Dry patch

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 ). 

FIG. 10 Formation of a dry patch (zone) in an unstable liquid film on a rigid substrate. 

The technique used to study dewetting dynamics on materials consists of making a flat, smooth elastomer surface. A hquid puddle is deposited within a 50-mm-diameter ring of 0.1-mm-thick plasticized adhesive paper adhering to the substrate. The adhesive paper acts as a spacer. A microscope slide is drawn over the liquid to obtain a liquid film of ca. 0.1-mm thickness. At this thickness, the liquid film is unstable, being much less than the equilibrium value, of ca. 1.5 mm calculated from Eq. (29). Nucleation of dry patches... 

The occurrence and growth of dewetted holes, or dry patches, is followed using a video camera rigged up to a low-power microscope. Using a video recorder, it is possible to analyze up to 24 frames/second. 

FIG. 12 Radius, r(t), of dry patches growing in an unstable TCP fihn on silicone mbber (RTV 615, General Electric Co.), of different shear modulus, p,. Teflon PFA is a hard fluoropolymer of similar surface free energy (y = 20 mJ-m ). 

A film can only break up into droplets after a disturbance the film locally thins to less than t)q)ically 1000 nm (see Fig. 6.40). In this region the interaction force (van der Waals, electrical double layer, for example) between the liquid-solid and liquid-air surface of the film becomes important. Attraction forces can rupture the thin film and a dry patch is nucleated. Such a film is called a non-wetting film. When the interaction between the two film interfaces is repulsive the so-called disjoining pressure (see also p. 162) of the film, i.e. the pressure difference between the film and bulk liquid, is negative. In the other case of negative disjoining pressures, it may also be called conjoining pressure. 

The nucleation and growth of dry patches depends on the viscosity of the liquid. Redon et al. [74] found recently that ... 

Similar calculations can be done for families of unbounded sessile profiles. An unbounded sessile profile can be visualized physically as the configuration of an axisymmetric dry patch, or that of a meniscus around a cylindrical rod with its axis perpendicular... 

Experimental data on falling films have suggested that this theory provides a very conservative measurement of minimum wetting film thickness as it is derived from the principle of the stability of dry patches rather than the breakdown of films. In general, it was observed that films could be maintained down to close to an order of magnitude lower than the above equations suggest, although under very contrived conditions. 1 Based on this, it can be assumed that... 

Even if the flow is evenly distributed at the top of tower, channeling might still develop as the fluid trickles down. When two thin films converge they tend for form a thick film and a dry patch, which results in a reduction in contact area. So redistributors are placed every 10-15 feet along the length of the tower. 

FIGURE 15.145 Forces applying at an idealized dry patch on a liquid film (from Hartley and Murgatroyd [367], with permission from Elsevier Science). 

A significant problem in the use of falling film evaporation systems is that of maintaining the integrity of the liquid film. The minimum wetting rate is required to rewet any dry patches formed on the film and evaporators should be operated above this minimum condition. A review on this topic is given by Hewitt and Hall-Taylor [271]. Typical of the earlier work in this area is that of Hartley and Murgartroyd [367], who considered the situation illustrated in Fig. 15.145. 

A balance is made between the dynamic force arising from the change of direction of the liquid film and the surface tension force causing the dry patch to grow. When the dynamic force is greater than the surface tension force, then the dry patch will disappear. This allows the calculation of minimum wetting rate if the contact angle (see Fig. 15.145) is known. 

At smaller values of h, the maximum disappears, and the solution can be interpreted as a pure vapor phase thickening near the solid wall. The value h = ho such that Ps ho) = 0 corresponds to the nominal interface position on the dry surface in equilibrium with the bulk liquid. Clearly, the surface is not literally dry, as even on the nominally dry patches the density must be close to bulk liquid density under the specified conditions. At still smaller values of h (which may be also negative) Ps h) sharply decreases to large negative values, and the above approximation is no longer valid. 

Irregular flow and pressure during the mold-filling sequence can cause waviness, dry patches and overloading. Since many advanced RTM resins are solid or highly viscous at room temperature (and therefore all machine hardware must be thoroughly heated), a compact injection machine needs to have custom-fitted heated mantles on the reservoirs, pumps, fittings and mixers to eliminate any cold spots. 

Internal dry patches Trying to impregnate more than one layer of mat at a time presence of dry patches can be confirmed by tapping surface with a coin. 

Fig. 8.13. Schematic representation of conditions for the stability of thin liquid films. Thermal fluctuations of the free liquid-air interface are (a) diminished and (b) amplified, resulting in stable film or decomposition of the film in liquid drops and dry patches, respectively. Left schematic representation of the disjoining pressure middle uniform film and a sketch of a capillary wave with a disjoining pressure indicating the sign and the magnitude right resulting film profile.

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