Lifshitz-van Der Waals Forces Research Articles

Analysis of the Wettability of Partially Fluorinated Polymers Reveals the Surprisingly Strong Acid-Base Character of Poly(vinylidene Fluoride)

The wettabilities of the partially fluorinated polymers (ethylene-tetrafluoroethylene copolymer (ETFE), ethylene- chlorotrifluoroethylene copolymer (ECTFE), and poly(vinylidene fluoride) (PVDF)) were investigated by contact angle measurements. Zisman plots for ETFE and ECTFE exhibited linear relationships, while the Zisman plot for PVDF show- ed a slight curvature, which was interpreted to indicate strong non-dispersive interactions between the surface and the contacting liquids. The Lifshitz-van der Waals forces of the fluoropolymers were estimated to increase in the order of ETFE < PVDF << ECTFE. An evaluation of the polar or interaction energies showed that PVDF, which possesses the most acidic hydrogens among the examined fluoropolymers, has the strongest acid-base interactions.

Microbial Biofouling: A Mechanistic Investigation

Microbial biofouling is important in a variety of applications including reverse osmosis membranes and in-situ bioremediation. The initial microbial adhesion plays the key role in microbial biofouling on abiotic surfaces, which can be explained in terms of microbial surface properties. This research investigated the interactions of three indigenous bacteria with the porous medium of silica sand and their corresponding initial attachment. Traditional and extended DLVO forces were calculated based on independently measured bacterial and medium surface thermodynamic properties and were related to bacterial attachment observations. Lifshitz–van der Waals, electrostatic and Lewis acid/base forces were found to be strongly dependent on the separation distance between bacteria and the medium surface. It was concluded that the electrostatic force served as the barrier preventing the bacterial strains from getting close to the medium. Once the bacterial strains overcame the barrier with the aid of hydrodynamic forces, Lifshitz–van der Waals force and electrostatic force dropped while Lewis acid/base force increased with the decrease of separation distance. Consequently, Lewis acid/base force became the dominating force controlling bacterial adhesion.

Variations of the Lifshitz–van der Waals force between metals immersed in liquids

We present a theoretical calculation of the Lifshitz-van der Waals force between two metallic slabs embedded in a fluid, taking into account the change of the Drude parameters of the metals when in contact with liquids of different index of refraction. For the three liquids considered in this work, water, CCl(3)F and CBr(3)F the change in the Drude parameters of the metal imply a difference of up to 15% in the determination of the force at short separations. These variations in the force are larger for liquids with a higher index of refraction.

Poisson Analysis of Streptococcal Bond Strengthening on Stainless Steel with and without a Salivary Conditioning Film

Poisson analysis of retract force-distance curves in atomic force microscopy (AFM) has yielded a new dimension to the decoupling of individual bond forces into a hydrogen bonding and nonspecific force component. Accordingly, bacterial adhesion forces have been decoupled into a hydrogen bonding and nonspecific Lifshitz-Van der Waals contribution. Due to the forced nature of AFM contact, the nonspecific force contribution has hitherto turned out to be repulsive in the analysis of bacterial adhesion forces on nonconducting surfaces. In this study, we present the results of a Poisson analysis of adhesion forces for streptococci adhering to a conducting surface. Adhesion forces measured between stainless steel, both in the absence and presence of an adsorbed salivary conditioning film, increased with increasing contact time between the streptococcal AFM probe and the surface. Concurrent with the increase in adhesion force, there was an increase in the number of minor force peaks in the retract force-distance curves. Poisson analyses of the adhesion forces indicated repulsive nonspecific Lifshitz-Van der Waals forces for streptococci adhering to saliva-coated stainless steel, but interestingly and for the first time, attractive nonspecific forces were revealed on stainless steel in the absence of a salivary conditioning film. We tentatively attribute this to attraction between the negatively charged streptococci and their positive image charges in the conducting material, which cannot develop in a nonconducting material or in the presence of a nonconductive protein layer on the stainless steel surface.

