Liquid spreading

Consider a drop of a pure Hquid resting on a plane solid surface of area ocq, such that the liquid covers an area a, the remainder of the space being filled by the vapour of the liquid. Let the specific 

Therefore, neglecting changes in gravitational potential energy, relation [7.18] is the condition for the liquid to spread completely over the whole of the solid surface. Should  

Since the liquid-vapour interface must have 7ly positive, there is a third possibility  

for a given solid-liquid pair, 9 is greater than tt/2 the liquid is said to be non-wetting with respect to the solid when n/2 0 0 the liquid is said to be partially wetting and when 0=0 the liquid is said to wet the solid. In practice, 9 is zero only when the liquid has already wetted the surface, so that a very thin layer of liquid is adsorbed to the surface. 

The angle of contact is the only directly measureable quantity which gives information about interfacial free energies. One method of determining 9 is to take a flat plate of the appropriate solid and partially immerse it in the required liquid. The plate is then rotated about an axis, lying in the plate and parallel to the liquid surface, until the liquid meets the solid on one surface without curvature of the liquid surface. 9 is then the angle of the plate from the horizontal on that side. Another method uses the sessile drop, discussed in section 7.11. 

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The terms wetting and nonwetting as employed in various practical situations tend to be defined in terms of the effect desired. Usually, however, wetting means that the contact angle between a liquid and a solid is zero or so close to zero that the liquid spreads over the solid easily, and nonwetting means that the angle is greater than 90° so that the liquid tends to ball up and run off the surface easily. 

A drop of liquid spreads over a horizontal surface. Obtain dimensionless groups of the variables which will influence the rale at which the liquid spreads. 

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. 

Water-sensitive papers are readily available in most countries and provide a convenient system for visually assessing spray drift performance. These papers are coated with bromoethyl blue, which turns from yellow to blue when contacted with water. " However, since any water can cause this change in color, care needs to be taken to prevent accidental exposure to sources of water other than the pesticide. Such cards do not work well under humid conditions, and are not appropriate for sampling droplets with diameter below 15 qm. Quantitative estimates of droplet size distributions must take account of the exponential increase in droplet volume as the droplet diameter increases. As droplets strike the paper, the liquid spreads over the surface and leaves a stain with a size that is dependent on the volume of the droplet. The apparent droplet size will be greater for large droplets than for small droplets, and the size determination must be corrected to avoid bias. 

Complete wetting of a solid is only possible if a drop of the liquid spreads spontaneously at the surface, i.e. for 9 = 0 or cos 9=1. The limiting value cos 6 = 1 is a constant for a solid and is named critical surface tension of a solid y... Therefore, only liquids with yl < Vc have the ability to spontaneously spread on surfaces and wet them completely. Tab. 4.2 gives an overview of critical surface tension values of different polymer surfaces [40]. From these data it can be concluded that polytetrafluoroethylene surfaces can only be wetted by specific surfactants with a very low surface tension, e.g. fluoro surfactants. 

Werner complex See coordination compound. ver-nor, kam,picks ) wet aahing org chem The conversion of an organic compound into ash (decomposition) by treating the compound with nitric or sulfuric acid. wet ash-ii) wettability chem The ability of any solid surface to be wetted when in contact with a liquid that is, the surface tension of the liquid is reduced so that the liquid spreads over the surface.. wed-a bil-od-e ... 

The contact angle 0 of a liquid on a solid is the reflection of its wetting power. If 0 = 0, the liquid spreads freely on the solid surface and wets it. The relationship between the contact angle of a liquid on a solid and the surface tensions in the presence of saturated vapor of the liquid, is given by the Young s equation, Eq. (2). 

Liquid Spreading coefficient Liquid Spreading coefficient... 

More fundamental objections to Young s equation center on the issue of whether the surface is in a true state of thermodynamic equilibrium. In short, it may be argued that the liquid surface exerts a force perpendicular to the solid surface, yLV sin 6. On deformable solids a ridge is produced at the perimeter of a drop on harder solids the stress is not sufficient to cause deformation of the surface. This is the heart of the objection. Is it correct to assume that a surface under this stress is thermodynamically the same as the idealized surface that is free from stress Clearly, the troublesome stress component is absent only when = 0, in which case the liquid spreads freely over the surface, and Figure 6.6 becomes meaningless. 

Young s equation is the basis for a quantitative description of wetting phenomena. If a drop of a liquid is placed on a solid surface there are two possibilities the liquid spreads on the surface completely (contact angle 0 = 0°) or a finite contact angle is established.1 In the second case a three-phase contact line — also called wetting line — is formed. At this line three phases are in contact the solid, the liquid, and the vapor (Fig. 7.1). Young s equation relates the contact angle to the interfacial tensions 75, 7l, and 7sl [222,223] ... 

Figure 7.15 Liquid spreading on a solid surface with a precursor film. ...

Wetting Young s modulus The process by which a liquid spreads over a solid surface. The modulus where stress/strain is both a ratio and is constant. (This is only at very low elongations in elastomers.)... 

Bearing in mind the difficulties that may arise it is important to have available reliable and easily understood tests for assessing the level of treatment. Since an effect of treatment is to increase the wettability of the surface the most simple test is to apply a liquid of low surface tension, in order to compare treated with untreated film. If the liquid spreads uniformly the surface may be judged suitable for printing if, on the other hand, it coalesces into discrete droplets, the wettability is insufficient. There are available for such tests commercial surface tension fluids, which simply are applied to the polyolefin and their behaviour noted. 

The angle of rotation and the element height therefore affect the extent of vapor and liquid spread in a structured packing. For this reason, element height is relatively short (typically 6 to 12 in) and the angle of rotation is around 90°. 

The uniformity of liquid spread in structured packings strongly depends on (67,146) texture of surface, presence and size of perforations, connection of packing elements in packing layers, and presence of wall wipers. 

Figure 9.3 Liquid spread profile in a packed column (20-in-lD column, packed with 6 ft of 2-in Pall rings, water study. Data from P. J, Hoek, Ph.D. thesis. University of Delft, The Netherlands, 1983.)...

The factors affecting the development of wall flow are the same as those that affect the liquid spread. These include height into the bed, packing type, packing size, and liquid flow rate (66,140). 

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