Porous Cellulose Thin Films as Sustainable and Effective Antimicrobial Surface Coatings.

In the present work, we developed an effective antimicrobial surface film based on sustainable microfibrillated cellulose. The resulting porous cellulose thin film is barely noticeable to human eyes due to its submicrometer thickness, of which the surface coverage, porosity, and microstructure can be modulated by the formulations and the coating process. Using goniometers and a quartz crystal microbalance, we observed a threefold reduction in water contact angles and accelerated water evaporation kinetics on the cellulose film (more than 50% faster than that on a flat glass surface). The porous cellulose film exhibits a rapid inactivation effect against SARS-CoV-2 in 5 min, following deposition of virus-loaded droplets, and an exceptional ability to reduce contact transfer of liquid, e.g., respiratory droplets, to surfaces such as an artificial skin by 90% less than that from a planar glass substrate. It also shows excellent antimicrobial performance in inhibiting the growth of both Gram-negative and Gram-positive bacteria (Escherichia coli and Staphylococcus epidermidis) due to the intrinsic porosity and hydrophilicity. Additionally, the cellulose film shows nearly 100% resistance to scraping in dry conditions due to its strong affinity to the supporting substrate but with good removability once wetted with water, suggesting its practical suitability for daily use. Importantly, the coating can be formed on solid substrates readily by spraying, which requires solely a simple formulation of a plant-based cellulose material with no chemical additives, rendering it a scalable, affordable, and green solution as antimicrobial surface coating. Implementing such cellulose films could thus play a significant role in controlling future pan- and epidemics, particularly during the initial phase when suitable medical intervention needs to be developed and deployed.

Hair surface interactions against different chemical functional groups as a function of environment and hair condition.

OBJECTIVE: The nature and magnitude of molecular interactions on hair surfaces underpin the design of formulated products, of which the application involves a competitive adsorption process between cationic surfactants, fatty alcohols and surface actives such as silicone. The knowledge of molecular interaction with hair surface will not only provide insight on the surface binding affinity but also offer an effective methodology in characterizing surface deposits. METHODS: Untreated and chemically treated hair samples were treated with either conditioner chassis alone (gel network) or conditioner chassis plus silicone (chassis/TAS). Hair surface interactions against four different chemical functional groups, namely methyl (-CH3 ), acid (-COOH), amine (-NH2 ) and hydroxyl (-OH), were quantified in both ambient and aqueous environment using Chemical Force Microscopy, a method based on atomic force microscopy (AFM). RESULTS: Surface adhesion on hair in ambient is dominated by capillary force that is determined by both the wettability of hair fibre (hydrophobic vs. hydrophilic), presence of any deposits and the chemical functionality of the AFM cantilever. Capillary force is diminished and replaced by electrostatic interaction when polar groups are present on both hair and AFM cantilever. A distinctively different force, hydrophobic interaction, plays a major role when virgin hair and hydrophobic functionalized AFM cantilever make contact in water. CONCLUSION: Results acquired by AFM cantilevers of different functional groups show that hydrophobic interaction is a key driver for deposition on virgin hair, whilst electrostatic interaction is the most important one for bleached hair. Interfacial conformation of chassis components upon deposition is determined by the hair surface properties. Our study highlights the possibility of a range of polar groups, not necessarily negatively charged, on the damaged hair. Unlike conventional surface chemical analysis method, it is possible to quantitatively evaluate the interfacial conformation of deposited surface actives on hair, which identifies the target moieties for conditioning products on different types of hair.

