Colloidal Probe Atomic Force Microscopy Research Articles

Contactless Rheology of Soft Gels Over a Broad Frequency Range

We report contactless measurements of the viscoelastic rheological properties of soft gels. The experiments are performed using a colloidal-probe atomic force microscope in a liquid environment and in dynamic mode. The mechanical response is measured as a function of the liquid gap thickness for different oscillation frequencies. Our measurements reveal an elastohydrodynamic coupling between the flow induced by the probe oscillation and the viscoelastic deformation of the gels. The data are quantitatively described by a viscoelastic lubrication model. The frequency-dependent storage and loss moduli of the polydimethylsiloxane gels are extracted from fits of the data to the model and are in good agreement with the Chasset-Thirion law. Our results demonstrate that contactless colloidal-probe methods are powerful tools that can be used for probing soft interfaces finely over a wide range of frequencies.

Ionic liquids on uncharged and charged surfaces: In situ microstructures and nanofriction

In situ changes in the nanofriction and microstructures of ionic liquids (ILs) on uncharged and charged surfaces have been investigated using colloid probe atomic force microscopy (AFM) and molecular dynamic (MD) simulations. Two representative ILs, [BMIM][BF4] (BB) and [BMIM][PF6] (BP), containing a common cation, were selected for this study. The torsional resonance frequency was captured simultaneously when the nanoscale friction force was measured at a specified normal load; and it was regarded as a measure of the contact stiffness, reflecting in situ changes in the IL microstructures. A higher nanoscale friction force was observed on uncharged mica and highly oriented pyrolytic graphite (HOPG) surfaces when the normal load increased; additionally, a higher torsional resonance frequency was detected, revealing a higher contact stiffness and a more ordered IL layer. The nanofriction of ILs increased at charged HOPG surfaces as the bias voltage varied from 0 to 8 V or from 0 to —8 V. The simultaneously recorded torsional resonance frequency in the ILs increased with the positive or negative bias voltage, implying a stiffer IL layer and possibly more ordered ILs under these conditions. MD simulation reveals that the [BMIM]+ imidazolium ring lies parallel to the uncharged surfaces preferentially, resulting in a compact and ordered IL layer. This parallel “sleeping” structure is more pronounced with the surface charging of either sign, indicating more ordered ILs, thereby substantiating the AFM-detected stiffer IL layering on the charged surfaces. Our in situ observations of the changes in nanofriction and microstructures near the uncharged and charged surfaces may facilitate the development of IL-based applications, such as lubrication and electrochemical energy storage devices, including supercapacitors and batteries.

Tunable Adhesion and Interfacial Structure of Layer‐by‐Layer Assembled Block co‐polymer Micelle and Polyelectrolyte Coatings

AbstractUnderstanding and tuning nanoscale structure is critical in developing new coatings and adhesives. In this work layer‐by‐layer assembly of block co‐polymer (BCP) micelles and oppositely charged polyelectrolytes produces structurally unique coatings with wet adhesion comparable to that of mussel adhesive proteins. Cationic (CAT) and anionic (ANI) BCPs, synthesized by atom transfer radical polymerization (ATRP), are used to create colloidally stable, self‐assembled, spherical BCP micelles. The assembly of BCP micelle and polyelectrolyte multilayers is monitored in situ where CAT‐ and ANI‐BCP micelles exhibit linear and super‐linear growth, respectively. Imaging of the surfaces reveals that CAT‐BCP micelles yield flat, uniform layers whereas ANI‐BCP micelle assemblies form islands that increase in surface area with each additional layer. The adhesion of these layers, measured by colloidal probe atomic force microscopy (CP‐AFM), shows that the distinct layers of CAT‐BCP micelle assemblies produce alternating high and low adhesion surfaces whereas ANI‐BCP micelle assemblies continually increase in adhesion with each additional bilayer. The unique behavior of each assembly demonstrates that both composition and structure play important roles in wet adhesion of submicron layers and that each can be tuned to target performance for different applications.

