A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Friction and Adhesion of Gecko-Inspired PDMS Flaps on Rough Surfaces

Geckos have developed a unique hierarchical structure to maintain climbing ability on surfaces with different roughness, one of the extremely important parameters that affect the friction and adhesion forces between two surfaces. Although much attention has been paid on fabricating various structures that mimic the hierarchical structure of a gecko foot, yet no systematic effort, in experiment or theory, has been made to quantify the effect of surface roughness on the performance of the fabricated structures that mimic the hierarchical structure of geckos. Using a modified surface forces apparatus (SFA), we measured the adhesion and friction forces between microfabricated tilted PDMS flaps and optically smooth SiO(2) and rough SiO(2) surfaces created by plasma etching. Anisotropic adhesion and friction forces were measured when sliding the top glass surface along (+y) and against (-y) the tilted direction of the flaps. Increasing the surface roughness first increased the adhesion and friction forces measured between the flaps and the rough surface due to topological matching of the two surfaces but then led to a rapid decrease in both of these forces. Our results demonstrate that the surface roughness significantly affects the performance of gecko mimetic adhesives and that different surface textures can either increase or decrease the adhesion and friction forces of the fabricated adhesives.     

Pull-off force measurements between rough surfaces by atomic force microscopy

Direct measurements of the pull-off (adhesion) forces between pharmaceutical particles (beclomethasone dipropionate, a peptide-type material, and lactose) with irregular geometry and rough polymeric surfaces (series of polypropylene coatings, polycarbonate, and acrylonitrile-butadiene-styrene) were carried out using the atomic force microscope. These measurements showed that roughness of the interacting surfaces is the significant factor affecting experimentally measured pull-off forces. A broad distribution of pull-off force values was noted in the measurements, caused by a varying adhesive contact area for a particle located on rough substrate. The possibility of multiple points of contact between irregularly shaped pharmaceutical particles and substrate surfaces is demonstrated with nanoindentations of the particle in a fluoro-polymer film. Force-distance curves showing the "sawtooth" pattern are additional evidence that particles make contact with substrates at more than one point. Reduced adhesion of 10- to 14-microm-diameter lactose and peptide material particles to the polypropylene coatings with a roughness of 194 nm was found in this study. Similar pull-off force versus roughness relationships are also reported for the model spherical particles, silanized glass particle with a size of 10 microm and polystyrene particle with a diameter of 9 microm, in contact with polypropylene coatings of varying roughness characteristics. It was found that the model recently proposed by Rabinovich et al. (J. Colloid Interface Sci. 232, 1-16 (2000)) closely predicts the pull-off forces for glass and lactose particles. On the other hand, the adhesion of the peptide material and polystyrene particle to polypropylene is underestimated by about an order of magnitude with the theoretical model, in which the interacting substrates are treated as rigid materials. The underestimate is attributed to the deformation of the peptide material and polystyrene particles.     

Detachment of Particles from Surfaces: An AFM Study

We have measured interactions between hydrophilic and hydrophobic surfaces in an aqueous medium at various pH and ionic strengths as well as in some organic solvents using atomic force microscopy and analyzed them in terms of particle adhesion and detachment from surfaces. In hydrophilic systems the forces observed were found to be well described by DLVO theory at large separation distances. Very long range hydrophobic forces were not observed in hydrophilic-hydrophobic systems. Nevertheless, the jump into contact was found to occur at distances greater that those predicted by just van der Waals attraction. The interaction between two hydrophobic surfaces was dominated by the long-range attraction due to hydrophobic forces. This interaction was found to be sensitive to the type of substrate as well as to the pH and electrolyte concentration. Measured pull-off forces showed poor reproducibility. However, average values showed clear trends and were used to estimate interfacial energies or work of adhesion for all systems studied by means of the Derjaguin approximation. These values were compared to those calculated by the surface tension component theory using the acid-base approach. Good qualitative agreement was obtained, giving support for the usefulness of this approach in estimating interfacial energies between surfaces in liquid media. A comparison of the measured adhesion force with hydrodynamic detachment experiments showed good qualitative agreement. Copyright 2001 Academic Press.     

