Surface Force Measurements Research Articles

Probing Hydrophobic Interactions between Polymer Surfaces and Air Bubbles or Oil Droplets: Effects of Molecular Weight and Surfactants.

Hydrophobic interaction plays an important role in numerous interfacial phenomena and biophysical and industrial processes. In this work, polystyrene (PS) was used as a model hydrophobic polymer for investigating its hydrophobic interaction with highly deformable objects (i.e., air bubbles and oil droplets) in aqueous solutions. The effects of polymer molecular weight, solvent (i.e., addition of ethanol to water), the presence of surface-active species, and hydrodynamic conditions were investigated, via direct surface force measurements using the bubble/drop probe atomic force microscopy (AFM) technique and theoretical calculations based on the Reynolds lubrication theory and augmented Young-Laplace equation by including the effect of disjoining pressure. It was found that the PS of low molecular weight (i.e., PS590 and PS810) showed slightly weaker hydrophobic interactions with air bubbles or oil droplets, as compared to glassy PS of higher molecular weight (i.e., PS1110, PS2330, PS46300, and PS1M). The hydrophobic interaction between PS and air bubbles in a 1 M NaCl aqueous solution with 10 vol % ethanol was weaker than that in the bare aqueous solution. Such effects on the hydrophobic interactions are possibly achieved by influencing the structuring/ordering of water molecules close to the hydrophobic polymer surfaces by tuning the surface chain mobility and surface roughness of polymers. It was found that the addition of three surface-active species, i.e., cetyltrimethylammonium chloride (CTAC), Pluronic F-127, and sodium dodecyl sulfate (SDS), to the aqueous media could suppress the attachment of the hydrophobic polymer and air bubbles or oil droplets, most likely caused by the additional steric repulsion due to the adsorbed surface-active species at the bubble/polymer/oil interfaces. Our results have improved the fundamental understanding of the interaction mechanisms between hydrophobic polymers and gas bubbles or oil droplets, with useful implications on developing effective methods for modulating the related interfacial interactions in many engineering applications.

Protein- and Cell-Resistance of Zwitterionic Peptide-Based Self-Assembled Monolayers: Anti-Biofouling Tests and Surface Force Analysis.

Peptide-based self-assembled monolayers (peptide-SAMs) with specific zwitterionic amino acid sequences express an anti-biofouling property. In this work, we performed protein adsorption and cell adhesion tests using peptide-SAMs with repeating units of various zwitterionic pairs of amino acids (EK, DK, ER, and DR). The SAMs with the repeating units of EK and DK (EK and DK SAMs) manifested excellent bioinertness, whereas the SAMs with the repeating units of ER and DR (ER and DR SAMs) adhered proteins and cells. We also performed surface force measurements using atomic force microscopy to elucidate the mechanism underlying the difference in the anti-biofouling property. Our measurements revealed that water-induced repulsion with a range of about 8 nm acts between EK SAMs (immobilized on both probe and substrate) and DK SAMs, whereas such repulsion was not observed for ER and DR SAMs. The strength of the repulsion exhibited a clear correlation with the protein- and cell-resistance of the SAMs, indicating that the interfacial water in the vicinity of EK and DK SAMs is considered as a physical barrier to deter protein and cells from their adsorption or adhesion. The range of the repulsion observed for EK and DK SAMs is longer than 8 nm, indicating that the hydrogen bonding state of the interfacial water with a thickness of 4 nm is modified by EK and DK SAMs, resulting in the expression of the anti-biofouling property.

Direct measurement of surface forces: Recent advances and insights

The direct measurement of forces between atomically smooth mica surfaces down to sub-nanometer separation was pioneered over 50 years ago and has yielded deep understanding of a range of interfacial effects, not least the forces that determine colloidal stability and self-assembly, the properties of highly confined fluids, and the molecular origin of friction and lubrication. Here, we describe recent advances, including the use of substrates other than mica, probing the shear properties of highly confined fluids including hydration layers, and the modulation of surface forces by surface-attached macromolecules and amphiphiles, together with microscopic imaging of the surface morphology. These advances enabled novel features such as external potential control of the interacting surfaces, new understanding of lubrication in aqueous and biological systems, the design of novel nanoparticles and surface assemblies for modulating frictional dissipation, and insight into the nature of long-ranged attraction between surfactant-hydrophobized surfaces. We conclude by briefly outlining future challenges and opportunities provided by such direct surface forces studies.

