Colloidal Probe Atomic Force Microscopy Research Articles

Study of the Influence of the Crystallographic Orientation of Cassiterite Observed with Colloidal Probe Atomic Force Microscopy and its Implications for Hydrophobization by an Anionic Flotation Collector.

In this study, the physicochemical behaviors of the (110), (100), as well as (001) of SnO2 were investigated by using high-resolution direct force spectroscopy. The measurements were conducted between a silica sphere and sample surfaces in 10 mmol/L KCl between pH 3.1 and 6.2 using colloidal probe atomic force microscopy (cp-AFM-hydrophilic). Dissimilar interactions were detected on different-oriented surfaces. The pH values where the force switched from positive to negative can be clearly distinguished and be ordered as SnO2(100) < SnO2(001) ≈ SnO2(110). By fitting the force curves in the Derjaguin–Landau–Verwey–Overbeck theory framework, anisotropic surface potentials were computed between the three sample surfaces following a similar trend as force interaction. To study the implication of crystallographic orientation to surfactant adsorption, we used Aerosol 22 (sulfosuccinamate) as an anionic collector for cassiterite flotation to functionalize the different samples at pH 3. The contact angle measurements, the topography visualizations by AFM, and the force measurement using cp-AFM with hydrophobized spheres (cp-AFM-hydrophobized) have shown that Aerosol 22 was adsorbed on the sample surfaces inhomogeneously. The adsorption followed the range of SnO2(110) > SnO2(100) > SnO2(001) in the concentration from 1 × 10–6 to 1 × 10–4 mol/L.

PH-Responsive PEG/PAA Multilayer Assemblies for Reversible Adhesion of Micro-Objects

Manipulating micro-objects plays a crucial role in a wide range of fundamental and applied research works. Here, we propose an original strategy based on the chemical modification of a substrate by hydrogen-bonded films elaborated by layer-by-layer (LbL) assemblies of poly(ethylene glycol) (PEG) and poly(acrylic acid) (PAA). First, the influence of the polymer molecular weight on the growth of the PEG/PAA multilayer was evaluated. Optical reflectometry analysis used to follow in situ the film buildup revealed a strong dependence of the deposited amount of polymer on the ratio of monomer units of each polymer (nPAA/nPEG). Then, colloidal probe atomic force microscopy (AFM) microscopy was carried out in an aqueous medium to monitor the adhesion forces of multilayer surfaces composed of N polymer layers. Pull-off forces were converted using the Johnson–Kendall–Roberts (JKR) model to access the thermodynamic work of adhesion. Results indicated that PEG/PAA multilayer films exhibit weak adhesion forces that are sensitive to the number of deposited polymer layers at pH 2. In addition, a progressive increase of the solution pH reduced the adhesion due to the destruction of the hydrogen-bonded multilayer film. To simulate the capture and the release of a micro-object, borosilicate particles acting as spherical micro-objects were adsorbed onto a PEG/PAA film. Once again, an increase of the solution pH led to desorption of particles, as shown by optical microscopy. Finally, an AFM tip functionalized by a PEG/PAA multilayer was used to achieve successful micromanipulation operations (capture and release) of a 10 μm diameter borosilicate sphere in an aqueous solution.

Friction at nanopillared polymer surfaces beyond Amontons’ laws: Stick-slip amplitude coefficient (SSAC) and multiparametric nanotribological properties

Frictional and nanomechanical properties of nanostructured polymer surfaces are important to their technological and biomedical applications. In this work, poly(ethylene terephthalate) (PET) surfaces with a periodic distribution of well-defined nanopillars were fabricated through an anodization/embossing process. The apparent surface energy of the nanopillared surfaces was evaluated using the Fowkes acid-base approach, and the surface morphology was characterized using scanning electron microscope (SEM) and atomic force microscope (AFM). The normal and lateral forces between a silica microparticle and these surfaces were quantified using colloidal probe atomic force microscopy (CP-AFM). The friction-load relationship followed Amonton’s first law, and the friction coefficient appeared to scale linearly with the nanopillar height. Furthermore, all the nanopillared surfaces showed pronounced frictional instabilities compared to the smooth sliding friction loop on the flat control. Performing the stick–slip amplitude coefficient (SSAC) analysis, we found a correlation between the frictional instabilities and the nanopillars density, pull-off force and work of adhesion. We have summarised the dependence of the nanotribological properties on such nanopillared surfaces on five relevant parameters, i.e. pull-off force fp, Amontons’ friction coefficient μ, RMS roughness Rq, stick–slip amplitude friction coefficient SSAC, and work of adhesion between the substrate and water Wadh in a radar chart. Whilst demonstrating the complexity of the frictional behaviour of nanopillared polymer surfaces, our results show that analyses of multiparametric nanotribological properties of nanostructured surfaces should go beyond classic Amontons’ laws, with the SSAC more representative of the frictional properties compared to the friction coefficient.

