Surface Force Measurements Research Articles

Zwitterion-Conjugated Protein Coatings for Enhanced Antifouling in Complex Biofluids: Underlying Molecular Interaction Mechanisms.

Biofouling can cause severe infections, device malfunctions, and failures in diagnostics and therapeutics. Proteins such as bovine serum albumin (BSA) have recently been used as coatings to resist biofouling because they combine surface anchoring and antifouling properties. However, their antifouling effectiveness will significantly deteriorate in complex biofluids with high salinity, limiting their practical applications. In this work, we developed a zwitterion-conjugated protein with enhanced antifouling capability by grafting zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC) onto BSA protein via a click reaction. This conjugated protein can easily anchor on various substrates, both inorganic and organic, and exhibits efficient and broad-spectrum fouling resistance to metabolites, proteins, and complex biofluids. Even in the complex fetal bovine serum with higher salinity, the BSA@MPC coating can also maintain 99% fouling resistance robustly, over 6-fold superior to native BSA-coated surfaces in antifouling capability. Direct surface forces measurement reveals that such outstanding antifouling properties of conjugated protein BSA@MPC could be attributed to the stable hydration layer on its surface and the steric repulsion from the antipolyelectrolyte behavior of zwitterionic MPC polymer in the high-salinity environment. Our findings advance the development of protein-based functional materials and provide valuable insights for designing novel antifouling surfaces for marine, food, and bioengineering applications.

Cation valency in water-in-salt electrolytes alters the short- and long-range structure of the electrical double layer

Highly concentrated aqueous electrolytes (termed water-in-salt electrolytes, WiSEs) at solid-liquid interfaces are ubiquitous in myriad applications including biological signaling, electrosynthesis, and energy storage. This interface, known as the electrical double layer (EDL), has a different structure in WiSEs than in dilute electrolytes. Here, we investigate how divalent salts [zinc bis(trifluoromethylsulfonyl)imide, Zn(TFSI)2], as well as mixtures of mono- and divalent salts [lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) mixed with Zn(TFSI)2], affect the short- and long-range structure of the EDL under confinement using a multimodal combination of scattering, spectroscopy, and surface forces measurements. Raman spectroscopy of bulk electrolytes suggests that the cation is closely associated with the anion regardless of valency. Wide-angle X-ray scattering reveals that all bulk electrolytes form ion clusters; however, the clusters are suppressed with increasing concentration of the divalent ion. To probe the EDL under confinement, we use a Surface Forces Apparatus and demonstrate that the thickness of the adsorbed layer of ions at the interface grows with increasing divalent ion concentration. Multiple interfacial layers form following this adlayer; their thicknesses appear dependent on anion size, rather than cation. Importantly, all electrolytes exhibit very long electrostatic decay lengths that are insensitive to valency. It is likely that in the WiSE regime, electrostatic screening is mediated by the formation of ion clusters rather than individual well-solvated ions. This work contributes to understanding the structure and charge-neutralization mechanism in this class of electrolytes and the interfacial behavior of mixed-electrolyte systems encountered in electrochemistry and biology.

Probing the interfacial behaviors of interfacially active and non-active asphaltenes and their impact on emulsion stability

HypothesisAsphaltenes subfractions with distinct interfacial behaviors may play different roles in stabilizing oil–water emulsions. ExperimentsIn this work, whole asphaltenes were separated into interfacially active asphaltenes (IAA) and interfacially non-active asphaltenes (INAA). Employing advanced nanomechanical techniques, we have explored the compositions, morphologies, sizes, adsorption, and interfacial behaviors of IAA and INAA. FindingsIAA exhibits a high and unevenly distributed oxygen content, distinguishing it from INAA. In toluene, the diameters of IAA and INAA are about 60 nm and 6 nm, respectively. When adsorbed irreversibly on mica surfaces, the thickness of the IAA and INAA film was measured at ∼5.5 nm or 1 nm, respectively; while in a toluene solution, the film thickness reached ∼46 nm and 3.1 nm for IAA and INAA, respectively. IAA demonstrates superior interfacial activity, and elastic/viscous moduli compared to INAA at the water-toluene interface. Quantified surface force measurements reveal that IAA stabilizes water droplets in toluene at a concentration of only 10 mg/L, while INAA requires a higher concentration of 100 mg/L. This work provides the first comprehensive investigation into the adsorption and interfacial behaviors of asphaltene subfractions and provides useful insights into the asphaltenes-stabilization mechanism of emulsions.

