We begin by discussing the methods for estimating the solid-liquid interfacial properties via thermodynamic measurements on bulk systems. We then discuss the contact angle on a uniform perfect solid surface where the Young equation is applicable. The observation that most surfaces are neither smooth nor uniform is addressed in Section X-5, where contact angle hysteresis is covered. Then we describe briefly the techniques used to measure contact angles and some results of these measurements. Section X-7 is taken up with a discussion of theories of contact angles and some practical empirical relationships.  [c.347]

Contact Angle Hysteresis  [c.355]

Although not fully understood, contact line hysteresis is generally attributed to surface roughness, surface heterogeneity, solution impurities adsorbing on the surface, or swelling, rearrangement or alteration of the surface by the solvent. The local tilting of a rough surface or the local variation in interfacial energies on a heterogeneous surface can cause the contact angle to vary. It is not yet clear whether, like other hysteretic phenomena (such as found in magnetism), contact angle hysteresis can be described by irreversible transitions or jumps between domains of equilibrium states [41]. Here we review some of the main features of heterogeneous or rough surfaces and their effect on contact angle measurements.  [c.355]

There have been numerous studies of heterogeneous surfaces [48-54]. Johnson and Dettre [48] and later Neumann and Good [49] showed that heterogeneous surfaces cause metastable equilibrium states of the system, allowing for multiple contact angles. An example of the magnitude and variability of contact angle hysteresis is shown in Fig. X-4 for titania slides coated with varying amounts of a surfactant. Qualitatively, the advancing angle is determined by the  [c.356]

Dettre and Johnson [37] (see also Good [64]) made calculations on a mathematical model consisting of sinusoidal grooves such as those in Fig. X-5 concentric with a spherical drop (i.e., gravity was neglected). Minimization of the surface free energy with the local contact angle defined by the Young equation led to a drop configuration such that the apparent angle d in Fig. X-5 was that given by Eq. X-31. The free energy contained barriers as the drop was deformed such that the liquid front moved over successive ridges. The presence of these small energy barriers suggested that hysteresis arose when the drop had insufficient vibrational energy to surmount them. Similar anal-  [c.358]

Lin et al. [70, 71] have modeled the effect of surface roughness on the dependence of contact angles on drop size. Using two geometric models, concentric rings of cones and concentric conical crevices, they find that the effects of roughness may obscure the influence of line tension on the drop size variation of contact angle. Conversely, the presence of line tension may account for some of the drop size dependence of measured hysteresis.  [c.359]

The third set of causes of contact angle hysteresis arise as a result of specific interactions between the liquid and solid phases. When a liquid traverses a solid, it may alter the molecular structure of the solid, thereby producing contact angle hysteresis. This occurs on polymer substrates where the solvent swells, solvates, or otherwise perturbs the surface [72,73]. One example is the large hysteresis of methylene iodide on agar gels (66-30°) [74]. This effect has been examined in some detail by Neumann and co-workers studying the effect of alkane swelling  [c.359]

Surfactant-coated surfaces may also rearrange on contact with a liquid as shown by Israelachvili and co-workers [77]. This mechanism helps to explain hysteresis occurring on otherwise smooth and homogeneous surfaces.  [c.360]

It is clear from our discussion of contact angle hysteresis that there is some degree of variability in reported contact angle values. The data collected in Table X-2, therefore, are intended mainly as a guide to the type of behavior to be expected. The older data comprise mainly results for refractory and relatively polar solids, while newer data are for polymeric surfaces.  [c.364]

There is appreciable contact angle hysteresis for many of the systems reported in Table X-2 the customary practice of reporting advancing angles has been followed.  [c.364]

The situation is complicated, however, because some of the drag on a skidding tire is due to the elastic hysteresis effect discussed in Section XII-2E. That is, asperities in the road surface produce a traveling depression in the tire with energy loss due to imperfect elasticity of the tire material. In fact, tires made of high-elastic hysteresis material will tend to show superior skid resistance and coefficient of friction.  [c.438]