Role of Interfacial Interactions in the Deposition of Colloidal Clay Particles in Porous Media

Colloidal clay particle transport under saturated conditions is believed to be controlled by its interactions with the surrounding environment. The dominating forces among these interactions are electrostatic forces that are determined by colloidal clay particle and porous medium surface charge density and Lifshitz–van der Waals forces that are determined by colloidal clay particle and porous medium surface thermodynamic properties. Electrostatic forces are greatly affected by solution chemistry in terms of solution ionic strength and pH. In this research, electrostatic and Lifshitz–van der Waals forces of natural colloidal clay particles with a model porous medium of silica sand were quantified at different ionic strength and pH conditions. At the same time, colloidal clay particle transport in the model medium of silica sand was conducted in a laboratory column. The maximum electrostatic forces, F EL (max), which occurred when the separation distance between colloidal clay particles and the porous medium was in the range of the sum of the double layer thicknesses of the colloidal clay particles and the porous medium, was found to be the determinant factor for colloidal clay particle deposition in the porous medium. Colloidal clay particle desorption in the porous media was related to the net effect of attractive Lifshitz–van der Waals forces and repulsive electrostatic forces, evaluated at the equilibrium distance where physical contact between the colloidal clay particle and silica sand actually occurred (i.e., affix force). Higher colloidal clay particle desorption was found to coincide with smaller affix force values.

Interaction forces between waterborne bacteria and activated carbon particles

Activated carbons remove waterborne bacteria from potable water systems through attractive Lifshitz–van der Waals forces despite electrostatic repulsion between negatively charged cells and carbon surfaces. In this paper we quantify the interaction forces between bacteria with negatively and positively charged, mesoporous wood-based carbons, as well as with a microporous coconut carbon. To this end, we glued carbon particles to the cantilever of an atomic force microscope and measured the interaction forces upon approach and retraction of thus made tips. Waterborne Raoultella terrigena and Escherichia coli adhered weakly (1–2 nN) to different activated carbon particles, and the main difference between the activated carbons was the percentage of curves with attractive sites revealed upon traversing of a carbon particle through the bacterial EPS layer. The percentage of curves showing adhesion forces upon retraction varied between 21% and 69%, and was highest for R. terrigena with positively charged carbon (66%) and a coconut carbon (69%). Macroscopic bacterial removal by the mesoporous carbon particles increased with increasing percentages of attractive sites revealed upon traversing a carbon particle through the outer bacterial surface layer.

Increased adhesion of Enterococcus faecalis strains with bimodal electrophoretic mobility distributions

Initial adhesion is a determinant in the development of microbial biofilms. It is influenced, amongst others, by the surface hydrophobicity and the electrostatic characteristics of the substratum and adhering organisms. Enterococcus faecalis strains, grown in pure cultures, generally display subpopulations with different electrokinetic features, reflected in a bimodal electrophoretic mobility distribution. Here, the initial adhesion kinetics of five heterogeneous and five homogeneous E. faecalis strains were followed in a parallel-plate flow chamber. After 4 h of flow, heterogeneous strains adhered in significantly higher numbers than homogeneous strains (7.3 × 10 6 and 1.9 × 10 6 cm −2, respectively), but the initial deposition rates were not significantly influenced (740 and 600 cm −2 s −1, respectively). Apparently, initial deposition of bacteria is mainly governed by attractive Lifshitz–Van der Waals forces that overwhelm the electrostatic repulsion energy barrier, thus resulting in similar initial deposition rates for the various bacterial populations investigated. In contrast, during later stages of adhesion, bacteria in heterogeneous cultures likely experience a lower electrostatic repulsion from already adhering bacteria than bacteria in homogeneous cultures, thus allowing a closer proximity of the bacteria with respect to each other, which ultimately leads to increased adhesion after 4 h.

Numerical calculation of the drag force applied to particle pushing

The interaction of solid particles with a solidifying interface is studied with numerical and analytical models for the drag and pushing forces. The numerical results of the drag force on a spherical particle and planar interface were compared with the forces given by both, the Stokes and modified Stokes equations. The pushing force was calculated employing the Casimir–Lifshitz–van der Waals force. The results show the range and degree of validity of the Stokes and modified Stokes equations for large and small separations. From this, it is shown that the particle–interface separations for steady pushing obtained numerically, for the same particle radius and solidification velocity. In addition, the critical velocities obtained with the present hybrid model are compared with experimental results published in the literature.

Surface properties of bamboo fiber and a comparison with cotton linter fibers

In order to understand the different touch senses from bamboo and cotton fibers, the surface properties of bamboo fiber, e.g., the surface free energy, the Lifshitz–van der Waals force, and Lewis acid and base components have been determined using the column wicking technique. Taking the traditional cotton linter fiber as a reference, this paper shows that both these fibers have the similar surface free energies. However, a big difference between these two fibers which has been importantly observed is that the bamboo fiber having greater Lewis acid component seems to be double than the cotton linter fiber. Since water has been found to have such surface property, it is suggested that the bamboo fiber touch in the skin of people may like the touch between water and skin of people. Meanwhile, other surface properties of these two fibers are compared.