Exploring the mechanisms of calcium carbonate deposition on various substrates with implications for effective anti-scaling material selection

The unexpected scaling phenomena have resulted in significant damages to the oil and gas industries, leading to issues such as heat exchanger failures and pipeline clogging. It is of practical and fundamental importance to understand the scaling mechanisms and develop efficient anti-scaling strategies. However, the underlying surface interaction mechanisms of scalants (e.g., calcite) with various substrates are still not fully understood. In this work, the colloidal probe atomic force microscopy (AFM) technique has been applied to directly quantify the surface forces between calcite particles and different metallic substrates, including carbon steel (CR1018), low alloy steel (4140), stainless steel (SS304) and tungsten carbide, under different water chemistries (i.e., salinity and pH). Measured force profiles revealed that the attractive van der Waals (VDW) interaction contributed to the attachment of the calcium carbonate particles on substrate surfaces, while the repulsive electric double layer (EDL) interactions could inhibit the attachment behaviors. High salinity and acidic pH conditions of aqueous solutions could weaken the EDL repulsion and promote the attachment behavior. The adhesion of calcite particles with CR1018 and 4140 substrates was much stronger than that with SS304 and tungsten carbide substrates. The bulk scaling tests in aqueous solutions from an industrial oil production process showed that much more severe scaling behaviors of calcite was detected on CR1018 and 4140 than those on SS304 and tungsten carbide, which agreed with surface force measurement results. Besides, high salinity and acidic pH can significantly enhance the scaling phenomena. This work provides fundamental insights into the scaling mechanisms of calcite at the nanoscale with practical implications for the selection of suitable anti-scaling materials in petroleum industries.

Measurement of surface forces between sphere and plane in water

Measurement of surface forces between sphere and plane in water

Alkali silica reaction in concrete - Revealing the expansion mechanism by surface force measurements

Alkali-silica reaction (ASR) is a major cause for concrete deterioration worldwide. However, the mechanism leading to cracking has not been identified yet. In this study, the extended Surface Force Apparatus (eSFA) has been used to determine the surface forces of alkali-silica solutions between two atomically smooth mica surfaces. The setup imitates the situation present in reactive concrete aggregates with negatively charged silicate surfaces and negatively charged polynuclear silica in solution. The distance of strong electrostatic repulsion increases with increasing concentration of dissolved silica, leading to the buildup of pressure up to 6 MPa. When the precipitation of ASR products in confined conditions is triggered by the addition of CaCl2 to the alkali-silica solution, the resulting solidification pressure forces the mica platelets apart and reaches 6–13 MPa. The eSFA experiments shows that solidification pressure is the mechanism leading to aggregate cracking and expansion of ASR-affected concrete.

Facile fabrication of non-fluorine polymer brush/loop surfaces for oil/water separation and self-cleaning applications

Polydimethylsiloxane (PDMS), a low surface energy material, is widely used in the preparation of various (super)hydrophobic materials. Nevertheless, the impact of PDMS chain architectures (i.e., linear, branched, difunctional and monofunctional PDMS) on surface properties, such as hydrophobicity and self-cleaning, has received limited attention. In this work, we constructed PDMS chains on stainless steel mesh/polyester fiber/silica surfaces to form both looped and linear topological structures. Surfaces with looped structure showed higher roughness and stronger hydrophobicity in air and water, compared to the surfaces with linear structures. Surface force measurements demonstrated that the silica surface with looped structure exhibited stronger hydrophobic forces in water than the surfaces with linear structure. The looped structure coatings also exhibited superior self-cleaning and faster water sliding compared to the linear structure coatings. Additionally, both looped and linear PDMS coated stainless steel mesh/polyester fibers showed efficient separation of oil/water mixture. This study sheds light on the influence of polymer topology on surface properties and advances the development of applications in oil/water separation and self-cleaning.

