Contact angle drop size effect

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. 

When plotted as a function of drop size (Fig. 9), the contact angle was found to decrease with increasing drop height. A different analysis of these data was performed in the original paper. In that case the maximum slope near the drop edge was used, as well as a direct inversion of the droplet shape. The data could be fit to an empirical 1/z function. In the present analysis we use the method of the effective contact angles defined earlier, together with Eq. (18). For the Flamaker constant A, we calculated a value of approximately -2 X 10 ° J. However, the best fit to Eq. (18) is for a pure exponential decay of the form ... 

Some of the commonly used techniques for measuring contact angle [215, 216, 217] are the sessile drop method, captive bubble method and Wilhelmy plate method. These techniques have been extensively used and well documented for characterisation of modified PE surfaces [218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230] for various applications. Whitesides et al. [231 ] studied the wetting of flame-treated polyethylene film having ionisable organic acids and bases at the polymer-water interface. The effect of the size of substituted alkyl groups in amide and ester moieties on the surface hydrophilicity was also studied [232]. The biocompatibility of the polyethylene film surface modified with various water-soluble polymers was evaluated using the same technique [233]. The surface properties of hy-perbranched polymers have been very recently reported [234]. 

The most commonly used is the sessile drop method, and most of the available literature data are for smooth, macroscopic, solid surfaces. Some special problems arise when one is interested in the wettability of finely divided solids. The small size of the particles makes measurement difficult, but, more importantly, surface roughness and particle shape can have a significant effect on the contact angle. 

J. Drehch, J. D. Miller, and R. J. Good, The effect of drop (bubble) size on advancing and receding contact angles for heterogeneous and rough sohd surfaces as observed with sessile-drop and captive-bubble techniques, J. Colloid Interface Set, 179,37-50 (1996). 

Fig. 2.13 Effect of drop size on the contact angle and sliding angle of hot polyethylene droplets on (a) a hydrophobic sol gel coating and (b) a fluoiinated self-assembled-monolayer on silicon wafer...

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