [36]LSMO 3 nm Silicon Ambient AFM [37]Gold coated 484 and 968 nm Silicon Ambient AFM [38]PSL 1000 nm [39]

van der Waals PVT 2.02 µm Gold Vacuum SEM [41]force PSL 2 µm Gold coated Vacuum SEM [42]

and SiO2 glassSolder 30 µm Aluminum Vacuum SEM [43]Glass 3–5 µm Vacuum SEM [45]PSL 20 µm Silicon Vacuum SEM [46]

Electrostatic Solder 30 µm Stainless Ambient Optical [48, 50]force steel microscopy

Capillary Glass, steel 2–3 mm Glass, steel Ambient Optical [54]force microscopy

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fect, it is challenging to perform manipulation operations involving non-conductiveparticles or non-conductive substrates.

The electrostatic force-based and capillary force-based manipulation techniquesdiscussed above both operate in the ambient environment, and both use an opticalmicroscope to locate the particles and monitor the manipulation process. Due to theintrinsic resolution limitation of an optical microscope, it is difficult to preciselymanipulate and position sub-micrometer size particles with these techniques. Inaddition, the electrostatic technique can only be applied to conductive particles, andelectric discharge might occur if the required voltage is high, which may damagethe particle [49]. The capillary force-based manipulation technique is applicable toparticles of any material, and it is very unlikely to cause any mechanical damageto the particles. However, the available capillary force for manipulation depends onthe water wettabilities of the particle and the manipulation probe [54].

Most of the AFM-based particle manipulations are accomplished by pushing aparticle to the destination on the substrate surface, while the other manipulationtechniques involve picking up the particle and depositing it back on the substrate.Particle manipulation through pushing on the substrate surface with AFM has sev-eral disadvantages. First, because the AFM can only push a particle along theline-scan direction, it is laborious to position the particle to a location in a differentdirection, and the manipulation range is limited by the AFM scan range. Second, itis hard to push a particle across a high barrier, a wide trench or other particles alongthe path, and it is impossible to deposit particles onto another substrate throughpushing.

For those manipulation techniques involving picking up particles with a probe,it is relatively effortless to position a particle on the desired location on the sameor even a different substrate. However, it is a challenge to re-deposit the particleback onto the substrate, since the same force previously used to pick up the particleprevents the particle from being detached from the probe. While researchers havepresented several particle placement procedures for van der Waals force-based [43],electrostatic force-based [51] and capillary force-based [52] manipulations, suchoperations are quite laborious and may have limited success. Sometimes the parti-cles also stick to the AFM tip during the pushing process in AFM-based manipula-tion. It is desirable to have a technique that can both pick up and deposit a particlein a quick and reliable manner.

3. Particle Adhesion Study: Contact Techniques

In addition to high-resolution imaging, AFM can also measure the interaction forcebetween the probe and a substrate with high precision, which makes it an idealinstrument for particle adhesion study. Numerous particle adhesion measurementshave been carried out with the AFM. Readers are referred to the paper of Segerenet al. [15] and the recent book of Drelich and Mittal [55] for detailed reviews onAFM-based microparticle adhesion studies. These AFM-based adhesion studies can

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be classified into two categories: (1) Adhesion study with the pull-off method;(2) Adhesion study through lateral pushing. Besides using AFM, the particle–substrate adhesion properties have also been explored with several custom-madetesting tools.

3.1. Particle Adhesion Study Through Pull-Off Test

The pull-off test is a straightforward method to measure the particle–substrate adhe-sion. A commonly used technique for pull-off study is the colloid probe technique(Fig. 1). First, an individual micro/nano-particle of interest is attached to the tip ofan AFM cantilever. Readers are referred to the recent review paper by Gan [56]for a detailed description of various techniques for particle attachment. The parti-cle is then brought into contact with and separated from a substrate. The maximumforce needed to detach the particle from the substrate is measured and the particle–substrate adhesion is evaluated. The colloid probe technique is a versatile techniquethat allows surface adhesion study under different conditions. In addition, the samecolloid probe can be used to perform a series of experiments.