Interfacial Properties of Chitosan in Interfacial Shear and Capsule Compression.

The time-dependent behavior of surface-active adsorption layers at the oil/water interface can dictate emulsion behavior at both the micro- and macroscale. In addition, self-healing behavior of the adsorption layer may benefit emulsion stability subject to large deformation under processing or during final application. We explore the behavior of chitosan, a known hydrophilic emulsifier, which forms nanoparticle aggregates when the concentration of acetate buffer exceeds 0.3 M. We observe a Pickering adsorption layer building and strain-dependent behavior of the chitosan at the medium chain triglyceride oil/water interface. We compare this to the behavior of identical chitosan layers coated on oil droplets via atomic force microscopy colloidal probe compression in both linear and oscillatory compressions. In both interfacial shear rheometry and the capsule compression, a thick, elastic layer with strong time-dependent recovery behavior is observed, suggesting that the layer has some self-healing capabilities.

Surface Forces between Nanomagnetite and Silica in Aqueous Ca2+ Solutions Studied with AFM Colloidal Probe Method

Dispersion and aggregation of nanomagnetite (Fe3O4) and silica (SiO2) particles are of high importance in various applications, such as biomedicine, nanoelectronics, drug delivery, flotation, and pelletization of iron ore. In directly probing nanomagnetite–silica interaction, atomic force microscopy (AFM) using the colloidal probe technique has proven to be a suitable tool. In this work, the interaction between nanomagnetite and silica particles was measured with AFM in aqueous Ca2+ solution at different pH levels. This study showed that the qualitative changes of the interaction forces with pH and Ca2+ concentrations were consistent with the results from zeta-potential measurements. The repulsion between nanomagnetite and silica was observed at alkaline pH and 1 mM Ca2+ concentration, but no repulsive forces were observed at 3 mM Ca2+ concentration. The interaction forces on approach were due to van der Waals and electrical double-layer forces. The good fitting of experimental data to the DLVO model and simulations supported this conclusion. However, contributions from non-DLVO forces should also be considered. It was shown that an increase of Ca2+ concentration from 1 to 3.3 mM led to a less pronounced decrease of adhesion force with increasing pH. A comparison of measured and calculated adhesion forces with a few contact mechanics models demonstrated an important impact of nanomagnetite layer nanoroughness.

Depletion Forces Induced by Mixed Micelles of Nonionic Block Copolymers and Anionic Surfactants.

Depletion forces were measured between a silica sphere and a silica plate in solutions containing nonionic Pluronic P123 poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymers and anionic sodium dodecyl sulfate (SDS) surfactants using colloidal probe atomic force microscopy. Prior research established synergistic depletion force enhancement in solutions containing SDS and unimeric Pluronic F108 block copolymers via formation of large pseudo-polyelectrolyte complexes. The current work addresses a more complex system where the polymer is above its critical micelle concentration, and surfactant binding alters not only the size and charge of the micelles but also the number of polymers per micelle. Force profiles were measured in 10 000 ppm P123 (1 wt %, corresponding to 1.72 mM based on average molar mass) solutions containing SDS at concentrations up to 64 mM and compared to micellar P123 solutions and to P123-free SDS solutions. Whereas force profiles in the SDS-free micellar P123 solutions were purely repulsive, P123/SDS complexation produced synergistic depletion force enhancement for SDS concentrations between 2 and 32 mM. The synergism that occurred within a finite SDS concentration range was explained by comparing the hydrodynamic size, molar mass, charge, and concentration of depletants in P123/SDS mixtures and their respective single-component solutions obtained with the aid of dynamic light scattering, static light scattering, and dodecyl sulfate ion-selective electrode measurements. These measurements showed that complexation produced effects that would be mutually counteracting with respect to depletion forces: decreasing the mixed micelle hydrodynamic diameter relative to SDS-free P123 micelles would tend to weaken depletion forces, while adding charge and decreasing the aggregation number of polymers per micelle (thereby increasing the number concentration of micellar depletants) would tend to strengthen depletion forces.

