Spreading pressure contact angle

We also describe the spreading of a thin surfactant laden aqueous film on a hydrophilic solid, i.e., one in which the dynamic contact angle is small. In such a case, the osmotic pressure gradient generated by the nonuniform distribution of surfactant micelles in the liquid film can drive fhe spreading process. The mofivation for this study comes from the need to understand the detergent action involved in the removal of an oily soil from a soiled surface. This paper presents an overview of our recent work. 

If the liquid resting on a solid is volatile, it can distribute itself along the surface of the solid through the vapour thus Hardy6 found that only those acids and alcohols which have a sensible vapour pressure distribute themselves along the surface, from a drop in one spot, so as to lubricate it in any reasonable time. It does not appear to be necessary, for this kind of spreading to occur to some extent, that the contact angle should be zero it was found that the coefficient of friction of paraffin wax was... 

Techniques for spreading monolayers of polar long chain compounds on mercury in a Langmuir type film balance, and for measuring their surface area-pressure properties, have been described by one of the present authors (3). Using these techniques, it has proved possible to measure continuously the change in contact angle of a water droplet superposed on the monolayer, as the film pressure is controllably varied. This has now been done for monolayers of the normal C12-C20 fatty acids and the normal primary Ci4-Ci8 alcohols on the mercury substrate. 

Becher and Becher (2) measured the spreading pressure, tt, of a series of surfactants on plant and synthetic surfaces. The surfactants were characterized by HLB (hydrophile-lipophile balance) values (3) similar to those of Jansen s adjuvants. The relationship between the spreading pressure and the contact angle can be expressed as tt = yL cos 0, where yL is the surface tension of the liquid and 6 is the contact angle between the liquid and a solid surface. Becher and Becher demonstrated that surfactants in their series which exhibited maximum spreading pres-... 

Figure 10 presents the interface shape of the rivulet for wall superheat as 0.5 K and Re = 2.5. Here also presented the data on pressure in liquid and heat flux density in rivulet cross-section. The intensive liquid evaporation in near contact line region causes the interface deformation. As a result the transversal pressure gradient creates the capillarity induced liquid cross flow in direction to contact line. Finally the balance of evaporated liquid and been bring by capillarity is established. This balance defines the interface shape and apparent contact angle value.For the inertia flow model, the solution is obtained from a non-stationary system of equations, i.e., it is time-dependable. In this case the disturbances in flow interface can create the wave flow patterns. The solutions of unsteady state liquid spreading on heat transfer surface without and with evaporation are presented on Fig. 11. When the evaporation is not included (for zero wall superheat) the wave pattern appears on the interface. When the evaporation includes, the apparent contact angle increase immediately and deform the interface. It causes the wave suppression due to increasing of the film curvature.

Figure 5.7. Attachment of spherical particles or droplets to (solid) surfaces, (a) h, no interaction (b) h of colloidal range interaction determined by the disjoining pressure across phase 2 (c) attachment (d) attachment of a rectangular particle (e) spreading of an attached drop until the contact angle is a (f) droplet deforms but does not wet (a = 180°) (g) complete wetting (h) partial wetting on a completely wetting film.

Water absorption by porous solids depends on the water-solid contact angle and on the liquid surface tension. The spreading of water in fabrics is the result of wetting the fiber surface, penetration into the fibers, and capillary pressure [20], The wetting of yarns depends on their surface energy and the interfiber space. 

In addition, if the contact angle is zero, by combining Equations (424) and (496), one obtains the surface (spreading) pressure as... 

For negligible spreading pressure (7rsv), by combining Equation (708) with the Young-Dupr equation, the general contact angle equation is obtained ... 

FIGURE 10.24 Contact angles (6) of liquids at an A-S surface (upper row) and at an A-W surface (lower row). The pictures of the drops are cross sections through the largest diameter. A is air, L is liquid, O is oil, S is solid, W is water (or an aqueous solution). Numbers are interfacial tensions (y) in mN m-1.17S is spreading pressure. The scale varies among the panels. 

FIGURE 13.19 Possible mechanisms involved in the rupture of aqueous foam films of thickness <5 induced by hydrophobic particles 1, 2 and 3 indicate subsequent stages, (a) Solid particle (b) and (c) oil droplet (d) oil droplet or composite particle. Thick arrows indicate spreading of oil or surfactant. A is air, W is water 6 is contact angle as measured in the water phase i] is viscosity of the water phase (subscript W) or the oil phase (subscript O) J7S is spreading pressure. 

Two methods can be used for the assessment of the 7s of divided solids contact-angle measurements and adsorption processes. The drawbacks of the contact-angle measurements are associated with surface roughness of the samples. As for the adsorption process, determination of the components of the surface free energy of the solid is based on interpretation of adsorption isotherms, either complete (calculation from spreading pressures) or only from the first linear part of the isotherm. In this respect, inverse gas chromatography (IGC), which appears to be the technique of choice (17), was extensively used in this study. 

Contact angle < 90° (Fig. 10.23a). This is the most interesting case. Upon melting, the liquid will spread and pull the plates closer as a result of two forces. The first component is due to negative pressure that develops in the meniscus as a result of its negative curvature, or... 

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