Deep channel viscometer

With Equations 7.71 and 7.72, the system of equations relevant to a given experimental setup can be solved to give the surface rheological coefficients. The deep channel viscometer of Mannheimer and Schechter (1970) is popular and also provides a very nice instance where the complete solution to the equations of motion is available. A sketch of the apparatus is shown in Figure 7.5. A cylindrical annulus is filled with two liquids A and B. The annulus is held stationary and the floor rotated with a constant angular velocity S2. Also, and are the inner and the outer radii of the annulus in the cylindrical coordinate system and designates the position of the interface. It is seen here that and V0B are the only nonzero velocity components in the two phases, that they are functions of r and z only, and that the pressures satisfy the hydrostatic equilibrium equations. The solution to the equations of motion subject to the conventional boundary conditions are... 

FIGURE 7.5 The deep channel viscometer. The longitudinal section of a radially symmetric figure is shown. 

Can the effect of surface active agents, as given in Section 7, describe the observations made in the deep channel viscometer experiments Discuss. 

The deep-channel viscometer could also be adapted for measurement of the nonlinear interfacial rheological behaviour of the film [52]. In this case several small tracer particles are placed on the fluid interface at different radial positions and the angular velocities are determined from measurements of the period of revolution. When used to measure viscoelastic properties, the deep-channel viscometer is operated in an oscillatory mode, in which case the floor of the viscometer is oscillated sinusoidally. Simultaneous measurements of the phase angle between the surface motion and the oscillating motion of the bottom dish, and the surface-to-floor amplitude ratio, may permit determination of the viscoelastic properties of the fluid interface, presuming knowledge of an appropriate rheological model [52]. 

The interfacial shear viscosities are measured by the deep channel viscous traction surface viscometer (5) at the Illinois Institute of Technology. The oil-water equilibrium tensions are measured by either the spinning drop or the du Nouy ring (6) method. 

Figure 3.70. Deep channel surface shear viscometer according to Mannheimer and Schechter (loc. cit.) (a) side view, (b) top view.

The reasons behind the specific choice of apparatus geometry can best be shown by a brief review of prior work. The earliest canal type surface viscometer was introduced by Dervician and Joly (8). In this apparatus, an insoluble monolayer is floated on a substrate fluid in a straight channel. The film is forced to flow through the channel by movement of a floating barrier. This motion is resisted principally by surface viscosity. Thus, the small force required to propel the film at a given speed may be measured and used to determine the surface viscosity of the film. A relatively complete theoretical treatment has been provided by Harkins and Kirkwood (5) for insoluble films with Newtonian surface viscosity in deep channels. Actual measurements are typically made in shallow channels, however, which are formed by floating the channel boundaries on the liquid surface. This method is not applicable to soluble surface films, which tend to diffuse through the substrate fluid and pass behind the barrier. Nevertheless, the most accurate values of surface viscosity available have been produced by this approach. 

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]. 

Wasan et al. (163) used a deep-channel viseometer in studying the interfaeial shear viseosity of Salem crudes/water with and without the addition of petroleum sulfonate and salts, as well as Illinois erude/brine with pen-tadecyl benzenesulfonate. They showed a decreased coalescence time with decreased shear viseosity. A viscous traction shear viscometer was used for fraetionated crude oil/brine by Pasquarelli and Wasan (164), who showed that increased interfacial shear viseosity is eorrelated with decreased coalescence. Later, Wasan eorrelated interfacial shear viscosity with film-drainage time to determine effective demulsifiers (178). 

Another convenient method for measuring the interfacial shear viscosity is to use the deep-channel surface viscometer (25), shown schematically in Figure 8. Basically it consists of two concentric brass cylinders (separated by a distance y ) lowered into a pool of liquid contained within a brass dish, to a depth at which the brass cylinders nearly touch the bottom of the dish. The dish is rotated with a known angular velocity, co , and the midchannel or centerline surface motion of the interface within the channel (formed... 

The deep-channel surface viscometer is a frequently used experimental method for measuring interfacial shear viscosity owing to its sensitivity (irish S 10 surface poises) and relatively simple analytical theory. The main drawback of this technique is the necessity of placing a small tracer particle within the interfacial flow field for tracking the central surface velocity. This may be particularly cumbersome with heavy-oil systems, for which the particle may require several hours or more to execute a complete revolution, as well as with liquid -liquid systems, for which the placement of the particle at the interface may be difficult. For more details, see Refs. 58 and 151-156. 

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