Dilational and shear rheology of adsorption layers at liquid interfaces

Interfacial shear rheology of protein–surfactant layers

Stimuli-sensitive emulsions stabilized by microgel particles consisting of poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) and being responsive to both pH and temperature have been investigated with respect to the visco-elastic properties of the interfacial layer Properties of the interfacial layer were probed by means of shear and dilatational rheology as well as by compression isotherms and are related to the microgel packing at the interface as visualized by cryogenic scanning electron microscopy The corresponding pH dependent emulsion stability is strongly correlated with the visco-elastic properties of the microgel covered oil-water interface At high pH when the microgels are charged, a structure of partially interconnected microgels is found that provides elastic, soft gel-like interfaces At low pH, however, the uncharged microgels are densely packed and the interface is rather brittle Obviously, these pH dependent visco-elastic properties of the microgel layer at the oil-water interface play a determining role in the stability of emulsion droplets and allow us to prepare very stable emulsions when the microgels are charged and to break the emulsion by changing the pH

Interfacial layers of stimuli-responsive poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) microgels characterized by interfacial rheology and compression isotherms

Complex fluid-fluid interfaces play an important role in a variety of application domains, from emulsion and foam stability, to thin films in biomedical applications, to coating flow phenomena. The current work reviews progress made in particular for interfaces where rheological stresses, peculiar to the interface, play an important role. The developments made in the area of constitutive modeling are briefly reviewed to clarify which material functions can be measured. For shear rheometry, progress in analyzing the flow field in the measurement device has been key, combined with advances in control over surface concentration and microstructural evaluation. For dilation/compressional rheometry much work has been done on separating changes in the surface tension from the extra rheological stresses. Finally, we discuss how “simple complex flows”, such as thin film and drainage flows, offer a first step up in complexity and seem to present a good benchmark problem for testing constitutive equations and the interplay between transport phenomena, interfacial rheology and the changes in state variables.

Tensiometry and rheology of complex interfaces

The torsion pendulum is not only a mainstay instrument in the world of precision measurement and gravitational physics, but is important in electrical science, biophysics, petrology, metallurgy, and various other fields of endeavor Whether used in the ‘‘static’’ (deflection) mode, the ‘‘dynamic’’ (oscillating) mode, or in some more complex configuration, instrumentation of this kind enables one to isolate and measure weak effects that would otherwise be difficult if not impossible to observe against the background gravitational field of the earth In this review, we present a brief history of fiber‐suspended apparatus and assess the fundamental limits of performance of the dumbbell pendulum We then inventory the different versions of such systems presently used by gravitational physicists and discuss the various interrogation techniques used to monitor the movement of the suspended test mass Next, we tabulate some of the applications for torsion instruments outside of gravitational physics, and close with a few comments on the direction of research in this area

Torsion balances, torsion pendulums, and related devices

We have constructed an apparatus which enables one to measure, simultaneously, the shear modulus, the viscosity, and the surface pressure of a monomolecular film on water under a hydrostatic compression. It is particularly suitable for measurements of the static shear modulus of insoluble monolayers. A capillary wave generator/detector system was also incorporated to determine the surface pressure. The trough is a Teflon cup 12.7 cm (5.4 in.) in inner diameter with sloping interior sides. As the cup is raised, while maintaining the water elevation fixed, the film is uniformly compressed. A rotor suspended at the center from a fine torsion fiber is used to measure the mechanical properties (surface viscosity and/or surface shear modulus). To measure the quasistatic shear properties, the cup is rotated through a small angle and the residual deflection of the rotor determined. Dynamic shear response is probed by studying transient torsional oscillations following a sudden angular displacement of the torsion ...

B. M. Abraham

Papers

Centro‐symmetric technique for measuring shear modulus, viscosity, and surface tension of spread monolayers