Contact angle hysteresis due to surface roughness
By means of the calculus of variations it is shown that even on rough surfaces the Young equation applies and that the Wenzel equation relies on a false assumption. Contact angle hysteresis, i .e. the difference in the apparent advancing and receding angles, for homogeneous rough solid substrates is due to the local slope of the solid surface at the three phase contactline.
The maximum advancing and receding contact angles were measured using the tilted plate method of Wolfram. The thermodynamic Young angle is the arithmetic mean between these advancing and receding angles. The hysteresis was constant for a given sample of a solid substrate. Furthermore, we define the rugosity of the surface as a function of this hysteresis.
By means of Young angle measurements, adsorption of Triton X-100 solutions as a function of the concentration is investigated. The adsorption at apolar surfaces (paraffin) is equal to that at the air/liquid interface. For more polar substrates, such as polyethylene (PE), this adsorption is lower and is nearly zero for polar substrates such as polyethylene-terephtalate (PET).
By coronization of PE, the rugosity as well as the polarity increases. Also, the surface is electrically charged, but these charges disappear slowly in time. The strength of this paper is that we take into account both the advancing and the receding contact angles, whereas in the literature the experimental receding values are neglected.
Key wordsContact angle hysteresis s urface roughness Wenzel’s equation adsorption at the solid surface coronization surface properties
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