Modulation of substrate van der Waals forces using varying thicknesses of polymer overlayers

Thin polymeric coatings are commonly used for altering surface properties and modulating the interfacial performance of materials. Possible contributions from the substrate to the interfacial forces and effects are, however, usually ignored and are not well understood, nor is it established how the coating thickness modulates and eventually eliminates contributions from substrates to the van der Waals (vdW) interfacial force. In this study we quantified, by colloid-probe atomic force microscope (AFM) and by theoretical calculations, the interfacial vdW contributions from substrates acting through ethanol plasma polymer (EtOHpp) coatings of a range of thicknesses on Au and Si bulk materials. In approach force curves against EtOHpp-coated Au substrates the magnitude of the vdW force decreased as the EtOHpp coating thickness increased to 18 nm and then plateaued with further increases in coating thickness, providing direct evidence for a contribution to the total interfacial vdW force from the Au substrate acting through thin coatings. The experimental observations accord with theoretical calculations of the thickness dependence of Hamaker coefficients derived from rigorous simulation using the Lifshitz theory. In addition, the measured forces agree well with theoretical predictions including correction for finite roughness. Thus, our experimental and theoretical results establish how the thickness of polymer thin film coatings modulates the total interfacial vdW force and how this can be used to tune the net vdW force so as to either contain a large substrate contribution or arise predominantly from the polymeric overlayer. Our findings enable rational design of coating thickness to tailor interfacial interactions and material performance.

Adhesion of Epithelial Cells to PNIPAm Treated Surfaces for Temperature-Controlled Cell-Sheet Harvesting.

Stimuli responsive polymer coatings are a common motive for designing surfaces for cell biological applications. In the present study, we have characterized temperature dependent adhesive properties of poly(N-isopropylacrylamide) (PNIPAm) microgel coated surfaces (PMS) using various atomic force microscopy based approaches. We imaged and quantified the material properties of PMS upon a temperature switch using quantitative AFM imaging but also employed single-cell force spectroscopy (SCFS) before and after decreasing the temperature to assess the forces and work of initial adhesion between cells and PMS. We performed a detailed analysis of steps in the force–distance curves. Finally, we applied colloid probe atomic force microscopy (CP-AFM) to analyze the adhesive properties of two major components of the extracellular matrix to PMS under temperature control, namely collagen I and fibronectin. In combination with confocal imaging, we could show that these two ECM components differ in their detachment properties from PNIPAm microgel films upon cell harvesting, and thus gained a deeper understanding of cell-sheet maturation and harvesting process and the involved partial ECM dissolution.

New method to measure interaction force between particle and air bubble/water droplet using a micro-Newton mechanics testing instrument

The interaction force between particle and air bubble/water droplet is the key to understand the underlying mechanism for mineral flotation. The most commonly used approach is colloidal probe atomic force microscopy (AFM), which usually operates in the nano-Newton range. This makes the quantification of the adhesion (capillary) force during retraction challenging, because the magnitude of the adhesion force is much higher than that of surface forces. In this short communication, a new instrument, the micro-mechanical testing machine (MMTM), was used to measure the interaction force between a micrometer glass bead and an air bubble/water droplet with both micro-Newton range and nano-Newton resolution. A single-axis capacitive microforce sensor based on interdigitated comb capacitors was equipped in the MMTM. Similar to the colloidal probe AFM, a hydrophilic glass bead with a diameter of 193 μm was adhered to the end of the cantilever. The MMTM allows the simultaneous recording of the data of force, displacement, and time at a fixed velocity. The results showed that a strong jump-into contact and adhesion force were observed between the particle and water droplet. In contrast, a weaker jump-into and smaller adhesion force were observed between the bubble and particle. Based on the theoretical equation of the adhesion force, the equilibrium contact angle of the glass bead used was back calculated to be approximately 39.58°, which is reasonable for its force behavior. The MMTM provides a flexible and alternative way to investigate the surface and interfacial forces in flotation systems.