Detailing molecular interactions of ionic liquids with charged SiO2 surfaces: A systematic AFM study

It is crucial to understand the behaviour and interfacial interactions as well as properties of ionic liquids (ILs) at electrode surfaces on the molecular level for developing IL-based electrochemical energy storage devices including supercapacitors and batteries. In this work, a colloid probe atomic force microscopy (CP-AFM) -based experimental approach is presented to determine the molecular interaction forces between ILs and differently charged SiO2 microspheres. The effects of structural variations in ILs and the nature surface charges of SiO2 on the molecular interaction force are systematically studied. The surface charges of SiO2 were achieved by grafting quaternary ammonium and –COOH, –NH2 groups. The determined molecular interaction force is found to be strongly dependent on the surface charge, in which, the force enhances at a more negatively charged surface. Furthermore, the ILs with longer alkyl chains on cations exhibit stronger molecular interaction forces with the charged SiO2. These reported experimental results on the molecular level provide new insights for model development and molecular simulations of ILs interacting with charged surfaces and guide the design of ILs-based supercapacitor and battery systems.

Molecular interactions of ionic liquids with SiO2 surfaces determined from colloid probe atomic force microscopy.

Ionic liquids (ILs) interact strongly with many different types of solid surfaces in a wide range of applications, e.g. lubrication, energy storage and conversion, etc. However, due to the nearly immeasurable large number of potential ILs available, identifying the appropriate ILs for specific solid interfaces with desirable properties is a challenge. Theoretical studies are highly useful for effective development of design and applications of these complex molecular systems. However, obtaining reliable force field models and interaction parameters is highly demanding. In this work, we apply a new methodology by deriving the interaction parameters directly from the experimental data, determined by colloid probe atomic force microscopy (CP-AFM). The reliability of the derived interaction parameters is tested by performing molecular dynamics simulations to calculate translational self-diffusion coefficients and comparing them with those obtained from NMR diffusometry.

Impact of surface free energy on electrostatic extraction of particles from a bed

HypothesisElectrostatic extraction of particles from a bed to a pendent droplet to form liquid marbles has previously been investigated with respect to particle conductivity, size and shape, however, interparticle forces have not been specifically interrogated. If cohesion is the dominant force within the particle bed, then particles will be more readily extracted with reduced surface free energy. ExperimentsGlass particles were surface-modified using various alkyltrichlorosilanes. The surface free energy was measured for each sample using colloid probe atomic force microscopy (AFM) and sessile drop measurements on similarly modified glass slides. The ease of electrostatic particle extraction of each particle sample to a pendent droplet was compared by quantifying the electric field force required for successful extraction as a function of the measured surface free energy. FindingsSurface free energy calculated from sessile droplet measurements and AFM were not in agreement, as work of adhesion of a liquid droplet on a planar substrate is not representative of the contact between particles. Ease of electrostatic extraction of particles was observed to generally decrease as a function of AFM-derived surface free energy, confirming this is a critical factor in electrostatic delivery of particles to a pendent droplet. Roughness was also shown to inhibit particle extraction.

Multiscale Characterization of the Mechanical Properties of Fibrin and Polyethylene Glycol (PEG) Hydrogels for Tissue Engineering Applications

AbstractShear rheology and atomic force microscopy (AFM) are used to characterize the stiffness of hydrogels in tissue engineering applications, with several studies reporting differences of several orders of magnitude in the elastic moduli determined by these two methods. This work compares the elastic properties of soft fibrin and polyethylene glycol (PEG) hydrogels used for stem cell applications, determined by AFM indentation with different probe sizes (from nano‐ to micrometer) to shear rheometry data. For all hydrogels, AFM nanoscale probing consistently yields higher elastic modulus (E) values and variability than micrometer‐probe indentation, while the shear modulus (G) values determined are the lowest. Colloidal probe AFM results are closer to rheology data for the stiffest samples, where E/G ratios converge to the theoretical Trouton ratio of 3. The results suggest that high polymer concentration hydrogels are better described by the affine elastic network theory, whereas low polymer concentration hydrogels deviate significantly from the Trouton ratio. Thus, for soft hydrogels relevant for stem cell culture, the assumption E = 3G is often invalid and care should be taken when comparing data from studies where different characterization methods are used in order to discern the impact of material properties on cell behavior.