Israelachvili and co-workers have addressed the effect of the state of the monolayer by studying a series of surfactant monolayers of variable surface density [53,54] with the surface force apparatus (described in Chapter VI). The lubricating behavior of surfactant monolayers can be characterized by three regimes illustrated in the pseudo-phase diagram for friction in Fig. XII-8. Solidlike materials slide along a shear plane via stick-slip or smooth motion. The friction is medium to low. Liquidlike materials are always close to equilibrium and the molecules have a rapid response time when sheared and friction is again low. Intermediate densities produce amorphous layers that have significant interdigitation between layers and yet slow relaxation times. This interpenetration produces adhesion hysteresis (see Section XII-8) and significant friction. As illustrated in Fig. XII-8, these regimes can be reached through variation of the surfactant surface coverage, temperature, load, velocity, or exposure to organic vapors.  [c.446]

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102].  [c.517]

Thus D(r) is given by the slope of the V versus P plot. The same distribution function can be calculated from an analysis of vapor adsorption data showing hysteresis due to capillary condensation (see Section XVII-16). Joyner and co-woikers [38] found that the two methods gave very similar results in the case of charcoal, as illustrated in Fig. XVI-2. See Refs. 36 and 39 for more recent such comparisons. There can be some question as to what the local contact angle is [31,40] an error here would shift the distribution curve.  [c.578]

As also noted in the preceding chapter, it is customary to divide adsorption into two broad classes, namely, physical adsorption and chemisorption. Physical adsorption equilibrium is very rapid in attainment (except when limited by mass transport rates in the gas phase or within a porous adsorbent) and is reversible, the adsorbate being removable without change by lowering the pressure (there may be hysteresis in the case of a porous solid). It is supposed that this type of adsorption occurs as a result of the same type of relatively nonspecific intermolecular forces that are responsible for the condensation of a vapor to a liquid, and in physical adsorption the heat of adsorption should be in the range of heats of condensation. Physical adsorption is usually important only for gases below their critical temperature, that is, for vapors.  [c.599]

Fig. XVll-19. Adsorption of CH4 on MgO(lOO) at 77.35 K. The vertical line locates each vertical step corresponds to the condensation of a monolayer. There was no hysteresis. Desorption points are shown as . (From Ref. 110.) Fig. XVll-19. Adsorption of CH4 on MgO(lOO) at 77.35 K. The vertical line locates each vertical step corresponds to the condensation of a monolayer. There was no hysteresis. Desorption points are shown as . (From Ref. 110.)
The question is not trivial such agreement is not assured in the case of systems showing hysteresis (see Section XVII-16), and it has been difficult to affirm it on rigorous thermodynamic grounds in the case of a heterogeneous surface.  [c.648]

Adsorption on Porous Solids—Hysteresis  [c.662]

Below the critical temperature of the adsorbate, adsorption is generally multilayer in type, and the presence of pores may have the effect not only of limiting the possible number of layers of adsorbate (see Eq. XVII-65) but also of introducing capillary condensation phenomena. A wide range of porous adsorbents is now involved and usually having a broad distribution of pore sizes and shapes, unlike the zeolites. The most general characteristic of such adsorption systems is that of hysteresis as illustrated in Fig. XVII-27 and, more gener-  [c.664]

Fig. XVII-28. Hysteresis loops in adsorption. Fig. XVII-28. Hysteresis loops in adsorption.
Explanations of hysteresis in capillary condensation probably begin in 1911 with Zsigmondy [198], who attributed the effect to contact angle hysteresis due to impurities this might account for behavior of the type shown in Fig. XVII-28a, but not in general for the many systems having retraceable closed hysteresis loops. Most of the early analyses and many current ones are in terms of a model representing the adsorbent as a bundle of various-sized capillaries. Cohan [199] suggested that the adsorption branch—curve abc in Fig. XVII-28Z)—represented increasingly thick film formation whose radius of curvature would be that of the capillary r, so that at each stage the radius of capillaries just filling would be given by the corresponding form of the Kelvin equation (Eq. III-19)  [c.665]