Reifenberger and co-workers [20, 22–26] reported several pull-off force mea-surements with AFM. For instance, Schaefer et al. [20] explored the influenceof surface roughness on the particle adhesion with the colloid probe technique.A polystyrene particle attached to an AFM cantilever was first brought into con-tact with a highly oriented pyrolytic graphite (HOPG) substrate with a certain loadand then it was pulled off from the substrate. The measured adhesion force wasmore or less constant and did not depend on the external load applied. The theo-retical adhesion force was calculated based on the JKR model, which turned outto be 50–175 times larger than the measured forces. The surface roughness of thesphere was then measured, and it was found that the sphere surface contained smallasperities of tens of nanometers in height. The authors argued that due to roughsphere surface, the actual contact area between the sphere and substrate was muchsmaller than predicted. Later, Gady et al. [22] explored the contributions of van derWaals interaction and electrostatic interaction in particle–substrate adhesion. The

Figure 1. Schematic of pull-off test performed using the colloid probe technique.

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non-contact force gradient between a polystyrene microsphere and a flat HOPGsubstrate was measured. They found that when particle–substrate separation waswithin 30 nm, the short range van der Waals force fitted well with the experimen-tal data. However, at larger separation the long-range electrostatic force dominatedthe particle–substrate interaction. Such force was believed to result from the elec-tron charge trapped in the polystyrene sphere. In a further study [23], they exploredthe contribution of electrostatic force in the interaction between polystyrene sphereand HOPG substrate with two techniques: direct force measurement and force gra-dient measurement. For electrostatic interaction, two models were presented withthe electron charge either uniformly distributed over the entire sphere or localizedwithin a region at the bottom of the sphere. From experimental data fitting theyconcluded that the dominant contribution to the electrostatic interaction was a lo-calized charge at the bottom of the sphere. The polystyrene sphere was soakedin methanol to reduce the charge, and a decrease of the interaction force was ob-served. Later, they further explored the jump-to-contact phenomenon between thepolystyrene sphere and HOPG substrate with different sphere sizes [26]. A strongdependence of the jump-to-contact distance on the particle radius was observed,and the experimental results fitted well with an interaction model containing bothvan der Waals force and a localized electrostatic force.

The colloid probe technique was used by Mizes [57] in toner particle adhe-sion study. Later, Bowen et al. [58] used this technique to explore the adhesionin liquids. Farshchi-Tabrizi et al. [59] used the technique to study the influence ofrelative humidity on the particle–substrate adhesion. Zhou et al. [60], Gotzingerand Peukert [61], George and Goddard [62] and Yang et al. [63] used this tech-nique to explore the influence of surface roughness on particle–substrate adhe-sion.

Also a few colloid probe-based adhesion studies have been carried out withcustom-made apparatuses. For instance, Heim et al. [64] measured the particle–particle adhesion with a custom-made setup in the ambient environment. Spher-ical silica particles with a radius of 0.5–5.5 µm was attached to the AFMcantilever, and the pull-off force and the rolling friction force between thissphere and another sphere on substrate were measured. The measured rollingfriction force was found to be only around one percent of the correspond-ing pull-off force. Ecke and co-workers [65, 66] developed a particle inter-action apparatus that enabled measurement of the normal and friction forcesbetween an individual particle and a substrate. Their system is similar to anAFM in terms of motion freedom and force-sensing capacity but cannot per-form surface imaging. The vertical and horizontal deflections of the cantileverwere monitored with a 2-D position sensitive sensor, which enabled measure-ment of both vertical and lateral forces. With the colloid probe technique, theymeasured the pull-off force and friction force between silica microspheres anda silicon wafer, and their results were comparable to the DMT model predic-tions.

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3.2. Particle Adhesion Study Through Lateral Pushing