Friction and adhesion forces of Bacillus thuringiensis spores on planar surfaces in atmospheric systems

The kinetic friction force and the adhesion force of Bacillus thuringiensis spores on planar surfaces in atmospheric systems were studied using atomic force microscopy. The influence of relative humidity (RH) on these forces varied for different surface properties including hydrophobicity, roughness, and surface charge. The friction force of the spore was greater on a rougher surface than on mica, which is atomically flat. As RH increases, the friction force of the spores decreases on mica whereas it increases on rough surfaces. The influence of RH on the interaction forces between hydrophobic surfaces is not as strong as for hydrophilic surfaces. The friction force of the spore is linear to the sum of the adhesion force and normal load on the hydrophobic surface. The poorly defined surface structure of the spore and the adsorption of contaminants from the surrounding atmosphere are believed to cause a discrepancy between the calculated and measured adhesion forces.     

Force measurements between sub-100 nm colloidal particles

Interparticle forces have been measured between polystyrene latex particles as small as 85 nm in diameter in KCl solutions. A variant of the differential electrophoresis technique, called particle force light scattering (PFLS), was used to measure forces between Brownian, nearly touching particles for diameters from 4500 nm down to 85 nm. The forces, some less than 0.1 pN, matched to within a factor of 2 with predictions from depletion and DLVO theory.     

Experimental humidity dependency of small particle adhesion on silica and titania

The humidity present in ambient atmosphere affects the adhesion of small particles by causing capillary bridge formation between the particle and the surface. Even in moderate relative humidities this, usually attractive, force can have a significant effect on adhesion behaviour of micro and sub-micro particles. We have directly measured the pull-off forces of initially adhered oxide particles on oxide surfaces with atomic force microscope in controlled atmosphere with adjustable humidity. We demonstrate the effect of the surface roughness resulting in two different regions of capillary formation and the particle shape having a strong effect on the humidity dependency of adhesion. The experimental results are explained by theoretical framework.     

Targeted particulate adhesion to cellulose surfaces mediated by bifunctional fusion proteins

The adhesion of particles to surfaces is an integral element in many commercial and biological applications. In this article, we report on the direct measurements of protein-mediated deposition and binding of particles to model cellulose surfaces. This system involves a family of heterobifunctional fusion proteins that bind specifically to both a red dye and cellulose. Amine-coated particles were labeled with a red dye, and a fusion protein was attached to these particles at various number densities. The strength of adhesion of a single particle to a cellulose fiber was measured using micropipette manipulation as a function of the specificity of the protein and its surface density and contact time. The frequency and force of adhesion were seen to increase with contact time in fiber experiments. The dynamics of adhesion of the functionalized particles to cellulose-coated glass slides under controlled hydrodynamic flow was explored using a flow chamber for two scenarios: detachment of bound particles and attachment of particles in suspension as a function of the shear rate and surface density of protein. Highly specific adhesion was observed. The critical shear rate for particle detachment was an increasing function of cellulose binding domain (CBD) density on particle surface. A rapid irreversible attachment of particles to cellulose was observed under flow. Using a family of proteins that were divalent for binding either the red dye or cellulose, we found that particle detachment occurred because of the failure of the cellulose-CBD bond. A comparison of fiber binding and particle detachment results suggests that forces of adhesion of particles to cellulose of up to 2 nN can be obtained with this molecular system through multiple interactions. This study, along with the adhesion simulations currently under development, forms the basis of particulate design for specific adhesion applications.     

Direct measurement of hydrophobic particle–bubble interactions in aqueous solutions by atomic force microscopy: Effect of particle hydrophobicity

Direct measurements of the interaction forces between a spherical silica particle and a small air bubble have been conducted in aqueous electrolyte solutions by using an atomic force microscope (AFM). The silica particle was hydrophobized with a silanating reagent, and the interaction forces were measured by using several particles with different surface hydrophobicities. In the measured force curves, a repulsive force was observed at large separation distances as the particle moved towards the bubble. The origin of the repulsive force was attributed to an electrostatic double-layer force because both the particle and bubble were negatively charged. After the repulsive force, an extremely long-range attractive force acted between the surfaces. These results indicate that the intervening thin water film between the particle and bubble rapidly collapsed, resulting in the particle penetrating the bubble.

The instability of the thin water film between the surfaces suggests the existence of an additional attractive force. By comparing the repulsive forces of the obtained force curves with the DLVO theory, the rupture thickness was estimated. The hydrophobicity of the particle did not significantly change the rupture thickness, whereas the pH of the solution is considered to be a critical factor.     

