Biomedical subjects

Rafael Tadmor

Publications and source records attributed to Rafael Tadmor.

8 recordsLinked to original sources

PubMed2004

Line energy and the relation between advancing, receding, and young contact angles.

The line energy associated with the triple phase contact line is a function of local surface defects (chemical and topographical); however, it can still be calculated from the advancing and receding contact angles to which those defects give rise. In this study an expression for the line energy associated with the triple phase contact line is developed. The expression relates the line energy to the drop volume, the interfacial energies, and the actual contact angle (be it advancing, receding, or in between). From the expression we can back calculate the equilibrium Young contact angle, theta0, as a function of the maximal advancing, thetaA, and minimal receding, thetaR, contact angles. To keep a certain maximal hysteresis between advancing and receding angles, different line energies are required depending on the three interfacial energies and the drop's volume V. We learn from the obtained expressions that the hysteresis is determined by some dimensionless parameter, K, which is some normalized line energy. The value of K required to keep a constant hysteresis (thetaA-thetaR) rises to infinity as we get closer to theta0 = 90 degrees.

Journal Article↗

PubMed2004

Large deformations during the coalescence of fluid interfaces.

Surface forces and shape changes were simultaneously measured during the approach and coalescence of two liquid-liquid and liquid-air interfaces. Large normal and lateral deformations were observed that are nevertheless consistent with a simple theoretical analysis of the long-range effects of short-range attractive van der Waals forces. The results imply that two fluidlike structures such as liquid droplets and soft biological cells can sense each other at much larger separations than previously assumed based on criteria taken from the interactions of hard particles.

Journal Article↗

PubMed2004

Correlation of AFM and SFA measurements concerning the stability of supported lipid bilayers.

Phospholipid bilayers were studied by means of atomic force microscopy (AFM) and a surface force apparatus (SFA). The stability of the supported bilayers was described by the amount of irregularities in the topography of the membrane by means of AFM and by the occurrence of hemifusion in the SFA, which is an indicator of defective bilayers. The bilayers, composed of lipids having the same headgroup but different chain lengths in the two leaflets, were prepared by Langmuir-Blodgett deposition and transferred at different surface pressures. The topography of the supported bilayers in aqueous solution, as imaged by AFM, revealed an increasing number of defects in the supported lipid membranes with decreased deposition pressure of the outer lipid layer. These defects, which appeared in the form of monolayer and bilayer (self-assembled) thick holes within the membrane, were energetically favorable over an evenly depleted bilayer. We found that the quantity of these defects (holes of </=0.5 micro m diameter and covering up to 30% of the surface area) correlated well with the stability of the bilayers as measured by SFA, a truly complementary instrument.

Elasticity↗

PubMed2004

Fourier transform infrared-probed O(3P) microreactor: demonstration with ethylene reactions in argon matrix.

To demonstrate the development of an oxygen atom microreactor in the form of liquid-helium-cooled solid argon matrix deposited on an infrared (IR) window, the oxidation of ethylene by mobile O atoms has been investigated. O atom diffusion through the argon matrix is confirmed and used to examine ethylene-oxygen atom reactions. In a bench-scale matrix isolation system probed with a Fourier transform infrared (FT-IR) spectrometer, matrices of solid Ar at 8-10 K doped with NO2 and ethylene have been prepared on a ZnSe window within an evacuated cryostat. The matrices have been photolyzed using 350-450 nm photons, and the reaction products resulting from the reaction of O(3P), one of the photolysis products of NO2, with ethylene have been identified using FT-IR and a Gaussian 98W simulation program. These products include oxirane, acetaldehyde, ethyl nitrite radical, and ketene. The temperature effect in the range of 10-30 K on the products formed has also been investigated. The reaction mechanisms are discussed and the viability of the solid Ar matrix being a low temperature microreactor to examine reaction mechanisms of mobile oxygen atoms is elaborated.

Argon↗

PubMed2003

Sliding friction with polymer brushes.

Using high-resolution shear force measurements, we examine in detail the frictional drag between rubbing surfaces bearing end-tethered polymeric surfactants (brushes). The drag attains a maximum on initial motion, attributed to elastic stretching of the chains, which falls by a cascade of relaxations to a value characteristic of kinetic friction. This has a very weak velocity dependence, attributed to chain moieties dragging within a self-regulating, mutual interpenetration zone. When sliding stops, the shear stress across the polymer layers decays logarithmically with time, consistent with the relaxation of a network of dangling ends.

Journal Article↗

PubMed2003

Thickness and refractive index measurements using multiple beam interference fringes (FECO).

We report on the use of optical interferometry employing fringes of equal chromatic order (FECO) in a surface force apparatus (SFA) to determine film thicknesses and refractive indices of confined media for a wide range of separations. In particular, we show how to calculate the surface separation (film thickness) based on two fringes whose contact position was not measured. We discuss the measurement accuracy, and though the theoretical accuracy is 1 A for all separations, we show that in practice, for large separations, it is very hard to get to this accuracy.

Journal Article↗

PubMed2002

Thin film rheology and lubricity of hyaluronic acid solutions at a normal physiological concentration.

Using a surface forces apparatus to measure forces, and optical (multiple beam) interferometry to measure surface shapes and separations (to +/-1 A), the normal, viscous, and shear (lubrication) forces between smooth mica surfaces in aqueous hyaluronic acid (HA) solutions were measured. The experimental conditions of loading pressures, pH, and HA concentration were set to closely correspond to physiological human knee-joint conditions. From the force and optical (refractive index) measurements, it was concluded that, like other negatively charged polyelectrolytes, HA does not naturally adsorb on the mica surface which is hydrophilic and weakly negatively charged at physiological conditions: the polymer solution exhibits the bulk viscosity (22.5 +/- 1.5 cP) for films thicker than about 0.4 miccrom of the polymer, whereas for thinner films, the viscosity decreases monotonically toward the value of the pure electrolyte solution (1 cP) as HA is extruded from between the surfaces. This is indicative of a repulsive "depletion" interaction of HA with each mica surface and to a weakly attractive polymer-mediated force between the two surfaces. Thus, free HA in synovial fluid is not expected to act as a good "boundary lubricant." Relaxation measurements on approaching and receding surfaces in HA solutions were also performed, and it is shown that the presence of HA in the bulk solution can improve "hydrodynamic" modes of lubrication, for example, by assuaging the compression stroke. The study includes information that is beneficial to researchers working with biomaterials viscosupplementation devices.

Adsorption↗

PubMed2002

Additional attraction between surfactant-coated surfaces.

A simple model that shows an additional attraction between solvated surfactant-coated systems is developed. The model simply calculates the van der Waals attraction between the solvated surfactant layers. This attraction was previously neglected as it was expected to have a small energetic contribution. This is indeed the case; however, despite the small energetic contribution the force is large. In other words, although the expression that we get is small in energy, it is large in force. This is particularly important for surface force balance measurements, where using the developed expression, some apparent discrepancies between measured and theoretical values may now have a possible explanation, and especially those associated with surfactant-coated surfaces. We apply the new expression to a given system, and compare with the experimental results.

Journal Article↗