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S Garoff

Publications and source records attributed to S Garoff.

10 recordsLinked to original sources
PubMed2005

Unsteady motion of receding contact lines of surfactant solutions: the role of surfactant re-self-assembly.

Re-self-assembly of surfactant molecules must occur at moving contact lines of soluble surfactant solutions. Molecules are transported into and out of the contact line region from four sources: the three interfaces meeting at the contact line and the fluid confined between the solid-liquid and liquid-vapor interfaces. As molecules move among these sources at the contact line, they must rearrange. The dynamics of this re-self-assembly has been shown to have a dominating effect on the structure of advancing contact lines, causing unsteady motion and complex structure of the contact line. It might be assumed that the re-self-assembly for receding contact lines leads to more steady contact line movement. However, in this article we show that for a wide variety of systems this is not true. Quasi-static distortions of the contact line occur as it retreats because of the inability of the surfactant to completely re-self-assemble at localized positions along the contact line.

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PubMed2005

Wetting by simple room-temperature polymer melts: deviations from Newtonian behavior.

The hydrodynamics near moving contact lines of two room-temperature polymer melts, polyisobutylene (PIB) and polystyrene (PS), are different from those of a third polymer melt, polydimethylsiloxane (PDMS). While all three fluids exhibit Newtonian behavior in rotational rheological measurements, a model of the hydrodynamics near moving contact lines which assumes Newtonian behavior of the fluid accurately describes the interface shape of a variety of PDMS fluids but fails to describe the interface deformation by viscous forces in PIB and PS. The magnitude of the deviations from the model and the distance along the liquid-vapor interface over which they are seen increase with increasing capillary number. We conclude that the wetting behaviors of PIB and PS are influenced by weak elasticity in these low molecular weight melts and that dynamic wetting is more sensitive to this elasticity than standard rheometric techniques.

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PubMed2004

Characterizing the microscopic physics near moving contact lines using dynamic contact angle data.

Directly probing the fluid flow and liquid-vapor interface shape in the microscopic immediate vicinity of the moving contact line can only be accomplished in very specific and isolated cases. Yet this physics is critical to macroscopic dynamic wetting. Here we examine the microscopic (or inner) physics of spreading silicone fluids using data of macroscopic dynamic contact angle versus Capillary number Ca=U mu/sigma. This dynamic contact angle is precisely defined so that it can be related back to the microscopic behavior through detailed theory. Our results indicate that the parameters describing the inner region have a detectable dependence on spreading velocity when this velocity exceeds a critical value. This dependence is not scaled (i.e., the data are not collapsed) by Ca, which suggests that an additional time scale must be present in the model of the inner region.

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PubMed2000

Using x-ray reflectivity to determine the structure of surfactant monolayers

Interactions among the multiple degrees of freedom of surfactant molecules cause fascinating richness in the structure of their monolayers. Beyond this scientific motivation for studying surfactant monolayers, the technological use of monolayers for interfacial control and molecular assembly demands a clear understanding of monolayer structure. X-ray and neutron reflectivity have become prime techniques for determining this structure. We present x-ray reflectivity data for a representative surfactant monolayer system and outline an objective procedure for obtaining the maximum amount of structural information possible. Our approach combines tight control of instrumental parameters, dynamically optimized Monte Carlo and simulated annealing to probe the chi(2) hypersurface, and a set of statistical criteria for accepting and rejecting fits. We justify our procedure through tests using simulated data. Results indicate that an ensemble of fits must be performed for each set of reflectivity data in order to survey the chi(2) hypersurface adequately. A single good fit may yield structural parameters which are quite misleading, yet physically plausible. Thus, one must never be satisfied with performing just a single fit. In cases for which multiple, statistically equivalent fits are obtained, the apparent ambiguity is substantially mitigated by averaging the parameters over the ensemble of good fits. We also introduce a method of dealing with cases for which a good fit may be extremely difficult to find. Our analysis procedures can be generalized to other monolayer or multilayer systems and are also applicable to neutron reflectivity.

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PubMed2000

Reconstruction of bowing point friction force in a bowed string

A method is presented for reconstructing the friction force and the velocity at the bowing point of a string excited by a rosined bow sliding transverse to the string. Two versions of the method of reconstruction are presented, each approximate in different ways, but both capable of sufficient accuracy to allow useful application to problems of understanding frictional interactions in this dynamical system. The method is illustrated with simulated data to verify its accuracy, and results are shown for two contrasting cases of observed stick-slip string motion. As has been found in other investigations, the friction force during sliding is not determined by the instantaneous sliding speed. The results seem to be compatible with a thermally based model of rosin friction.

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PubMed1999

Elongation of confined ferrofluid droplets under applied fields.

Ferrofluids are strongly paramagnetic liquids. We study the behavior of ferrofluid droplets confined between two parallel plates with a weak applied field parallel to the plates. The droplets elongate under the applied field to reduce their demagnetizing energy and reach an equilibrium shape where the magnetic forces balance against the surface tension. This elongation varies logarithmically with aspect ratio of droplet thickness to its original radius, in contrast to the behavior of unconfined droplets. Experimental studies of a ferrofluid-water-surfactant emulsion confirm this prediction.

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PubMed1996

Microscopic and Macroscopic Dynamic Interface Shapes and the Interpretation of Dynamic Contact Angles.

We have studied shapes of dynamic fluid interfaces at distances </=1700 µm from the moving contact line at capillary numbers (Ca) ranging from 10(-3) to 10(-1). Near the moving contact line where viscous deformation is important, an analysis valid to O(1) in Ca describes the shape of the fluid interface. Static capillarity should describe the interface shape far from the contact line. We have quantitatively determined the extent of the regions described by the analysis with viscous deformation and by a static shape as a function of Ca. We observe a third portion of the interface between the two regions cited above, which is not described by either the analysis with viscous deformation or a static shape. In this third region the interface shape is controlled by viscous and gravitational forces of comparable magnitude. We detect significant viscous deformation even far from the contact line at Ca approximately > 0.01. Our measured dynamic contact angle parameter extracted by fitting the analysis with viscous deformation to the shape near the moving contact line coincides with the contact angle of the static-like shape far from the contact line. We measure and explain the discrepancy between this dynamic contact angle parameter and the apparent contact angles based on meniscus or apex heights. Our observations of viscous effects at large distances from the contact line have implications for dynamic contact angle measurements in capillary tubes.

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