Role of chemical interactions in bacterial adhesion to polymer surfaces

Development of biomaterial-related infections is attracting an increasing interest due to the significant percentage of implant failure in the hospital care. Recent literature puts in evidence the dependence of the infection risk on the different biomaterials used, because of the different interactions between material surface and micro-organisms. Despite this, the mechanisms underlying the adhesion of bacteria to the biomaterial surface are still unclear. Aim of this work is to study the initial events of the processes responsible for the bacterial adhesion on polymers in order to prevent the development of bacterial infections and the consequent failure and replacement of biomedical devices. Electrostatic and Lifshitz–van der Waals forces are usually considered responsible for the interactions at the biomaterial interface. A new term that involves Lewis acid–base interactions is here introduced to better describe the bacterial adhesion to the polymer surface. Two requirements are needed to test this hypothesis: the development of an ideal polymeric surface in terms of chemical and morphological properties and the choice of a specific bacterial strain to be utilized as “probe”. Experiments were worked out using an Escherichia coli (Gram−) strain that represent one of the principal isolates from infected biomaterial implants and its adhesion was investigated on polymers having different acid/basic character. The findings indicate that the bacterial adhesion is influenced by the chemical properties of the polymeric surface. These results may be interpreted taking into account a mechanism in which the acid/base (Lewis) interaction plays an important role.

Direct adhesion measurements of pharmaceutical particles to gelatin capsule surfaces

Scanning probe microscopy (SPM) was used to measure directly the adhesion of individual lactose particles to the surface of gelatin capsules employed in dry powder inhalant drug delivery systems. In this study, SPM shows that gelatin capsule surfaces with high surface heterogeneity and high-contrast friction exhibit high adhesion and that gelatin capsule surfaces with low surface heterogeneity and low-contrast friction exhibit low adhesion. The adhesion of lactose particles to gelatin capsules was also determined by measuring the retention of lactose particles in the capsules. The adhesion trend obtained with individual lactose particles using the colloidal probe technique agrees with the macroscopic retention results. The adhesion appears to be proportional to the particle size for homogeneous capsule surfaces. In dry powder inhalation products, the Lifshitz-van der Waals forces and acid-base interactions appear to be the principal forces contributing to particle-surface adhesion. The physicochemical nature of the capsule surface seems to dictate the spatial variation of adhesion across the surface. The SPM results clearly show that the surface physicochemical properties depend on the gelatin and mold release agent utilized in the manufacture of gelatin capsules. One of the practical implications of this study is that extraneous surface contamination of gelatin capsules by chemical processing aids such as mold release agents appears to be a key factor affecting the respirable fraction in dry powder inhalation products.

Surface Aggregation ofCandida albicanson Glass in the Absence and Presence of AdheringStreptococcus gordoniiin a Parallel-Plate Flow Chamber: A Surface Thermodynamical Analysis Based on Acid–Base Interactions

Adhesive interactions between yeasts and bacteria are important in the maintenance of infectious mixed biofilms on natural and biomaterial surfaces in the human body. In this study, the extended DLVO (Derjaguin–Landau–Verwey–Overbeek) approach has been applied to explain adhesive interactions betweenC. albicansATCC 10261 andS. gordoniiNCTC 7869 adhering on glass. Contact angles with different liquids and the ζ potentials of both the yeasts and bacteria were determined and their adhesive interactions were measured in a parallel-plate flow chamber.Streptococci were first allowed to adhere to the bottom glass plate of the flow chamber to different seeding densities, and subsequently deposition of yeasts was monitored with an image analysis system, yielding the degree of initial surface aggregation of the adhering yeasts and their spatial arrangement in a stationary end point. Irrespective of growth temperature, the yeast cells appeared uncharged in TNMC buffer, but yeasts grown at 37°C were intrinsically more hydrophilic and had an increased electron-donating character than cells grown at 30°C. All yeasts showed surface aggregation due to attractive Lifshitz–van der Waals forces. In addition, acid–base interactions between yeasts, yeasts and the glass substratum, and yeasts and the streptococci were attractive for yeasts grown at 30°C, but yeasts grown at 37°C only had favorable acid–base interactions with the bacteria, explaining the positive relationship between the surface coverage of the glass by streptococci and the surface aggregation of the yeasts.