OBJECTIF: La nature et l'intensité des interactions moléculaires mesurables à la surface d'un cheveu, caractérisent l'effet de la formulation du produit initialement appliqué sur le cheveu. L'application du produit et son effet sur le cheveu repose sur un mécanisme d'adsorption complexe combinant l'effet de différents éléments tels que des surfactants cationiques, des alcools gras et des agents de surface tel que le silicone. L'étude et l'analyse des interactions moléculaires à la surface du cheveu permettent non seulement de déterminer l'affinité adhésive de surface, mais aussi d'offrir une méthode efficace pour caractériser les dépôts de surface. MÉTHODE: Des cheveux initialement traités et non-traités chimiquement, ont été analysés après l'application d'un après-shampoing chassis (structure en gel) ou d'un après-shampoing chassis avec du silicone (chassis/TAS). Les interactions entre la surface du cheveu et quatre groupes fonctionnels chimiques - méthyle (-CH3 ), acide (-COOH), amine (-NH2) et hydroxy (-OH) ont été quantifiées à l'air et en milieu aqueux par microscopie à force chimique, une méthode basée sur la microscopie à force atomique (AFM). RÉSULTATS: L'adhésion de surface sur cheveu à l'air est dominée par la force capillaire qui est déterminée par la mouillabilité de la fibre capillaire (hydrophobe vs. hydrophile), la présence de dépôts, et la fonction chimique du cantilever. La force capillaire diminue et est remplacée par des interactions électrostatiques quand des groupes polaires sont présents à la fois sur le cheveu et le cantilever. Une autre force - l'interaction hydrophobe, joue un rôle majeur quand un cheveu non-traité / vierge et un cantilever de fonction hydrophobe se rencontrent en milieu aqueux. CONCLUSIONS: Les résultats obtenus à partir de cantilevers de différentes fonctions chimiques, montrent que l'interaction hydrophobe joue un rôle clé dans l'application de produit capillaire sur cheveux non-traité tandis que les interactions électrostatiques sont prédominantes dans le cas de cheveux traités chimiquement. La conformation interfaciale des composés chassis avant déposition est déterminée par les propriétés de surface du cheveu. Notre étude souligne la présence potentielle de différents groupes polaires, pas nécessairement chargés négativement, sur la surface de cheveux endommagés. A l'inverse des méthodes conventionnelles d'analyse chimique de surface, il est possible d'évaluer quantitativement la conformation interfaciale de dépôts d'agents actifs présents à la surface du cheveu, ce qui permet d'identifier les zones cibles pour l'application d'après-shampoing sur différents types de cheveux.

Characteristics of respiratory microdroplet nuclei on common substrates.

To evaluate the role of common substrates in the transmission of respiratory viruses, in particular SARS-CoV-2, uniformly distributed microdroplets (approx. 10 µm diameter) of artificial saliva were generated using an advanced inkjet printing technology to replicate the aerosol droplets and subsequently deposited on five substrates, including glass, polytetrafluoroethylene, stainless steel, acrylonitrile butadiene styrene and melamine. The droplets were found to evaporate within a short timeframe (less than 3 s), which is consistent with previous reports concerning the drying kinetics of picolitre droplets. Using fluorescence microscopy and atomic force microscopy, we found that the surface deposited microdroplet nuclei present two distinctive morphological features as the result of their drying mode, which is controlled by both interfacial energy and surface roughness. Nanomechanical measurements confirm that the nuclei deposited on all substrates possess similar surface adhesion (approx. 20 nN) and Young's modulus (approx. 4 MPa), supporting the proposed core-shell structure of the nuclei. We suggest that appropriate antiviral surface strategies, e.g. functionalization, chemical deposition, could be developed to modulate the evaporation process of microdroplet nuclei and subsequently mitigate the possible surface viability and transmissibility of respiratory virus.

Surface-Grafted Poly(ionic liquid) that Lubricates in Both Non-polar and Polar Solvents.

We show that a surface-grafted polymer brush, 1-n-butyl-3-vinyl imidazolium bromide-based poly(ionic liquids), is able to reduce the interfacial friction by up to 66% and 42% in dodecane and water, respectively. AFM-based force spectroscopy reveals that the polymer brush adopts distinctively different interfacial conformations: swollen in water but collapsed in dodecane. Minimal surface adhesion was observed with both polymer conformations, which can be attributed to steric repulsion as the result of a swollen conformation in water or surface solvation when the hydrophobic fraction of the polymer was exposed to the dodecane. The work brings additional insight on the polymer lubrication mechanism, which expands the possible design of the polymer architecture for interfacial lubrication and modification.

