Oscillatory surface shear

Figure 1.12 Stress-strain relationship for a typical oscillatory surface shear viscoelasticity measurement.

The main classic theory on the oscillatory rheology of dispersions of thin-walled capsules is due to Oldroyd [80]. In a latter paper, apart from the interfacial tension, Oldroyd introduced surface shear viscosity, surface shear elasticity, dilatational viscosity, and dilatational elasticity [33]. 

As we have seen in Section 6.6.1 such confined liquids may behave quite differently from the bulk lubricant. Near the surfaces, the formation of layered structures can lead to an oscillatory density profile (see Fig. 6.12). When these layered structures start to overlap, the confined liquid may undergo a phase transition to a crystalline or glassy state, as observed in surface force apparatus experiments [471,497-500], This is correlated with a strong increase in viscosity. Shearing of such solidified films, may lead to stick-slip motions. When a critical shear strength is exceeded, the film liquefies. The system relaxes by relative movement of the surfaces and the lubricant solidifies again. 

Rheology of various polymer layered-silicate nanocomposites - intercalated, exfoliated and end-tethered exfoliated (prepared by in-situ polymerization from reactive groups tethered to the silicate surface), have been performed in a conventional melt-state rheometer in both oscillatory and steady shear modes. These experimental studies have provided insight into the relaxation of polymer chains when confined by the layers of inorganic silicates, as well as the role of shear in orienting the layered nanocomposites. 

Fig. 12. Brush height z(t) versus time t/r in the presence of an oscillatory shear for the two cases presented in Fig. 11. The solid curve is for the upper (moving) surface and the dotted line is for the lower surface. From ref. [79].

Finally, there also remains the possibility that the extra normal force observed by Klein et al. [16] may arise for other reasons than an increase in the brush height. Since the experiments are in an open system, solvent entering and leaving the gap between the two curved surfaces may contribute to the observed extra normal force. This could explain the fact that there is a critical shear rate for the extra normal force to appear. Sufficed to say, the origin of the excess normal force under an oscillatory shear remains an open issue. Further experiments and simulations are needed to clarify the origin of this effect. 

Oscillatory measurements using the cone-and-plate viscometer are sometimes carried out to demonstrate the elastic behavior of a viscoelastic fluid [10]. The fluid in the viscometer is subjected to an oscillatory strain imposed on the bottom surface while the response of the shearing stress is measured on the top surface. If the phase shift between the input strain and the output stress is 90°, the sample is purely viscous if it is 0°, the sample is completely elastic. A measured phase shift between 0° and 90° demonstrates that the fluid is viscoelastic. 

Another important aspeet that we have not discussed in this paper is the dynamic properties of the polymer surface layers such as their behavior under steady shear [42] or under an oscillatory stress [43]. This is obviously essential for adhesion studies and theoretical work in this direction is certainly needed. 

Various characterization methods both in vitro and in vivo can provide information to understand, predict, and improve the performance of drug delivery systems. Selection of methods depends on the material properties and their applications. Viscoelastic properties can be measured using both DMA and oscillatory shear rheometry. DSC is a most useful method of measuring thermal transitions. Various microscopic methods are available to obtain the microstrac-ture and shape of the materials. Amorphous and crystaUine materials have different packing patterns of molecules, and these properties can be determined from XRD or density measurements. Surface properties such as surface elemental composition and material porosity can be obtained from various spectroscopic methods as well as from BET measurements. The biocompatibility of the material can be determined from both in vitro and in vivo assays. In vitro dissolution testing can be utilized to correlate with the in vivo performance of polymeric drug delivery systems. All these characterization methods can provide valuable information... 

Shear stress Shear stress is exerted on the valve by blood flowing across the face of the valve tissue. In basic terms, shear stress is defined as the component of the stress parallel to the surface of interest. The aortic valve experiences completely different shear profiles on each side of the leaflet, tightly correlating with the preferential calcification of the fibrosa [16, 53]. Shear stress at the leaflet surface is experienced in a cyclical manner—due to the blood flow during the typical cardiac cycle. The ventricularis is subjected to unidirectional shear stress as blood is ejected from the ventricle to the aorta whereas the fibrosa experiences oscillatory shear stress. Oscillatory shear stress at the fibrosa has been directly associated with valve dysfunction and CAVD [16, 53, 54]. Aboelkassem et al [55] derived a ma-... 

---