Drying surface tension

Release stiction has been recognized as a problem since the late 1980s, especially for surface micromachined structures. During drying, surface tension from the liquid-vapor interface causes a downward force on the structural layer. If the layer touches the substrate, it is prone to stick onto the surface. It is hypothesized that etch products and/or contaminants in the rinse water can then precipitate out of solution during drying and cause a bond that is stronger (e.g., a chemical bond)... 

If liquids are involved in the transfer, for example, the particles are suspended in a liquid and then this liquid is deposited onto the grid and allowed to dry, surface tension effects can also massively disturb the homogeneity of deposit. One way to avoid this is to remove the liquid without allowing it to dry. For example, the protein nanofibers in the SPM image of Figure 9.15b deposited themselves voluntarily when a drop of solution containing them was placed in contact with the mica. The drop was then removed with a pipette and the surface was further washed with deposited water droplets and then removed intact. In this way molecule/surface affinities have been used [81] to make transfers from liquid to surface both predictable and homogeneous. 

General hydrodynamic theory for liquid penetrant testing (PT) has been worked out in [1], Basic principles of the theory were described in details in [2,3], This theory enables, for example, to calculate the minimum crack s width that can be detected by prescribed product family (penetrant, excess penetrant remover and developer), when dry powder is used as the developer. One needs for that such characteristics as surface tension of penetrant a and some characteristics of developer s layer, thickness h, effective radius of pores and porosity TI. One more characteristic is the residual depth of defect s filling with penetrant before the application of a developer. The methods for experimental determination of these characteristics were worked out in [4]. 

A modification of the foregoing procedure is to suspend the plate so that it is partly immersed and to determine from the dry and immersed weights the meniscus weight. The procedure is especially useful in the study of surface adsorption or of monolayers, where a change in surface tension is to be measured. This application is discussed in some detail by Gaines [57]. Equation 11-28 also applies to a wire or fiber [58]. 

The circular cross section of the polymer blobs does not prove that the polymer existed in solution as a tangled coil (although this is the case). The shape displayed by the particles in the photograph is probably due in part to surface tension occurring during the drying of the sample. 

The equation above suggests that one approach would be to use a pore Hquid that has a low surface tension. Indeed, two-step acid—base or acid—acid catalyzed sHica gels have been made, aged in ethanol or water, washed with various aprotic solvents, and finally evaporatively dried at 323 K for 48 hours and then at 383 K for 48 hours (43). The aprotic solvents used and their corresponding surface tension in N/m at room temperature (shown in... 

Fig. 3. Two-dimensional schematic illustrating the distribution of Hquid between the Plateau borders and the films separating three adjacent gas bubbles. The radius of curvature r of the interface at the Plateau border depends on the Hquid content and the competition between surface tension and interfacial forces, (a) Flat films and highly curved borders occur for dry foams with strong interfacial forces, (b) Nearly spherical bubbles occur for wet foams where...

Inks. The main components of the inks ate typically water, colorants, and humectants. Additives ate used to control drying time, waterfastness, lightfastness, and consistency of drop formation. Water is an excellent vehicle for ink jet because of its high surface tension and safety in all environments. 

Humectants and low vapor pressure cosolvents are added to inhibit drying of ink in the no22les. Surfactants or cosolvents that lower surface tension are added to promote absorption of ink vehicle by the paper and to prevent bleed. For improvements in durabiUty, additional materials such as film-forming polymers have been added. Ink developments are providing ink-jet prints with improved lightfastness, waterfastness, and durabiUty. As a result, such prints are beginning to rival the quaUty of electrophotographic prints. 

The excellent chemical resistance and physical properties of PVA resins have resulted in broad industrial use. The polymer is an excellent adhesive and possesses solvent-, oil-, and grease-resistant properties matched by few other polymers. Poly(vinyl alcohol) films exhibit high tensile strength, abrasion resistance, and oxygen barrier properties which, under dry conditions, are superior to those of any other known polymer. The polymer s low surface tension provides for excellent emulsification and protective coUoid properties. 

Supercritical and Freeze Drying. To eliminate surface tension related drying stresses in fine pore materials such as gels, ware can be heated in an autoclave until the Hquid becomes a supercritical fluid, after which drying can be accompHshed by isothermal depressurization to remove the fluid (45,69,72) (see Supercritical fluid). In materials that are heat sensitive, the ware can be frozen and the frozen Hquid can be removed by sublimation (45,69). 

Capillarity. The outer surface of porous material has pore entrances of various sizes. As surface Hquid is evaporated during constant rate drying, a meniscus forms across each pore entrance and interfacial forces are set up between the Hquid and material. These forces may draw Hquid from the interior to the surface. The tendency of Hquid to rise in porous material is caused pardy by Hquid surface tension. Surface tension is defined as the work needed to increase a Hquid s surface area by one square meter and has the units J/m. The pressure increase caused by surface tension is related to pore size ... 

E] Gas absorption aud desorption from water aud organics plus vaporization of pure liquids for Raschig riugs, saddles, spheres, aud rods, dp = nominal pacldug size, Cp = dry pacldug surface area/volume, = wetted pacldug surface area/volume. Equations are dimensionally consistent, so any set of consistent units can be used. <3 = surface tension, dynes/cm. 

To check the efficacy of grease removal, the alkali solution is rinsed away or neutralised by dipping in dilute acid. If, after removal from the acid, the draining metal surface remains wetted evenly all over for 30-60 s (or until it dries by evaporation), hydrophobic soils have been removed. Traces of grease cause the surface to de-wet, and surface tension draws the water into separate droplets. This is the water-break test. Traces of grease which remain when the work is plated do not prevent electrodeposition, but are detrimental to adhesion and corrosion resistance. 

In air-water flow the different flow patterns occur simultaneously in different micro-channels. Although the gas core may occupy almost the entire cross-section of the triangular channel, making the side walls partially dry, the liquid phase always remained continuous due to the fact that the liquid was drawn into the triangular corners by surface tension. 

---