The Influence of Mechanical Deformations on Surface Force Measurements

Surface Force Balance (SFB) experiments have been performed in a dry atmosphere and across an ionic liquid, combining the analysis of surface interactions and deformations, and illustrate that the mechanical deformations of the surfaces have important consequences for the force measurements. First, we find that the variation of the contact radius with the force across the ionic liquid is well described only by the Derjaguin–Muller–Toporov (DMT) model, in contrast with the usual consideration that SFB experiments are always in the Johnson–Kendall–Roberts (JKR) regime. Secondly, we observe that mica does not only bend but can also experience a compression, of order 1nm with 7μm mica. We present a modified procedure to calibrate the mica thickness in a dry atmosphere, and we show that the structural forces measured across the ionic liquid cannot be described by the usual exponentially decaying harmonic oscillation, but should be considered as a convolution of the surface forces across the liquid and the mechanical response of the confining solids. The measured structural force profile is fitted with a heuristic formulation supposing that mica compression is dominant over liquid compression, and a scaling criterion is proposed to distinguish situations where the solid deformation is negligible or dominant.

Unraveling the Interaction of Water-in-Oil Emulsion Droplets via Molecular Simulations and Surface Force Measurements

Water-in-oil emulsions widely exist in various chemical and petroleum engineering processes, and their stabilization and destabilization behaviors have attracted much attention. In this work, molecular dynamic (MD) simulations were conducted on the water-in-oil emulsion droplets with the presence of surface-active components, including a polycyclic aromatic compound (VO-79) and two nonionic surfactants: the PEO5PPO10PEO5 triblock copolymer and Brij-93. At the surface of water droplets, films were formed by the adsorbate molecules that redistributed during the approaching of the droplets. The redistribution of PEO5PPO10PEO5 was more pronounced than that of Brij-93 and VO-79, which contributed to lower repulsion during coalescence. The interaction forces during droplet coalescence were also measured using atomic force microscopy. Jump-in phenomenon and coalescence were observed for systems with VO-79, Brij-93, and a low concentration of Pluronic P123. The critical force before jump-in was lowest for the low concentration of Pluronic P123, consistent with the MD results. Adhesion was measured when separating water droplets with a high concentration of Pluronic P123. By correlating theoretical simulations and experimental force measurements, this work improves the fundamental understanding on the interaction behaviors of water droplets in an oil medium in the presence of interface-active species and provides atomic-level insights into the stabilization and destabilization mechanisms of water-in-oil emulsion.

Template-Stripped Ultra-Smooth Aluminum Films (0.2 nm RMS) for the Surface Forces Apparatus.

We present a method for the fabrication of ultra-smooth (0.2 nm RMS), aluminum substrates through template stripping (TS). The method relies on the use of mica as a template in combination with thermal evaporation of Al at high (>10 nm/s) rates under vacuum (≤1 × 10-7 Torr). As a reactive metal, Al is usually not considered a viable option for TS off oxide templates. However, under these conditions, the adhesion between the Al film and mica is poor, enabling the removal of the template under water without any mica residue. We verify the absence of mica using atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle measurements. We establish the suitability of these films for surface forces measurements. Multiple-bean interferometry in transmission yields high quality fringes allowing for the measurement of force-distance curves. The adhesion the films to mica is significantly higher than the adhesion of thermally evaporated Al (0.9 nm RMS). Preliminary results suggest that the TS-Al surface displays a higher corrosion resistance. The fabrication method will enable important experiments on this widely used material.

Phytoglycogen Nanoparticles: Nature-Derived Superlubricants.

Phytoglycogen nanoparticles (PhG NPs), a single-molecule highly branched polysaccharide, exhibit excellent water retention, due to the abundance of close-packed hydroxyl groups forming hydrogen bonds with water. Here we report lubrication properties of close-packed adsorbed monolayers of PhG NPs acting as boundary lubricants. Using direct surface force measurements, we show that the hydrated nature of the NP layer results in its striking lubrication performance, with two distinct confinement-controlled friction coefficients. In the weak- to moderate-confinement regime, when the NP layer is compressed down to 8% of its original thickness under a normal pressure of up to 2.4 MPa, the NPs lubricate the surface with a friction coefficient of 10-3. In the strong-confinement regime, with 6.5% of the original layer thickness under a normal pressure of up to 8.1 MPa, the friction coefficient was 10-2. Analysis of the water content and energy dissipation in the confined NP film reveals that the lubrication is governed by synergistic contributions of unbound and bound water molecules, with the former contributing to lubrication properties in the weak- to moderate-confinement regime and the latter being responsible for the lubrication in the strong-confinement regime. These results unravel mechanistic insights that are essential for the design of lubricating systems based on strongly hydrated NPs.

