PubMed Health⌕ Search

Biomedical subjects

J Israelachvili

Publications and source records attributed to J Israelachvili.

At least 19 recordsLinked to original sources
PubMed2003

Friction between two weakly adhering boundary lubricated surfaces in water.

The sliding of adhesive surfactant-bearing surfaces was studied with a surface forces apparatus nanotribometer. When the surfaces are fully immersed in an aqueous solution, the dynamic behavior is drastically different and more varied than under dry conditions. In solution, the shear stress exhibits at least five different velocity regimes. In particular, the sliding may proceed by an "inverted" stick-slip over a large range of driving velocities, this regime being bounded by smooth (kinetic) sliding at both lower and higher driving velocities. The general behavior of the system was studied in detail, i.e., over a large range of experimental conditions, and theoretically accounted for in terms of a general model based on the kinetics of formation and rupture of adhesive links (bonds) between the two shearing surfaces with an additional viscous term.

Journal Article↗
PubMed2001

Dynamic phase transitions in confined lubricant fluids under shear.

A surface force apparatus was used to measure the transient and steady-state friction forces between molecularly smooth mica surfaces confining thin films of squalane, C30H62, a saturated, branched hydrocarbon liquid. The dynamic friction "phase diagram" was determined under different shearing conditions, especially the transitions between stick-slip and smooth sliding "states" that exhibited a chaotic stick-slip regime. The apparently very different friction traces exhibited by simple spherical, linear, and branched hydrocarbon films under shear are shown to be due to the much longer relaxation times and characteristic length scales associated with transitions from rest to steady-state sliding, and vice versa, in the case of branched liquids. The physical reasons and tribological implications for the different types of transitions observed with spherical, linear, and branched fluids are discussed.

Journal Article↗
PubMed2001

Direct observation of shear-induced orientational phase coexistence in a lyotropic system using a modified x-ray surface forces apparatus.

The second generation x-ray surface forces apparatus (XSFA-II) allows for the first time simultaneous in situ small-angle x-ray scattering and surface force measurements. We have used the XSFA-II to monitor shear-induced orientational transitions in a lyotropic model lubricant system. Upon applying small shear amplitudes (approximately 20 micrometer) to a relatively thick (approximately 800 micrometer) film, we observed evidence for the formation of an orientational boundary layer at the shearing surface. Time-resolved x-ray diffraction revealed the gradual transition to shear-favored orientation by growth of the boundary layer.

Journal Article↗
PubMed1999

Polymer-cushioned bilayers. II. An investigation of interaction forces and fusion using the surface forces apparatus.

We have created phospholipid bilayers supported on soft polymer "cushions" which act as deformable substrates (see accompanying paper, Wong, J. Y., J. Majewski, M. Seitz, C. K. Park, J. N. Israelachvili, and G. S. Smith. 1999. Biophys. J. 77:1445-1457). In contrast to "solid-supported" membranes, such "soft-supported" membranes can exhibit more natural (higher) fluidity. Our bilayer system was constructed by adsorption of small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles onto polyethylenimine (PEI)-supported Langmuir-Blodgett lipid monolayers on mica. We used the surface forces apparatus (SFA) to investigate the long-range forces, adhesion, and fusion of two DMPC bilayers both above and below their main transition temperature (T(m) approximately 24 degrees C). Above T(m), hemi-fusion activation pressures of apposing bilayers were considerably smaller than for solid-supported bilayers, e.g., directly supported on mica. After separation, the bilayers naturally re-formed after short healing times. Also, for the first time, complete fusion of two fluid (liquid crystalline) phospholipid bilayers was observed in the SFA. Below T(m) (gel state), very high pressures were needed for hemi-fusion and the healing process became very slow. The presence of the polymer cushion significantly alters the interaction potential, e.g., long-range forces as well as fusion pressures, when compared to solid-supported systems. These fluid model membranes should allow the future study of integral membrane proteins under more physiological conditions.

Aluminum Silicates↗
PubMed1996

Role of hydration and water structure in biological and colloidal interactions.

The conventional explanation of why hydrophilic surfaces and macromolecules remain well separated in water is that they experience a monotonically repulsive hydration force owing to structuring of water molecules at the surfaces. A consideration of recent experimental and theoretical results suggests an alternative interpretation in which hydration forces are either attractive or oscillatory, and where repulsions have a totally different origin. Further experiments are needed to distinguish between these possibilities.

Solvents↗
PubMed1994

Higher order self-assembly of vesicles by site-specific binding.

The association of lipid molecules into spherical vesicles in solution as a result of non-specific intermolecular forces constitutes a primary self-assembly process. Such vesicles can undergo a secondary self-assembly into higher order structures in a controlled and reversible manner by means of site-specific ligand-receptor (biotin-streptavidin) coupling. Cryoelectron microscopy shows these structures to be composed of tethered, rather than adhering, vesicles in their original, unstressed state. In contrast, vesicles aggregated by nonspecific, such as van der Waals, forces are deformed and stressed, producing unstable structures. Vesicle association by site-specific binding provides a practical mechanism for the production of stable, yet controllable, microstructured biomaterials.

Adhesiveness↗
PubMed1994

Direct visualization of cavitation and damage in ultrathin liquid films.