Tribological Characteristics of Single-Layer h-BN Measured by Colloidal Probe Atomic Force Microscopy

The tribological characteristics of single-layer (1L) hexagonal-boron nitride (h-BN) were systematically investigated using colloidal probe atomic force microscopy, with an aim to elucidate the feasibility as a protective coating layer and solid lubricant for micro- and nanodevices. The experiments were performed to detect the occurrence of failure of 1L h-BN for up to 10,000 cycles under various normal forces. The failure of 1L h-BN did not occur for 10,000 cycles under a 10 μN normal force, corresponding to a contact pressure of about 0.34 GPa. However, the complete failure of 1L h-BN occurred faster with an increasing normal force from 20 to 42 μN. It was observed that the SiO2/Si substrate was locally exposed due to defect formation on the 1L h-BN. The Raman spectroscopy measurement results further suggest that the failure was associated with the compressive strain on 1L h-BN. The friction of 1L h-BN before failure was orders of magnitude smaller than that of a SiO2/Si substrate. The overall results indicate the feasibility of atomically thin h-BN as a protective coating layer and solid lubricant. In particular, the results of this work provide fundamental tribological characteristics of pristine h-BN as a guide, which may be helpful in other practical deposition methods for atomically thin h-BN with enhanced tribological characteristics.

A bioinspired mucoadhesive restores lubrication of degraded cartilage through reestablishment of lamina splendens

Adsorbed lubricious films composed of biomacromolecules are natively present at all articulating interfaces in the human body where they provide ultralow friction and maintain normal physiological function. Biolubrication gets impaired due to diseases such as osteoarthritis, in which cartilage damage results from alterations in synovial fluid and lamina splendens composition. Osteoarthritis is treated with hyaluronic acid (HA) orally or via intra-articular injection, but due to the poor adsorption of HA on the cartilage surface in the absence of adhesive molecules, pain relief is temporary. Here, we describe how natural lubrication on degraded cartilage surface can be restored with the help of a bioinspired mucoadhesive biopolymer chitosan catechol (Chi-C). Quartz crystal microbalance was used to mimic the formation of lamina splendens in vitro, known as synovial fluid conditioning films (SyCF), and colloidal probe atomic force microscopy was used to measure their nanoscale frictional properties. Clear evidence of glycoprotein (PRG4) recruitment by Chi-C increased the softness of SyCF, which also improved nanoscale lubrication in vitro, decreasing the friction coefficient from 0.06 to 0.03. At the macroscale, cartilage damage induced by Chondroitinase ABC increased the coefficient of friction (COF) from 0.07 ± 0.04 (healthy tissue) to 0.15 ± 0.03 (after tissue damage) in the presence of synovial fluid after sliding for 50 min. After Chi-C treatment of damaged cartilage, the COF fell to 0.06 ± 0.03, which is comparable to healthy cartilage. Chi-C did not adversely affect the metabolic activity of human chondrocytes. This study provides new key insight into the potential for restoring biolubrication through the use of muco-adhesive molecules.

Interaction of Different Charged Polymers with Potassium Ions and Their Effect on the Yield Stress of Highly Concentrated Glass Bead Suspensions.

The interaction of different charged polymers, namely anionic polycarboxylate superplasticizer (PCE) and neutral polyethylene glycol (PEG) with potassium ions, and their effect on the yield stress of highly concentrated glass bead suspension (GBS), were studied under different concentrations of potassium ions ([K+]). It was found that, compared to the neutral PEG, the negatively charged PCE can be adsorbed on glass beads (GB), and then decreases the yield stress of GBS. The increasing concentration of free polymer in the interstitial liquid phase with the increased polymer dosage leads to the higher yield stress of GBS, which may be caused by the higher depletion force. In addition, this effect is also related to the charge density of the polymer and the [K+] in the solution. Along with the increase in [K+], the yield stress of GBS increases significantly with the addition of PCE, but this cannot be observed with PEG, which indicates that potassium ions can interact with negatively charged PCE instead of the neutral PEG. At last, the interparticle forces between two single GB with adsorbed PCE in solutions containing [K+] and PCE were measured by colloidal probe atomic force microscopy to better understand the interaction of the charged polymer with counterions.

Swelling Behavior, Interaction, and Electrostatic Properties of Chitosan/Alginate Dialdehyde Multilayer Films with Different Outermost Layer.

In this study, self-cross-linked chitosan/alginate dialdehyde multilayer films, capped with either alginate dialdehyde (6 layers) or chitosan (7 layers), were fabricated using the layer-by-layer method. The disruption of the electrostatic equilibrium when exposing the fabricated layers to acidic and alkaline conditions causes swelling within the film and independently in the outermost layer, showing dependence on the ionic strength. Spectroscopic ellipsometry and quartz crystal microbalance with dissipation monitoring were employed to examine the swelling behavior. Atomic force microscopy colloidal probe measurements were conducted to assess the surface forces between the multilayer films at different pH and ionic strengths. Finally, the electrostatic properties of the multilayer films were examined at different pH and ionic strengths using zeta potential measurements. The results suggest that stimuli-responsiveness and overall swelling behavior of the polysaccharide multilayer films significantly depend on the outermost layer, an effect that should expectedly become more pronounced the thinner the film becomes.