Primitive model electrolytes in the near and far field: Decay lengths from DFT and simulations

Inspired by recent experimental observations of anomalously large decay lengths in concentrated electrolytes, we revisit the Restricted Primitive Model (RPM) for an aqueous electrolyte. We investigate the asymptotic decay lengths of the one-body ionic density profiles for the RPM in contact with a planar electrode using classical Density Functional Theory (DFT) and compare these with the decay lengths of the corresponding two-body correlation functions in bulk systems, obtained in previous Integral Equation Theory (IET) studies. Extensive Molecular Dynamics (MD) simulations are employed to complement the DFT and IET predictions. Our DFT calculations incorporate electrostatic interactions between the ions using three different (existing) approaches: one is based on the simplest mean-field treatment of Coulomb interactions (MFC), while the other two employ the Mean Spherical Approximation (MSA). The MSAc invokes only the MSA bulk direct correlation function, whereas the MSAu also incorporates the MSA bulk internal energy. Although MSAu yields profiles that are in excellent agreement with MD simulations in the near field, in the far field, we observe that the decay lengths are consistent between IET, MSAc, and MD simulations, whereas those from MFC and MSAu deviate significantly. Using DFT, we calculated the solvation force, which relates directly to surface force experiments. We find that its decay length is neither qualitatively nor quantitatively close to the large decay lengths measured in experiments and conclude that the latter cannot be accounted for by the primitive model. The anomalously large decay lengths found in surface force measurements require an explanation that lies beyond primitive models.

Modulating surface interactions for regenerable separation of oil-in-water emulsions

Developing antifouling coatings is of both fundamental and practical significance, but challenging, for membrane separation of oil-in-water (O/W) emulsions with charged surfactants, which has been conventionally achieved through the short-range hydration interaction from hydrophilic membrane surfaces as a physical barrier. Herein, we report a mussel-inspired antifouling coating formed by carboxyl and quaternary ammonium moieties bearing adjustable surface charge property, employing tunable long-range electrostatic interaction to achieve adaptive antifouling performance in response to the varying electrical characteristic of emulsions. The surface force measurement results showed that modulating the surface electrical properties of the coatings through varying the solution pH could effectively alter their electrostatic interactions with emulsions from attraction to repulsion, in combination with hydrodynamic interactions, to enable emulsion repellence of the coatings. Such functional coatings were further applied on a PVDF membrane with an enhanced water permeability and reusability achieved to separate both the positively and negatively charged emulsions. By properly modulating the membrane surface charge, the modified membrane demonstrated its adaptive antifouling performance for efficient and universal water treatment. This work provides useful insights into the antifouling mechanisms of membranes to O/W emulsions, as well as developing functional materials with tunable surface interactions for various engineering applications.

Surface Forces and Structure in a Water-in-Salt Electrolyte.

Water-in-salt electrolytes are a fascinating new class of highly concentrated aqueous solutions with wide electrochemical stability windows that make them viable as aqueous battery electrolytes. However, the high ion concentration of water-in-salt electrolytes means that these systems are poorly understood when compared to more dilute electrolyte solutions. Here, we present direct surface force measurements across thin films of a water-in-salt electrolyte at several concentrations. We find that the electrolyte adopts a layered structure at charged interfaces composed of a nanostructure of a hydrated cation and nonaqueous anion-rich domains. These observations will aid in the interpretation of capacitance and double-layer behavior of water-in-salt electrolytes with consequences for their use in energy storage devices.