Dynamics and Morphology of Holes in Dewetting of Thin Films

The formation and growth of holes in apolar nonslipping Newtonian thin films subjected to long-range Lifshitz–van der Waals forces are investigated based directly on numerical solutions of the thin-film equation. The nonphysical divergences of the hydrodynamic model and of the van der Waals force at the three-phase contact line are removed by inclusion of the short-range Born repulsion. Three distinct regimes of the hole growth after its appearance are identified: (1) a short, unsteady phase in which the dynamic contact angle and velocity change rapidly, followed by (2) a long, quasi-steady phase with slow logarithmic changes, and finally (3) a hindered phase in which rims of the neighboring holes overlap and lead to formation of equilibrium drops. The cross section of the rim surrounding a growing hole is noncircular and asymmetric, with higher slopes near the contact line. A slight depression is created ahead of the moving rim, but the regions of the film away from the rim remain undisturbed. For very viscous (e.g., polymeric) liquids displaying small contact angles, a nonlinear regime of hole growth (radius ∝ timeq, with the exponentq≈ 0.7–0.9) is obtained for realistic time scales.

Measurement of residual stress effects by means of single-particle composite tests

The effects of thermal residual stresses on adhesion have been quantitatively examined by means of a single-particle composite technique. Residual stresses were varied systematically by controlling the molding time-temperature profile for single, surface-modified glass spheres embedded in a thermoplastic polymeric matrix. Interfacial failure was induced by tensile straining of single-particle composites until debonding at the geometric poles was observed between the matrix and spherical particle. The interfacial strength was calculated and is reported as a function of residual hoop stresses, the magnitude of the latter determined by the processing conditions, i.e. different residual stress states within the matrix. A preliminary study was conducted using two interfaces: an octylsilane/poly(vinyl butyral) interface and a di-aminosilane/poly(vinyl butyral) interface. The former is a weak bond with only Lifshitz-van der Waals forces operating across the interface, and the latter a strong bond featuring a molecular penetration mechanism. Both interfaces were deleteriously affected by increases in residual stresses, with the former having the highest level of residual stress with complete interfacial strength loss. Experimental requirements for a more quantitative study are described.

Influence of Relative Humidity of Storage Air on the Adhesion and Autoadhesion of Micronized Particles to Particulate and Compacted Powder Surfaces

The influence of the relative humidity of storage air on the median adhesion force at the interface of lactose monohydrate and salmeterol xinafoate has been investigated by the centrifuge technique using particle-on-particle and particle-on-surface experiments. The relative humidity of storage air can strongly affect the median adhesion force. Condensed moisture was found to result in strong irreversible solid bridge bonds or in weak solid bridge bonds that are of comparable strength to Lifshitz–van der Waals forces obtained under conditions of low relative humidity up to 35%. The effect was dependent on the structure of the substrate surface, the solubility of the particles adhered, and the solubility of the surface material. The use of different substrate surfaces such as particles and compacted powder disks of identical materials led to different results. Hence, if the influence of relative humidity of storage air on the adhesion between particles in, for example, an interactive mixture is to be predicted, the adhesion measurements have to be performed using particle-on-particle adhesion force measurements. Relative humidity of the air greater than 75% results in irreversible, strong forces acting at the interface of the particles. Below this humidity level, capillary forces can be removed by storing the mixtures at a very low relative humidity of about 5% for at least 72 h.

Components of the surface free energy of low rank coals in the presence of n-alkanes

Measurements of contact angles for water, glycerol, formamide and diiodomethane on the surfaces of four low rank coals (31.1, 31.2, 32.1 and 32.2) covered with n-alkanes (n-hexane, n-undecane and n-hexadecane) were made. Using the approaches of van Oss et al., the geometric mean of the interfacial free energy, and Young's equation, the components of the surface free energy of coals and coal precovered with n-alkane films were calculated. Also the total surface free energy was determined from the so-called equation of state. Using the values of the components of surface free energy or total free energy of the coal, the spreading of the n-alkanes at the coal/water interface and the free energy of interaction between coal particles in the water phase were determined. On the basis of these results we have stated that, on the coal surface in air, a very stable, n-alkane film, resulting from a wetting process by spreading is formed, which first of all reduces the apolar component of the surface free energy of the coal and can partially block the polar component. However, it was found that n-alkanes cannot completely spread over a coal surface immersed in water, and the contact angle depends on the type of n-alkane. It was also found that the presence of an n-alkane film on a coal surface decreased the attractive interactions between the coal particles in the water phase, owing to Lifshitz-van der Waals forces and increased interactions resulting from acid-base forces in comparison to ‘pure’ coal.