Molecular Understanding of Fouling Induction and Removal: Effect of the Interface Temperature on Milk Deposits.

Molecular details concerning the induction phase of milk fouling on stainless steel at an elevated temperature range were established to better understand the effect of temperature on surface fouling during pasteurization. The liquid-solid interface that replicates an industrial heat exchanger (≤75°C), including four stages (preheating, heating, holding, and cooling), was investigated using both a quartz crystal microbalance (QCM-D) and a customized flow cell. We found that the milk fouling induction process is rate-limited by the synergistic effects of bulk reactions, mass transfer, and surface reactions, all of which are controlled by both liquid and surface temperatures. Surface milk foulant becomes more rigid and compact as it builds up. The presence of protein aggregates in the bulk fluid leads to a fast formation of surface deposit with a reduced Young's modulus. Foulant adhesion and cohesion strength was enhanced as both interfacial temperature and processing time increased, while removal force increased with an increasing deposit thickness. During cleaning, caustic swelling and removal showed semilinear correlations with surface temperature (TS), where higher TS reduced swelling and enhanced removal. Our findings evidence that adsorption kinetics, characteristics of the foulant, and the subsequent removal mechanism are greatly dependent on the temperature profile, of which the surface temperature is the most critical one.

Lateral diffusion of single poly(ethylene oxide) chains on the surfaces of glassy and molten polymer films.

Fluorescence correlation spectroscopy was used to show that the temperature-dependent diffusion coefficient of poly(ethylene oxide) (PEO) adsorbed on polystyrene and different poly(alkyl methacrylate) (PAMA) films in aqueous solution exhibited a maximum close to (but below) the surface glass transition temperature, Tgs, of the film. This elevated diffusion was observed over a small range of temperatures below Tgs for these surfaces, and at other temperatures, the diffusion was similar to that on silicon, although the diffusion coefficient for PEO on polystyrene at temperatures above Tgs did not completely decrease to that on silicon, in contrast to the PAMA surfaces. It is concluded that the enhanced surface mobility of the films near the surface glass transition temperature induces conformational changes in the adsorbed PEO. The origin of this narrow and dramatic increase in diffusion coefficient is not clear, but it is proposed that it is caused by a coupling of a dominant capillary mode in the liquid surface layer with the polymer. Friction force microscopy experiments also demonstrate an unexpected increase in friction at the same temperature as the increase in diffusion coefficient.

Probing fibronectin adsorption on chemically defined surfaces by means of single molecule force microscopy.

Atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) and a quartz crystal microbalance (QCM) were respectively employed to probe interfacial characteristics of fibronectin fragment FNIII8-14 and full-length fibronectin (FN) on CH3-, OH-, COOH-, and NH2-terminated alkane-thiol self-assembled monolayers (SAMs). Force-distance curves acquired between hexahistidine-tagged FNIII8-14 immobilised on trisNTA-Ni2+ functionalized AFM cantilevers and the OH and COOH SAM surfaces were predominantly 'loop-like' (76% and 94% respectively), suggesting domain unfolding and preference for 'end-on' oriented binding, while those generated with NH2 and CH3 SAMs were largely 'mixed type' (81% and 86%, respectively) commensurate with unravelling and desorption, and 'side-on' binding. Time-dependent binding of FN to SAM-coated QCM crystals occurred in at least two phases: initial rapid coverage over the first 5 min; and variably diminishing adsorption thereafter (5-70 min). Loading profiles and the final hydrated surface concentrations reached (~ 950, ~ 1200, ~ 1400, ~ 1500 ng cm-2 for CH3, OH, COOH and NH2 SAMs) were consistent with: space-filling 'side-on' orientation and unfolding on CH3 SAM; greater numbers of FN molecules arranged 'end-on' on OH and especially COOH SAMs; and initial 'side-on' contact, followed by either (1) gradual tilting to a space-saving 'end-on' configuration, or (2) bi-/multi-layer adsorption on NH2 SAM.

