Dewetting speed

The speed of wetting has been measured by running a tape of material that is wetted either downward through the liquid-air interface, or upward through the interface. For a polyester tape and a glycerol-water mixture, a wetting speed of about 20 cm/sec and a dewetting speed of about 0.6 cm/sec has been reported [37]. Conversely, the time of rupture of thin films can be important (see Ref. 38). 

However, on rigid substrates, the growth of dry zones is accompanied by a rim of excess liquid with width X (Fig. 10). As the dewetting proceeds, X increases. For short times and < K, the growth of dry patches is controlled only by surface tension forces and the dewetting speed is constant. A constant dewetting speed of 8 mm-s has been measured when a liquid film of tricresyl phosphate (TCP) dewets on Teflon PFA, a hard fluoropoly-mer of low surface free energy (p. = 250 MPa, 7 = 20 mJ-m ). 

We therefore have qualitative evidence for the dependence of the dewetting speed on the elastic properties of the substrate. Dependence of wetting on the elastic modulus was previously suggested in the case of thin substrates [31], It may be conjectured that cross-linking affects the surface properties of the elastomer and, therefore, wettability. However,... 

FIG. 13 Linear regression between the dewetting speed, U, and where p, is the shear modulus of the silicone rubber (RTV 615, General Electric Co.). 

The theory of viscoelastic braking in liquid spreading exposes the various possibilities that may exist for controlling wetting or dewetting speeds by changing solid rather than liquid properties. Applications may exist in the fields of contact lenses, printing, and vehicle tire adhesion. 

Figure 11.5 Dependence of the polymer dewetting pattern on molecular weight and dewetting speed.

The molecular-kinetic theory predicts a maximum wetting speed vmax and a minimum dewetting speed vmin. At speeds larger than vmax gas bubbles form. This was indeed observed. For water this value is vmax 5 — 10 m/s. 

Fig. 1.21. Graph of speed as a function of dynamical contact angle according to the theory of hydrodynamics. For V < Vm, there exists an angle d, and hence a dynamical meniscus. For V > Vm, hydrodynamic instability occurs a liquid film is drawn along behind. Vd is the dewetting speed of this film, discussed later...

In the case (m) of intercalated films, the rim must distort the soft material around it. Consequently, it becomes rather flattened and viscous dissipation still predominates. It is no longer localised only in the wedge fronts, but occurs throughout the volume. It increases with the size of the rim, whereas the driving force 5 remains constant the dewetting speed thus decreases in time. Experiments on intercalated Aims are currently underway at the Institut Curie in Prance (Martin). 

The values of Go and y are known and for the elastomer of Young s modulus of 2.1 MPa, [/o = 8 X 10 mm-s [12]. We can then evaluate 8 at ca. 20 mn. This value is perhaps a little high but of the same order of magnitude as earlier estimated [6]. Thus, despite some necessary approximations and simplifying hypotheses, we arrive at a semiquantitative explanation of the relationship between dewetting and therefore, presumably, wetting speed and the molecular structure of the elastomeric substrate. 

Figure 1. Optical micrograph ofa receding edge of a chloroform solution of polystyrene. The solution is in the upper right comer, wheras the dewetted part is the lower left part of the picture. The receding speed ofthe three phase line is approx. 200 pm/s. The picture was taken in the refection mode, since silicon...

Figure 1. Dynamic surface dewetting (a) first step contact between polymer and water surfaces, (b) second step polymer surface lift with constant speed and dewetting starts and (c) third step break of the water column.

Figure 2. Typical curve of polyethylene dewetting (surface diameter 50 mm lift speed 0.05 mm/s). At point A, the sample is just in simple contact with the water. After that, the polymer is lifted (part A-B). When the water column starts to break (point B), the normal strength is decreased. The break continues from point B to C. At the end (point C) the final measured normal strength is again equal to its initial value (i.e. 0 mN).

We will explain some rather unusual phenomena. As an example, dewet-ting at high speed can generate shock waves Likewise, in liquid films millions of times more viscous than water, holes can open up at such high velocities ( m/s) that high-speed cameras are required to capture the phenomenon. 

The two approaches are in fact quite similar. The difference is that in the repelling method, water is added to the monomer solution for the dewetting process, and in the dipping method, the sample with applied undiluted monomer is dipped into a water reservoir accurate control of the dipping speed is achieved through a motorized stage. If the monomer film dewets in air, the monomer remains on the hydrophilic domains of the substrate. This is the case with the withdrawal method [15,16] in which the patterned substrate is completely immersed in the monomer and pulled out at a fixed angle and a controlled speed into ambient air. 

Figure 14.15 Photographs of various dewetted PEDOT/PSS illustrating factors affecting dewetting, (a] PEDOT 1 1, 700 nm FDTS SAM. (b) PEDOT 1 1, 500 nm FDTS SAM, oxygen-plasma-treated surface, (c) PEDOT 1 1, 300 nm FDTS SAM. [d] PEDOT 1 3, 250 nm FDTS SAM. (e) PEDOT 1 1, 500 nm FDTS SAM, 30 nm Si02 mesa, (f) 5 pm FDTS SAM gap, dip-coated by PEDOT 1 3. (g) PEDOT 1 1 lines printed with a jet frequency of 4 Hz, printing speed of 0.1 and 0.3 mm s, 500 nm FDTS SAM, 30 nm mesa, (h) PEDOT 1 1 lines printed with a jet frequency of 4 Hz, printing speed of 0.1 and 0.3 mm s , 500 nm FDTS SAM, 80 nm mesa. (Images taken from Ref. 72.)...

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