Atomic Force Microscopy Investigation of the In Situ‐Formed Oxide Layer at the Interface of Al2O3−C/Steel Melt in Terms of Adhesion Force and Roughness in a Model System

Herein, the in situ‐formed layers on carbon‐bonded alumina refractories immersed in liquid 42CrMo4 steel at 1650 °C are investigated with regard to their macro‐ and microroughness, wetting properties, as well as adhesive interactions. Colloidal probe (CP) atomic force microscopy (AFM) are used to describe the adhesion in a water‐based model system. To take the interaction between melt and the in situ layer into account, the adhesive force is measured via drop probe AFM using a model system with mercury droplets. For poorly wetted in situ layer samples, adhesion forces increase with smaller hold times in the melt due to a less homogeneous layer and vice versa if only van der Waals forces act. In contrast, the influence of the immersion time in the melt and the lattice parameter of the crystals on the layer is of minor importance for melt droplet−layer adhesion.

Biomimetic functional hydrogel particles with enhanced adhesion characteristics for applications in fracture conformance control

Fracture conformance control in reservoir can effectively improve water channel to increase crude oil displacement efficiency of subsequent waterflooding. Inspired by the remarkable underwater wet adhesion of mussel byssus, hydrogel particles which can adhere stably on the fracture rock surface in reservoir conditions could achieve long-lasting reservoir control effect. In this work, the size-controllable biomimetic functional hydrogel particles were prepared by mechanical shearing after bulk hydrogel was constructed by catechol-functionalized polyacrylamide and phenolic resin crosslinking agent. The influence of solution salinity on the aggregation and adhesion of hydrogel particles was investigated via scanning electron microscope (SEM), colloidal probe atomic force microscope (AFM) and quartz crystal microbalance with dissipation (QCM-D). The results showed that hydrogel particles maintained well-dispersed state in low-salinity water, while exhibited significant adhesion and adsorption capacity to the silica surface in simulated reservoir salinity water. This is of great importance to the practical applications that the hydrogel particles would not show enhanced adhesion to rock surfaces until the reservoir salinity water was met, which was beneficial to the in-depth migration of hydrogel particles to achieve effective deep reservoir profile control. Furthermore, the visible micro-model was designed and applied to evaluate profile control effect of hydrogel particles, and the results showed that hydrogel particles could withstand water flushing and adhere stably to the fracture surface. The waterflooding sweep efficiency was increased remarkably from 20.3% ± 2.0% to 38.8% ± 2.0%. This work would help better understand the function mechanism of hydrogel particles in reservoir control and provide novel and efficient method for the practical application in enhanced oil recovery.

Effects of solution pH and polyethylene oxide concentrations on molybdenite–molybdenite, molybdenite–kaolinite, and molybdenite–quartz interaction forces: AFM colloidal probe study

Molybdenite (MoS2), a major resource for Mo, is often associated with some gangue minerals, such as kaolinite and quartz. The flotation separation of fine-grained molybdenite from gangue minerals is always difficult due to the low efficiency of capturing fine particle by bubbles and the heterogeneous agglomeration with gangue particles. In this study, a fundamental investigation was carried out on molybdenite–molybdenite, molybdenite–kaolinite, and molybdenite–quartz interaction forces with varying solution pH and polyethylene oxide (PEO) concentrations using the atomic force microscopy (AFM) colloidal probe technique. And the results of force measurements were interpreted in comparison with the zeta potential distributions, adsorption kinetics experiments and quantum–mechanical calculations. pH 6 solution was optimal for the selective aggregation of molybdenite particles when a stronger adhesion force between molybdenite and molybdenite can be obtained, while a weaker adhesion force existed in the kaolinite–molybdenite and quartz–molybdenite systems. And adding 1–2 ppm of PEO to the solution was the best to improve the flocculation of molybdenite particles and inhibit the adhesion of kaolinite and quartz particles to molybdenite particles in pH 6 solution. Adsorption kinetics experiments and adsorption energy calculations showed the highest adsorption performance of PEO on molybdenite surface, which was attributed to the interaction of sulfur and hydrogen.