Tribocorrosion behaviour of pure titanium in bovine serum albumin solution: A multiscale study.

Tribocorrosion behaviour of pure titanium in phosphate buffer saline (PBS) solution has been investigated systematically as a function of surface chemistry and bovine serum albumin (BSA) content in the solution. A ball-on-disk tribometer coupled with an electrochemical cell was used to study the effect of electrochemical conditions (i.e. anodic and cathodic applied potentials, as well as at open circuit potential) on the tribocorrosion response of titanium. It was found that the main material loss is due to mechanical wear caused by plastic deformation. The mechanical wear was higher under anodic conditions than under cathodic, partially due to an increased presence of debris particles at the sliding interface that act as third bodies. The effect of BSA on the interaction between alumina and titanium, as well as the behaviour of third bodies during the mechanical wear, were investigated in the nanoscale level using atomic force microscopy based force spectroscopy. It was found that the presence of BSA affects tribocorrosion in various ways. Firstly, it increases the repassivation rate of the oxide film by inhibiting the cathodic reactions and accelerating the anodic reactions. Secondly, it increases the mechanical wear by increasing the adhesion of debris onto the sliding interface, while at anodic conditions it increases the rolling efficiency of the debris particles that further enhances the mechanical wear.

Graphene oxide substrates with N-cadherin stimulates neuronal growth and intracellular transport.

Intracellular transport is fundamental for neuronal function and development and is dependent on the formation of stable actin filaments. N-cadherin, a cell-cell adhesion protein, is actively involved in neuronal growth and actin cytoskeleton organization. Various groups have explored how neurons behaved on substrates engineered to present N-cadherin; however, few efforts have been made to examine how these surfaces modulate neuronal intracellular transport. To address this issue, we assembled a substrate to which recombinant N-cadherin molecules are physiosorbed using graphene oxide (GO) or reduced graphene oxide (rGO). N-cadherin physisorbed on GO and rGO led to a substantial enhancement of intracellular mass transport along neurites relative to N-cadherin on glass, due to increased neuronal adhesion, neurite extensions, dendritic arborization and glial cell adhesion. This study will be broadly useful for recreating active neural tissues in vitro and for improving our understanding of the development, homeostasis, and physiology of neurons. STATEMENT OF SIGNIFICANCE: Intracellular transport of proteins and chemical cues is extremely important for culturing neurons in vitro, as they replenish materials within and facilitate communication between neurons. Various studies have shown that intracellular transport is dependent on the formation of stable actin filaments. However, the extent to which cadherin-mediated cell-cell adhesion modulates intracellular transport is not heavily explored. In this study, N-cadherin was adsorbed onto graphene oxide-based substrates to understand the role of cadherin at a molecular level and the intracellular transport within cells was examined using spatial light interference microscopy. As such, the results of this study will serve to better understand and harness the role of cell-cell adhesion in neuron development and regeneration.

Effect of the electrochemical characteristics of titanium on the adsorption kinetics of albumin.

An electrochemical quartz crystal microbalance (EQCM) was used to examine the electrochemical behaviour of pure titanium in phosphate buffered saline (PBS) and PBS-containing bovine serum albumin (BSA) solutions, and the associated adsorption characteristics of BSA under cathodic and anodic applied potentials. It was found that the electrochemical behaviours of bulk titanium substrate and titanium-coated QCM sensors are slightly different in PBS buffer solution, which is attributed to the difference in their surface roughness. The oxide film formed on the surface of the QCM sensor during potentiostatic tests was found to affect its electrochemical behaviour, while cathodic cleaning is not sufficient to have it removed. Lastly, the excessive amount of electrons on the titanium surface upon application of a cathodic potential could result in the desorption of BSA due to electrostatic repulsion and protein dehydration. In contrast, application of anodic potential charges the titanium surface positively and can facilitate protein adsorption when the surface is not saturated with protein.