The Surface Forces Apparatus technique was used to observe, at the nanoscopic level, the rapid growth and disappearance of vapor cavities between two moving solid surfaces while simultaneously monitoring their effects on the deformations and damage produced on the surfaces. Both approach-separation and shear motions were studied. In both geometries, as the relative velocity between the two surfaces increases, they deform 'elastohydrodynamically'; in some regions the surfaces become flattened (corresponding to the build up of positive or compressive pressure in the adjacent liquid) while in other regions the surfaces become pointed (corresponding to negative or tensile pressure). Vapor cavities nucleate at these pointed regions once the relative velocity of the two surfaces exceeds a certain critical value. The sudden nucleation and growth of a cavity in a thin liquid film is seen to be a much more violent event than its subsequent collapse, and surface damage due to cavitation occurs during the inception rather than during the much smoother collapse of cavities. These studies are the first to directly visualize cavity formation and associated damage in ultrathin lubricating films, and provide a fairly detailed picture of both of these mechanisms and the relationship between them. The picture that emerges in complex and different from the currently accepted model, in which damage is caused during the collapse of cavities rather than their growth. The results should have a bearing on all phenomena where cavitation and cavitation damage occur in thin lubricating films.

Aluminum Silicates↗
PubMed1993

Role of calcium in the adhesion and fusion of bilayers.

The interaction forces and fusion mechanisms of mixed zwitterionic-anionic phospholipid bilayers were measured with the surface forces apparatus. The bilayers were 3:1 mixtures of either dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol (DMPC/DMPG) or dilauroylphosphatidylcholine and dilauroylphosphatidylglycerol (DLPC/DLPG), and experiments were carried out in NaCl solutions with and without CaCl2. In NaCl solutions, the forces between either mixed bilayer system were consistent with the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory of repulsive electrostatic and attractive van der Waals forces, and fusion did not occur. At high pH (> 6) and in high (20 mM) NaCl concentrations, a short-range hydration force extending about 13 A was evident, indicative of Na+ binding to the surfaces. In the presence of this large hydration repulsion, the interbilayer adhesion was abolished. When CaCl2 was added to the bathing solutions in the presence or absence of NaCl, the bilayers phase separate into small domains, coinciding with the occurrence of a large, long-range attractive force. Fusion occurred readily between the more fluid domains. The phase separations and fusion events could be directly visualized by observing the shapes of the optical fringes used to measure the surface separation and the change in surface profiles with time. The ease of fusion between mixed bilayers in the presence of calcium correlated closely with the strength of the long-range attractive force. This force is attributed to the additional hydrophobic force between domains or domain boundaries due to the exposure of excess hydrophobic groups resulting from the Ca(2+)-induced condensation of the PG- headgroups.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium↗
PubMed1985

Direct measurements of forces between phosphatidylcholine and phosphatidylethanolamine bilayers in aqueous electrolyte solutions.

We report direct measurements of the full interbilayer force laws (force vs. distance) between bilayers of various phosphatidylcholines and phosphatidylethanolamine in aqueous solutions. Bilayers were first deposited on molecularly smooth (mica) surfaces and the interbilayer forces then measured at a resolution of 1 A. Three types of forces were identified: attractive van der Waals forces, repulsive electrostatic (double-layer) forces, and (at short range) repulsive steric hydration forces. Double-layer forces, which arise from ion binding, were insignificant in monovalent salt solutions, e.g., NaCl up to 1 M, but were already present in solutions containing millimolar levels of CaCl2 and MgCl2, giving rise to forces in excellent agreement with theory. Ca2+ binds more strongly than Mg2+, and both bind less to lecithin bilayers in the fluid state (T greater than Tc). The plane of charge coincides with the location of the negative phosphate groups, while the effective plane of origin of the van der Waals force is 4-5 A farther out. In water, the adhesion energies are in the range 0.10-0.15 erg/cm2 for lecithins and approximately 0.8 erg/cm2 for phosphatidylethanolamine. The adhesion energies vary on addition of salt due to changes in the repulsive double-layer and hydration forces rather than to a change in the attractive van der Waals force. The short-range repulsive forces which balance the van der Waals force at separations of 10-30 A are due to a combination of hydration and steric repulsions, the latter arising from thermal motions of head groups and thickness fluctuations of fluid bilayers (above Tc). It is also concluded that bilayer fusion is not simply related to the interbilayer force law.

Calcium Chloride↗
PubMed1975

ESR spectral analysis of the molecular motion of spin labels in lipid bilayers and membranes based on a model in terms of two angular motional parameters and rotational correlation times.

Electron spin resonance (ESR) spectral line shapes are calculated for a nitroxide spin-labeled molecule undergoing rapid restricted rotations (twisting) about its long molecular axis while simultaneously tumbling within a cone. Explicit expressions are derived for the hyperfine splittings and g-values, as well as for the secular contributions to the motionally modulated linewidths. The present model is useful for analyzing the restricted twisting and tumbling motions, and rotational correlation times, of spin-labeled molecules in bilayers. Simulated spectra compare well with experimental spectra of lecithin bilayers marked with cholestane spin label, over a wide temperature range.

Cholestanes↗