Influence of Hydrophobicity on Accuracy of Goniometry Method Used for Static Contact Angle Calculation

To obtain the applicability of goniometry method in contact angle measuring,the influence of hydrophobicity on its accuracy was systematically investigated.Water drop profiles,with contact angle ratings from 5°up to 175°,were numerically generated based on the Young-Laplace equation;meanwhile,by using contact angle meter and digital camera,15and 10images of water drop on surfaces of different materials,varying from nearly superhydrophilic to superhydrophobic,were acquired for each of the equipment.Moreover,the goniometry method was used to calculate the static contact angle of the obtained images by selected testers.The results indicated that,for one image,the calculation time varied from 6sto 14s,and its contact angles were different when it was calculated by different operators or in different calculations:the maximum difference between its contact angles obtained by different operators was 15.65°,and it was 28.10°between different calculations by a same operator.The contact angle calculation errors of the images acquired by the digital camera were larger than those of the images acquired by the contact angle meter.The errors by contact angle calculation increased approximately with real contact angle;the calculated results of the superhydrophobic surfaces were 2.40°~26.86°smaller than their real values,meaning that the goniometry method was not suitable for static contact angle calculations in this case.It is concluded that,if high accuracy in experiments is required,a contact-angle-calculation method that can choose an algorithm intelligently from three algorithms,i.e.the circle fitting algorithm,ellipse fitting algorithm,and axisymmetric drop shape analysis-profile( ADSA-P)algorithm,according to the surface hydrophobicity and drop volume,should be selected.Meanwhile,if the requirement of accuracy is low and the number of drop images needed to be processed is small,the goniometry method can be selected to calculate the static contact angle,and at the same time,skilled operators should be selected.