Self-assembling of chimeric mussel-inspired bio-adhesives originated from Mytilus californianus and Anabaena flos-aquae: A new approach to develop underwater adhesion

Bio-adhesives play a pivotal role in a wide range of medical applications. However, there are some problems about their application in different pH values and low adhesion force under wet conditions. Here, we report new recombinant fusion protein achieved by mussel foot proteins (Mfps) of Mytilus Californianus and gas vesicle protein A (GvpA) of Anabaena flos-aquae by genetic engineering methods. Transmission electron microscopy verified that these chimeric proteins self-assembled into β-sheet rich fibres because of GvpA amyloid structure. Also, in comparison to other studies, their adhesion forces were significantly increased, especially in the alkaline environments based on Mfp-3 and Mfp-5, confirmed by the colloidal probe atomic force microscope. This study illustrates that copolymer of Mfp-5-GvpA:GvpA-Mfp-3 can be used as a sturdy underwater adhesive with tolerance to auto-oxidation, especially at basic conditions.

Controlling supraparticle shape and structure by tuning colloidal interactions

HypothesisAssembly of colloids in drying colloidal suspensions on superhydrophobic surface is influenced by the colloidal interactions, which determine the shape and interior structure of the assembled supraparticle. The introduction of salt (electrolyte) into the assembly system is expected to influence the colloid interactions and packing during the evaporation process. Hence, both the outer shape and internal structure of supraparticles should be controlled by varying salt concentrations. ExperimentsSuspensions of electrostatically stabilized polystyrene particles with specified salt concentrations were chosen as model systems to conduct the evaporation on a superhydrophobic surface. A systematic study was performed by regulating the concentration and valency of salt. The morphology and interior of supraparticles were carefully characterized with electron scanning microscopy, while the colloidal interaction was established using colloidal probe atomic force microscopy. FindingsSupraparticles displayed a spherical-to-nonspherical shape change due to the addition of salts. The extent of crystallization depended on salt concentration. These changes in shape and structure were correlated with salt-dependent single colloid interaction forces, which were not previously investigated in detail in radially symmetric evaporation geometry. Our findings are crucial for understanding assembly behavior during the drying process and offer guidance for preparing complex supraparticles to meet specific applications requirement.

Polyelectrolyte-Assisted Dispersions of Reduced Graphite Oxide Nanoplates in Water and Their Gas-Barrier Application.

Dispersion of graphene and related materials in water is needed to enable sustainable processing of these 2D materials. In this work, we demonstrate the capability of branched polyethylenimine (BPEI) and polyacrylic acid (PAA) to stabilize reduced graphite oxide (rGO) dispersions in water. Atomic force microscopy colloidal probe measurements were carried out to investigate the interaction mechanisms between rGO and the polyelectrolytes (PEs). Our results show that for positive PEs, the interaction appears electrostatic, originating from the weak negative charge of graphene in water. For negative PEs, however, van der Waals forces may result in the formation of a PE shell on rGO. The PE-stabilized rGO dispersions were then used for the preparation of coatings to enhance gas barrier properties of polyethylene terephthalate films using the layer-by-layer self-assembly. Ten bilayers of rGOBPEI/rGOPAA resulted in coatings with excellent barrier properties as demonstrated by oxygen transmission rates below detection limits [<0.005 cm3/(m2 day atm)]. The observed excellent performance is ascribed to both the high density of the deposited coating and its efficient stratification. These results can enable the design of highly efficient gas barrier solutions for demanding applications, including oxygen-sensitive pharmaceutical products or flexible electronic devices.