Probing the surface forces between air bubbles and bitumen via direct force Measurements: Effects of aqueous chemistry

The surface interactions of air bubbles and other components (e.g., particles, oil droplets) significantly influence the operation efficiency of a variety of engineering processes such as oily water treatment, bitumen extraction and flotation. Herein, the surface interactions of air bubbles with two types of Athabasca bitumen samples in aqueous solutions of varying pH, salinity, type of cations and in the presence of surfactants have been systematically characterized using a bubble probe atomic force microscope (AFM) technique. AFM imaging has revealed that exposure to high concentrations of NaCl and CaCl2 solutions or alkaline environments causes roughening of the bitumen surfaces. The results of surface force measurements demonstrate that the interaction and attachment behaviors between bubbles and bitumen are significantly affected by ionic strength, solution pH, and the presence of surfactants. The experimental force measurement results could be accurately described by a theoretical model that incorporates Reynolds lubrication theory and augmented Young-Laplace equation, with the inclusion of disjoining pressure. In low-salinity conditions, the bubble-bitumen interaction is dominated by electric double layer (EDL) repulsion, which prevents surface attachment. This effect is more pronounced at elevated pH conditions. In high-salinity solutions, however, the EDL interactions are significantly reduced, and the hydrophobic interaction becomes the dominant factor, overcoming the van der Waals repulsion and leading to the attachment of bubbles to bitumen surfaces. Raising aqueous pH weakens the bubble-bitumen hydrophobic interaction, whereas the introduction of calcium ions strengthens this interaction, resulting in enhanced surface attachment. Interestingly, even in high-salinity conditions, the presence of a small number of surfactants can inhibit bubble-bitumen contact, mainly caused by reduced hydrophobic attraction and increased steric repulsion. This work provides valuable nanoscale insights into how bubbles and bitumen interact in intricate aqueous environments, and the results show practical implications for controlling similar interfacial phenomena in various engineering processes.

Effect of solution pH and polyethylene oxide concentration on surface/interface properties, flocculation and rheology of concentrated monodisperse ultrafine synthetic tailings slurry

The effects of polyethylene oxide (PEO) dosage and solution pH on the flocculation, rheological and surface/interfacial properties of the slurry were investigated by sedimentation, rheology, adsorption and surface force experiments. The results demonstrate that the initial settling rate (ISR) of the tailings is at its lowest in strongly acidic solutions. The ISR reaches its peak in strongly alkaline solutions. The PEO dosage exhibits a modest impact on the yield stress of the concentrate in strong acid solutions, while it wields a notable influence on the yield stress in neutral and strongly alkaline solutions. Quartz crystal microbalance with dissipation (QCM-D) measurements reveal that strong alkaline solutions enhance the adsorption of PEO chains on silica surfaces. Strongly acidic solutions partially inhibit the adsorption of PEO chains. Surface force measurement results indicate that PEO chains can bridge the tailings particles in concentrated suspensions through hydrogen bonding, and consequently elevating the suspension's yield stress.

Flow Structures and Aerodynamic Behavior of a Small-Scale Joined-Wing Aerial Vehicle under Subsonic Conditions

Flow behavior and aerodynamic performance of a small-scale joined-wing unmanned aerial vehicle (UAV) was studied experimentally and numerically under various pitch and yaw angle combinations in subsonic flow conditions. Selected numerical results are compared against experimental results obtained using surface oil flow visualizations and force measurements, with additional simulations expanding the range of combined pitch and yaw configurations. Under zero-yaw conditions, increasing the pitch angle leads to the formation of symmetric ogive vortex roll-ups close to the fuselage and their significant interactions with the fore-wing. Additionally, contributions to lift and drag coefficients under zero-yaw conditions by the key UAV components have been documented in detail. In contrast, when the UAV is subjected to combined pitch and yaw, no clear evidence of such ogive vortex roll-ups can be observed. Instead, asymmetric flow separations occur over the fuselage’s port side and resemble bluff-body flow behavior. Additionally, these flow separations become more complex, and they interact more with the fuselage and fore- and aft-wings when the yaw angle increases. Lift and drag variations due to different pitch and yaw angle combinations are also documented. Finally, rolling and yawing moment results suggest that the present UAV possesses adequate flight stability unless the pitch and yaw angles are high.

Multifunctional Underwater Adhesive Film Enabled by a Single-Component Poly(ionic liquid).