Besides pull-off measurements, the particle–substrate adhesion has been exploredthrough laterally pushing with AFM probes. For instance, Sheehan and Lieber[67] measured the force needed to move a molybdenum oxide nano-crystal onsingle-crystal molybdenum disulfide surfaces through lateral pushing with AFM.A good linear correlation between the measured static friction force and the nano-crystal area was observed. Dickinson and co-workers [68–70] studied the effectof relative humidity and applied stress on the particle–substrate adhesion forceswith AFM. Sodium chloride microparticles deposited on a sodalime glass substratewere pushed with an AFM tip in the ambient environment. They found that theload applied to the particle depended on the scan speed of the AFM tip. At fastscan the load applied to the particle was larger than at slow scan, however, the par-ticle was detached at slow scan rather than at fast scan. They concluded that thefailure of the particle–substrate interface was not only due to the application of acritical stress that related to the intrinsic strength but rather as a result of progres-sive crack growth at stresses below the ultimate strength of the interface. They alsofound that the nominal shear strength at the particle–substrate interface strongly de-pended on the relative humidity. A chemically enhanced crack growth approach wasused to estimate the crack speed, which explained the dependence of the interfacialshear strength on the relative humidity. They also estimated the work of adhesionbetween the particle and substrate in terms of electrostatic and van der Waals con-tributions. Ritter et al. [71] performed controlled translational manipulation of latexnanospheres on a HOPG surface with a custom-made scanning force microscope.The oscillating voltage exciting the cantilever vibration was varied during the scanto determine the critical voltage needed to displace a nanosphere. The measuredthreshold voltage value was found to depend on the diameter of the sphere. Theupper limit of the friction force was estimated to be 2–13 nN. The authors also ob-served that the spheres tended to slide rather than roll while being pushed laterally.Later, Yuan and Lenhoff [72] measured the mobility of amidine latex particles ona mica surface with AFM. By repeatedly scanning a particle with increasing lateralforce in line-scan mode, the force needed to displace the particle was obtained. Twotypes of particles with different diameters were studied, and the average measuredfriction force was found to be proportional to the particle diameter. The adhesionforce between the nanoparticle and the substrate was estimated from the measuredfriction force, and it was of the same order of magnitude as the van der Waals forcefrom theoretical analysis.

Ding et al. [46] explored the particle adhesion with a custom-made nanoma-nipulator in the vacuum chamber of an SEM. Micro-size PSL spheres settled ona silicon substrate were laterally pushed with an AFM probe attached to the ma-nipulator (Fig. 2). The response of a particle to an increasing lateral pushing forcewas obtained. The existence of a rolling resistance moment was observed, and thework of adhesion between the particle and substrate was calculated, which was inagreement with the value from theoretical predictions.

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Figure 2. Lateral pushing of a PSL microsphere settled on a silicon substrate with an AFM cantilever.

Figure 3. Schematic of (a) electric field detachment and (b) centrifugal detachment techniques forparticle adhesion study.

4. Particle Adhesion Study: Non-contact Techniques

Besides contact measurements, there are also several other adhesion study tech-niques such as electric field detachment, centrifugal detachment and mechanicalvibration techniques that do not involve direct probing of individual particles.

4.1. Electric Field Detachment

Electric field detachment method is a non-contact adhesion study technique whereelectric force is used to remove particles from a surface. In an electric field detach-ment measurement, a pair of flat electrodes is separated by a certain gap, and theparticles of interest are deposited on the surface of one electrode (Fig. 3a). A dcvoltage is applied between the two electrodes and its magnitude is gradually in-creased. When the Coulomb force exerted on a particle by the electric field exceeds

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the van der Waals and electrostatic interactions between the particle and the sub-strate, this specific particle will be detached from the substrate. This technique canbe used to explore the influences of particle size, electric charge, shape and surfaceroughness on particle–substrate adhesion.

The electric field detachment technique has been commonly used in exploringthe adhesion properties of charged toner particles. For example, Feng and col-leagues [73–75] performed theoretical study on the electric field detachment ofnon-uniformly charged toner particles with finite element method. Hays [76, 77],Hays and Sheflin [78], Mizes et al. [79] and Takeuchi [80] used this technique toexperimentally explore the adhesion of toner particles to various substrates. Severaltechniques have been used to monitor the detachment of charged toner particlesfrom the substrate. For example, Mizes et al. [79] used a light reflection method todetect the surface coverage of both electrodes, while Takeuchi [80] monitored thecurrent flow between the electrodes.

The electric field detachment technique can also be modified to measure the ad-hesion of a single particle. For instance, Takeuchi [80] used a small metal sphere(1 mm in diameter) as the top electrode that could be positioned above a singleparticle on the substrate. A CCD camera was used to monitor the jump of the par-ticle. With this technique he explored the adhesion of hundreds of toner particlesindividually.

4.2. Centrifugal Detachment

Centrifugal detachment technique is a commonly used method to study the adhe-sion of a group of particles to a surface. In a centrifugal detachment measurement,a substrate with particles deposited on its surface is rotated at successively higherand higher speed (Fig. 3b), and the number of particles detached at a certain ro-tation speed is monitored. The corresponding centrifugal force is calculated fromthe particle size and the rotation velocity, and this force equals the adhesion forcebetween the particle and the substrate. To eliminate air resistance, sometimes thecentrifugal detachment measurements are carried out in vacuum [80].