Fundamental study on surface force measurements using QCM

Fundamental study on surface force measurements using QCM

Fundamental study on surface force measurements using QCM

Fundamental study on surface force measurements using QCM

Scattering Morphology Resolved Total Internal Reflection Microscopy (SMR-TIRM) Of Colloidal Spheres

Nanometer to micrometer scale colloidal particles are regularly found in applications in which surface forces dominate behavior. Consequently, a wide range of surface force measurement tools have been developed to probe interactions as a function of physiochemical properties. One tool, Total Internal Reflection Microscopy (TIRM), is an exceptionally sensitive probe of both conservative and non-conservative surface forces. A recent variant of TIRM called Scattering Morphology Resolved (SMR) TIRM utilizes the morphology of scattered light in concert with the integrated intensity to measure the position and orientation of a colloidal particle. Although the target of SMR-TIRM is the field of non-spherical “anisotropic” particles, spherical particles have been found to scatter evanescent waves with surprising morphology. Herein, we present experiments and simulations of the scattering morphology of a spherical particle. The morphology was probed as a function of particle size, incident beam polarization, and particle separation distance. We found that spherical particles scattered light with a noncircular morphology. Moreover, we found the morphology depended upon both the scaled particle size with respect to the incident beam wavelength and the incident beam polarization. Although the scattering morphology from the sphere was surprisingly complex, we did not find that these effects would alter the interpretation of scattering as a function of particle separation distance.

Multilayered non‐uniform atomic force microscope piezoelectric microcantilever control and vibration analysis considering different excitation based on the modified couple stress theory

Atomic force microscope (AFM) is one of the most powerful tools for surface scanning, force measurement, and nano-manipulation. To improve its performance, vibration and control of AFM micro-cantilever (MC) should be studied. Hysteresis, as an undesired phenomenon affecting vibration amplitude and phase, is also another important issue to be examined. In this paper, vibration analysis and control of a ZnO non-uniform multi-layered piezoelectric MC has been investigated in non-contact mode. A modified couple stress theory has been used to obtain the strain energy for modeling the MC. In order to control the amplitude, a sliding mode controller (SMC) has been utilized on AFM, due to its application in uncertain and nonlinear systems. For applying the control signal, two methods of piezo and base actuation are studied. The results are compared with proportional integral derivative (PID) control method and it is demonstrated that SMC method reduces the control input close to the surface and increases the accuracy near the surface. In addition to MC control, hysteresis amplitude and phase differences are investigated by applying the Prandtl-Ishlinskii model. Also, surface topography is studied with hysteresis. The simulations show backward phase difference and an increase in amplitude, accordingly.

Role of interfacial water in determining the interactions of proteins and cells with hydrated materials

Water molecules play a crucial role in biointerfacial interactions, including protein adsorption and desorption. To understand the role of water in the interaction of proteins and cells at biological interfaces, it is important to compare particular states of hydration water with various physicochemical properties of hydrated biomaterials. In this review, we discuss the fundamental concepts for determining the interactions of proteins and cells with hydrated materials along with selected examples corresponding to our recent studies, including poly(2-methoxyethyl acrylate) (PMEA), PMEA derivatives, and other biomaterials. The states of water were analyzed by differential scanning calorimetry, in situ attenuated total reflection infrared spectroscopy, and surface force measurements. We found that intermediate water which is loosely bound to a biomaterial, is a useful indicator of the bioinertness of material surfaces. This finding on intermediate water provides novel insights and helps develop novel experimental models for understanding protein adsorption in a wide range of materials, such as those used in biomedical applications.

Binding mechanism of a de novo coiled coil complex elucidated from surface forces measurements

We used the Surface Forces Apparatus to elucidate the interaction mechanism between grafted 5 heptad-long peptides engineered to spontaneously form a heterodimeric coiled-coil complex. The results demonstrated that when intimate contact between peptides is reached, binding occurs first via weakly interacting but more mobile distal heptads, suggesting an induced-fit association process. Precise control of the distance between peptide-coated surfaces allowed to quantitatively monitor the evolution of their biding energy. The binding energy of the coiled-coil complex increased in a stepwise fashion rather than monotonically with the overlapping distance, each step corresponding to the interaction between a quantized number of heptads. Surface forces data were corroborated to surface plasmon resonance measurements and molecular dynamics simulations and allowed the calculation of the energetic contribution of each heptad within the coiled-coil complex.

