Ultrasonic shear waves

Fig. 3.7.1. Experimental arrangement for studying the reflexion of ultrasonic shear waves at a solid-nematic interface.

The viscosity coefficients may also be determined by studying the reflexion of ultrasonic shear waves at a solid-nematic interface. The technique was developed by Martinoty and Candau. A thin film of a nematic liquid crystal is taken on the surface of a fused quartz rod with obliquely cut ends (fig. 3.7.1). A quartz crystal bonded to one of the ends generates a transverse wave. At the solid-nematic interface there is a transmitted wave, which is rapidly attenuated, and a reflected wave which is received at the other end by a second quartz crystal. The reflexion coefficient, obtained by measuring the amplitudes of reflexion with and without the nematic sample, directly yields the effective coefficient of viscosity. 

Figure 14 Ultrasonic shear waves at the interface between two media (r, > i .).

One measures this rate by observing the increase in attenuation of the ultrasonic shear wave as the area of contact grows. Growth continues until the substrate is covered with the polygonal units rather than flattened spheres. 

Alves CH, Machado JC. Measurement of plasma clotting time using ultrasonic shear waves. Physiol Meas 1994 15 309-16. 

Use of Ultrasonic Shear Waves for the Study of Liquid Interfaces... 

This chapter is concerned with the study of liquid interfaces. Liquid interfaces would mostly mean droplets, vesicles, or bubbles. Bubbles and "nanobubbles," in particular, have been studied extensively in the context of slippage and the long-range hydrophobic attraction.Today they are studied by numerous groups independently of these specific applications. This chapter only covers a rather narrow subtopic of this research field. We show in the second part that nanobubbles look like stiff objects when investigated with ultrasonic shear waves. 

As the mechanical properties of the final product often depend on the orientation of the polymer chain, it is very important to characterize the orientation of the polymer chain both quickly and accurately in nondestructive fashion. When Edwards and Thomas [55] used the propagation velocity of an ultrasonic shear wave to detect anisotropic behavior in the mechanical properties of a solid, the results obtained indicated that this method was quite sensitive for semicrystalline polymers but much less effective for amorphous polymers. 

Determination of the viscosity coefficients from the mechanical wave propagation and attenuation in the ordered nonatic phase is probably the closest to the first principles methods. The shear impedance technique is based on measuring the reflection and attenuation of ultrasonic shear waves [90-92]. The conqtlex shear impedance of the nematic sample, Zn = Rn + iXn, is determined from the complex... 

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