The Adsorption of Triton X-100 at the Air-Aqueous Solution Interface

Measurements of the surface tension of aqueous solutions of Triton X-100 (TX-100) were made at different temperatures. Isotherms and thermodynamic adsorption parameters were determined from the surface tensions. Also, the equation of state for the monolayer film of TX-100 at the water-air interface was analyzed. A linear dependence was found between log Γ and log C in the region of low concentration of TX-100 in solution, and three different methods used to calculate the free energy of adsorption gave slightly different results. In the region of low concentration the Π(A - A 0 ) values were equal to 1kT. At high concentration, there were attractive Lifshitz-van der Waals forces between the apolar parts of the TX-100 molecules and repulsive acid-base forces between the heads of the TX-100 molecules in the adsorbed monolayer film.

Stability of Monodisperse Zinc Sulfide Colloidal Dispersions

The stability of monodisperse, spherical colloidal particles of zinc sulfide, in the presence of NaCl and CaCl 2 solutions, has been studied in this work. The so-called extended DLVO theory of stability is used to explain the data. In this model, it is proposed that Lewis acid-base (AB) interactions have to be considered for better explaining the stability of ZnS colloidal dispersions. Theoretical interaction energy-distance curves are computed and compared to experimental determinations of the stability of the suspensions, obtained from time evolution of both their optical absorbance and particle diameter. Previously, the ζ potential of the particles and their surface free-energy components were determined as a function ofelectrolyte concentration, using, respectively, electrophoretic mobility measurements and the thin-layer wicking method. The effect of NaCl concentration on the ζ potential of the particles is typical of indifferent electrolytes, whereas Ca 2+ cations appear to specifically interact with the ZnS surface. The stability of the suspensions is lowest for concentrations around 10 -2 M, whereas higher concentrations seem to stabilize the suspensions. After calculation of the surface free-energy components of the particles, potential energy of interaction curves are computed for different interparticle distances. A comparison is carried out between the predictions of both classical and extended DLVO models and experimental stability data. A good qualitative agreement between theoretical and experimental results is found when the latter model is used. The inclusion of (Lewis) acid-base interactions between the particles is thus a useful tool to adequately describe the stability of ZnS suspensions. The results support the previous findings (van Oss, C. J. ; et al. Clays Clay Min. 1990, 38, 151) on the suitability of adding acid-base (Lewis) forces to electrostatic and Lifshitz-van der Waals forces to have a powerful theory capable of predicting many aspects of the behavior of colloidal suspensions.

Trajectory Modeling of Non-Brownian Particle Flotation Using an Extended Derjaguin-Landau-Verwey-Overbeek Approach

Pressure flotation of nitrocellulose (NC) fines was quantified by tralectory analysis. A model was formulated that incorporates recently developed extended DLVO (ExDLVO) theory and the short-range hydrodynamic interactions of a particle near a bubble surface. A repulsive Lifshitz-van der Waals (LW) force was found to exist between particle and bubble. Attractive long-range hydrophobic forces between NC particles and bubbles were responsible for particle adhesion. Model predictions of flotation efficiency compared well with experimental results

Hydrophobic, hydrophilic and other interactions in epitope-paratope binding

Macroscopic, non-covalent, aspecific interactions between hydrophilic biopolymers, particles and cells in aqueous media tend to be repulsive; they are caused by Lifshitz-van der Waals (LW), Lewis acid-base (AB) and electrostatic (EL) forces. Microscopic scale specific interactions, e.g. between epitopes and paratopes, are also non-covalent and caused by attractive LW, AB and EL forces, which locally must be able to overcome the long- to medium-range macroscopic aspecific repulsive forces. Thus epitopes and paratopes need to be able to attract each other over a distance of at least 3 nm. The medium- and long-range specific attractive forces are mainly of hydrophobic (AB) and of EL origin; in aqueous media the medium- and long-range LW attractions are usually much weaker. It has been shown that hydrophobic (AB) interactions are as often enthalpic as entropic. Upon expulsion of interstitial water of hydration between epitope and paratope, a strong interfacial bond ultimately arises which is mainly caused by LW forces.