Tremendous efforts have been devoted to exploiting synthetic wet adhesives for real-life applications. However, developing low-cost, robust, and multifunctional wet adhesive materials remains a considerable challenge. Herein, a wet adhesive composed of a single-component poly(ionic liquid) (PIL) that enables fast and robust underwater adhesion is reported. The PIL adhesive film possesses excellent stretchability and flexibility, enabling its anchoring on target substrates regardless of deformation and water scouring. Surface force measurements show the PIL can achieve a maximum adhesion of 56.7 mN·m-1 on diverse substrates (both hydrophilic and hydrophobic substrates) in aqueous media, within ∼30 s after being applied. The adhesion mechanisms of the PIL were revealed via the force measurements, and its robust wet adhesive capacity was ascribed to the synergy of different non-covalent interactions, such as of hydrogen bonding, cation-π, electrostatic, and van der Waals interactions. Surprisingly, this PIL adhesive film exhibited impressive underwater sound absorption capacity. The absorption coefficient of a 0.7 mm-thick PIL film to 4-30 kHz sound waves could be as high as 0.80-0.92. This work reports a multifunctional PIL wet adhesive that has promising applications in many areas and provides deep insights into interfacial interaction mechanisms underlying the wet adhesion capability of PILs.

Removal of colloidal impurities by thermal softening-coagulation-flocculation-sedimentation in steam assisted gravity drainage (SAGD) produced water: Performance, interaction effects and mechanism study

Effective removal of colloidal impurities from steam-assisted gravity drainage (SAGD) produced water is essential for enhancing water recycling in industry. Response surface methodology (RSM) was employed to optimize the thermal softening-coagulation-flocculation-sedimentation process with softeners (Ca(OH)2, MgO and Na2CO3), poly-DADMAC as coagulant, and cationic polyacrylamide (PAM) as flocculant, and assess the interaction effects of operational variables for sludge volume index (SVI) and the removal efficiency of turbidity (TSS), particulate hardness, silica, total organic carbon (TOC) and total inorganic carbon (TIC) in synthetic SAGD produced water. The optimal conditions at 0.93 desirability were 67 mg/L poly-DADMAC dose, 14 min mixing time with softeners only, 200 rpm coagulation speed, and 16 min flocculation time. At this condition, the predicted maximum removal of turbidity, TSS, particulate hardness, silica, TOC and TIC were 99.2 %, 99.1 %, 99.4 %, 27.0 %, 69.0 %, and 30.3 %, respectively, and the value of SVI was 38.1 mL/g. Our results indicated that poly-DADMAC dose and mixing time with softeners only were the most influential factors for the treatment process. Furthermore, the temperature effect on removal mechanisms was investigated at optimal conditions under room temperature and high temperature (80 °C). The results of floc characterization and surface force measurement indicated that temperature could facilitate the removal of colloidal impurities via forming larger and denser flocs and changing their surface composition. The results also provided confirmation that adsorption and subsequent bridging are the main removal mechanisms in the coagulation-flocculation process. This study illuminates the importance of the interactions among operational variables and provides in-depth insights into the mechanism for the removal of colloidal impurities under high temperature.

Using micro-milled surface topography and force measurements to identify tool runout and mechanistic model coefficients

Using micro-milled surface topography and force measurements to identify tool runout and mechanistic model coefficients

Ternary Synergy of Lys, Dopa, and Phe Results in Strong Cohesion of Peptide Films.

Synergistic interactions between 3,4-dihydroxyphenylalanine (Dopa, Y*), cationic residues, and the aromatic rings have been recently highlighted as influential factors that enhance the underwater adhesion strength of mussel foot proteins and their derivatives. In this study, we report the first ever evidence of a cation-catechol-benzene ternary synergy between Y*, lysine (Lys, K), and phenylalanine (Phe, F) in adhesive peptides. We synthesized three hexapeptides containing a different combination of Y*, K, and F, i.e., (KY*)3, (KF)3, and (KY*F)2, respectively, exploring the relationship between the cohesive performance and molecular architecture of peptides. The peptide with the (KY*F)2 sequence displays the strongest underwater cohesion energy of 10.3 ± 0.3 mJ m-2 from direct nanoscale surface force measurements. Combined with molecular dynamics simulation, we demonstrated that there are more bonding interactions (including cation-π, π-π, and hydrogen bond interactions) in (KY*F)2 compared to the other two peptides. In addition, peptide (KY*F)2 still shows the strongest cohesive energies of 7.6 ± 0.7 and 3.7 ± 0.5 mJ m-2 in acidic and high-ionic strength environments, respectively, although the cohesive energy decreases compared to the value in pure water. Our results further explain the underwater cohesion mechanisms combining multiple interactions and offer insights on designing Dopa containing underwater adhesives.