Mizes et al. [79] used the centrifugal detachment technique in their toner particleadhesion study. They used a high-resolution CCD camera to monitor the particleson a small area of the substrate to track the status of the same particles remainingon the surface after each higher and higher rotation speed. Rimai and co-workers[81, 82] used this technique to investigate the contributions of van der Waals inter-action and electrostatic force on the adhesion of spherical and irregular polymermicroparticles. They found that the van der Waals interaction between chargedspherical polymer particles and a photoreceptor substrate was so strong that only asmall percentage of particles were removed even at the highest centrifugal acceler-ation and lowest charge. The substrate surface was then coated with a monolayerof zinc stearate powder, which dramatically reduced the particle–substrate van derWaals interaction and facilitated particle removal. Their work showed that the vander Waals interaction is more significant than electrostatic force in this charged

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particle–substrate interaction, but it can also be significantly reduced by modifyingthe surface condition of the substrate. Very recently, Takeuchi [80] used the cen-trifugal technique to explore the contributions of van der Waals force, electrostaticforce and capillary force in toner particle adhesion. This technique was also usedby Zhou et al. [60] to explore the influence of particle charge and surface roughnesson the toner particle–substrate interaction.

4.3. Mechanical Vibration

Mechanical vibration is another non-contact technique that researchers use to ex-plore the adhesion property between a particle and a substrate. Cetinkaya and co-workers [83–93] have carried out intensive research in this area. They demonstratednon-contact nanoparticle removal from a flat substrate and also from trenches andpinholes with pulsed-laser induced plasma and shock waves [83, 85–88, 93]. Later,they reported laser induced plasma particle removal in the presence of a thin liquidfilm [92]. Recently, they demonstrated that the rotational motion of an adhered par-ticle could be excited through acoustic base excitation [89–91]. A silicon substratewith microparticles deposited on its surface was mounted on top of an ultrasonictransducer. The mechanical vibration of a particle was excited and its motion wasmonitored with a laser Doppler vibrometer (Fig. 4). The rotational motion of theparticle was detected and its frequency was measured. The rotational natural fre-quency of the adhered particle was related to the adhesion bond and the work ofadhesion of the particle–substrate system. The work of adhesion calculated from

Figure 4. Schematic of non-contact mechanical vibration experimental configuration for parti-cle–substrate adhesion study (courtesy of Dr. Cetinkaya).

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the measured rotational natural frequency was in good agreement with the analyti-cally predicted value.

Recently, Eglin et al. [94] demonstrated the manipulation of microparticles withan inertial force. Micro-size stainless steel or glass particles were deposited onto asubstrate which was attached to a piezoelectric plate. A periodic parabolic wave-form signal was applied to the piezoelectric plate, which induced periodic displace-ment of the plate. Due to the inertia, the particles slid on the substrate surface as theplate suddenly reversed its displacement direction. The inertia force applied ontothe particle under sudden substrate acceleration was analyzed, and the force neededto move the first particle was found to be 54 ± 17 nN. They also measured the fric-tion force between the glass particle and silicon substrate with the colloid probetechnique, and the results agreed well with the estimated inertial force.

4.4. Comparison of Different Particle Adhesion Study Techniques

The contact technique explores the adhesion property between a single particle anda substrate, while the non-contact technique such as electric field detachment orcentrifugal detachment method provides the average and distribution of the adhe-sion forces of a group of particles from a single measurement. Furthermore, contactmeasurement only studies the particle–substrate adhesion property in the region ofcontact, which may not give representative adhesion information for non-sphericalparticles. In most non-contact measurements the particle–substrate adhesion at var-ious contact configurations are obtained simultaneously, so they can provide betterinsight into the overall particle adhesion properties. In addition, some of the non-contact techniques allow the study of electrostatic charge effect, an effect whichcannot be studied with contact techniques. Readers are referred to the works ofMizes et al. [79] and Takeuchi [80] for comparisons of several contact and non-contact adhesion study techniques in toner particle adhesion research.