Influence of adsorbed polymers on the removal of mineral particles from a planar surface

 Experimental results on the role of adsorbed polymers on the particle adhesion are presented. Both Brownian (silica particles) and non-Brownian (glass beads) particles were used. The particles were deposited onto the internal surface of a glass parallelepiped cell, and then submitted to increasing laminar flow rates. The pH and the ionic strength of the electrolytes were fixed. The adhesive force was related to the hydrodynamic force required to dislodge 50% of the initially attached beads. We found that high molecular weight PEO had little effect on the adhesion of small silica beads due to the low affinity of the polymer for silica or glass surfaces. On the contrary, PEO greatly enhanced the adhesion of bigger glass beads forced to deposit on the capillary surface because of gravity. The increase was all the more pronounced as the molecular weight of the polymer was high. The effect of high molecular weight cationic copolymers on the adhesion of silica particles was drastic. The maximal force (1500 pN) applied by the device could not enable any particle detachment even using polymers of low cationicity rate (5%), showing the efficiency of electrostatic attractions. When copolymers were adsorbed on both surfaces (particles and plane), the adhesive force exhibited a maximum at intermediate coverage of particles. This optimum was related to the optimum flocculation concentration classically observed in flocculation of suspensions by polymers. Received: 16 February 1996 Accepted: 10 September 1996     

Chemical Force Microscopy Study of Adhesion and Friction between Surfaces Functionalized with Self-Assembled Monolayers and Immersed in Solvents

Adhesive and frictional forces between surfaces modified with self-assembled monolayers (SAMs) and immersed in solvents were measured with chemical force microscopy as functions of surface functionality and solvent. Si/SiO2 substrates were modified with SAMs of alkylsiloxanes (SiCl3(CH2)n-X), and gold-coated AFM tips were modified with SAMs of alkylthiolates (HS-(CH2)n-X). SAMs of alkylsiloxanes terminated in a methyl or oxidized vinyl group; SAMs of alkanethiolates terminated in a methyl or carboxyl group. Adhesive and frictional forces were measured in hexadecane, ethanol, 1,2-propanediol, 1,3-propanediol, and water. The work of adhesion (W) was calculated with the Johnson-Kendall-Roberts theory of adhesive contact. The JKR values agreed well with values derived from the Fowkes-van Oss-Chaudhury-Good surface tension model and from contact angle results. Calculated values of W for all combinations of contacting surfaces and solvents spanned two orders of magnitude. W correlated with the surface tension of the solvent for hydrophobic/hydrophobic interactions; hydrophilic/hydrophilic and hydrophobic/hydrophilic interactions were more complex. Friction forces were fit to a modified form of Amonton's law. For any solvent, friction coefficients were largest for the hydrophilic/hydrophilic contacting surfaces. The friction coefficient for any contacting pair was largest in hexadecane. In polar solvents, friction coefficients scaled with solvent polarity only for hydrophobic/hydrophobic contacting pairs. Copyright 1999 Academic Press.     

AFM interaction study of alpha-alumina particle and c-sapphire surfaces at high-ionic-strength electrolyte solutions

Ionic strength dependence of interaction and friction forces between hydrophilic alpha-alumina particles and c-sapphire surfaces (0001) were investigated under basic pH conditions using the colloidal probe method. The compression of the double layer could be seen from force-distance curves as the ionic strength of the solution increased. The forces were repulsive at all ionic strengths measured, even though the interaction distance changed drastically. No jump to contact occurred. The interaction distance decreased from about 20 nm in 10(-3) M KCl solution to about 7 nm in the 1 M KCl case. The lubricating effect of hydrated cations on the lateral friction force was demonstrated at high electrolyte concentrations. This was attributed to more hydrated cations being present in the solution. The friction behavior was closely related to the short-range repulsive forces between the alpha-alumina surfaces at pH 11.     

Effect of roughness on particle adhesion in aqueous solutions: a study of Saccharomyces cerevisiae and a silica particle

An atomic force microscope (AFM) has been used to quantify the adhesion of living cells Saccharomyces cerevisiae on three different silica surfaces with defined roughness. The effects of support roughness on the adhesion forces of a smooth silica particle were studied in addition. A living single cell was immobilized at the apex of a tipless AFM cantilever using a key-lock mechanism. Adhesion was quantified from the force-distance data measured on a smooth silica substrate and two substrates coated with hydrophilic monodisperse silica particles with 110 and 240 nm in diameter to study the effect of roughness on particle adhesion. The AFM technique gives unique insight into the primary colonization event of biofilm formation. The new knowledge helps substantially to design surface coatings relevant for biotechnology, medicine and dentistry.     