Detecting Early-Stage Cohesion Due to Calcium Silicate Hydration with Rheology and Surface Force Apparatus.

Extremely robust cohesion triggered by calcium silicate hydrate (C-S-H) precipitation during cement hardening makes concrete one of the most commonly used man-made materials. Here, in this proof-of-concept study, we seek an additional nanoscale understanding of early-stage cohesive forces acting between hydrating model tricalcium silicate (C3S) surfaces by combining rheological and surface force measurements. We first used time-resolved small oscillatory rheology measurements (SAOSs) to characterize the early-stage evolution of the cohesive properties of a C3S paste and a C-S-H gel. SAOS revealed the reactive and viscoelastic nature of C3S pastes, in contrast with the nonreactive but still viscoelastic nature of the C-S-H gel, which proves a temporal variation in the cohesion during microstructural physicochemical rearrangements in the C3S paste. We further prepared thin films of C3S by plasma laser deposition (PLD) and demonstrated that these films are suitable for force measurements in the surface force apparatus (SFA). We measured surface forces acting between two thin C3S films exposed to water and subsequent in situ calcium silicate hydrate precipitation. With the SFA and SFA-coupled interferometric measurements, we resolved that C3S surface reprecipitation in water was associated with both increasing film thickness and progressively stronger adhesion (pull-off force). The lasting adhesion developing between the growing surfaces depended on the applied load, pull-off rate, and time in contact. These properties indicated the viscoelastic character of the soft, gel-like reprecipitated layer, pointing to the formation of C-S-H. Our findings confirm the strong cohesive properties of hydrated calcium silicate surfaces that, based on our preliminary SFA measurements, are attributed to sharp changes in the surface microstructure. In contact with water, the brittle and rough C3S surfaces with little contact area weather into soft, gel-like C-S-H nanoparticles with a much larger surface area available for forming direct contacts between interacting surfaces.

A Novel Method for Soft Contact Sensing Based on Electrical Impedance Sensitivity Images

This paper presents a distributed soft contact sensing method based on a sensitivity mapping function, which relates the change in measured voltages to that in the elastomer conductivity due to contact force acting on its surface. The sensitivity-image-based sensing system uses a small number of boundary electrodes with a multiplexer to create different electric-field patterns to generate a series of sensitivity images for machine learning, significantly reducing the number of training data typically obtained with single-point indentation measurements. The mapping function, which does not rely on the knowledge of the electric and conductivity fields during online sensing, can be trained with only a small amount of measured data in the order of the square of electrode number. The proposed method has been experimentally evaluated on two 16-electrode prototypes trained with 129 and 80 data for two typical applications, which are tactile perception on a flat surface and contact force measurement in a model knee joint, respectively. The former verifies the measurement accuracy of the contact position and force magnitude while the latter demonstrates the application of this method for measuring the internal joint forces between two curved surfaces.

Colloidal Systems in Concentrated Electrolyte Solutions Exhibit Re-entrant Long-Range Electrostatic Interactions due to Underscreening.

Surface force measurements have revealed that at very high electrolyte concentrations as well as in neat and diluted ionic liquids and deep eutectic solvents, the range of electrostatic interactions is far greater than the Debye length. Here, we explore the consequences of this underscreening for soft-matter and colloidal systems by investigating the stability of nanoparticle dispersions, the self-assembly of ionic surfactants, and the thickness of soap films. In each case, we find clear evidence of re-entrant properties due to underscreening at high salt concentrations. Our results show that underscreening in concentrated electrolytes is a general phenomenon and is not dependent on confinement by macroscopic surfaces. The stability of systems at very high salinity due to underscreening may be beneficially applied to processes that currently use low-salinity water.