Among the five adhesion study techniques discussed, the pull-off test is the onlytechnique that directly measures the adhesion force between a particle and a sub-strate. While the particles are also being detached from the substrate in electric fielddetachment and centrifugal detachment measurements, the corresponding adhesionforces are not directly measured but calculated from the applied voltage or the rota-tion speed. In lateral push test and mechanical vibration test, the particle–substrateadhesion force is not measured. Instead, the particle–substrate adhesion property isdeduced from the measurement of other parameters such as the friction force, therolling resistance moment, or the rotational vibration frequency.

The pull-off study with a colloid probe is the most extensively used technique inparticle adhesion study. Compared with other techniques discussed, it has severaladvantages. First, with this technique essentially any desired magnitude of force canbe applied to pull a particle off the substrate. Second, this technique has high force-sensitivity. Third, it can be used to measure the adhesion between the same particleand different substrates of varied surface conditions, or between the same particleand the same substrate but under different environmental conditions. As will be

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discussed in the next section, the colloid probe technique has been widely used inexploring the influence of certain parameters such as surface roughness and relativehumidity on particle–substrate adhesion. Fourth, an external force can be applied tothe particle in contact with substrate before pull-off to explore the influence of thepre-load force on adhesion. However, this technique also has several disadvantages.First, it is difficult to charge the attached particle for electrostatic adhesion studysince the AFM probe is not an electrical insulator. Second, it is laborious to preparea colloid probe. Third, at present it is rather challenging to attach a particle of lessthan 1 µm in diameter to an AFM tip [56], which limits the application of thistechnique in nanoparticle adhesion study.

Similar to the pull-off test, the lateral push test also explores the adhesion ofindividual particles. It is a non-destructive technique that typically does not causeany damage to the particle. However, for lateral push tests performed in the SEM,the electron beam charging issue limits the types of particles and substrates thatcan be studied. For lateral push tests performed with AFM in the ambient, its forcesensitivity is lower than the colloid probe technique since the lateral stiffness of anAFM cantilever probe is much higher than its normal stiffness.

The electric field detachment method can be used to explore the role of elec-trostatic charge in particle–substrate adhesion, which is a major advantage overcontact-based techniques. On the other hand, this technique can only measure theadhesion of charged particles since uncharged particles cannot be lifted off by elec-tric field. Also, if the charged particles adhere strongly to the substrate, a highvoltage is needed to detach them from the substrate. And applying high voltagebetween a small gap may result in air breakdown unless the setup is enclosed in avacuum environment.

Similar to the electric field detachment technique, the centrifugal detachmenttechnique enables the determination of the adhesion forces of a large number ofparticles. Moreover, it can be used to study the adhesion for both charged and un-charged particles. However, centrifugal detachment tests are more time-consumingto perform than electric field detachment tests [79]. In addition, this technique isonly applicable to relatively large size particles. Since the adhesion force is propor-tional to the particle radius [9, 10] and the centrifugal force is proportional to theparticle volume, with decreasing particle size higher velocity is required to removethe particle. For instance, Rimai et al. [82] reported that charged toner particles(∼7.1 µm diameter) adhered so strongly to a photoreceptor substrate that only asmall percentage could be removed even at highest centrifugal acceleration and low-est charge. According to Segeren et al. [15], for small particles (typically <5 µm)the centrifugal force will not be high enough to exceed the adhesion force.

The mechanical vibration is a non-contact and non-destructive technique for par-ticle adhesion study. While only the vibration of a single particle can be measuredat a time, the measuring beam spot can be rapidly positioned to another particle onthe substrate for another measurement. However, this technique is also difficult to

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be applied to nano-size particles since the measurements are carried out under anoptical microscope.

Overall, with current techniques it is relatively difficult to explore the adhesionbetween a nano-size particle and a substrate due to limitations in particle attach-ment, force sensing and specimen observation, etc.

5. Parameters Influencing Particle Adhesion

According to the adhesion models such as the JKR and DMT models, the particle–substrate adhesion depends only on the geometries and mechanical properties ofthe particle and the substrate. In reality, the particle–substrate adhesion is also in-fluenced by several other factors such as the surface roughnesses of the particle andsubstrate, the electrostatic charge and charge distribution, and the relative humidityof the environment. In recent years there have been many theoretical and exper-imental studies to explore the influence of such parameters on particle–substrateadhesion.

