Hysteresis, contact angle

Advancing and receding angles are clearly shown at the front and the back of the drop on the tilted surface. 

Due to the gravity field (mg sin adl, m = mass of the drop, g = acceleration due to gravity), the drop will shde until the difference between the work of dewetting and wetting balances the gravity force. 

Hysteresis can be demonstrated by measuring the force on a plate that is continuously immersed in the liquid. When the plate is immersed, the force will decrease due to buoyancy. When there is no contact angle hysteresis, the relationship be- 

Equation (2.28) suggests that the contact angle of a water droplet on a solid surface is a constant determined only by the surface tensions that are in turn dependent only on the material properties involved. This is counterintuitive. In fact, it is contrary to our experience from daily life  

Advancing and receding contact angles. When a substrate is tilted, a water droplet on it will show different contact angles at different points on the boundary. 

Controlling pressure difference across a water-to-air interface may vary the radius of curvature of the water meniscus from a negative to a positive value. Source Adapted from Cheng, D.M., Y.J.P. Choe, and H.R. Jiang. 2008. Journal of Microelectromechanical Systems, 17(4), 962-973. With permission.) 

Although the existence of contact angle hysteresis has been recognized for at least 100 years, the root of the evU has not always been understood. In addition to the physicochemical adsorption process already mentioned, which leads to differences in advancing and receding contact angles, it is recognized 

FIGURE 17.4. The cxjntact angle is a relatively easy piece of information to obtain (with a httle practice). The more common systems of measurement include the sessile drop (a), the captive bubble (b), the sessile bubble (c), and the tilting plate (d). 

The Effects of Surface Roughness ou Coutact Augles aud Wettiug 

The theoretical discussion of contact angle and wetting to this point has assumed implicitly that the solid surface in question is a smooth, ideal plane. In fact, of course, very few solid surfaces even begin to approach such a state. The finest polished glass surface, for example, will usually have asperities of 5 nm or more. Commonly encountered polished surfaces, will be much rougher by factors of 10-1000. The earliest, and still most useful, quantitative attempt to correlate the observed contact angle of a liquid on a solid with the surface roughness is the Wenzel relationship which proposes a thermodynamic relationship such that 

Equation (17.3) may be taken as a fundamental definition of the effect of surface roughness on wetting and spreading phenomena. 

We also suppose the pressures across the liquid-vapor interface are equal. From Equation (4.1) the Hquid-vapor interface has zero mean curvature. That is  

The contact line, CL, of the Hquid-vapor interface and the solid surface is given by 

From (4.2) the local contact angle at the contact line is given by cos0y for (x,y, 0) e CL. 

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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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