Changes in the interaction characteristics of polyelectrolyte complex covered silica surfaces

The adsorption of polyelectrolyte complexes, PEC, made from the cationic poly (diallyldimethylammonium) chloride (PDADMAC) and the anionic maleic acid-co-propene copolymer (MA-P) on a Si-wafer surface has been studied. The application of highly diluted colloidally dispersed PEC solutions led to the deposition of single PEC particles onto the surface of the Si-wafer. The interaction forces of the heterogeneously covered surface were monitored by direct force measurements with an atomic force microscope (AFM) in the force volume mode. On the surface of a single PEC particle drastic changes in the interaction forces were found in comparison with the unmodified Si-wafer: in all force vs. distance curves a strong increase of the adhesion was measured that can be attributed to the formation of electrostatic bonds between the negatively charged Si₃N₄-tip and the cationic excess charge of the PEC. Additionally, the behavior during approach of both surfaces has been distinct: at pH 6.1 we see a long range electrostatic attraction between the tip and the PEC particle. The attraction becomes even stronger at pH 4.1, because of an increased positive net charge. Generally, a heterogeneous surface with a wide variety of interaction features can be created by the adsorption of PEC particles.     

New SFA techniques for studying surface forces and thin film patterns induced by electric fields

We describe two ways to measure normal and/or lateral forces between two surfaces in a surface forces apparatus (SFA) while an electric field is applied between the surfaces. The first method involves depositing thin conductive layers on the exposed substrate (usually mica) sheets; the second involves using the optically reflecting silver layers on the back surfaces of the sheets as the electrodes. Two types of experiments were performed using these new techniques: (1) measuring the effects of an electric field on the rheology of an approximately 40-microm-thick film of zeolite particles suspended in silicone oil and (2) a dynamic study of electric field-induced pattern formation of a thin polymer film. In the first study, under an electric field of strength approximately 106 V/m the shear force or effective viscosity of the colloid suspension was found to be two orders of magnitude higher than in the absence of the field, when the expected bulk value was measured. In the dynamic study, the initially uniform film transformed into a 2-D honeycombed network of depressed cells bounded by elevated ridges that grew slowly with time in a way consistent with previously derived theories. The new techniques should be applicable to studies of other systems and interactions, such as double-layer forces, micro- and nanoelectrorheology, electric field-induced ordering of particles, and the effects of electric fields on adhesion, friction, and lubrication.     

Adhesion forces between hybrid colloidal particles and concanavalin A

Hybrid particles of poly(methyl methacrylate) and carboxymethylcellulose, PMMA/CMC, were attached to atomic force microscopy cantilevers and probed against concanavalin A (ConA) films formed either on Si wafers or on CMC substrate. Regardless of the substrate, the approach curves showed different inclinations, indicating that the probe first touches a soft surface and then a hard substrate. The distance corresponding to the soft layer was estimated as 20 +/- 10 nm and was attributed to the CMC layers attached to the hybrid particles surfaces. Probing PMMA/CMC particles against ConA adsorbed onto Si wafers yielded retract curves with a sawlike pattern. The average range of adhesion forces (maximum pull-off distance) and mean adhesion force were estimated as 100 +/- 40 nm and -11 +/- 7 nN, respectively, evidencing multiple adhesions between CMC sugar residues and ConA. However, upon probing against ConA adsorbed onto CMC substrates, the mean pull-off distance and mean adhesion force were reduced to 37 +/- 18 nm and -3 +/- 1 nN, respectively, indicating that the ConA molecules immobilized onto CMC films are less available to interact with the hybrid particle than the ConA molecules adsorbed onto Si wafers. Another set of experiments, where PMMA/CMC particle probed against ConA-covered Si wafers in the presence of mannose, showed that the addition of mannose led to a considerable decrease in the mean adhesion force from -11 +/- 7 to -3 +/- 1 nN. Two hypotheses have been considered to explain the effect caused by mannose addition. The first suggested the desorption of ConA from the substrate so that the hybrid particle would probe bare Si wafer (weak adhesion). The second proposed the adsorption of mannose onto the ConA layer so that mannose layer would probe against another mannose layer, leading to low adhesion forces. In situ ellipsometry and capillary electrophoresis have been applied to check the hypotheses.     