5.1. Surface Roughness

The surface roughness has a significant effect on the particle–substrate adhesion. Incontact mechanics adhesion models both the particle and the substrate are assumedto have perfectly smooth surfaces. In real practice, however, both the particles andthe substrate inevitably have some degree of surface roughness. Because of the sur-face roughness, the actual contact area between the particle and the substrate isdecreased and the separation between the two mating surfaces is increased, whichresults in dramatic decrease of the adhesion force. As a result, discrepancies be-tween the measured adhesion forces and the theoretically predicted values havebeen frequently reported.

The influence of surface roughness on particle–substrate adhesion has attractedsignificant research attention. Rabinovich et al. [95, 96] performed a detailed studyon the effect of substrate surface roughness on the adhesion between a spherical par-ticle and a flat substrate. They proposed a model that more accurately described thesurface asperities with the measured root-mean-square roughness and peak-to-peakdistance [95]. They also experimentally explored the adhesion between surfaces ofcontrolled nanoscale roughness [96], and reported that a root-mean-square rough-ness as small as 1.6 nm could reduce the adhesion force by nearly a factor of five.Further increasing the surface roughness, however, did not significantly reduce theadhesion force. Their model was able to predict the particle–substrate adhesionwithin 50% of the experimental values, which was a significant improvement asmost of the previous models underestimated the adhesion force by 10–50 times.According to the authors, the precise prediction of the adhesion between surfacesmainly depends on how to obtain a realistic value of the radii of the asperities onthe surfaces.

Cooper et al. [97] also developed a theoretical model to characterize van derWaals interactions between a rough, deformable spherical particle and a flat,

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smooth, hard surface. The particle was modeled as a spherical colloid uniformlycovered with hemispherical asperities with constant radii and uniform separations.They also experimentally measured the adhesion between a PSL particle and a sili-con substrate with the colloid probe technique, and reported that their model couldpredict the adhesion force more accurately than the classical theories. Later theyreported another study where the adhesion force between a micro-size polystyreneparticle and a silicon substrate was measured in an aqueous solution with pH rang-ing from 2 to 10 [98]. A strong dependence of the measured adhesion force on thesolution pH value was observed. The adhesion force measured in acidic conditionagreed well with theoretical value from the van der Waals adhesion model, andwas roughly 15 times greater than at more basic conditions. The authors attributedthe decrease of the adhesion force at basic condition to a change of the substratemorphology due to the etching away of surface oxide of the silicon substrate. Sub-sequent AFM topography study on the silicon substrate did reveal an increase ofsubstrate surface roughness with the increase of solution pH value. Later, they pre-sented a particle–surface adhesion model that included the geometry and surfacemorphology of both the particle and the substrate [99, 100], which allowed the pre-diction of adhesion interactions between surfaces of arbitrary shapes. They found agood agreement between the predictions of this model and their previously reportedexperimental results [97, 98].

Beach et al. [101] characterized the adhesion between rough pharmaceutical par-ticles and rough surfaces with the colloid probe technique. They observed that for aparticle of specific geometry and surface roughness, its adhesion with the substratecould be increased or decreased by varying the surface roughness of the substrate.They compared their experimental results with the adhesion force predicted with theRabinovich et al.’s model [95, 96], and found that the model predicted the pull-offforces for the lactose and silanized glass particles and underestimated the adhe-sion forces of the peptide and PS particles by an order of magnitude. The authorsattributed the underestimate to the particle deformation during contact and the com-plex particle surface topography. Later, Tormoen et al. [102] used the colloid probetechnique to measure the adhesion force between a 5 µm glass sphere and a cali-bration grating containing a regular array of triangular ridges with 3 µm spacingsand <10 nm peak radii. A significant variation of the measured adhesion force wasobserved as the probe was positioned at different locations over the grating ridges,and three contact scenarios between the probe and ridges were discussed. Similarobservation was reported by Segeren et al. [15], who found that the adhesion forcereached a maximum value at the point where the roughness of the substrate matchedthe roughness of the microparticle surface.

Using both centrifugal detachment and colloid probe techniques, Mizes et al.[79] studied the influence of surface roughness on particle adhesion with toner par-ticles of varied roughness. Similar studies were performed by Zhou et al. [60],also using both centrifugal detachment and colloid probe techniques. The adhesionforces between a smooth polystyrene microparticle to several substrates with dif-

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ferent roughnesses were measured, and a significant reduction in adhesion forcewith increasing surface roughness was observed. The adhesion forces between atoner particle of irregular shape and several substrates were also measured, whichturned out to be smaller than those of the smooth particles. Later, Gotzinger andPeukert [61] studied the adhesion between spherical alumina particles and ce-ramic substrates of varied surface roughness. By coating silicon wafers with silicananoparticles of 30, 110 and 240 nm in diameters, they were able to prepare sub-strates with precisely-controlled surface roughnesses. Their experimental resultsagreed with the values predicted with the model of Rabinovich et al. [95, 96].