The section cd can be regarded as due to relatively large cone-shaped pores that would fill and empty without hysteresis. At the end of section cd, then, all pores should be filled, and the adsorbent should hold the same volume of any adsorbate. See Ref. 200 for a discussion of this conclusion, sometimes known as the Gurvitsch rule.  [c.666]

The phenomenon of contact angle hysteresis has been recognized and studied for a long time. Numerous researchers have studied its origin experimentally and theoretically [36, 37]. The general observation is that the contact angle measured for a liquid advancing across a surface exceeds that of one receding from the surface. An everyday example is found in the appearance of a raindrop moving down a windowpane or an inclined surface [38-40]. This difference, known as contact angle hysteresis, can be quite large, as much as 50° for water on mineral surfaces. This can be quite important in coating processes.  [c.355]

Generally, a contact line traversing a heterogeneous surface will become pinned to the patches, producing a lower contact angle (for a review of the theories of pinning, see Ref. 55). There are energy barriers to surpass as the contact line moves across these pinning points. The ability to cross these energy barriers and hence the magnitude of the contact angle hysteresis is very sensitive to ambient vibrations [56, 57], hence the variability in the literature. As a test of the forces pinning the contact line to heterogeneities, Nadkami and Garoff [58] measured the distortion of a contact line traversing a single 20-100-/im defect produced by melting a polymethylmethacrylate (6 = 90°) particle on a polystyrene (9 = 83°) surface. The contact line on a vertical Wilhelmy plate follows  [c.357]

There is a great need for model heterogeneous surfaces to study contact angle hysteresis. One completely random heterogeneous surface has been created very recently by Decker and Garoff [57], By coating a surface with a fluorinated surfactant then degrading it with ultraviolet illumination in the presence of water vapor, they produce a surface having randomly heterogeneous surface energy. This is an important advance in the ability to study hysteresis in a consistent manner. Self-assembled monolayers [44] represent another system of variable heterogeneity.  [c.358]

Another approach to the systematic study of contact angle hysteresis is to produce surfaces of known patterning. Studying capillary rise theoretically on a surface having vertical strips of alternating surface energy, Gaydos and Neumann [60] determined that the minimum patch size necessary to produce contact angle hysteresis was surprisingly large, approximately one micrometer. In measurements on a surface having vertical line defects. Marsh and Cazabat [61] found that the healing length from the defect to the unperturbed contact line varied linearly with the width of the vertical defect up to a saturation value when the defect is approximately 0.75 mm in width. When the strips are turned horizontally, Li and Neumann [62] conclude that the advancing angle will approach the minimum free energy state while the receding angle will be subject to vibrations and can attain one of many metastable states. This helps to explain the relative reproducibility of the advancing contact angle as compared to the receding angle. These studies illustrate the usefulness of models and experiments on surfaces having carefully controlled geometries.  [c.358]

More recently, emphasis has been given to adhesion between the polymer and substrate as a major explanation for friction [33], the other contribution being from elastic hysteresis (see Section XII-2E). The adhesion may be mostly due to van der Waals forces, but in some cases there is a contribution from electrostatic charging. Yethiraj and co-workers have carried out simulations of polymers combining Monte Carlo methods with density functional theory [34,35], They show segmental depletions and enhancements near solid walls, which may help provide a molecular explanation for friction and adhesion in polymers. Brochard-Wyart and de Gennes have shown that the stretching transition polymers attached to a surface will experience at a critical shear rate will result in significant slippage and a consequently reduced friction [36].  [c.442]