Graphite superlubricity enabled by triboinduced nanocontacts

Colloidal probe Atomic Force Microscopy allows to explore sliding states of vanishing friction, i.e. superlubricity, in mesoscopic graphite contacts. Superlubricity is known to appear upon formation of a triboinduced transfer layer, originated by material transfer of graphene flakes from the graphitic substrate to the colloidal probe. Previous studies suggest that friction vanishes due to crystalline incommensurability at the newly formed interface. However this picture still lacks several details, such as the roles of the tribolayer roughness and of loading conditions. Hereafter we gain deeper insight into the tribological response of micrometric silica beads sliding on graphite under ambient conditions. We show that the tribotransferred flakes behave as lubricious nanoasperities with a twofold role. First, they decrease the silica-graphite true contact area, in fact causing a breakdown of adhesion and friction by one order of magnitude. Second, they govern mechanical dissipation through the specific energy landscape experienced by the topographically-highest triboinduced nanoasperity. Remarkably, such contact junctions can undergo a load-driven atomic-scale transition from continuous superlubric sliding to dissipative stick-slip, that agrees with the single-asperity Prandtl-Tomlinson model. Superlubricity in mesoscopic silica-graphite junctions may therefore arise from the load-controlled competition between interfacial crystalline incommensurability and contact pinning effects at one dominant nanoasperity.

Operando observation of concentrated SiO2 suspensions by optical coherent tomography during flow curve measurements: The relationship between polymer dispersant structures and surface interactions

HypothesisFlow curve measurement is commonly used to characterize the flow behavior of concentrated suspensions. However, dynamic changes in the suspension inner microstructures under highly sheared conditions have not been correctly understood even though they strongly affect the measured shear stress. We hypothesize that the real particle dynamics during shearing could be effectively revealed by a systematic investigation that combines macroscopic flow curve measurements with operando microstructural observation employing an optical coherent tomography (OCT) apparatus and surface interaction measurements with the colloidal probe atomic force microscopy (AFM) method. ExperimentsThe model system was spherical SiO2/toluene suspensions stabilized by polyethyleneimine (PEI) partially complexed with different fatty acids. Inner structures of the suspensions during flow curve measurements were observed by the OCT technique. The surface-surface interactions in toluene were analyzed using the colloidal probe AFM method. FindingsOperando OCT observations revealed that during flow curve measurements, the suspensions can have completely different microscopic flow modes depending on the fatty acid species complexed to PEI and the solid concentrations. These microscopic flow modes could not be recognized using the typical flow curve measurements alone. The different flow modes can be explained by surface interactions measured by the colloidal probe AFM method.

How latex film formation and adhesion at the nanoscale correlate to performance of pressure sensitive adhesives with cellulose nanocrystals.

Emulsion polymerized latex-based pressure-sensitive adhesives (PSAs) are more environmentally benign because they are synthesized in water but often underperform compared to their solution polymerized counterparts. Studies have shown a simultaneous improvement in the tack, and peel and shear strength of various acrylic PSAs upon the addition of cellulose nanocrystals (CNCs). This work uses atomic force microscopy (AFM) to examine the role of CNCs in (i) the coalescence of hydrophobic 2-ethyl hexyl acrylate/n-butyl acrylate/methyl methacrylate (EHA/BA/MMA) latex films and (ii) as adhesion modifiers over multiple length scales. Thin films with varying solids content and CNC loading were prepared by spin coating. AFM revealed that CNCs lowered the solids content threshold for latex particle coalescence during film formation. This improved the cohesive strength of the films, which was directly reflected in the increased shear strength of the EHA/BA/MMA PSAs with increasing CNC loading. Colloidal probe AFM indicated that the nano-adhesion of thicker continuous latex films increased with CNC loading when measured over small contact areas where the effect of surface roughness was negligible. Conversely, the beneficial effects of the CNCs on macroscopic PSA tack and peel strength were outweighed by the effects of increased surface roughness with increasing CNC loading over larger surface areas. This highlights that CNCs can improve both cohesive and adhesive PSA properties; however, the effects are most pronounced when the CNCs interact favourably with the latex polymer and are uniformly dispersed throughout the adhesive film. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Self-Assembled Permanent Micro-Magnets in a Polymer-Based Microfluidic Device for Magnetic Cell Sorting.

