Films deformation time

At a quasi-elastic impact the film thinning velocity is much smaller than the particle velocity. Thus dh/dt is very much less than unity and can be neglected in deriving Eq. (11.5). It is easy to obtain the well-known formula of Evans (1954) for the film deformation time from Eq.(11.6),... 

Fig. 3.68. Change in film deformation with time (a) and with temperature lowering (b) film from NaDoS...

The initial distance Hq is large compared with h, the thickness of the film at time t. The change in time, At, is the time it takes to reach a critical thickness for film rupture. Several versions of this equation exist that include internal circulation within the drop, rigid yet deformable interfaces, and complete interface mobility [64, 65]. 

Williams plotted the film-thinning time for deformable and nondeformable droplets against droplet radius. While an increase in droplet size increases the time required for thinning of a deformable droplet, nondeformable droplets experience a reduction in film thinning time as their size increases. It is interesting also to note the square relationship on thinning rate wifii nondeformable droplets and an inverse square relationship for deformable droplets. Clearly, increasing the apphed field across a system with deformable droplets could result in a reduction in coalescence efficiency. 

In Section 4.2.2 the central role of atomic diffusion in many aspects of materials science was underlined. This is equally true for polymers, but the nature of diffusion is quite different in these materials, because polymer chains get mutually entangled and one chain cannot cross another. An important aspect of viscoelastic behavior of polymer melts is memory such a material can be deformed by hundreds of per cent and still recover its original shape almost completely if the stress is removed after a short time (Ferry 1980). This underlies the use of shrink-fit cling-film in supermarkets. On the other hand, because of diffusion, if the original stress is maintained for a long time, the memory of the original shape fades. 

It has been also shown that when a thin polymer film is directly coated onto a substrate with a low modulus ( < 10 MPa), if the contact radius to layer thickness ratio is large (afh> 20), the surface layer will make a negligible contribution to the stiffness of the system and the layered solid system acts as a homogeneous half-space of substrate material while the surface and interfacial properties are governed by those of the layer [32,33]. The extension of the JKR theory to such layered bodies has two important implications. Firstly, hard and opaque materials can be coated on soft and clear substrates which deform more readily by small surface forces. Secondly, viscoelastic materials can be coated on soft elastic substrates, thereby reducing their time-dependent effects. 

Fig. 1.18 A film of silicone oil of 1 mm thickness is flowing along a vertically oriented planer sheet of PMMA. In a tagging experiment, a horizontal slice of 2 mm thickness is marked and its deformation is recorded as a function of the separation time A between the...

In many applications, we exert forces on polymer products for extended periods of time. In such cases it is important that they do not deform beyond acceptable limits. We address this concern through creep testing, which may take days, weeks, or even months to perform. We can also change the rate at which we perform certain tests. Thus we might test the polymer resin to be used in a crash helmet at high speed to mimic an impact. In contrast, the film used in a plastic bag would be better tested at slower speeds, which are more representative of what it would experience during use. 

Figure 4 Evolution of the orientation during and after the deformation of PS films at different temperatures above Tg. Time-resolved birefringence results with up to a 2 ms time resolution were obtained using the transmission method. Reproduced with permission from Messe et al. [15]. Copyright Elsevier 2001.

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