The Counterion Effect of Imidazolium-Type Poly(ionic liquid) Brushes on Carbon Dioxide Adsorption.

Imidazolium-based poly(ionic liquid) brushes were attached to spherical silica nanoparticles bearing various functionalities by using a surface-initiated atom transfer radical polymerization ("grafting from" technique). A temperature-programmed desorption process was applied to evaluate and analyze the carbon dioxide adsorption performance of the synthesized polymer brushes. The confined structure of the surface-attached polymer chains facilitates gas transport and adsorption, leading to an enhanced adsorption capacity of carbon dioxide molecules compared with pure polymer powders. Temperature-programmed desorption profiles of the synthesized polymer brushes after carbon dioxide adsorption reveal that the substituent groups on the nitrogen atom at the 3-position of the imidazole ring, as well as the associated anions significantly affect the adsorption capacity of functionalized poly(ionic liquid) brushes. Of the tested samples, amine-functionalized poly(ionic liquid) brushes associated with hexafluorophosphate ions exhibit the highest carbon dioxide adsorption capacity of 2.56 mmol g-1 (112.64 mg g-1 ) at 25 °C under a carbon dioxide partial pressure of 0.2 bar.

Adsorption of Fibronectin Fragment on Surfaces Using Fully Atomistic Molecular Dynamics Simulations.

The effect of surface chemistry on the adsorption characteristics of a fibronectin fragment (FNIII8⁻10) was investigated using fully atomistic molecular dynamics simulations. Model surfaces were constructed to replicate self-assembled monolayers terminated with methyl, hydroxyl, amine, and carboxyl moieties. It was found that adsorption of FNIII8⁻10 on charged surfaces is rapid, specific, and driven by electrostatic interactions, and that the anchoring residues are either polar uncharged or of opposing charge to that of the targeted surfaces. On charged surfaces the presence of a strongly bound layer of water molecules and ions hinders FNIII8⁻10 adsorption. In contrast, adsorption kinetics on uncharged surfaces are slow and non-specific, as they are driven by van der Waals interactions, and the anchoring residues are polar uncharged. Due to existence of a positively charged area around its cell-binding region, FNIII8⁻10 is available for subsequent cell binding when adsorbed on a positively charged surface, but not when adsorbed on a negatively charged surface. On uncharged surfaces, the availability of the fibronectin fragment's cell-binding region is not clearly distinguished because adsorption is much less specific.

Blob Size Controls Diffusion of Free Polymer in a Chemically Identical Brush in Semidilute Solution.

The diffusion of rhodamine-labeled poly(ethylene glycol) (r-PEG) within surface-grafted poly(ethylene glycol) (s-PEG) layers in aqueous solution at 18 °C was measured by fluorescence correlation spectroscopy. The diffusion coefficient of r-PEG within s-PEG was controlled by the grafting density, σ, and scaled as σ-1.42±0.09. It is proposed that a characteristic blob size associated with the grafted (brush) layer defines the region through which the r-PEG diffusion occurs. The diffusion coefficients for r-PEG in semidilute solution were found to be similar to those in the brushes.

Influence of Cobalt Ions on Collagen Gel Formation and Their Interaction with Osteoblasts.

