F.C Goodrich

The theory of absolute surface shear viscosity II. The rotating disk problem

Abstract The numerical results of a rigorous mathematical analysis of fluid flow in the neighborhood of a rotating disk inserted into a liquid interface supporting an adsorbed surface film are presented. The theory (described in detail elsewhere) takes full account of the coupling of interfacial and substrate fluid flows, which is to say that it removes the deficiency present in previous theories of surface viscometry which assumed that these flows are independent. It is shown that in the low surface viscosity region the results are sensitively dependent upon the coupling of interfacial and substrate motion. Finally we demonstrate that the rotating disk viscometer is a poor experimental design for the determination of low surface viscosity.

A new surface viscometer of high sensitivity. I. Theory

A new design for a surface viscometer is proposed in which a film covered interface is driven by making contact with a rotating ring inserted into a narrow gap in the wall of a cylindrical vessel. The fluid motion is analyzed theoretically in laminar flow approximation, assuming Newtonian behavior in both the substrate and the interface. From our analysis, we find that the surface viscosity can be measured by comparing the period of revolution of a floating dust particle inserted into the interface near the cylindrical axis with the rotational period of the ring. Errors caused by the finite depth of the vessel and the finite thickness of the knife edge are negligible, and the only geometrical parameter needed is the radius of the cylinder. At the air/water interface, surface viscosities η in the range 10−5 ≤ η ≤ 0.1 surface poise may be measured quantitatively.

A new surface viscometer of high sensitivity. II. Experiments with stearic acid monolayers

Abstract A working model of the surface viscometer analyzed theoretically in Part I is described and details of its calibration and operation are reviewed. Experimental and theoretical flow patterns are compared and shown to be in excellent agreement. Some experiments with stearic acid monolayers spread on water at 22°C and pH = 7.0 are reported. We find that the surface shear viscosity of these monolayers is very low until high surface concentrations are achieved, when an abrupt rise in surface viscosity is coincident with a simultaneous abrupt rise in surface pressure. Over all ranges of surface concentration studied the rheology of stearic acid monolayers is Newtonian.

Interfacial conformational modifications in a polypeptide

Abstract Poly-γ-methyl- l -glutamate (PMG) spread as a monolayer at the air/water interface is capable of existing either in the α (helix coil) or in the β (pleated sheet) conformations. By suitable choice of a spreading solvent, the investigator may control which conformation is dominantly present in the interface. The β conformer is characterized by a high surface viscosity which fails with rising temperature, while the a conformer at an equivalent surface concentration exhibits a relatively low surface viscosity which increases irreversibly with temperature. We interpret these results to mean that the a helices are partly denatured by the surface environment, while the intermolecular hydrogen bonds in the β structure are weakened by rising temperature. In no case examined by us in a complete denaturation of the a helices to the β structure observed.

The theory of absolute surface shear viscosity V. The effect of finite ring thickness

Abstract In the theory of Couette surface viscometers it is convenient to idealize a rotating ring thrust into a fluid interface supporting an adsorbed surface film as making a knife-edge contact of zero thickness. The validity of this assumption is examined by calculating from theory the flow patterns to be expected when the ring makes a contact of finite thickness with the interface. The zero thickness idealization is found to be valid for films of moderate to large surface viscosity, but it fails badly for films of low surface viscosity and places a limit upon the sensitivity of the ring method.

The theory of absolute surface shear viscosity. IV. The double ring couette surface viscometer

Abstract The hydrodynamic problem of two concentric, counter rotating rings making a knife-edge contact with a viscous surface film supported by a semi-infinite, viscous substrate is discussed. Plots of interfacial velocity profiles are presented, and the relation between the torque on the outer, stationary ring and the surface viscosity is given in the form of interpolation polynomials which permit the calculation of the surface viscosity from experimental values of the torque.

The Kinetics of Growth of an Aerosol in a Flow Reactor: I. Experimental

Abstract A novel and inexpensive design for an aerosol generator is described. The apparatus reliably and reproducibly generates nearly monodisperse aerosols in a size range convient for study by light scattering. With its use, we have examined the rate of growth of dibutyl phthalate aerosol droplets nucleated by sodium chloride particles in a supersaturated jet.

The Theory of Absolute Surface Shear Viscosity. III. The Rotating Ring Problem

The problem of steady, laminar flow in the vicinity of a rotating ring inserted into a semi-infinite fluid interface is solved when the liquid surface is covered with an insoluble film possessing an intrinsic surface viscosity. The results are presented as interpolation polynomials which permit calculation of the absolute surface viscosity of the film from measured values of the torque exerted on the ring.

The kinetics of growth of an aerosol in a flow reactor. II. Theoretical

Abstract The data of Part I concerning the rate of growth of dibutyl phthalate aerosol droplets by condensation from a supersaturated atmosphere are interpreted successfully over the early stages of the growth process in terms of a collision hypothesis based upon the kinetic theory of gases. Over the later stages of growth, however, it is necessary to postulate a diffusion controlled rate law. Under either hypothesis, the accommodation coefficient of vapor molecules striking the liquid surface is unity.

A new surface viscometer of high sensitivity. III. Stearic acid at the oil/water interface

Abstract In a theoretical section the hydrodynamic theory of the knife-in-wall surface shear viscometer is extended to the oil/water interface. The theory is restricted to laminar flow of a Newtonian interface undergoing shear strains only. In a subsequent experimental section, the theory is applied to absorption films of stearic acid at the cyclohexane/water interface. At a fixed concentration of stearic acid in cyclohexane, the interfacial shear viscosity is shown to vary as changes are made in either the pH of the aqueous substrate or the temperature of the system. As the hydrogen ion concentration decreases, the interfacial viscosity rises from pH 5.5 to a dramatic maximum at pH 9.0. At pH > 9.0 the interface rapidly becomes less viscous as the pH is further increased. The change in the interfacial state at pH 9 is also coincident with a precipitous drop in the interfacial tension. When the aqueous phase is held at pH 9.0 and the system slowly warmed, an abrupt decrease in the interfacial viscosity occurs at 21.7°C. This “melting” point proves to be quite reproducible and is independent of the direction of temperature change. At T 22.0°C the viscosity remains fairly constant with T.