While friction does not correlate directly with adhesion (there are many examples of high-friction surfaces having low adhesion and vice versa), it appears to relate well to adhesion hysteresis. Adhesion hysteresis is the difference in adhesive energy measured on loading and unloading. Isrealachvili and co-workers have found a relationship between adhesion hysteresis and friction measured for model surfactant layers on molecularly smooth mica in a surface forces apparatus [33,34]. This is illustrated in Fig. XII-12, where the friction traces for two identical surfactant monolayers in air and decane vapors are shown with the adhesion load curves. In air there is significant adhesion hysteresis (unloading, yp > loading, 7 ) corresponding to measurable friction. They correlate the friction force F to move a distance D  [c.451]

Silicon surfactants enable water to wet hydrophobic surfaces while not inhibiting its tendency to wet hydrophilic surfaces. Such surfactants are called superspreaders if the addition of a small amount (-0.1%) allows water to spread into a thin wetting film on a hydrophobic surface within tens of seconds [24]. While most wetting studies involve microscopic observation of the spreading front, several groups have adapted the Wilhelmy plate (see Section II-6C) technique to measure the interfacial tension in wetting liquids or wetting tension [25, 26]. Ellipsometric studies [6, 8] and x-ray reflectivity [18] have provided more quantitative analysis of the layer structure. A problem in wetting arises due to contact angle hysteresis (see Section X-5). Usually it is the advancing angle that is important, but in the case of sheep dips and other such processes it is the receding angle as the animal is removed from the bath that impacts the degree of retention of the dip.  [c.467]

The foregoing analysis regards the porous solid as equivalent to a bundle of capillaries of various size, but apparently not very much error is introduced by the fact that such solids in reality consist of interconnected channels, provided that all pores are equally accessible (see Refs. 41, 42) by this it is meant that access to a given-sized pore must always be possible through pores that are as large or larger. An extreme example of the reverse situation occurs in an ink bottle pore, as illustrated in Fig. XVI-3. These are pores that are wider in the interior dian at the exit, so that mercury cannot enter until the pressure has risen to the value corresponding to the radius of the entrance capillary. Once this pressure is realized, however, the entire space fills, thus giving an erroneously high apparent pore volume for capillaries of that size. Such a situation should also lead to a hysteresis effect, that is, on reducing the pressure, mercury would leave the entrance capillary at the appropriate pressure, but the mercury in the ink-bottle part would be trapped. Indeed, there is often some hysteresis in that  [c.579]

Everett [209] has pointed out that the bundle of capillaries model can be outrageously wrong for real systems so that the results of the preceding type of analysis, while internally consistent, may not give more than the roughest kind of information about the real pore structure. One problem is that of ink bottle pores (Fig. XVI-3), which empty at the capillary vapor pressure of the access charmel but then discharge the contents of a larger cavity. Barrer et al. [210] have also discussed a variety of geometric situations in which filling and emptying paths are different. In a rather different perspective, Seri-Levy and Avnir [210a] have shown that hysteresis can occur with adsorption on a fractal surface due to pores that are highly irregular ink bottle in shape.  [c.667]

A concluding comment might be made on the temperature dependence of adsorption in such systems. One can show by setting up a piston and cylinder experiment that mechanical work must be lost (i.e., converted to heat) on carrying a hysteresis system through a cycle. An irreversible process is thus involved, and the entropy change in a small step will not in general be equal to q/T. As was pointed out by LaMer [220], this means that second-law equations such as Eq. XVII-107 no longer have a simple meaning. In hysteresis systems, of course, two sets of st values can be obtained, from the adsorption and from the desorption branches. These usually are not equal and neither  [c.668]

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a.  [c.669]

See pages that mention the term Hysteresis : [c.174]    [c.213]    [c.213]    [c.360]    [c.363]    [c.452]    [c.580]    [c.618]    [c.667]    [c.668]   
Physical chemistry of surfaces (0) -- [ c.0 ]

Engineering materials Ч.1 (2002) -- [ c.256 ]

Industrial ventilation design guidebook (2001) -- [ c.207 ]