Surface Rheometer Research Articles

Equilibrium and out-of-equilibrium dynamics in a molecular layer of azopolymer floating on water studied by Interfacial Shear Rheology

We report the details of the construction and calibration of a sensitive surface rheometer, inspired by an instrument described in the literature, adapted to the study of photosensitive polymeric materials. By high resolution video tracking of the motion of a floating magnetized needle we are able to measure the viscoelastic complex shear modulus G of a Langmuir monolayer with an accuracy of 5*10 − 6 N/m. This instrument is then employed for the rheological characterization of a Langmuir monolayer of a photosensitive azobenzene polymer, which can be brought out of equilibrium by a suitable photoperturbation. The shear modulus is measured as a function of temperature, illumination power and wavelength. The reversible rheological changes induced in the film by photo-perturbation are monitored during time, observing a transition from a predominantly elastic (G mainly real) to a viscoelastic regime (real and imaginary parts of G comparable). These results are confirmed by a comparison with independent measurements performed using other rheological techniques. Finally a discussion is made, taking into account the results of a recent X-ray photon correlation spectroscopy (XPCS) experiment on the same polymer in equilibrium and out of equilibrium.

Deformation of a Capsule in a Spinning Drop Apparatus

This paper studies the rheological properties of ultrathin aminomethacrylate membranes that are synthesized at the interface between oil and water. We have measured the kinetics of the cross-linking reactions, the zero shear modulus, and the nonlinear rheological properties of those two-dimensional networks. The measurements were performed at a planar interface using a special surface rheometer. Once stabilized, the membranes exhibit rubber-elastic properties. In addition, we have formed small emulsion droplets using the same type of interfacial polymerization. In order to get more information on the mechanical properties of the resulting microcapsules, we have developed a new experimental technique that consists in measuring the deformation of a capsule in a spinning drop apparatus. A theoretical analysis of the deformation process allows an interface apparent elastic Young modulus to be calculated. From a comparison of the surface shear modulus with the Young modulus, it is then possible to estimate the surface Poisson ratio, which is here approximately zero. These results point to the existence of area compressible anisotropic membranes.

Influence of Interfacial Properties of Lipophilic Surfactants on Water-in-Oil Emulsion Stability

The purpose of this study is to investigate interfacial properties of lipophilic nonionic surfactants of the sorbitan fatty acid ester type (Spans 20, 80, 83, and 85) with the aim of selecting appropriate emulsifiers for water-in-oil (W/O) emulsions under different conditions. Interfacial tension and rheology were investigated at a planar mineral oil/double distilled water interface using a Wilhelmy plate method and an oscillatory ring surface rheometer, respectively. The effects of bulk concentration, temperature and the presence of salt in the aqueous phase on the interfacial properties of surfactant films were determined. The relationship between interfacial film properties and W/O emulsion stability in the presence of salt was assessed by visual observation of phase separation on storage. All the surfactants exhibited viscoelastic properties with the exception of Span 85. Interfacial association of inverse micelles and/or surfactant multilayer formation are probably responsible for the observed viscoelasticity. Addition of sodium chloride to the aqueous phase and increase in temperature influenced the viscoelastic properties. Emulsion stability to phase separation decreased in the presence of salt. The interfacial rheology data were predictive of emulsion stability for the systems studied.

Viscoelastic swirling flow with free surface in cylindrical chambers

The flow of a viscoelastic liquid driven by the steadily rotating bottom cover of a cylindrical cup is investigated. The flow field and the shape of the free surface are determined at the lowest significant orders of the regular domain perturbation in terms of the angular velocity of the bottom cap. The meridional field superposed on a primary azimuthal field shows a structure of multiple cells. The velocity field and the shape of the free surface are strongly effected by the cylinder aspect ratio and the elasticity of the liquid. The use of this flow configuration as a free surface rheometer to determine the first two Rivlin-Ericksen constants is shown to be promising.

Rheological studies of asphaltene films adsorbed at the oil/water interface

The compressibilities of crude oil/water interfaces have been measured for Brega, Kuwait and Tia Juana crude oils by an adaptation of the pendant drop method. Addition to the oils of a commercial dispersant (BP1100X) which contains an oil-soluble non-ionic surfactant resulted in increased compressibilities, the rate of increase with amount of BP1100X added being inversely related to the asphaltene content of the oils. The viscoelastic properties of the interfaces were studied with a surface rheometer working at low shearing stresses. Creep curves showed rapid elastic deformation and slow irrecoverable flow. The flow curves which were irregular indicated very high surface viscosities which appeared to increase with applied shear stress. It is suggested that thick films of asphaltene particles were responsible. The addition of 4% v/v BP1100X to the oils produced no effect for Kuwait and Tia Juana crudes but Brega oil interfaces showed an increased elasticity. Interfaces formed between a dispersion of asphaltenes in n-heptane/toluene and water exhibited both rapid and retarded elasticities as well as Newtonian flow.