Small changes in PDMMLA structure influence the adsorption behavior of ECM proteins and syndecan-4 on PDMMLA derivative surfaces: Experimental validation by tensiometric surface force measurements

Cardiovascular diseases are the leading cause of death around the world according to the World Health Organization. In-stent restenosis is an inflammatory response of the immune system to endovascular stent implantation in atherosclerotic patients. Biocompatible and biodegradable polymers are of great interest in this field in order to limit the side effects of stent treatments. Poly([R,S]-3,3-dimethylmalic acid) (PDMMLA) is a new biodegradable statistical polyester which presents promising properties as a stent coating. In this work, we studied by dynamic tensiometry, the adhesion of extracellular matrix proteins (bovine serum albumin, fibronectin, fibrinogen, and vitronectin) and plasma membrane proteoglycan (syndecan-4) on three PDMMLA derivatives with different hydrophilicity levels. The results show that proteins have different adhesion profiles and affinity on these surfaces. They show similar behavior on the most hydrophilic surface, making hydrophilic, ionic and hydrogen type bonds. Then we compared each protein’s individual profile to that of a mixture of all studied proteins. The comparison shows that vitronectin and syndecan-4 are the quantitatively dominating proteins adsorbed by specific interactions. Based on the results from previous studies, this work allowed us to identify the most important PDMMLA surface as a promising biomaterial for bioactive stent-coating.

Control of bubble ζ-potentials to improve the kinetics of bubble-particle interactions

Derjaguin and Dukhin (1961) developed a kinetic model for the bubble-particle interactions in flotation using the classical DLVO theory. According to the model, the energy barrier (E1) to flotation should increase with the square of particle ζ-potential due to the increase in the repulsive electrical double-layer (EDL) forces in the thin liquid films (TLFs) of water confined between the two dissimilar surfaces. In the present work, surface force measurements have been conducted by monitoring the changes in local curvatures of the air bubbles approaching flat silica surfaces of varying contact angles, while controlling the bubble ζ-potentials in alkyltrimethylammonium chloride (CnTACl) solutions of varying chain lengths and concentrations. The results show that the kinetics of film thinning and contact angle formation increased with decreasing energy barriers. It has been found, however, that E1 = 0 is not the sufficient condition for maximum kinetics, as the bubble-particle interaction continued to improve with further increases in the negative disjoining pressure in the TLFs. These findings can be attributed to the increase in the hydrodynamic forces in TLFs due to the increases in the attractive EDL and hydrophobic forces. The fundamental information obtained in the present work may be useful for developing novel flotation reagents and designing more efficient flotation machines.

Optimization of mica surface hydroxylation in water vapor plasma monitored by optical emission spectroscopy

Mica, a mineral consisting of alumino-silicate layers, can be cleaved to obtain atomically smooth surfaces, and it is often used as a model substrate for surface force measurements. Due to mica's chemical inertness, covalent surface modifications are not straightforward. By applying a water vapor plasma treatment, reactive silanol groups can be generated on the surface. Up to now, the optimization of the plasma process was a time-consuming trial-and-error process. Furthermore, no clear correlation between the plasma parameters and the hydroxyl surface density was elucidated. To overcome such limitations, we invoke in-situ optical emission spectroscopy (OES). A correlation between the pressure-normalized OES hydroxyl intensity and the surface density of hydroxyl groups was found, which allows real-time optimization of the plasma conditions. The density of silanol groups was quantified via chemical derivatization X-ray photoelectron spectroscopy (XPS) using a fluorinated monochlorosilane. An apparent upper limit for the hydroxyl surface density of 2.7 SiOH/nm2 was found—the limitation being imposed by packing restraints of the derivatizing reagent. While real surface density may be underestimated by derivatization, the obtained maximum value is an order of magnitude higher than obtained from wet-chemical hydroxylation, and, it is above the hitherto highest reported values for water vapor plasma treatment.

A Study on Surface Force Measurement for Contacted Arbitral Two Surfaces —Effect of Surface Roughness, and Indirect Surface Force Measurement Method—

The present paper is intended to show effect of surface roughness on surface force measurement and to demonstrate a newly-proposed indirect surface force measurement using two probes. In the case of hydrogen-terminated and octadecyl-trichloro-silane-coated Si surfaces with micro-mounds, measured surface forces is strongly affected by micro-roughness. The surface force, which varies from 2 μN to 180 μN, is proportional to the logarithm of the curvature radius of the micro-mound. Meanwhile, in the proposed indirect measurement method of surface force, surface force of arbitrary two surface is calculated on the basis of modified JKR and Fowkes theories with surface forces measured by two kinds of probes. The authors prepared a specialized OTS-coated probe as well as a standard glass probe to verify validity of the indirect surface force measurement. For a case of surface force between Si and APTES surfaces, the indirectly measured value reasonably corresponds to the directly measured value. It is confirmed that the proposed method is effective for surface force evaluation for arbitrary two surfaces without probe coating.