Recently, George and Goddard [62] studied the adhesion of a rough particle anda flat substrate with the colloid probe technique. To account for the real surfaceroughness, the authors proposed a model that represented the surfaces with period-ically close-packed spherical caps of equal radii. The surface topographies of theparticle and substrate were experimentally characterized with AFM, and the radiiof the spherical caps in the model were chosen to ensure that the surface had thesame root-mean-square roughness and the same density of asperities as the actualcontact area. A roughly linear relationship between the measured adhesion forceand the effective contact radius was found. And the adhesion force predicted withtheir proposed model was consistent with the predictions from contact mechanicsmodels. Very recently, Yang et al. [63] used the colloid probe technique to measurethe adhesion forces between glass spheres of different sizes and a silicon surface,and explored the effect of random surface roughness on the measured adhesionforces. They used several models to account for the surface roughness in the pull-off force calculation, and reported that using normal surface roughness parameterswas insufficient in correcting the adhesion force because the surfaces of the colloidprobes had multi-scale surface roughness. A multi-scale sequential contact modelwas proposed, which was shown to be more accurate in adhesion force correction.

5.2. Electrostatic Charge

The electrostatic charge present on a particle surface introduces electrostatic forceand thus affects the adhesion of the particle to a substrate. Such interaction dependson both the amount of the charge and its distribution.

Charged particles are commonly encountered in certain industrial processes suchas electrophotography and electrostatic powder coating. Because of its practicalimportance, the influence of electrostatic charge on the adhesion of toner parti-cles in electrophotography process has been intensively studied by a number ofresearchers. For instance, Feng and colleagues [73–75, 103] theoretically analyzedthe magnitude of the adhesion force of charged toner particles. They showed thatthe electrostatic force between a charged toner particle and a substrate could varyover an order of magnitude, depending on the distribution of the electrostatic chargeon the particle surface. Hays [76, 77], Hays and Sheflin [78], Rimai and co-workers[81, 82, 104] and Mizes and colleagues [57, 79] experimentally explored the ad-

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hesion of charged toner particles, and their results support a model of non-uniformdistribution of the electrostatic charge on the toner particle surface.

5.3. Relative Humidity

In the ambient environment, the relative humidity is known to affect the adhesionbetween a particle and a surface. The condensation of water vapor may form a capil-lary bridge in the contact region between a hydrophilic particle and substrate, whichwill introduce capillary force and increase the particle adhesion to the substrate.

The effect of capillary force on particle–substrate adhesion has been exploredanalytically by several researchers [105–107], and also a number of experimentalworks have been reported studying such effect. For instance, Fuji et al. [108] inves-tigated the relationship between the adhesion force and relative humidity for silicaparticles with the colloid probe technique. Surface treatment was performed on thesilica particles to control their wettability from hydrophilic to hydrophobic. They re-ported that the adhesion force between hydrophilic particles maintained a relativelyconstant value at low humidity but increased rapidly at high relative humidity. Forhydrophobic particles, there was no increase in adhesion force even at saturatedvapor pressure. They attributed the increase of adhesion force to the formation ofa capillary bridge between hydrophilic particles and the substrate at high humid-ity. Recently, Takeuchi [80] reported that when the relatively humidity was higherthan 50%, the capillary force started to contribute to the adhesion between toner par-ticles and a substrate. As the relative humidity was higher than 70%, the capillaryforce became the most dominant force. The influence of humidity on adhesion ofnanoscale contacts has also be experimentally studied by Nalaskowski et al. [109],Biggs et al. [110] and Farshchi-Tabrizi et al. [59].