Towards Nanomechanical Properties from Pipette Ion Currents

Recent studies have correlated biological functions and diseases of various organs and tissues in the human body with their mechanical properties. In addition, interest in extracellular vesicles has surged as they have been found to play a crucial role in intercellular communications and regulatory functions. Both developments have fueled interest in the field of nanoscale biomechanics. A recently-developed technique called microaspiration (MA), which characterizes the mechanical properties of ‘soft’ particles, is well-suited for this task. MA operates by applying a negative pressure to aspirate samples through a micropipette (translocation) while monitoring both optical and electrical (ion current) data. Both sets of data are then used to derive mechanical properties by applying appropriate models. It has been demonstrated that MA can be used to derive mechanical properties from translocation events for both biological and polymer samples. To translate the technique down to the nanoscale an extensive and systematic study is required. As such, polydimethylsiloxane (PDMS) emulsions are excellent standard test samples due to their well-known material properties, and their tunable size range which covers both microscale and nanoscale. Results from MA experiments can also be correlated with those derived from colloidal probe atomic force microscopy (AFM). This comparative study aims to comprehensively adapt this mechanical characterization tool to the nanoscale.

Single particle triboelectrification of Titan sand analogs

Sand electrification is important for aeolian sediment transportation on terrestrial bodies with silicate sand as the main sediment composition. However, it has not been thoroughly studied for icy bodies such as Titan with organic sand as the main dune-forming material. We used the colloidal probe atomic force microscopy (AFM) technique to study triboelectric charging processes using Titan and Earth sand analogs. We found that it is easy to generate triboelectric charges between naphthalene (a simple aromatic hydrocarbon), polystyrene (an aromatic hydrocarbon polymer), and borosilicate glass (Earth silicate sand analog). Strong electrostatic forces can be measured after contact and/or tribocharging. In contrast, tholin, a complex organic material, does not generate any detectable electrostatic forces with contact or tribocharging within the detection limit of the instrument. If Titan sand behaves more like tholin, this indicates that the tribocharging capacity of Titan sand is much weaker than Earth silicate sand and much less than previously measured by Méndez Harper et al. (2017), where only simple organics were used for Titan sand analogs. Thus, triboelectrification may not contribute to increasing interparticle forces between sand particles on Titan as much as on Earth. Interparticle forces generated by other electrostatic processes or other interparticle forces such as van der Waals and capillary cohesion forces could be the dominant interparticle forces that govern Titan sand formation and sediment transportation on the surface. Titan sand is also unlikely to produce large electrical discharge through tribocharging to affect future missions to Titan's surface.

Wetting Transition on Liquid-Repellent Surfaces Probed by Surface Force Measurements and Confocal Imaging.

Superhydrophobic surfaces in the Cassie-Baxter wetting state retain an air layer at the surface which prevents liquid water from reaching into the porous surface structure. In this work we explore how addition of ethanol, which reduces the surface tension, influences the wetting properties of superhydrophobic and smooth hydrophobic surfaces. Wetting properties are measured by dynamic contact angles, and the air layer at the superhydrophobic surface is visualized by laser scanning confocal microscopy. Colloidal probe atomic force microscopy measurements between a hydrophobic microsphere and the macroscopic surfaces showed that the presence of ethanol strongly affects the interaction forces. When the macroscopic surface is superhydrophobic, attractive forces extending up to a few micrometers are observed on retraction in water and in 20 vol % ethanol, signifying the presence of a large and growing gas capillary. Submicrometer attractive forces are observed between the probe particle and a smooth hydrophobic surface, and in this case a smaller gas capillary is formed. Addition of ethanol results in markedly different effects between superhydrophobic and hydrophobic surfaces. In particular, we show that the receding contact angle on the superhydrophobic surface is of paramount importance for describing the interaction forces.

Colloidal Depletion and Structural Force Synergism or Antagonism in Solutions of Mutually Repelling Polyelectrolytes and Ionic Surfactants.