Surface rheological properties of hyaluronic acid solutions.

Using an oscillating ring surface rheometer, surface shear rheological studies of hyaluronic acid solutions at physiological pH have demonstrated the elastico-viscous nature of the surface films. The properties of these surface films change with time and are shown to be related to bulk concentration, ionic strength and pH. This ageing behaviour can be explained on the basis of molecular conformational changes and molecular segmental kinetics. The results are discussed in relation to the postulated function of hyaluronic acid in synovial fluid.

An experimental study of the rheological behavior of surface films

AbstractA constitutive equation has been developed to relate surface stresses to the deformational history of an interface. To verify the applicability of the equation to real surface films, experimental studies involving spread and adsorbed monolayers have been conducted using the deep‐channel surface rheometer operated both under the conditions of oscillatory and constant floor motion. The results reported here are the first obtained using an oscillatory system.Experimental results indicate that both spread and adsorbed monolayers can exhibit viscoelastic behavior which may be represented by the model. Furthermore, it has been found that for three long chain, fatty acid films, spread on a pH 6.1 aqueous substrate, surface shear viscosity and surface elasticity increase with film pressure and molecular weight. The observed rheological behavior was found to be strongly pH dependent.

A constitutive equation for a viscoelastic interface

AbstractA constitutive equation has been developed to relate surface stresses to the deformational history of an interface. This equation is designed to provide a unified description of complex interfacial rheological phenomena. New and as yet unreported behavior such as surface normal stresses and viscoelastic contributions to the dynamic surface tension are predicted.A monolayer of poly(vinyl alcohol) adsorbed at an air/water interface has been studied using a deep channel surface rheometer and the observed viscoelastic behavior shown to be fitted by the model.

The rheology of some interfacial adsorbed films of macromolecules II. Stress relaxation phenomena

With the use of a surface rheometer especially designed for the purpose, stress-relaxation phenomena have been studied in unimolecular films of bovine serum album in, pepsin, poly-a, L-lysine (d.p. 15) and sodium arabinate at the hydrocarbon/water interface. The behaviour of the films has been analyzed in terms of the constituent rheological elements and the relaxation spectra have been obtained. From the dependence of the relaxation spectra on temperature, the activation energies for the relaxation of the films have been found. The relationship of these energies to the hydrogen bond energy (which, it was shown in part I, is largely responsible for the observed visco-elasticity) has been discussed.

The gelation of high polymers at the air-water interface

Monolayers of various synthetic polymers were spread at the air-water interface and they were subjected to a constant shear stress by means of a surface rheometer. The types of the strain time curve thus obtained were peculiar to a given film and represented by the corresponding mechanical model. From the rheological characteristics of films, they were arranged into five groups. The gelation area, i. e. the smallest area at which films begin to aquire any viscoelasticity during compression, was also specific to a given film. It was found that films of crystalline polymer gelify at an area larger than close-packed area. The force-area curves of polymer films were discussed from the rheological standpoint.

Rheology of Surface Films. III. A New Type of Surface Rheometer using Electromagnetic Driving Technique

Abstract In order to establish the standard set of surface rheometry, which involves the rheological measurement of films at liquid surface, a new type of surface rheometer was devised using electromagnetic driving technique. The advantage of this apparatus lies not only in its ease of operation and simplicity of construction, but also in its accessibility to various sorts of measurement. In addition, the wide range of measurement and remote control of operation provide a practical advantage for the experiment of surface films.

Rheology of Surface Films. II. A Surface Rheometer for Measuring the Film Creep under Constant Shear Stress

Abstract A surface rheometer was presented which enables the creep of surface films to be measured under constant shear stress. The main construction consists of a ring and a suspending torsion wire. Automatic adjustment of the torsion head driving, which made distorsion of the wire to be constant throughout the creep test, was accomplished by means of a phototube relay mechanism. The creep curve and its analysis data of ovalbumin monolayer were also presented.