Dilated dynamic condition

A general description of the fundamental relationships governing the dynamic response of linear viscoelastic materials may be found in several sources (28, 37, 93). In general, sinusoidally applied strains (stresses) result in sinusoidal stresses (strains) that are out of phase. Measurements may be made under uniaxial, shear, or dilational loading conditions, and the resultant complex moduli or compliance and loss-phase angle are computed. Rotating radius vectors are usually taken to represent the... 

The main reason why volumetric capacity is normally estimated in a relatively crude manner, as a proportion of swept volume , is that derivations of transport efficiency are generally secondary to assessments of the density condition of the solid in the conveyed state. The product as fed is in a dynamic condition, so the best simple assessment is to assume a loose poured measurement of product density. This value can be adjusted up or down, to take account of the speed of the feed screw and the assessed packing characteristics of the flow channel from the hopper. Material carried within the normal feeder speed range of 15-100 r/min tends to be relatively dense at low speeds but more dilated at the faster speed. 

The Marangoni elasticity can be determined experimentally from dynamic surface tension measurements that involve known surface area changes. One such technique is the maximum bubble-pressure method (MBPM), which has been used to determine elasticities in this manner (24, 26). In the MBPM, the rates of bubble formation at submerged capillaries are varied. This amounts to changing A/A because approximately equal bubble areas are produced at the maximum bubble pressure condition at all rates. Although such measurements include some contribution from surface dilational viscosity (23, 27), the result will be referred to simply as surface elasticity in this work. 

Wasan and his research group focused on the field of interfacial rheology during the past three decades [15]. They developed novel instruments, such as oscillatory deep-channel interfacial viscometer [20,21,28] and biconical bob oscillatory interfacial rheometer [29] for interfacial shear measurement and the maximum bubble-pressure method [15,29,30] and the controlled drop tensiometer [1,31] for interfacial dilatational measurement, to resolve complex interfacial flow behavior in dynamic stress conditions [1,15,27,32-35]. Their research has clearly demonstrated the importance of interfacial rheology in the coalescence process of emulsions and foams. In connection with the maximum bubble-pressure method, it has been used in the BLM system to access the properties of lipid bilayers formed from a variety of surfactants [17,28,36]. 

In the present work, we summarize recent observations on the coalescence behavior of several oil-water-surfactant systems. We are particularly interested in the behavior exhibited by low tension systems and the role of bulk phase viscous effects and dynamic interfacial properties such as interfacial shear and dilatational viscosities. Also, a new experimental approach for conducting coalescence studies is described which involves tests in an inclined spinning drop device. This approach allows for coalescence tests under controlled conditions and provides an efficient method for quickly screening and evaluating different surfactant systems. 

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