Depletion and structural forces were measured between a silica sphere and plate in solutions containing sodium polyacrylate (Na-PAA) anionic polyelectrolyte and sodium dodecyl sulfate (SDS) anionic surfactant using colloidal probe atomic force microscopy, at high pH where the two species are electrostatically repelling from each other and from the silica surfaces. Measurements were performed for a range of SDS and Na-PAA concentrations to span conditions where only one of the species or both of the species would exert a detectable depletion or structural force when present in a single-component solution. In mixed solutions, conditions were identified (i) where depletion attraction was synergistically enhanced or antagonistically weakened relative to single component solutions; (ii) where the range of the depletion attraction was significantly extended and the repulsive structural force barrier was eliminated, due to simultaneous depletion of both species over different length scales; and (iii) where one species was the dominant depletant and forces in mixtures were indistinguishable from those in a single component solution of the dominant depletant. Force measurements were interpreted with the aid of pyrene solubilization assays of SDS micellization and dynamic light scattering investigation of the state of assembly of the polyelectrolyte or surfactant. The variety of colloidal force effects were attributed to ionic strength and excluded volume effects of Na-PAA on SDS micellization, ionic strength effects of SDS on Na-PAA chain clustering in solution, and ionic strength effects on the counterion contribution to polyelectrolyte osmotic pressure. While prior studies have shown that depletion force synergism occurs when polymers and surfactants form mixed complexes, this work shows that it can occur in noncomplexing mixtures as well, and it indicates the variety of effects that should be taken into account when attempting to predict forces in such mixtures.

Enhancement of stratification of colloidal particles near a substrate induced by addition of non-adsorbing polymers

Polymers are widely mixed in colloidal dispersions to make better materials. It is important to understand the effects of the addition of polymers. In the present work, we studied micro structure of the coated surface of the mixture by using a purely theoretical method (an integral equation theory). To corroborate the theoretical results, our original analysis method for colloidal-probe atomic force microscopy was also applied. It was found that the addition of non-adsorbing polymers enhances the stratification of the colloidal particles near the substrate. Meanwhile, the polymers themselves are not stratified unlike the colloidal particles.

Integral equation theory based method to determine number density distribution of colloidal particles near a substrate using a force curve from colloidal probe atomic force microscopy

Colloidal particles dispersed in a liquid form a layer-like structure at a solid-liquid interface. Amano et al. have developed a method to calculate the number density distribution of colloidal particles on a substrate from a colloidal probe atomic force microscopy (CP-AFM) force curve (Amano et al., J. Phys. Chem. B. 122 (2018) 4592). This method assumes that the probe-particle pair potential is rigid. In this paper, we propose a new method to calculate the number density distribution which does not use the assumption. The new method is based on Ornstein-Zernike equations and hyper-netted chain approximation, and the number density distribution is obtained numerically. The new method shows better performance than the previous method. The new method can also calculate the pair potential between colloidal particles and a substrate from a CP-AFM force curve.

Interfacial Forces at Layered Surfaces: Substrate Electrical Double-Layer Forces Acting through Ultrathin Polymer Coatings.

Manipulating the surface properties of materials via the application of coatings is a widely used strategy to achieve desired interfacial interactions, implicitly assuming that the interfacial forces of coated samples are determined exclusively by the surface properties of the coatings. However, interfacial interactions between materials and their environments operate over finite length scales. Thus, the question addressed in this study is whether interactions associated with bulk substrate materials could act through thin coatings or, conversely, how thick a coating needs to be to completely screen subsurface forces contributed by underlying substrates. Plasma polymer layers were deposited on silicon wafer substrates from ethanol vapor, with identical chemical composition, ultrasmooth surfaces, and varying thicknesses. Using colloid-probe atomic force microscopy, electrical double-layer forces were determined in solutions of various ionic strengths and fitted using the Derjaguin-Landau-Verwey-Overbeek theory. For the thicker ethanol plasma polymers, the fitted surface potentials reflected the presence of surface carboxylate groups and were invariant with thickness. In contrast, for coatings <18 nm thick, the surface potentials increased steadily with decreasing film thickness; the measured electrical double-layer forces contained contributions from both the coating and the substrate. Theoretical calculations were in agreement with this model. Thus, our observations indicate that the higher surface potential of the underlying SiO2 surface can influence the interactions between a colloid particle and the multilayer structure if coatings are sufficiently thin. Such superposition needs to be factored into the design of coatings aimed at the control of material interactions via surface forces.