Interphase Protein Layers Formed on Self-Assembled Monolayers in Crowded Biological Environments: Analysis by Surface Force and Quartz Crystal Microbalance Measurements.

We investigated a viscous protein layer formed on self-assembled monolayers (SAMs) in crowded biological environments. The results were obtained through force spectroscopic measurements using colloidal probes and substantiated by exhaustive analysis using a quartz crystal microbalance with an energy dissipation technique. A hydrophobic SAM of n-octanethiol (C8 SAM) in bovine serum albumin (BSA) solution is buried under an adlayer of denatured BSA molecules and an additional viscous interphase layer that is five times more viscous than the bulk solution. C8 SAMs in fetal bovine serum induced a formation of a thicker adsorbed protein layer but with no observable viscous interphase layer. These findings show that a fouling surface is essentially inaccessible to any approaching molecules and thus has a new biological and physical identity arising from its surrounding protein layers. In contrast, the SAMs composed of sulfobetaine-terminated alkanethiol proved to be sufficiently protein-resistant and bio-inert even under crowded conditions due to a protective barrier of its interfacial water, which has implications in the accurate targeting of artificial particles for drug delivery and similar applications by screening any non-specific interactions. Finally, our strategies provide a platform for the straightforward yet effectual in vitro characterization of diverse types of surfaces in the context of targeted interactions in crowded biological environments.

Facile and scalable surface functionalization approach with small silane molecules for oil/water separation and demulsification of surfactant/asphaltenes-stabilized emulsions

Oil contaminations have been challenging issues in a wide range of engineering and environmental fields. Herein, we employ a small silane molecule, γ-mercaptopropyldi(trimethylsiloxy)methylsilane (MD(SH)M), to functionalize fiber-based substrates (i.e., stainless steel mesh, cotton fiber) for oil decontamination processes including oil–water separation, oil absorption and demulsification. The functionalized stainless-steel mesh (SSM@PM) can completely separate water and oils of high/low density without prewetting by oil. The functionalized cotton fibers (CF@PM) can absorb different oils from water without being wetted by water and maintain excellent oil absorption performance even after 10 cyclic reuses. Besides, CF@PM shows good demulsification performance to water-in-oil emulsions stabilized by span 80 and asphaltenes. The underlying interaction mechanisms are investigated using force measurements. The CF@PM shows much stronger repulsion to water droplets in air as compared to unmodified cotton fiber, and SSM@PM has strong attraction to oil droplets under water but almost no attraction to water droplets in oil, accounting for the effective oil/water separation capacity. Such strong attraction between oil and MD(SH)H-modified surfaces can be attributed to the hydrophobic interaction as confirmed by surface force measurements. This work provides a facile surface functionalization approach with great potential applications in oil/water separation and demulsification of surfactant- or asphaltenes-stabilized emulsions.

Bench-scale oil fouling/antifouling tests under high temperature and high pressure conditions and the underlying interfacial interaction mechanisms

Oil fouling is commonly encountered in many environmental and engineering fields and remains a challenging issue. The fouling issue in steam-assisted gravity drainage (SAGD) process for extracting bitumen from oil sands deposits has received much attention from oil industry. In view of low capital investment and exceptional antifouling performance, adding antifoulants has emerged as a promising strategy for addressing fouling issues. Developing an efficient method to evaluate their antifouling performance under production conditions is urgently needed. In this work, we report a novel bench-scale test method for quantitatively evaluating the antifouling performance of antifoulants under high temperature and high pressure (HTHP), close to SAGD operation conditions. The effects of antifoulant dosage and stirring rate on the antifouling performance are systematically investigated. The underlying fouling and antifouling mechanisms are characterized through direct molecular and surface force measurements. It is found that the commercial antifoulant exhibits exceptional antifouling performance at the dosage range of 20–35 ppm, and increasing stirring rate enhances the antifouling behavior. The force measurement results indicate that the antifoulant can adsorb to organics and target substrate surfaces, performing the antifouling function by lowering the hydrophobic interaction between organics and target substrates. This work provides a novel and feasible method for characterizing fouling phenomena and evaluating antifoulants under HTHP conditions and improves the understanding of fundamental interaction mechanisms underlying the fouling and antifouling phenomena.