Angle Hysteresis

Probably the most apparent and common phenomenon observed in measurements of contact angle of water on a polymer surface is the discrepancy found between advancing and receding contact angles. This discrepancy, found in both sessile 

Intrinsic hysteresis is a direct result of surface configuration change, which occurs as a result of wetting the surface with water. The plates were purposely immersed to a deeper immersion depth in the second cycle to observe the extent of intrinsic hysteresis. Significant surface configuration change affects the calculated contact angles on immersion and emersion, which violates the assumption of 

In general, dynamic hysteresis is smaller with smaller cross-sectional area because the effect of buoyancy is less. Thus, the dynamic hysteresis is not a measure of surface characteristics. Intrinsic hysteresis is a measure of the surface dynamic stability of the surface. 

CORRELATION BETWEEN WILHELMY (DYNAMIC) AND SESSILE DROPLET (STATIC) CONTACT ANGLES 

The sessile droplet contact angle measurement is a simple and accurate method to obtain information pertinent to the surface energy of a sample. The Wilhelmy balance method, on the other hand, is a very useful method to investigate the surface dynamic aspect of a sample, which will be described in the following sections. The instability of some of plasma-treated polymer surface observed by the Wilhelmy balance method is also described in Chapter 30. 

FIGURE 2.7 The solid surface in (a) is turned to obtain a horizontal vapor (V)-liquid (L) interface in (b). (c) The advancing contact angle is shown, and (d) shows the receding contact angle (e) The measured values of Xare shown to illustrate the hysteresis.  

It has been argued previously that an ordinary glass surface is completely wet by water. A casual experiment will show this is not the case. Since glass has a high surface energy, it is wet by almost all substances, and consequently small amoimts 

FIGURE 2.8 The advancing and receding contact angles of water on titania-coated glass after treatment with trimethyloctadecylammonium chloride are shown as a function of coating treatments with 1.1% polydibutyl titanate. Reprinted with permission from Dettre and Johnson (1965). Copyright 1965 American Chemical Society. 

FIGURE 2.9 The advancing and receding contact angles of water are plotted against the carbon number of n-alkyl platinum for amines adsorbed (a) from aqueous solutions, (b) from melts, and (c) from cetane solutions. Data from Baker et al. (1952) and Shafrin and Zisman (1949, 1954). 

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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. 

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. 

Fig. 13. Molion of a liquid interline across a sharp edge, (a) Close-up of movement over the sharp edge show.s that the liquid must exhibit the appropriate contact angle against the surface in front of it in order to advance over the edge, (b) Contact angle hysteresis resulting from the sharp edge.

Fractional coverage (% Phase state y,v by JKR method (mJ/m-) Contact angle hysteresis (mJ/m ) ... 

Table 4 Contact Angle and Contact Angle Hysteresis of Modified Polyethylene ...

The effect of thermal aging on polyethylene and isotactic polypropylene have been studied by Konar et al. [49]. They used contact angle, contact angle hysteresis, and XPS to characterize the modified surfaces of the polymers. Hysteresis increased with aging temperature. In the case of polyethylene, thermal aging led to a significant increase in adhesion strength of polyethylene with aluminium, but the increase in the case of polypropylene was much less marked. 

It has been shown that contact angle hysteresis might arise as a result of inhomogeneities of the surface wetted by the liquid phase or surface roughness [70]. When surface roughness plays a considerable role, the observed contact angle may depend on the exact position of the contact line with respect to the microscopic or... 

This same technique should be helpful in understanding wetting properties important in the oil industry since wetting is very dependent on mineral surface energies. The use of contact angle hysteresis information may allow a better understanding of the effects of surface heterogeneities of natural mineral samples. The dynamic Wilhelmy plate technique is ideally suited for such experiments ... 

The MAMs were found to be closely packed. NEXAFS and FTIR spectroscopy studies revealed that the molecular tilting angles relative to the surface normal varied from 4° to 21° as a function of Ax. In wetting studies it was found that the highest water contact angles ( 131°) with the lowest contact angle hysteresis... 

However, no matter how rough the surface, the forces will be the same as those that exist between a solid and a liquid. The surface roughness may show contact angle hysteresis if one makes the drop move, but this will arise from other parameters (e.g., wetting and dewetting). Further, in practice, the surface roughness is not easy to define. A fractal approach has been used to achieve a better understanding (Feder, 1988 Birdi, 1993). 

What do you think are the causes of the contact angle hysteresis observed in this experiment ... 

An informative model for contact angle hysteresis is obtained by postulating the surface... 

FIG. 6.10 Contact angle hysteresis (a) weighing a meniscus in a Wilhelmy plate experiment versus the depth of immersion of the plate (b) both the advancing and receding contact angles are equal (c) 6a > dr. 

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