For biochemical and medical applications, particle manipulation may need to beperformed in a liquid environment. A number of particle–substrate adhesion studieshave been carried out in liquids. For example, Resch et al. [32] first demonstratedthe manipulation of gold nanoparticles in a liquid environment with a scanningforce microscope. Butt [111] and Preuss and Butt [112, 113] investigated the ad-hesion between a particle and an air bubble in an aqueous electrolyte with AFM.Mulvaney et al. [114] measured the interaction force between a silica sphere and anoil droplet in water with AFM. Bowen et al. [58] explored the adhesion of a silicasphere to a silica surface in a liquid under different surface conditions with the col-loid probe technique. The surfaces of the sphere and substrate were ethanol washedor treated with oxygen plasma, and the measurements were carried out in sodiumchloride solutions at pH 3 and 8. They found that at pH 3 the adhesion forcesmeasured from ethanol-washed surface agreed with the theoretical values but theforces measured from plasma-treated surfaces were at least an order of magnitudelower than the theoretical values. At pH 8, the adhesion forces measured from bothethanol-washed and plasma-treated surfaces were significantly larger than the the-oretical values. While the authors were not able to provide an explanation for such

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discrepancy, it showed that the particle–substrate adhesion in a liquid is complexand not fully understood.

5.4. Discussion

Both particle and substrate surface roughnesses significantly affect the particle–substrate van der Waals interaction. As Rabinovich et al. [96] reported, a root-mean-square roughness of 1.6 nm reduces the adhesion force by a factor of five.The electrostatic force plays a significant, and in some cases a dominant, role inthe adhesion between a charged particle and a substrate. Depending on the surfaceroughness, amount of electrostatic charge and the charge distribution, either van derWaals force or electrostatic force can dominate the adhesion of a charged particle.For example, Rimai et al. [82] reported that the interaction between the chargedspherical polymer particles and photoreceptor substrate was dominated by the vander Waals interaction, which was later reduced by increasing the substrate surfaceroughness to have electrostatic force dominate the interaction. Zhou et al. [60] alsodemonstrated that van der Waals force could be minimized by increasing the parti-cle or substrate surface roughness, which allowed the electrostatic force to dominatethe adhesion of charged toner particles. Mizes et al. [79] reported that charging ir-regularly shaped toner particles increased the particle–substrate adhesion by morethan an order of magnitude.

The influence of relative humidity on the particle adhesion depends on the waterwettablities of both the particle and substrate surfaces. For hydrophobic surfaces,the relative humidity has little effect on their adhesion properties. For hydrophilicsurfaces, the relative humidity also does not significantly affect the adhesion at lowrelative humidity (<50%), but the capillary force will dominate the adhesion at highrelative humidity. For instance, Takeuchi [80] reported that the adhesion force dueto water bridge contributes to the adhesion when the relative humidity is over 50%,and it becomes the predominant force at a relative humidity over 70%. Biggs et al.[110] reported an increase in pull-off force by a factor of between 5 and 7 whenthe relative humidity was over 60%. According to Pakarinen et al. [105], at a givenrelative humidity the magnitude of the capillary force also depends on the particleshape.

While several analytical models considering the influence of surface roughness,electrostatic charge and capillary force have been developed and experimentallyverified, a model that integrates all these parameters has yet to be developed. Onedifficulty is the lack of knowledge of the surface electrostatic charge distribution.While the surface roughnesses of the particles and substrates can be accurately mea-sured, at present it is difficult to map the distribution of electrostatic charge ona charged particle surface.

6. Conclusion

Micro/nano-particle manipulation and adhesion studies are an important researcharea that has attracted intense research attention. Various particle manipulation

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protocols have been proposed and extensive experiments have been performedwith commercial or custom-made instruments. Relatively precise positioning ofmicro/nano-particles on various surfaces has been demonstrated and successful as-sembly of 2-D and 3-D patterns through particle manipulation has been reported.However, each of the present techniques has certain limitations in one or more ofaspects regarding, for example, real-time monitoring of the manipulation process,particle re-deposition, particle size and positioning accuracy. A system that can au-tomatically assemble nanoparticles into desired patterns at desired locations withnanometer scale positioning accuracy has yet to be developed.

The adhesion between a micro/nano-particle and a substrate is a complex phe-nomenon. A number of theoretical models have been proposed in recent yearsthat include parameters such as surface roughness and humidity which are ignoredin classical models. Numerous experiments have been performed to explore theparticle–substrate adhesion, with mixed results in comparison with the theoreticalpredictions. Considering the limitations of available particle adhesion measure-ment techniques and analytical models, further developments in both theoreticalmodeling and experimental techniques are needed for full characterization of theparticle–substrate adhesion.

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micro/nano-particle manipulation and adhesion studies

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