How to impose stick boundary conditions in coarse-grained hydrodynamics of Brownian colloids and semi-flexible fiber rheology

Long-range hydrodynamics between colloidal particles or fibers is modelled by the fluid particle model. Two methods are considered to impose the fluid boundary conditions at colloidal surfaces. In the first method radial and transverse friction forces between particle and solvent are applied such that the correct friction and torque follows for moving or rotating particles. The force coefficients are calculated analytically and checked by numerical simulation. In the second method a collision rule is used between colloidal particle and solvent particle that imposes the stick boundary conditions exactly. The collision rule comprises a generalisation of the Lowe-Anderson thermostat to radial and transverse velocity differences.     

Residence time, loading force, pH, and ionic strength affect adhesion forces between colloids and biopolymer-coated surfaces

Exopolymers are thought to influence bacterial adhesion to surfaces, but the time-dependent nature of molecular-scale interactions of biopolymers with a surface are poorly understood. In this study, the adhesion forces between two proteins and a polysaccharide [Bovine serum albumin (BSA), lysozyme, or dextran] and colloids (uncoated or BSA-coated carboxylated latex microspheres) were analyzed using colloid probe atomic force microscopy (AFM). Increasing the residence time of an uncoated or BSA-coated microsphere on a surface consistently increased the adhesion force measured during retraction of the colloid from the surface, demonstrating the important contribution of polymer rearrangement to increased adhesion force. Increasing the force applied on the colloid (loading force) also increased the adhesion force. For example, at a lower loading force of approximately 0.6 nN there was little adhesion (less than -0.47 nN) measured between a microsphere and the BSA surface for an exposure time up to 10 s. Increasing the loading force to 5.4 nN increased the adhesion force to -4.1 nN for an uncoated microsphere to a BSA surface and to as much as -7.5 nN for a BSA-coated microsphere to a BSA-coated glass surface for a residence time of 10 s. Adhesion forces between colloids and biopolymer surfaces decreased inversely with pH over a pH range of 4.5-10.6, suggesting that hydrogen bonding and a reduction of electrostatic repulsion were dominant mechanisms of adhesion in lower pH solutions. Larger adhesion forces were observed at low (1 mM) versus high ionic strength (100 mM), consistent with previous AFM findings. These results show the importance of polymers for colloid adhesion to surfaces by demonstrating that adhesion forces increase with applied force and detention time, and that changes in the adhesion forces reflect changes in solution chemistry.     

Micromechanical adhesion force measurements between tetrahydrofuran hydrate particles

Adhesion forces between tetrahydrofuran (THF) hydrate particles in n-decane were measured using an improved micromechanical technique. The experiments were performed at atmospheric pressure over the temperature range 261-275 K. The observed forces and trends were explained by a capillary bridge between the particles. The adhesion force of hydrates was directly proportional to the contact force and contact time. A scoping study examined the effects of temperature, anti-agglomerants, and interfacial energy on the particle adhesion forces. The adhesion force of hydrates was found to be directly proportional to interfacial energy of the surrounding liquid, and to increase with temperature. Both sorbitan monolaurate (Span20) and poly-N-vinyl caprolactam (PVCap) decreased the adhesion force between the hydrate particles.     

Adhesion of single polyelectrolyte molecules on silica, mica, and bitumen surfaces

In a recent study (Energy Fuels 2005, 19, 936), a partially hydrolyzed polyacrylamide (HPAM) was used as a process aid to recover bitumen from oil sand ores. It was found that HPAM addition at the bitumen extraction step not only improved bitumen recovery but also enhanced fine solids settling in the tailings stream. To understand the role of HPAM, single-molecule force spectroscopy was employed for the first time to measure the desorption/adhesion forces of single HPAM molecules on silica, mica, and bitumen surfaces using an atomic force microscope (AFM). Silicon wafers with an oxidized surface layer and newly cleaved mica were used, respectively, to represent sand grains and clays in oil sands. The force measurements were carried out in deionized water and in commercial plant process water under equilibrium conditions. The desorption/adhesion forces of HPAM obtained on mica, silica, and bitumen surfaces were approximately 200, 40, and 80 pN in deionized water and approximately 100, 50, and 40 pN in the plant process water, respectively. The measured adhesion forces together with the zeta potential values of these surfaces indicate that the polymer would preferentially adsorb onto clay surfaces rather than onto bitumen surfaces. It is the selective adsorption of HPAM that benefits both bitumen recovery and tailings settling when the polymer was added directly to the bitumen extraction process at an appropriate dosage.