Metals on metal implants have long been used in arthroplasties because of their robustness and low dislocation rate. Several relatively low-corrosion metals have been used in arthroplasty, including 316L stainless steel, titanium, and cobalt-chromium-molybdenum alloy. Debris from these implants, however, has been found to cause inflammatory responses leading to unexpected failure rates approaching 10% 7 years surgery. Safety assessment of these materials traditionally relies on the use of simple two-dimensional assays, where cells are grown on the surface of the material over a relatively short time frame. It is now well-known that the composition and stiffness of the extracellular matrix (ECM) have a critical effect on cell function. In this work, we have evaluated how cobalt ions influence the assembly of type I collagen, the principle component of the ECM in bone. We found that cobalt had a significant effect on collagen matrix formation, and its presence results in local variations in collagen density. This increase in heterogeneity causes an increase in localized mechanical properties but a decrease in the bulk stiffness of the material. Moreover, when collagen matrices contained cobalt ions, there was a significant change in how the cells interacted with the collagen matrix. Fluorescence images and biological assays showed a decrease in cell proliferation and viability with an increase in cobalt concentration. We present evidence that the cobalt ion complex interacts with the hydroxyl group present in the carboxylic terminal of the collagen fibril, preventing crucial stabilizing bonds within collagen formation. This demonstrates that the currently accepted toxicity assays are poor predictors of the longer-term biological performance of a material.

Salt Dependence of the Tribological Properties of a Surface-Grafted Weak Polycation in Aqueous Solution.

The nanoscopic adhesive and frictional behaviour of end-grafted poly[2-(dimethyl amino)ethyl methacrylate] (PDMAEMA) films (brushes) in contact with gold- or PDMAEMA-coated atomic force microscope tips in potassium halide solutions with different concentrations up to 300 mM is a strong function of salt concentration. The conformation of the polymers in the brush layer is sensitive to salt concentration, which leads to large changes in adhesive forces and the contact mechanics at the tip-sample contact, with swollen brushes (which occur at low salt concentrations) yielding large areas of contact and friction-load plots that fit JKR behaviour, while collapsed brushes (which occur at high salt concentrations) yield sliding dominated by ploughing, with conformations in between fitting DMT mechanics. The relative effect of the different anions follows the Hofmeister series, with I- collapsing the brushes more than Br- and Cl- for the same salt concentration.

Effect of zinc oxide film morphologies on the formation of Shewanella putrefaciens biofilm.

Zinc oxide (ZnO) films were prepared on aluminum substrate by a hydrothermal method to investigate the effect of their surface characteristics, including morphology and hydrophobicity, on the corresponding antibiofilm performance. The surface characteristics of the prepared ZnO films were examined by a comprehensive range of methodologies, suggesting that films of distinctive surface morphologies were successfully formed. Subsequently, their antibiofilm activities, using Shewanella putrefaciens as a model bacterium, were assessed. Surface measurements confirmed that the ZnO films equipped with a nanoscopic needlelike surface feature are more hydrophobic than those possessing densely packed microflakes. The reduced number of live cells and presence of biofilm, confirmed by optical and electron microscopy results, suggest that the former films possess an excellent antibiofilm performance. It is believed that the engineered nanoscopic needle feature might penetrate the cell membrane when they are in contact, allowing the effective substance of ZnO antibacterial ingredients to diffuse into the embedded bacteria. Furthermore, such surface characteristics might perturb the integrity of the cell membrane causing the intracellular substance is leaked from the cells. As such, the combinatorial effects of nanoscopic feature resulted in an inhibited growth of S. putrefaciens biofilm on ZnO film.

Nanotribological Investigation of Polymer Brushes with Lithographically Defined and Systematically Varying Grafting Densities.