Magnetophoresis-based microfluidic devices offer simple and reliable manipulation of micro-scale objects and provide a large panel of applications, from selective trapping to high-throughput sorting. However, the fabrication and integration of micro-scale magnets in microsystems involve complex and expensive processes. Here we report on an inexpensive and easy-to-handle fabrication process of micrometer-scale permanent magnets, based on the self-organization of NdFeB particles in a polymer matrix (polydimethylsiloxane, PDMS). A study of the inner structure by X-ray tomography revealed a chain-like organization of the particles leading to an array of hard magnetic microstructures with a mean diameter of 4 µm. The magnetic performance of the self-assembled micro-magnets was first estimated by COMSOL simulations. The micro-magnets were then integrated into a microfluidic device where they act as micro-traps. The magnetic forces exerted by the micro-magnets on superparamagnetic beads were measured by colloidal probe atomic force microscopy (AFM) and in operando in the microfluidic system. Forces as high as several nanonewtons were reached. Adding an external millimeter-sized magnet allowed target magnetization and the interaction range to be increased. Then, the integrated micro-magnets were used to study the magnetophoretic trapping efficiency of magnetic beads, providing efficiencies of 100% at 0.5 mL/h and 75% at 1 mL/h. Finally, the micro-magnets were implemented for cell sorting by performing white blood cell depletion.

Exploring the roles of roughness, friction and adhesion in discontinuous shear thickening by means of thermo-responsive particles

Dense suspensions of colloidal or granular particles can display pronounced non-Newtonian behaviour, such as discontinuous shear thickening and shear jamming. The essential contribution of particle surface roughness and adhesive forces confirms that stress-activated frictional contacts can play a key role in these phenomena. Here, by employing a system of microparticles coated by responsive polymers, we report experimental evidence that the relative contributions of friction, adhesion, and surface roughness can be tuned in situ as a function of temperature. Modifying temperature during shear therefore allows contact conditions to be regulated, and discontinuous shear thickening to be switched on and off on demand. The macroscopic rheological response follows the dictates of independent single-particle characterization of adhesive and tribological properties, obtained by colloidal-probe atomic force microscopy. Our findings identify additional routes for the design of smart non-Newtonian fluids and open a way to more directly connect experiments to computational models of sheared suspensions.

Geometrical Confinement Modulates the Thermoresponse of a Poly(N-isopropylacrylamide) Brush

The structure of grafted-from poly(N-isopropylacrylamide) (PNIPAM) brushes is investigated as a function of confining stress and system temperature using neutron reflectometry (NR), numerical self-consistent field theory, and the reanalysis of colloid probe atomic force microscopy (AFM) data from the literature. For NR experiments, confinement is achieved using a custom-made sample environment, and the corresponding reflectometry data are analyzed using a novel “distribution model.” The NR and AFM experiments probe similar temperature–stress combinations and generally find qualitative agreement, with some variations highlighting path-dependent (isostress vs isothermal, respectively) behavior. All techniques indicate that confinement removes the critical transition point in the thermoresponse of PNIPAM and results in the brush assuming a block-like volume fraction profile with a uniform internal structure. The PNIPAM brushes recover from such treatment, regaining their thermoresponse upon resolvation. Understanding the structure of responsive polymer brushes under confinement is essential, as brush applications are often accompanied by a surface-normal force (i.e., lubrication) and brush properties are dependent on their structure.

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.