Use, Abuse, Misuse and Proper use of Contact Angles: A Critical Review

What is presented here is not a review in the ordinary sense, but instead a very personal way to look at some aspects of the contact angle topic, emphasizing the many misleading concepts presented in the scientific literature, even in highly cited papers. It is also emphasized that such misleading ideas may lead to some "new discoveries" which, particularly in the extremely competitive world of modern research, may appear more an opportunity for citation than a real breakthrough. We proceed by examining some cases, particularly significant because of the number of their citations in the literature, which illustrate various misconceptions encompassing the confusion between sliding angle and contact angle hysteresis, the inaccurate definition of superhydrophobicity, the uncriticized occurrence of meaningless negative contact angles, the so-called petal effect presented as a novel phenomenon different from common composite surfaces, or the lotus leaf described as a chemically hydrophilic surface with a superhydrophobic behaviour attributable to only its geometrical structure. An attempt is made not only to criticize some misleading ideas, definitions and proposals, but also to suggest the corresponding correct approach.