Following controlled photodeprotection of a 2-nitrophenylpropyloxycarbonyl-protected (aminopropyl)triethoxysilane (NPPOC-APTES) film and subsequent derivatization with a bromoester-based initiator, poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes with various grafting densities were grown from planar silicon substrates using atom transfer radical polymerization (ATRP). The grafting density correlated closely with the extent of deprotection of the NPPOC-APTES. The coefficient of friction for such PMPC brushes was measured by friction force microscopy in water and found to be inversely proportional to the grafting density due to the osmotic pressure that resists deformation. Deprotection of NPPOC-APTES via near-field photolithography using a range of writing rates enabled the fabrication of neighboring nanoscopic polymeric structures with dimensions ranging from 100 to 1000 nm. Slow writing rates enable complete deprotection to occur; hence, polymer brushes are formed with comparable thicknesses to macroscopic brushes grown under the same conditions. However, the extent of deprotection is reduced at higher writing rates, resulting in the concomitant reduction of the brush thickness. The coefficient of friction for such polymer brushes varied smoothly with brush height, with lower coefficients being obtained at slower writing rate (increasing initiator density) because the solvated brush layer confers greater lubricity. However, when ultrasharp probes were used for nanotribological measurements, the coefficient of friction increased with brush thickness. Under such conditions, the radius of curvature of the tip is comparable to the mean spacing between brush chains, allowing the probe to penetrate the brush layer leading to a relatively large contact area.

Experimental and computational examination of anastellin (FnIII1c)-polymer interactions.

Using a combination of experimental and computational approaches, the interaction between anastellin, a recombinant fragment of fibronectin, and representative biomaterial surfaces has been examined. Anastellin and superfibronectin have been seen to exhibit antiangiogenic properties and other properties that may make it suitable for consideration for incorporation into biomaterials. The molecular interaction was directly quantified by atomic force microscope (AFM)-based force spectroscopy, complemented by adsorption measurements using quartz crystal microbalance (QCM). Using AFM, it was found that the anastellin molecule facilitates a stronger adhesion on polyurethane films (72.0 pN nm-1 ) than on poly (methyl methacrylate) films (68.6 pN nm-1 ). However, this is not consistent with the QCM adsorption measurements, which show no significant difference. Molecular dynamics simulations of the behavior of anastellin on polyurethane in aqueous solution were performed to rationalize the experimental data, and show that anastellin is capable of rapid adsorption to PU while its secondary structure is stable upon adsorption in water. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 737-745, 2017.

Quantifying mineral surface energy by scanning force microscopy.

Fundamental understanding of the wettability of carbonate formations can potentially be applied to the development of oil recovery strategies in a complex carbonate reservoir. In the present study, surface energies of representative carbonate samples were evaluated by direct quantitative force measurements, using scanning force microscopy (SFM) at sub-micron scale, to develop a reliable method to predict reservoir wettability. Local adhesion force measurements were conducted on appropriate calcite and dolomite samples and performed in air as well as in the presence of polar and nonpolar fluids. This study demonstrated that, by comparing measurements of adhesion forces between samples of the same mineral in different fluids, it is feasible to determine the surface energy of a given mineral as well as its polar and nonpolar components. The derived values are in agreement with literature. A proof-of-principle protocol has been established to quantify surface energy using SFM-based adhesion measurements. This novel methodology complements the conventional contact angle measurement technique, where surface energy can only be examined at large length scale. The reported methodology has great potential for further optimization into a new standard method for fast and accurate surface energy determination, and hence provides a new tool for reservoir rock wettability characterization.

Effect of extracellular polymeric substances on the mechanical properties of Rhodococcus.

The mechanical properties of Rhodococcus RC291 were measured using force spectroscopy equipped with a bacterial cell probe. Rhodococcal cells in the late growth stage of development were found to have greater adhesion to a silicon oxide surface than those in the early growth stage. This is because there are more extracellular polymeric substances (EPS) that contain nonspecific binding sites available on the cells of late growth stage. It is found that EPS in the late exponential phase are less densely bound but consist of chains able to extend further into their local environment, while the denser EPS at the late stationary phase act more to sheath the cell. Contraction and extension of the EPS could change the density of the binding sites, and therefore affect the magnitude of the adhesion force between the EPS and the silicon oxide surface. By treating rhodococcal EPS as a surface-grafted polyelectrolyte layer and using scaling theory, the interaction between EPS and a solid substrate was modelled for the cell approaching the surface which revealed that EPS possess a large capacity to store charge. Changing the pH of the surrounding medium acts to change the conformation of EPS chains.